lock-free, concurrent, key/value index with efficient memory-mapped persistence and fast transient storage models
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readme.md

CL-CTRIE

CL-CTRIE is a common-lisp implementation of the CTrie unordered map data-structure described in the paper 'Concurrent Tries with Efficient Non-Blocking Snapshots, (c) ACM 2-25-2012' by Prokopec, Bronson, Bagwell, and Odersky.

Overview

A brief overview of general ctrie concepts and existing implementations is available through its page on wikipedia, of which the following is a brief excerpt:

The Ctrie data structure is a non-blocking concurrent hash array mapped trie based on single-word compare-and-swap instructions in a shared-memory system. It supports concurrent LOOKUP, INSERT and REMOVE operations. Just like the hash array mapped trie, it uses the entire 32-bit space for hash values thus having low risk of hashcode collisions... Ctries have been shown to be comparable in performance with concurrent skip lists, concurrent hash tables and similar data structures in terms of the lookup operation... However, they are far more scalable than most concurrent hash tables where the insertions are concerned. Most concurrent hash tables are bad at conserving memory - when the keys are removed from the hash table, the underlying array is not [reduced in size]. Ctries have the property that the allocated memory is always a function of only the current number of keys in the data-structure. Ctries have logarithmic complexity bounds of the basic operations... with a low constant factor due to [large dispersal ratio, (32^n arcs at level n)]. Ctries support a lock-free, linearizable, constant-time SNAPSHOT operation... This is a breakthrough in concurrent data-structure design, since other existing concurrent data-structures do not support snapshots. [This provides the means to support features such as] lock-free, linearizable iterator, size and clear operations. [This is superior to other] existing concurrent data-structures [which require the use of global locks [for exclusive, blocking semantics for update access] permitting... [no concurrent readers or writers] during any [update, insert, remove or other modifications]. In particular, Ctries have an O(1) ITERATOR creation operation, O(1) CLEAR operation, O(1) DUPLICATE operation and an amortized O(log n) operation for SIZE-RETRIEVAL.

Platform

Currently the lisp platform supported by cl-ctrie is SBCL version 1.0.55 or greater hosted on x86/x86-64 architecture. Support could easily be entended to include other common-lisp implementations that offer atomic compare-and-swap functionality, notably LispWorks 5.x/6.x, which is also well instrumented with lock-free, atomic primitives, although this is not necessarily a high priority for the initial development cycle.

Status

All tests should succeed; parallelism has been tested for 1, 2, 4, and 8 threads on an 8-core system (Dual Intel(R) Xeon(R) CPU E5462 @ 2.80GHz), SBCL version 1.0.57.56-2273f3a, and Mac OS X Server version 10.6.8.

Starting test run on Wednesday, September 5, 2012 09:07:32 AM EDT
-----------------------------------------------------------------

CHECK-ALET-FSM: 7 assertions passed, 0 failed.
CHECK-ATOMIC-CLEAR: 6 assertions passed, 0 failed.
CHECK-ATOMIC-UPDATE: 1 assertions passed, 0 failed.
CHECK-BULK-INSERT/DROP: 1048579 assertions passed, 0 failed.
CHECK-BULK-INSERT/LOOKUP: 1048578 assertions passed, 0 failed.
CHECK-BYTE-VECTOR-HEX-STRING-ROUNDRIP: 10 assertions passed, 0 failed.
CHECK-CATCH-CASE: 128 assertions passed, 0 failed.
CHECK-CTRIE-SMOKE-TEST: 31 assertions passed, 0 failed.
CHECK-DEPTH-AND-SIMPLE-EXTENSION: 8 assertions passed, 0 failed.
CHECK-EXTENSION/RETRACTION/LNODE-CHAINING: 14 assertions passed, 0 failed.
CHECK-FBIND: 3 assertions passed, 0 failed.
CHECK-FLAG-ARC-POSITION: 165 assertions passed, 0 failed.
CHECK-FLAG-COMPUTATION: 12 assertions passed, 0 failed.
CHECK-LNODE-INSERTED/REMOVED: 174 assertions passed, 0 failed.
CHECK-LNODE-LENGTH/ENLIST: 8 assertions passed, 0 failed.
CHECK-LNODE-SEARCH: 4 assertions passed, 0 failed.
CHECK-PARALLEL-INSERT-PARALLEL-DROP: 4194316 assertions passed, 0 failed.
CHECK-PARALLEL-INSERT-PARALLEL-LOOKUP: 4194312 assertions passed, 0 failed.
CHECK-SIMPLE-INSERT/LOOKUP: 170 assertions passed, 0 failed.
CHECK-SIMPLE-INSERT/LOOKUP/DROP: 255 assertions passed, 0 failed.
CHECK-TABLE-ABSTRACTION-FIXTURES: 18 assertions passed, 0 failed.
CHECK-TIMING-COLLECTION-FIXTURES: 18 assertions passed, 0 failed.

TOTAL: 10,486,817 assertions passed, 0 failed, 0 execution errors.

Ideosyncrasies

Perhaps most ideosyncrasies of this common-lisp ctrie implementation as compared with the original, written in Scala, result from the efforts I have taken to, where feasible, adopt an approach emphasizing a more functional oriented decomposition of the algortithm, written in a manner that is more closely representative of ideomatic, common-lisp coding style. For example, rather than expose a general purpose GCAS and RDCSS api, these protocols are incorporated into ctrie-specific abstractions. For GCAS the exposed api includes: INODE-READ INODE-MUTATE and INODE-COMMIT and for RDCSS: ROOT-NODE-ACCESS ROOT-NODE-REPLACE and ROOT-NODE-COMMIT. The liberties I have taken have the intended benefit of providing an interface that is much easier to digest, understand, remember, and work with (at least for me) than direct exposure in imperative style of the intricate mechanations that underlie the ctrie algorithm. On the other hand, the further one strays from a direct translation of the original (verified) ctrie implementation, the greater the likelihood of introducing bugs into an environment (lock-free concurrent data structure development) in which bugs can be extremely subtle and notoriously difficult to detect. I have attempted to strike an appropriate balance between these conflicting concerns, and I intend to mitigate the risk, at least in part, through continued development of an extensive arsenal of regression tests and benchmarking facilities.

In addition, there are a few differences in the feature set that is provided -- such as a suite of mapping operators in leiu of a Java-style Iterator. For the most part I expect that these changes will be preferable to developers accustomed to a more 'lispy' coding style.

For additional insight into the specifics unique to this ctrie implementation, an abbreviated reference to a number of internal details may be found in the section [Internal Reference](Internal Reference) of this document, or, of course, by referring to the comprehensive documentation that is provided as part of this distribution.

Source Files

The following outline provides a general description of the constituent source files of the cl-ctrie repository and their respective purpose.

Required

The following files are the core sources currently necessary for correct ctrie operation.

  • ctrie-package.lisp: Package Definition
  • ctrie.lisp: Structural Implementation
  • ctrie-util.lisp: Supporting Utilities

Supplemental

The following files define extended functionality, management, and analysis facilities supporting the development of CL-CTRIE.

  • ctrie-cas.lisp: SBCL CAS extensions
  • ctrie-lambda.lisp: Experiments with Stateful Protocols
  • ctrie-doc.lisp: Automated Documentation Support
  • ctrie-test.lisp: Test and Performance Measurement

Performance

To date, the principal concern has been to fully achieve a working and robust implementation of the CTRIE algorithm, with only the occaisional profiling run from time to time in order to at least get a general sense of where the hot-spots are and peace of mind that there are at least no glaring bottlenecks that seem out of hand. A basic profiling exercise of the following activities is reported below.

1. construct a new ctrie instance
2. perform one-million insertions of (fixnum) key/value pairs
3. perform one-million lookups verifying retrieval of every value
4. perform one-million removals verifying proper contraction of the structure
5. verify the resulting ctrie is identical to when initially created.
  seconds  |     gc     |    consed   |    calls   |  sec/call  |  name  
--------------------------------------------------------------
     1.331 |      1.001 | 332,691,664 |  2,198,622 |   0.000001 | MAKE-CNODE
     1.123 |      0.000 |       1,904 |  3,179,578 |   0.000000 | CTHASH
     0.871 |      0.000 |      44,608 |  5,344,350 |   0.000000 | ROOT-NODE-ACCESS
     0.847 |      0.182 |  69,549,200 |  2,198,622 |   0.000000 | %MAKE-CNODE
     0.820 |      0.000 |         576 | 12,582,934 |   0.000000 | FLAG
     0.796 |      0.000 |         352 | 12,582,934 |   0.000000 | FLAG-ARC-POSITION
     0.611 |      0.000 |         832 |  2,130,976 |   0.000000 | CTEQUAL
     0.551 |      0.000 |      33,600 | 12,549,110 |   0.000000 | FLAG-PRESENT-P
     0.529 |      0.000 |         720 |  5,344,350 |   0.000000 | FIND-CTRIE-ROOT
     0.496 |      0.044 | 108,904,336 |  2,198,622 |   0.000000 | MAKE-REF
     0.295 |      0.102 |  33,284,592 |  1,048,576 |   0.000000 | MAKE-SNODE
     0.243 |      0.000 |         144 |  2,198,622 |   0.000000 | CTSTAMP
     0.225 |      0.000 |         256 |  2,097,152 |   0.000000 | LEAF-NODE-KEY
     0.216 |      0.118 |  32,803,552 |  1,048,576 |   0.000000 | CNODE-EXTENDED
     0.189 |      0.000 |      32,848 |  2,097,152 |   0.000000 | LEAF-NODE-VALUE
     0.146 |      0.000 |       1,872 |  1,048,576 |   0.000000 | SNODE
     0.018 |      0.000 |           0 |     33,824 |   0.000001 | MAKE-INODE
     0.016 |      0.000 |           0 |     34,257 |   0.000000 | RESURRECT
     0.007 |      0.000 |   1,048,304 |     33,824 |   0.000000 | MAKE-TNODE
     0.000 |      0.127 |  50,427,456 |  1,048,576 |   0.000000 | CTRIE-GET
     0.000 |      0.000 |           0 |          1 |   0.000000 | CTRIE-SIZE
     0.000 |      0.000 |       5,104 |  1,048,576 |   0.000000 | CTRIE-PUT
     0.000 |      0.000 |           0 |          1 |   0.000000 | CTRIE-EMPTY-P
     0.000 |      0.000 |           0 |          1 |   0.000000 | CTRIE-MAP
     0.000 |      0.000 |      42,544 |  1,048,602 |   0.000000 | CTRIE-DROP
     0.000 |      0.000 |           0 |          1 |   0.000000 | CTRIE-MAP-KEYS
     0.000 |      0.000 |           0 |  1,116,226 |   0.000000 | MAP-NODE
     0.000 |      0.000 |           0 |     33,824 |   0.000000 | ENTOMB
     0.000 |      0.000 |           0 |     67,620 |   0.000000 | CNODE-UPDATED
     0.000 |      0.000 |     268,496 |  4,160,480 |   0.000000 | %INSERT
     0.000 |      0.000 |      39,024 |  2,164,798 |   0.000000 | INODE-COMMIT
     0.000 |      0.000 |     342,688 |  4,160,556 |   0.000000 | %REMOVE
     0.000 |      0.000 |         544 |     33,796 |   0.000000 | CLEAN-PARENT
     0.000 |      0.000 |   1,375,952 |     33,824 |   0.000000 | %MAKE-INODE
     0.000 |      0.000 |     262,352 |  4,194,304 |   0.000000 | %LOOKUP
     0.000 |      0.000 |       3,504 |  1,048,576 |   0.000000 | CNODE-TRUNCATED
     0.000 |      0.000 |           0 |         26 |   0.000000 | MAP-CNODE
--------------------------------------------------------------
     9.328 |      1.574 | 631,167,024 | 90,110,445 |            | Total

Documentation

Comprehensive, HTML based documentation may be found at the project relative pathname doc/api/index.html. In order to enable support for supplemental documentation related features, such as the ability to dynamically regenerate the provided documentation files incorporating all updates and changes as may be present in the source code, a (lightly) enhanced distribution of CLDOC is required and may be obtained on github.

The following sections provide a compact overview of the user api and a reference to some internal definitions of interest.

User API Reference

The user api of cl-ctrie should be largely familiar to the average common-lisp programmer. Nearly all exported symbols of the CL-CTRIE package begin with the prefix "ctrie" and thus can be convenientely incorporated via USE-PACKAGE or equivalent package definition. The following definitions comprise a quick reference to the the public user api:


[structure] CTRIE ()

A CTRIE structure is the root container that uniquely identifies a CTRIE instance, and contains the following perameters which specify the definable aspects of each CTRIE:

  • READONLY-P if not NIL prohibits any future modification or cloning of this instance.
  • TEST is a designator for an equality predicate that will be applied to disambiguate and determine the equality of any two keys. It is recommened that this value be a symbol that is fboundp, to retain capability of externalization (save/restore). At present, though, this is not enforced and a function object or lambda expression will also be accepted, albeit without the ability of save/restore.
  • HASH is a designator for a hash function, which may be desirable to customize when one has specific knowledge about the set of keys which will populate the table. At this time, a 32-bit hash is recommended as this is what has been used for development and testing and has been shown to provide good performance in practice. As with TEST it is recommended that HASH be specified by a symbol that is fboundp.
  • ROOT is the slot used internally for storage of the root inode structure that maintains the reference to the contents of the ctrie proper. The ctrie-root must only be accessed using the RDCSS ROOT NODE PROTOCOL defined by the top-level entry-points ROOT-NODE-ACCESS and ROOT-NODE-REPLACE

[function] MAKE-CTRIE (&REST ARGS &KEY NAME ROOT (READONLY-P NIL) (TEST 'EQUAL) (HASH 'SXHASH) &ALLOW-OTHER-KEYS)

CREATE a new CTRIE instance. This is the entry-point constructor intended for use by the end-user.

[function] CTRIE-P (OBJECT)

Returns T if the specified object is of type ctrie.

[function] CTRIE-TEST (CTRIE)

Returns the test of the specified ctrie

[function] CTRIE-HASH (CTRIE)

Returns the hash of the specified ctrie

[function] CTRIE-READONLY-P (CTRIE)

Returns and (with setf) changes the readonly-p of the specified ctrie

[function] CTRIE-PUT (CTRIE KEY VALUE)

Insert a new entry into CTRIE mapping KEY to VALUE. If an entry with key equal to KEY aleady exists in CTRIE, according to the equality predicate defined by CTRIE-TEST then the priorbmapping will be replaced by VALUE. Returns VALUE representing the mapping in the resulting CTRIE

[function] CTRIE-GET (CTRIE KEY)

Find the entry in CTRIE whose key is KEY and returns the associated value and T as multiple values, or returns NIL and NIL if there is no such entry. Entries can be added using SETF.

[function] CTRIE-DROP (CTRIE KEY)

Remove KEY and it's associated value from CTRIE. Returns as multiple values the value associated with KEY and T if there was such an entry, otherewise NIL and NIL

[macro] CTRIE-DO ((KEY VALUE CTRIE &KEY ATOMIC) &BODY BODY)

Iterate over the contents of CTRIE in the manner of dolist. For each (key . value) pair present in CTRIE, BODY (implicit PROGN) will be evaluated with the symbols specified for KEY and VALUE will be bound to the respective entry constituents. A special variable ACCUM (initially NIL) is available for accumulation of a result value which will be returned after the completion of the CTRIE-DO call. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the entries present in CTRIE.

;;;  EXAMPLE: (ctrie-do (k v ctrie)
;;;             (format t "~&~8S => ~10S~%" k v))

[function] CTRIE-MAP (CTRIE FN &KEY ATOMIC &AUX ACCUM)

Applies a function two arguments, FN, to each (key . value) pair present in CTRIE. During the extent of CTRIE-MAP, a special variable ACCUM (initially NIL) is available for accumulation of a result value which will be returned after the completion of the CTRIE-MAP call. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the entries present in CTRIE

[function] CTRIE-MAP-KEYS (CTRIE FN &KEY ATOMIC)

Applies a function one argument, FN, to each key present in CTRIE. During the extent of CTRIE-MAP-KEYS, a special variable ACCUM (initially NIL) is available for accumulation of a result value which will be returned after the completion of the CTRIE-MAP-KEYS call. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the keys present in CTRIE

[function] CTRIE-MAP-VALUES (CTRIE FN &KEY ATOMIC)

Applies a function one argument, FN, to each value present in CTRIE. During the extent of CTRIE-MAP-VALUES, a special variable ACCUM (initially NIL) is available for accumulation of a result value which will be returned after the completion of the CTRIE-MAP-VALUES call. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the values present in CTRIE

[function] CTRIE-KEYS (CTRIE &KEY ATOMIC)

Construct and return a list containing all keys present in CTRIE. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the keys present in CTRIE

[function] CTRIE-VALUES (CTRIE &KEY ATOMIC)

Construct and return a list containing all values present in CTRIE. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the values present in CTRIE

[function] CTRIE-SIZE (CTRIE &KEY ATOMIC &AUX (ACCUM 0))

Return the number of entries present in CTRIE. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of CTRIE

[function] CTRIE-CLEAR (CTRIE)

Atomically clear all entries stored in CTRIE, returning it to a condition which returns T when tested with predicate CTRIE-EMPTY-P

[function] CTRIE-PPRINT (CTRIE &OPTIONAL (STREAM T))

Pretty-print a representation of CTRIE as an alist containing all (key . value) pairs found in it. Atomicity is not guaranteed

[function] CTRIE-TO-ALIST (CTRIE &KEY ATOMIC)

Return an alist containing all (key . value) pairs found in CTRIE. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the entries in CTRIE

[function] CTRIE-TO-HASHTABLE (CTRIE &KEY ATOMIC HASH-TABLE &AUX (ACCUM (OR HASH-TABLE (MAKE-HASH-TABLE :TEST (CTRIE-TEST CTRIE) :HASH-FUNCTION (CTRIE-HASH CTRIE) :SYNCHRONIZED ATOMIC))))

Return a hash-table containing all (key . value) pairs found in CTRIE. If ATOMIC is non-NIL, the operation will be performed on a read-only atomic snapshot of CTRIE, which guarantees a consistent, point-in-time representation of the entries in CTRIE. If HASH-TABLE is specified, it will be used as the destination hash-table. Otherwise, a new hash-table will be created with hash-function and test identical to that of CTRIE. It will be created as synchronized if ATOMIC is not NIL

[function] CTRIE-FROM-HASHTABLE (HASH-TABLE &KEY CTRIE)

Return a CTRIE containing all (key . value) pairs found in HASH-TABLE. If CTRIE is specified, it will be used as the destination ctrie. Otherwise, a new ctrie will be created with test identical to that of HASH-TABLE. The hash-function will NOT be preserved, as that information does not appear to be recoverable from a hash-table once created

[function] CTRIE-FROM-ALIST (ALIST &KEY CTRIE)

Return a CTRIE containing all (key . value) pairs found in ALIST. If CTRIE is specified, it will be used as the destination ctrie. Otherwise, a new ctrie will be created. Atomicity is not guaranteed

[function] CTRIE-EMPTY-P (CTRIE)

Return T if CTRIE contains no entries, otherwise NIL. This function is O(1) and is much more efficient than testing for CTRIE-SIZE of 0

[function] CTRIE-MAX-DEPTH (THING)

Compute the maximum length of any arc. Useful as a diagnostic

[function] CTRIE-MIN-DEPTH (THING)

Compute the minimum length of any arc. Useful as a diagnostic

[function] CTRIE-SNAPSHOT (CTRIE &KEY READ-ONLY)

Atomically create a clone of CTRIE that may be operated upon independently without affecting or being affected by operations or content of the original CTRIE. If READ-ONLY is NIL (the default), the new CTRIE will be a READABLE/WRITABLE 'fork' of CTRIE (see CTRIE-FORK) otherwise, the clone will be READABLE only, which has considerable performance benefits in some circumstances, as the arcs will not require REFRESH and should be the preferred mode when updates (writability) of the clone are not required

[function] CTRIE-FORK (CTRIE)

Atomically create a READABLE and WRITABLE clone of CTRIE that may be operated upon independently without affecting or being affected by the original CTRIE.

[macro] CTRIE-LAMBDA (&ONCE CTRIE &REST REST)

Pandoric Object and Inter-Lexical Communication Protocol this macro builds the most unencumbered and widely applicable 'purist edition' Of our PLAMBDA based form. Even as such, a lot of care has been given to many subtle ways it has been refined to offer the most convenient and natural tool possible.

   ;;; (plambda (#<CLOSURE (LAMBDA (&REST ARGS)) {100929EB1B}> )
   ;;;
   ;;; DISPATCHING to FUNCTIONAL MAPPING:
   ;;;   (IF (REST ARGS)
   ;;;          (APPLY ARG (REST ARGS))
   ;;;          (FUNCALL ARG #'IDENTITY)) =>
   ;;; ------------------------------------------------------------
   ;;; INITIALIZING PLAMBDA
   ;;; ------------------------------------------------------------
   ;;;   IT => #S(CTRIE
   ;;;               :READONLY-P NIL
   ;;;               :TEST EQUAL
   ;;;               :HASH SXHASH
   ;;;               :STAMP #<CLOSURE (LAMBDA # :IN CONSTANTLY) {10092B516B}>
   ;;;               :ROOT #S(INODE
   ;;;                        :GEN #:|ctrie2196|
   ;;;                        :REF #S(REF
   ;;;                                :STAMP @2012-08-19T13:34:58.314457-04:00
   ;;;                                :VALUE #S(CNODE :BITMAP 0 :ARCS #())
   ;;;                                :PREV NIL)))
   ;;;   PLIST => (:CONTAINER #<CLOSURE (LAMBDA #) {100929EACB}> 
   ;;;             :TIMESTAMP @2012-08-19T13:34:58.314464-04:00)
   ;;;   STACK => (#<CLOSURE (LAMBDA #) {100929EACB}>)
   ;;; 
   ;;; ------------------------------------------------------------
   ;;;  #<CLOSURE (LAMBDA (&REST #:ARGS55)) {100929EACB}>
   ;;;```


_[generic-function]_ `CTRIE-LAMBDA-CTRIE  (CTRIE-LAMBDA-OBJECT)`

> Returns and (with setf) changes the ctrie of the specified ctrie-lambda-object



_[function]_         `CTRIE-LAMBDA-SPAWN  (SELF &KEY READ-ONLY)`

> Causes the atomic clone of enclosed ctrie structure and builds a new
lexical closure to operate on it.  Does not bother to reproduce fancy
(expensive) object, class, bindings, but provides almost identical
functionality.  May be used to more efficintly distribute workload
in parallel


_[macro]_            `DEFINE-CTRIE  (NAME CTRIE &REST ARGS &KEY (OBJECT T) SPEC)`

> Define a 'functional' __CTRIE-LAMBDA__ that combines all the the
capabilities of the raw data structure with behavior and semantics
one would expect of any other ordinary common-lisp function.  The
resulting symbol defined as 'name will be bound in three distinct
namespaces: the `SYMBOL-VALUE` will be bound to the LAMBDA CLOSURE
object, `SYMBOL-FUNCTION` (fdefinition) will be FBOUND to the
compiled function, and the corresponding '(SETF NAME) form will be
SETF-BOUND.  the syntax for invoking NAME is as in a LISP1; i.e., no
'funcall' is required (but still works if you prefer).
Calling `(NAME key)` returns the value mapped to key, or `NIL` just
as if by `(CTRIE-GET ctrie-name key).` Analogously when used as a
setf-able place such as by `(setf (NAME key) value)` it has the
equivalent behavior to the operation `(CTRIE-PUT ctrie-name key
value).` Use of this type of binding technique has some really
convenient effects that I've quickly started to become quite fond
of.  One such idiom, for example, `(mapcar MY-CTRIE '(key1 key2 key3
key4 ...))` returns a list containing all the mapped values
corresponding to the respective keys.  One additional feature that
I've found extremely useful is included _under the hood:_ Invoking
MY-CTRIE on an object of type FUNCTION will not search the ctrie for
an entry having that function ast its key, but will instead APPLY
that function to the actual CTRIE structure wrapped within the
closure.  Thus, `(MY-CTRIE #'identity)` will return the underlying
ctrie as just an ordinary instance of a CTRIE STRUCTURE.  
There are many other functions this is handy with, like
`(MY-CTRIE #'ctrie-size)` `(MY-CTRIE #'ctrie-to-hashtable)`
etc.  Some additional examples are provided below.

;;; (define-ctrie my-ctrie) ;;; => MY-CTRIE ;;; ;;; (describe 'my-ctrie) ;;; ;;; CL-CTRIE::MY-CTRIE ;;; [symbol] ;;;
;;; MY-CTRIE names a special variable: ;;; Value: #<CLOSURE (LAMBDA # :IN MAKE-CTRIE-LAMBDA) {100F73261B}> ;;;
;;; MY-CTRIE names a compiled function: ;;; Lambda-list: (&REST ARGS1) ;;; Derived type: FUNCTION ;;;
;;; (SETF MY-CTRIE) names a compiled function: ;;; Lambda-list: (VALUE KEY) ;;; Derived type: (FUNCTION (T T) *) ;;; ;;; ;;; (my-ctrie :HONG-KONG :FOOY) ;;; => :FOOY ;;; ;;; (my-ctrie :HONG-KONG) ;;; => :FOOY ; T ;;; ;;; (map 'list #'eval (mapcar #`(my-ctrie ,a1 ,a1) (iota 12))) ;;; => (0 1 2 3 4 5 6 7 8 9 10 11) ;;; ;;; (mapcar my-ctrie (iota 12)) ;;; => (0 1 2 3 4 5 6 7 8 9 10 11)



_[macro]_            `CTRIE-ERROR  (CONDITION &REST ARGS)`

> Signal a CTRIE related condition.


_[condition]_        `CTRIE-ERROR (ERROR)`

> Abstract superclass of CTRIE related conditions.


_[condition]_        `CTRIE-STRUCTURAL-ERROR (CTRIE-ERROR)`

> Condition designating that the CTRIE data structure
 has been determined to be invalid.


_[condition]_        `CTRIE-OPERATIONAL-ERROR (CTRIE-ERROR)`

> Condition for when an operational failure or
inconsistency has occurred.


_[condition]_        `CTRIE-OPERATION-RETRIES-EXCEEDED (CTRIE-OPERATIONAL-ERROR)`

> Condition indicating an operation has failed the
 maximum number of times specified by the special-variable
 *retries*


_[condition]_        `CTRIE-NOT-IMPLEMENTED (CTRIE-ERROR)`

> Condition designating functionality for which the
 implementation has not been written, but has not been deliberately
 excluded.


_[condition]_        `CTRIE-NOT-SUPPORTED (CTRIE-ERROR)`

> Condition designating functionality that is
deliberately not supported.


_[condition]_        `CTRIE-INVALID-DYNAMIC-CONTEXT (CTRIE-OPERATIONAL-ERROR)`

> Condition indicating an operation was attempted
 outside the dynamic extent of a valid enclosing WITH-CTRIE form


_[condition]_        `CTRIE-GENERATIONAL-MISMATCH (CTRIE-STRUCTURAL-ERROR)`

> Condition indicating an operation encountered an
 outdated or inconsistent node during its attempted traversal


_[function]_         `CTRIE-MODIFICATION-FAILED  (REASON &KEY OP PLACE)`

> Signal a modification failure with the appropriate attendant metadata.


_[condition]_        `CTRIE-MODIFICATION-FAILED (CTRIE-OPERATIONAL-ERROR)`

> This condition indicates an unhandled failure of an attempt to
       perform stateful modification to CTRIE.  The most common case in
       which this might occur is when such an attempt is mode on a CTRIE
       designated as READONLY-P.  In any case, this condition represents an
       exception from which processing cannot continue and requires
       interactive user intervention in order to recover.


* * * * * *

#### Internal Reference (abridged)

The following reference describes some selected internal
implementation details of interest.  Under normal circumstances it
should not be necessary to interact with these unexported symbols
unless developing an extension to cl-ctrie, but are presented here for
the sake of convenience, in order to provide better insight into the ctrie
structure in general, and to help illuminate significant aspects of this
implementation in particular.  As mentioned above, [comprehensive
documentation](doc/api/index.html) of all symbols is also provided,
and should be considered the authoratative reference to the CL-CTRIE
implementation.

* * * * * * *


_[special-variable]_ `*CTRIE*  (NIL)`

> Within the dynamic extent of a CTRIE operation this variable will
be bound to the root-container CTRIE operand.  It is an error if an
operation is defined that attempts access to a CTRIE without this
binding, which is properly established by wrapping the operation in
an appropriate WITH-CTRIE form.


_[special-variable]_ `*RETRIES*  (16)`

> Establishes the number of restarts permitted to a CTRIE operation
established by a WITH-CTRIE form before a condition of type
CTRIE-OPERATION-RETRIES-EXCEEDED will be signaled, aborting the
operatation, and requiring operator intervention to resume
processing.


_[special-variable]_ `*TIMEOUT*  (2)`

> Establishes the duration (in seconds) allotted to a CTRIE operation
established by a WITH-CTRIE form before a condition of type
CTRIE-OPERATION-TIMEOUT-EXCEEDED will be signaled, aborting the
operatation, and requiring operator intervention to resume
processing.


_[special-variable]_ `*HASH-CODE*  (NIL)`

> Special variable used to store the hash-code that corresponds to the
current operation.  Used as a means of improving efficiency by eliminating
needless recomputation of the hash function, which is the most expensive
part of most user-level ctrie operations.  If this value is not set, then
the hash will simply be computed on demand and processing will continue
unaffected.  Use of this variable is simply an optional performace
optimization techniqie.


_[macro]_            `MULTI-CATCH  (TAG-LIST &BODY FORMS)`

> Macro allowing catch of multiple tags at once and
  finding out which tag was thrown.
  * RETURNS: (values RESULT TAG)
     -  RESULT is either the result of evaluationg FORMS or the value
        thrown by the throw form.
     -  TAG is NIl if evaluation of the FORMS completed normally
        or the tag thrown and cought.
  * EXAMPLE:
  ```
      ;;; (multiple-value-bind (result tag)
      ;;;            (multi-catch (:a :b)
      ;;;                 ...FORMS...)
      ;;;              (case tag 
      ;;;                 (:a ...)
      ;;;                 (:b ...)
      ;;;                 (t ...)))
  ```


_[macro]_            `CATCH-CASE  (FORM &REST CASES)`

> User api encapsulating the MULTI-CATCH control-structure in a
  syntactic format that is identical to that of the familiar CASE
  statement, with the addition that within the scope of each CASE
  clause, a lexical binding is established between the symbol IT and
  the value caught from the throw form.


_[structure]_        `CTRIE ()`

> A CTRIE structure is the root container that uniquely identifies a CTRIE
instance, and  contains the following perameters which specify the
definable aspects of each CTRIE:

- `READONLY-P` if not `NIL` prohibits any future modification or
cloning of this instance.
- `TEST` is a designator for an equality predicate that will be
applied to disambiguate and determine the equality of any two
keys. It is recommened that this value be a symbol that is fboundp,
to retain capability of externalization (save/restore). At present,
though, this is not enforced and a function object or lambda
expression will also be accepted, albeit without the ability of
save/restore.
- `HASH` is a designator for a hash function, which may be
desirable to customize when one has specific knowledge about the set
of keys which will populate the table.  At this time, a 32-bit hash
is recommended as this is what has been used for development and
testing and has been shown to provide good performance in
practice. As with `TEST` it is recommended that `HASH` be specified
by a symbol that is fboundp.
- `ROOT` is the slot used internally for storage of the root inode
structure that maintains the reference to the contents of the ctrie
proper.  The ctrie-root must only be accessed using the _RDCSS ROOT
NODE PROTOCOL_ defined by the top-level entry-points `ROOT-NODE-ACCESS`
and `ROOT-NODE-REPLACE`


_[function]_         `CTRIE-P  (OBJECT)`

> Returns T if the specified object is of type ctrie.


_[function]_         `CTRIE-HASH  (CTRIE)`

> Returns the hash of the specified ctrie



_[function]_         `CTRIE-TEST  (CTRIE)`

> Returns the test of the specified ctrie



_[function]_         `CTRIE-READONLY-P  (CTRIE)`

> Returns and (with setf) changes the readonly-p of the specified ctrie



_[function]_         `CTHASH  (KEY)`

> Compute the hash value of KEY using the hash function defined by
the CTRIE designated by the innermost enclosing WITH-CTRIE form.


_[function]_         `CTEQUAL  (X Y)`

> Test the equality of X and Y using the equality predicate defined
by the CTRIE designated by the innermost enclosing WITH-CTRIE form.


_[macro]_            `WITH-CTRIE  (&ONCE CTRIE &BODY BODY)`

> Configure the dynamic environment with the appropriate condition
handlers, control fixtures, and instrumentation necessary to execute
the operations in BODY on the specified CTRIE. Unless specifically
documented, the particular configuration of this dynamic environment
should be considered an implementation detail and not relied upon. A
particular exception, however, is that within the dynamic extent of
a WITH-CTRIE form, the code implementing a CTRIE operation may
expect that the special variable `*CTRIE*` will be bound to the root
container of subject CTRIE.  See also the documentation for
`*CTRIE*`


_[function]_         `FLAG  (KEY LEVEL &OPTIONAL USE-CACHED-P)`

> For a given depth, LEVEL, within a CTRIE, extract the correspondant
sequence of bits from the computed hash of KEY that indicate the
logical index of the arc on the path to which that key may be found.
If USE-CACHED-P is non-NIL and the special-variable `*HASH-CODE*` is
non-NIL as well, the hash will not be recomputed, but instead the
value bound to `*HASH-CODE*` will be used as an optimization to
reduce unnecessary recomputation of hash function -- an expensive
operation.  This value is NOT checked and assumed to be valid and
appropriate to the given situation -- care must be taken to use this
cached value correctly.  When in doubt, recomputing hash may be a
performance penalty, but is guaranteed to always work in any
situation.  Note that the logical index of the arc is most likely
not the same as the physical index where it is actually located --
for that see `FLAG-ARC-POSITION`


_[function]_         `FLAG-PRESENT-P  (FLAG BITMAP)`

> Tests the (fixnum) BITMAP representing the logical index of all
arcs present in a CNODE for the presence of a particular arc whose
logical index is represented by FLAG.


_[function]_         `FLAG-ARC-POSITION  (FLAG BITMAP)`

> Given FLAG representing the logical index of an arc, and BITMAP
representing all arcs present, compute a physical index for FLAG in
such a manner as to always ensure all arcs map uniquely and
contiguously to the smallest vector that can contain the given
arcs.


_[function]_         `FLAG-VECTOR  (&OPTIONAL (CONTENT 0))`

> FLAG-VECTOR is a bit-vector representation of the (fixnum)
BITMAP. It is currently not used for any calculation, however it is
included within each CNODE as a convenience because it makes it
immediately clear from visual inspection which logical arc indexes
are represented in the node. For example, from the bit-vector
`#*10010000000000000000000000000000` one can easily see that the first
and fourth positions are occupied, and the rest empty.


_[structure]_        `REF ()`

> Atomically Stamped Reference structure _[Herlithy, TAOMP]_ that
encapsulates the mutable slots within an inode. Any specific `REF`
structure is, itself, never mutated.  Using the `REF` structure
as basis of inode implementation provides the capability to 'bundle'
additional metadata in an inode while still providing atomic compare
and swap using a single comparison of the aggregate `REF` instance.
 - `STAMP` defines a slot containing implementation-specific metadata
   that may be maintained internally as a means of tracking inode
   modification and update behavior.  It should not be referenced by
   user code, and the format of its contents should not be relied apon.
 - `VALUE` defines a slot that contains a reference to the MAIN-NODE
   that the enclosing inode should be interpreted as 'pointing to'
 - `PREV` defines a slot which, during the `INODE-COMMIT` phase of the
   _GCAS INODE PROTOCOL_ maintains a reference to the last valid
   inode state, which may be restored, if necessary, during the
   course of the `INODE-READ` / `INODE-COMMIT` arbitration process


_[function]_         `REF-P  (OBJECT)`

> Returns T if the specified object is of type ref.


_[function]_         `REF-STAMP  (REF)`

> Returns the stamp of the specified ref



_[function]_         `REF-VALUE  (REF)`

> Returns the value of the specified ref



_[function]_         `REF-PREV  (REF)`

> Returns and (with setf) changes the prev of the specified ref



_[structure]_        `FAILED-REF (REF)`

> A `FAILED-REF` is a structure that is used to preserve the linkage to
prior inode state following a failed GCAS.  Any inode access that
detects a `FAILED-REF` will immediately invoke a commit to restore the
inode to the state recorded in `FAILED-REF-PREV`


_[function]_         `FAILED-REF-P  (OBJECT)`

> Returns T if the specified object is of type failed-ref.


_[type]_             `LEAF-NODE  NIL`

> A LEAF-NODE represents a terminal value in a CTRIE arc.
LEAF-NODEs always contain a unit-payload of CTRIE data
storage; For example, an SNODE contains a key/value pair.


_[type]_             `BRANCH-NODE  NIL`

> a BRANCH-NODE represents a single arc typically contained
within A CNODE.


_[type]_             `MAIN-NODE  NIL`

> A MAIN-NODE is a node that typically represents a specific
depth or level within the ctrie and is referenced
by its own unique inode.


_[structure]_        `INODE ()`

> An INODE, or 'Indirection Node' is the mutable structure
representing the link between other 'main-node' structures found in
a ctrie.  An inode is the only type of node that may change the
value of its content during its lifetime.  In this implementation,
all such values as may change are encapsulated within a `REF`
substructure.  Each inode also contains a generational descriptor
object, comparible by identity only, which is used to identify the
state of synchronization between the inode and the current
'generation' indicated by the root inode at the base of the
CTRIE. As an inode may not change its 'gen identity' during its
lifetime, this disparity with the generation of the root node will
immediately result in the replacement of the inode with a new one
properly synchronized with the root's `GEN` object. In this
implementation, `GEN` objects are implemented by GENSYMS -- unique,
uninterned symbols which inherently provide very similar symantics
to those required of the generational descriptor.
- `GEN` defines a slot containing a generational descriptor object
- `REF` defines a slot containing a `REF` struct that encapsulates
  the mutable content within an INODE


_[function]_         `INODE-P  (OBJECT)`

> Returns T if the specified object is of type inode.


_[function]_         `INODE-GEN  (INODE)`

> Returns the gen of the specified inode



_[function]_         `INODE-REF  (INODE)`

> Returns and (with setf) changes the ref of the specified inode



_[function]_         `MAKE-INODE  (LINK-TO &OPTIONAL GEN STAMP PREV)`

> Construct a new INODE that represents a reference to the value
provided by argument LINK-TO, optionally augmented with a specified
generational descriptor, timestamp, and/or previous state


_[macro]_            `GCAS-COMPARE-AND-SET  (OBJ EXPECTED NEW EXPECTED-STAMP
                                           NEW-STAMP PREV)`

> A thin, macro layer abstraction over the basic compare-and-swap
primitive which provides a consistent interface to the underlying
inode structure and manages additional metadata, providing
reasonable defaults when they are not specified.


_[function]_         `INODE-READ  (INODE)`

> INODE-READ provides the top-level interface to the inode _GCAS ACCESS_
api, which is the mechanism which must be used to gain access to the
content of any NON-ROOT inode. For access to the root inode, refer
to the RDCSS inode api `ROOT-NODE-ACCESS`. Returns as four values,
the MAIN-NODE, the STAMP, the PREVIOUS STATE (if any), and the REF
structure encapsulated by the inode.


_[function]_         `INODE-MUTATE  (INODE OLD-VALUE NEW-VALUE)`

> INODE-MUTATE provides the top-level interface to the inode _GCAS
MODIFICATION_ api, which is the mechanism which must be used to
effect any change in a NON-ROOT inode.  For modification of the
root-inode, refer to the `ROOT-NODE-REPLACE` _RDCSS ROOT NODE
PROTOCOL_ Returns a boolean value which indicates the success or
failure of the modification attempt.


_[function]_         `INODE-COMMIT  (INODE REF)`

> INODE-COMMIT implements the _GCAS COMMIT_ protocol which is invoked
as necessary by the `INODE-READ` and `INODE-MUTATE` entry-points.  It is
not meant to be invoked directly, as this would most likely result
in corruption. Returns the `REF` structure representing the content of
whatever root inode wound up successfully committed -- either the
one requested, or one represented by a previous valid state.  In order
to coexist with the _RDCSS ROOT NODE PROTOCOL_ this GCAS COMMIT
implementation is augmented with RDCSS ABORTABLE READ semantics
by a forward reference to a RDCSS-aware `ROOT-NODE-ACCESS` in order
to safely compare INODE's generational descriptor with the one found
in the root inode of the subject CTRIE.


_[function]_         `SNODE  (KEY VALUE)`

> Construct a new SNODE which represents the mapping from
domain-element KEY to range-element VALUE.


_[structure]_        `SNODE ()`

> SNODE, i.e., 'Storage Node', is the LEAF-NODE structure ultimately
used for the storage of each key/value pair contained in the CTRIE.
An SNODE is considered to be immutable during its lifetime.
 - `KEY` defines the slot containing an element of the map's domain.
 - `VALUE` defines the slot containing the range-element mapped to `KEY`


_[function]_         `SNODE-P  (OBJECT)`

> Returns T if the specified object is of type snode.


_[function]_         `SNODE-KEY  (SNODE)`

> Returns the key of the specified snode



_[function]_         `SNODE-VALUE  (SNODE)`

> Returns the value of the specified snode



_[structure]_        `LNODE ()`

> LNODE, i.e., 'List Node', is a special structure used to enclose
SNODES in a singly-linked chain when the hash-codes of the
respective SNODE-KEYS collide, but those keys are determined to be
unique by the `CTRIE-TEST` function defined for that ctrie.  An
LNODE (and therefore a chain of LNODEs) is considered to be
immutable during its lifetime.  The order of the list is
implemented (arbitrarily) as most recently added first, analogous to
`CL:PUSH`
 - `ELT` defines the slot containing an enclosed SNODE
 - `NEXT` defines a slot referencing the next LNODE in the chain, or
   `NIL` if no further LNODES remain.


_[function]_         `LNODE-P  (OBJECT)`

> Returns T if the specified object is of type lnode.


_[function]_         `LNODE-ELT  (LNODE)`

> Returns the elt of the specified lnode



_[function]_         `LNODE-NEXT  (LNODE)`

> Returns the next of the specified lnode



_[function]_         `ENLIST  (&REST REST)`

> Construct a chain of LNODE structures enclosing the values supplied.
It is assumed (elsewhere) that each of these values is a valid SNODE
structure.


_[function]_         `LNODE-REMOVED  (ORIG-LNODE KEY TEST)`

> Construct a chain of LNODE structures identical to the chain starting
with ORIG-LNODE, but with any LNODE containing an SNODE equal to KEY
removed.  Equality is tested as by the predicate function passed as
the argument TEST. The order of nodes in the resulting list will
remain unchanged.


_[function]_         `LNODE-INSERTED  (ORIG-LNODE KEY VALUE TEST)`

> Construct a chain of LNODE structures identical to the chain starting
with ORIG-LNODE, but ensured to contain an LNODE enclosing an SNODE mapping
KEY to VALUE.  If the given KEY equal to a key already present somewhere
in the chain (as compared with equality predicate TEST) it will be
replaced.  Otherwise a new LNODE will be added. In either case, the LNODE
containing `(SNODE KEY VAlUE)` will be the first node in the resulting
list


_[function]_         `LNODE-SEARCH  (LNODE KEY TEST)`

> Within the list of lnodes beginning with LNODE, return the range value
mapped by the first SNODE containing a key equal to KEY as determined
by equality predicate TEST, or `NIL` if no such key is found.  As a
second value, in order to support storage of `NIL` as a key, return `T` to
indicate that the KEY was indeed found during search, or `NIL` to indicate
that no such key was present in the list


_[function]_         `LNODE-LENGTH  (LNODE)`

> Return the number of LNODES present in the chain beginning at LNODE


_[structure]_        `TNODE ()`

> A TNODE, or 'Tomb Node', is a special node structure used to preserve
ordering during `CTRIE-DROP` (`%remove`) operations.
Any time a TNODE is encountered during the course of a `CTRIE-GET` (`%lookup`)
operation, the operative thread is required to invoke a `CLEAN` operation
on the TNODE it has encountered and throw to `:RESTART` its lookup activity
over again.  A TNODE is considered to be immutable and may not change its
value during its lifetime.
 - `CELL` defines a slot which contains the entombed node structure.
    Only LNODE and SNODE type nodes are ever entombed


_[function]_         `TNODE-P  (OBJECT)`

> Returns T if the specified object is of type tnode.


_[function]_         `TNODE-CELL  (TNODE)`

> Returns the cell of the specified tnode



_[structure]_        `CNODE ()`

> A CNODE, or 'Ctrie Node' is a MAIN-NODE containing a vector of up
to `2^W` 'arcs' -- i.e., references to either an SNODE or INODE
structure, collectively referred to as `BRANCH-NODES.` Each CNODE
also contains a (fixnum) bitmap that describes the layout of which
logical indices within the total capacity of `2^W` arcs are actually
allocated within that node with BRANCH-NODES physically present.
For more specific details on these BITMAP and ARC-VECTOR
constituents, refer to the following related functions: `FLAG`
`FLAG-PRESENT-P` `FLAG-VECTOR` and `FLAG-ARC-POSITION. The CNODE
structure is considered to be immutable and arcs may not be added or
removed during its lifetime.  The storage allocated within a CNODE
is fixed and specified at the time of its creation based on the
value of BITMAP during initialization
 - `BITMAP`
 - `ARCS` 


_[function]_         `CNODE-P  (OBJECT)`

> Returns T if the specified object is of type cnode.


_[function]_         `MAKE-CNODE  (&OPTIONAL (BITMAP 0))`

> Construct a CNODE with internal storage allocated for the number of
arcs equal to the Hamming-Weight of the supplied BITMAP parameter.
If no BITMAP is provided, the CNODE created will be empty -- a state
which is only valid for the level 0 node referenced by the root of
the CTRIE.  This constructor is otherwise never called directly, but
is invoked during the course of higher-level operations such as
`CNODE-EXTENDED` `CNODE-UPDATED` `CNODE-TRUNCATED` and `MAP-CNODE`


_[function]_         `CNODE-EXTENDED  (CNODE FLAG POSITION NEW-ARC)`

> Construct a new cnode structure that is exactly like CNODE, but
additionally contains the BRANCH-NODE specified by parameter NEW-ARC
and logical index FLAG at the physical index POSITION within its
vector of allocated arcs.  The BITMAP of this new CNODE will be
calculated as an adjustment of the prior CNODE's BITMAP to reflect
the presence of this additional arc.  In addition, the physical
index within the extended storage vector for the other arcs present
may also change with respect to where they were located in the prior
CNODE.  In other words, the physical index of a given arc within the
compressed CNODE storage vector should never be relied upon
directly, it should always be accessed by calculation based on its
LOGICAL index and the current CNODE BITMAP as described in more
detail by the documentation for the functions `FLAG` `FLAG-VECTOR` and
`FLAG-ARC-POSITION`


_[function]_         `CNODE-UPDATED  (CNODE POSITION REPLACEMENT-ARC)`

> Construct a new cnode structure identical to CNODE, but having the
BRANCH-NODE physically located at POSITION within the storage
vector replaced by the one specified by REPLACEMENT-ARC.  Unlike
`CNODE-EXTENDED` and `CNODE-TRUNCATED` the allocated storage and
existing BITMAP of this CNODE will remain unchanged (as this is
simply a one-for-one replacement) and correspondingly, no reordering
of other nodes within the storage vector will occur


_[function]_         `CNODE-TRUNCATED  (CNODE FLAG POS)`

> Construct a new cnode structure that is exactly like CNODE, but
with the arc at logical index FLAG and physical storage vector
location POS removed.  The new CNODE will have an updated bitmap
value that is adusted to reflect the removal of this arc, and the
position of other arcs within the storage vector of the new CNODE
will be adjusted in a manner analogous to that of `CNODE-EXTENDED`
More details on this process may be found by referring to the
documentation for the functions `FLAG` `FLAG-VECTOR` and
`FLAG-ARC-POSITION`


_[function]_         `MAP-CNODE  (FN CNODE)`

> Construct a new cnode structure that is exactly like CNODE, but
with each arc (BRANCH-NODE) present in CNODE replaced by the result
of applying FN to that arc.  I.e., a simple functional mapping from
the old CNODE by FN.  As with `CNODE-UPDATED` the allocated storage and
BITMAP of the resulting CNODE will remain unchanged from the
original, and no physical reordering of nodes within the storage
vector will occur


_[function]_         `CNODE-CONTRACTED  (CNODE LEVEL)`

> The _CONTRACTION_ of a CNODE is an ajustment performed when a CNODE
at depth other than level 0 contains only a single SNODE arc.  In
such a case, that SNODE is entombed in a new TNODE, which is
returned as the result of the CNODE contraction. In all other cases
the CNODE is simply returned as-is.  A CONTRACTION represents the
first of the two-step _ARC RETRACTION PROTOCOL_ that effects the reclaimation
of allocated storage no longer used and the optimization of of lookup
efficiency by compacting CTRIE depth and thereby the number of levels
which must be traversed.  For further information, refer to the function
`CNODE-COMPRESSED` which implements the second stage of this protocol,
completing the process.


_[function]_         `CNODE-COMPRESSED  (CNODE LEVEL)`

> The _COMPRESSION_ of a CNODE is the second step of the _ARC
RETRACTION PROTOCOL_ completing a retraction that has been initiated
by `CNODE-CONTRACTED`.  The CNODE compression is computed by
generating a replacement cnode structure that is similar to CNODE,
but with any entombed inode arcs created during contraction simply
replaced by the SNODE that had been entombed. This is called the
_RESURRECTION_ of that SNODE. After all entombed inode arcs of a
cnode have been collapsed into simple SNODE leaves, if the resulting
CNODE has been compressed so far as to contain only a single SNODE
leaf, it is subjected to another CONTRACTION before it is returned
as the result of the compression. Otherwise it is simply returned
and represents a complete iteration of the _ARC RETRACTION PROTOCOL_


_[function]_         `CLEAN  (INODE LEVEL)`

> CLEAN is the basic entry-point into the arc retraction protocol. Given an
arbitrary, non-root inode referencing a CNODE that can be compressed,
update that inode to reference the result of that compression.  Otherwise
INODE will remain unaffected.


_[function]_         `CLEAN-PARENT  (PARENT-INODE TARGET-INODE KEY LEVEL)`

> During a `CTRIE-DROP` (`%remove`) operation, if the result of a KEY/VALUE
removal is an arc consisting of an `ENTOMBED` inode (one referencing a TNODE), then,
if that arc remains accessible from the parent of a CNODE containing it, generate
the compression of that CNODE and update its parent INODE with the result.


_[structure]_        `RDCSS-DESCRIPTOR ()`

> An RDCSS-DESCRIPTOR object represents a 'plan' for a proposed RDCSS
(restricted double compare single swap) operation. The use of this
descriptor object provides the means to effect an atomic RDCSS in
software, requiring only hardware support for single-word CAS, which is
preferable because it is commonly available on curent consumer hardware.
 - `OV`        designates a slot containing the OLD (current) root inode.
               If the swap is unsuccessful, the resulting ctrie will revert
               to this structure as the root inode. 
 - `OVMAIN`    designates a slot containing the CNODE that is referenced
               by the OLD (current) root inode.
 - `NV`        designates a slot containing a fully assembled replacement
               root inode referencing a valid CNODE. This pair will become
               the root inode and level 0 MAIN-NODE of the ctrie if the
               swap is successful.
 - `COMMITTED` designates a flag which, when not NIL, indicates that the
               RDCSS plan defined by this descriptor has completed
               successfully


_[function]_         `RDCSS-DESCRIPTOR-P  (OBJECT)`

> Returns T if the specified object is of type rdcss-descriptor.


_[function]_         `RDCSS-DESCRIPTOR-OV  (RDCSS-DESCRIPTOR)`

> Returns the ov of the specified rdcss-descriptor



_[function]_         `RDCSS-DESCRIPTOR-OVMAIN  (RDCSS-DESCRIPTOR)`

> Returns the ovmain of the specified rdcss-descriptor



_[function]_         `RDCSS-DESCRIPTOR-NV  (RDCSS-DESCRIPTOR)`

> Returns the nv of the specified rdcss-descriptor



_[function]_         `RDCSS-DESCRIPTOR-COMMITTED  (RDCSS-DESCRIPTOR)`

> Returns and (with setf) changes the committed of the specified rdcss-descriptor



_[function]_         `ROOT-NODE-ACCESS  (CTRIE &OPTIONAL ABORT)`

> ROOT-NODE-ACCESS extends `FIND-CTRIE-ROOT`,
implementing the _RDCSS ROOT NODE PROTOCOL_ for access to root inode of
CTRIE.  In particular, it ensures that if, instead of an inode, the
root of CTRIE contains an RDCSS descriptor of a proposed root-node
update, that it will immediately invoke `ROOT-NODE-COMMIT` to act
on that descriptor and return an INODE struct that is the result of
the completed commit process. `ROOT-NODE-ACCESS` only provides access
to the root inode _STRUCTURE_ and in particular it does not provide
safe access to the _CONTENT_ of that inode. In order to access those
contents, the root inode returned by `ROOT-NODE-ACCESS` must be further
processed by `INODE-READ` in order to still properly comply with the
underlying GCAS protocol implementation requirements common to all
inodes


_[function]_         `ROOT-NODE-REPLACE  (CTRIE OV OVMAIN NV)`

> ROOT-NODE-REPLACE implements the _RDCSS ROOT NODE PROTOCOL_ for
replacement of the ROOT INODE of a CTRIE structure with another one
that contains new or alternative values, achieving the end-result
effectively the same as if by mutation.  The replacement of the root
inode is accomplished in two, basic conceptual stages. First, an
`RDCSS-DESCRIPTOR` object which specifies in full the current state
and all desired changes in the proposed resulting state.  Thus,
whether any individual replacement attempt succeeds or fails, either
result is guarenteed to represent a valid state. An attempt is then
made to atomically swap this RDCSS-DESCRIPTOR with the current CTRIE
root inode.  Note that, although it contains all of the information
representing two, distinct, root inode states, the RDCSS-DESCRIPTOR
is not, itself, a valid root inode.  That is the reason why all
access to the root inode must be accomplished using this specialized
_RDCSS ROOT NODE PROTOCOL_ rather than just the _GCAS INODE
PROTOCOL_ alone, as is done with all other non-root inodes.  Once an
atomic compare-and-swap of an RDCSS-DESCRIPTOR object with the root
inode completes successfully, `ROOT-NODE-COMMIT` is invoked which
will attempt to complete the second step of this protocol.  The
result of that commit will be one or the other of the two valid
states defined in the RDCSS-DESCRIPTOR object.  If another thread
concurrently attempts access to a root node holding an
RDCSS-DESCRIPTOR object rather than an INODE, it will invoke
`ROOT-NODE-COMMIT` itself, possibly prempting our own attempt, but
guaranteeing nonblocking access to a valid root node by any concurrent
thread


_[function]_         `ROOT-NODE-COMMIT  (CTRIE &OPTIONAL ABORT)`

> rdcss api to complete a root-node transaction


_[function]_         `CTRIE-SNAPSHOT  (CTRIE &KEY READ-ONLY)`

> Atomically create a clone of CTRIE that may be operated upon
independently without affecting or being affected by operations or
content of the original CTRIE.  If READ-ONLY is NIL (the default),
the new CTRIE will be a READABLE/WRITABLE 'fork' of CTRIE (see
`CTRIE-FORK`) otherwise, the clone will be READABLE only, which has
considerable performance benefits in some circumstances, as the arcs
will not require `REFRESH` and should be the preferred mode when
updates (writability) of the clone are not required


_[function]_         `CTRIE-CLEAR  (CTRIE)`

> Atomically clear all entries stored in CTRIE, returning it to a condition
which returns `T` when tested with predicate `CTRIE-EMPTY-P`


_[function]_         `CTRIE-PUT  (CTRIE KEY VALUE)`

> Insert a new entry into CTRIE mapping KEY to VALUE.  If an entry
with key equal to KEY aleady exists in CTRIE, according to the
equality predicate defined by `CTRIE-TEST` then the priorbmapping
will be replaced by VALUE. Returns `VALUE` representing the
mapping in the resulting CTRIE


_[function]_         `%INSERT  (INODE KEY VALUE LEVEL PARENT STARTGEN)`

>    -  The detailed specifics required to perform an insertion into a
CTRIE map are defined by and contained within the `%INSERT` function,
which is not part of the USER API and should never be invoked
directly.  The procedures required for interaction with `%INSERT` are
managed entirely by the upper layer entry-points and should be
entirely invisible to the developer using CL-CTRIE.  The alogorithm is
intricate and requires quite some effort to figure out based on the
papers and documentation. For that reason this attempt is made to
properly document the process -- not to encourage anyone to fiddle
with it...

> 1.  A given call to %insert carries with it no guarantee that it will
actually succeed.  Further, there is no guarantee it will do anything
at all -- including ever returning to the caller having invoked it.
This is because there are a number of circumstances that may possibly
interfere with insertion in a lock-free concurrent data-structure and
in order to minimize the cost incurred by aborted attempts and
restarts, there is no effort wasted on careful condition handling or
recovery.  Instead, as soon as it is recognised that a particular
attempt will not succeed, Control is thrown via non-local exit
unceremoneously the entire call stack and restarting each time from
the very beginning. There are quite a few places within the process
where you will find these nonlocal exits. Originally I had in mind to
incorporate some additional insrumentation to track and gather
statistics the log the specifics of all this, and there are a number
of extension points provided.  For more details refer to the
`MULTI-CATCH` and `CATCH-CASE` control structure documentation.

> 2.  %insert ALWAYS begins at an INODE.  This is not surprising, of
course, since the root of the tree (where all inserts begin) is an
INODE, and because INODES are the only nodes that provide
mutability. When we which to effect change to the CTRIE regardless of
where or what type, It may only be accomplished by mutating the parent
INODE immediately above it to reference content that must be freshly
regenerated each time changes are required.  Once it is established
that the %insert always begins at an inode, we can reason further
about the sequence of events that follow by considering some of the
node oriented invariants for CTRIEs specified variously in the
academic literature.  First, it has been clearly defined that an INODE
structure may only reference three kinds of structure that we
collectively refer to as MAIN-NODES.  These are the three potential
cases which we will consider next.

> 3.  Consider first the TNODE. If indeed we follow an INODE and
discover it directly leads us to a TNODE, or 'Tomb Node'. This tells
us, first, that we have arrived at a dead-end, second that we must
assist with the 'compression' of this arc by invoking the `CLEAN`
operation on the tombed INODE's parent.  Finally, there is nothing
further we can do so we THROW to :RESTART.

> 4.  If traverse the INODE and arrive at an LNODE, we are also at the
end of the ARC, but if it is due to hash collision then the algorithm
then indeed it may be correct.  In this case we attempt to 'insert'
ourself in te LNODE chain and then invoke INODE mutate to atomic
commit and then THROW to :RESTART

> 5.  As a simple instance of the general case, we may arrive at a CNODE
with vacant arc that represents the index specified by the bits of our
KEY's hash code that are active for this level within the ctrie.  When
this is the case, we construct a replacement CNODE augmented with our
key/value pair as an SNODE LEAF at the physical position within the
CNODES storage vector appropriately translated from the logical arc
index as described by the documentation of the functions
`FLAG-ARC-POSITION` and `FLAG-VECTOR` If we successfully mutate the
parent inode by completing an atomic replacement of the old cnode with
the one we constructed, then our insertion has succeeded and we return
the range value now successfully mapped by KEY in order to indicate
our success.  Otherwise we THROW to :RESTART.

> 6.  If we find the logical index of our 'arc' in this CNODE is not
empty and available as we did above, there are exactly two other
possibly find there; we know this because it is required to be a
BRANCH-NODE -- either an snode leaf storage or an inode referencing a
MAIN-NODE that represents the next layer of the CTRIE.  We describe
various posible cases and the define a procedure specified for each
below.

> 7.  If we find that the node PRESENT at this index is an INODE, then
this is the simplest of the possible cases.  Conceptually, what we
intend to do is continue to follow our arc, descending to the next
level of the CTRIE structure that is referenced by that inode.  In
practice, however, we are required to consider the possibility that
the generational descriptor object the inode contains may not be
consistent with STARTGEN, which is the one current in the root INODE
of this CTRIE.  This may be the case, for example, as the result of
some past cloning/snapshot operation if we are the first since then to
traverse this inode. (Remember that the refresh of generational
descriptor occurs lazily on an as-needed basis in order to avoid
overhead incurred by eager traversals which often turn out to have
been unnecessary).  In consideration of this we proceeed as follows: -
If the inode generational descriptor is consistent with STARTGEN, we
simply continue along our arc by recursively invoking `%insert` on
that INODE.  If that function call returns successfully with a VALUE,
then the insertion was successful, and we also then return, passing
along that value.  Thus the result is communicated back through the
caller chain, eventually arriving back as the result of the original
CTRIE-PUT entry-point.  - Otherwise if the generational descriptor is
not consistent with STARTGEN we attempt an atomic `INODE-MUTATE` on
the PARENT INODE of this CNODE that effects the replacement of that
inode within it by one FRESHLY CREATED by `REFRESH` and ensured to be
consistent with STARTGEN.  If this succeeds, we invoke %insert
recusively and proceed in the same manner, since, effectively, we are
now in a state equivalent to the one described above.  - If the
INODE-MUTATE of the prior step did NOT succeed, then we are out of
options and throw to :RESTART the insertion process from the beginning
all over again.

> 8.  If we find that the node PRESENT at this index is an SNODE,
then our situation becomes a little bit more complex and
there are a few more contingencies we must be prepared to
address.

> 9.  Once again, looking at the simplest first, when an insert
operation encounters a leaf-node somewhere along the descent of it's
'own' arc, one potential case is that it found the node it was
looking for -- one that contains a key that satisfies the test
predicate defined for the dynamic extent of the current operation,
`CTEQUAL,` when compared to the `KEY` currently being `%INSERTED.`
If the equality test is satisfied then the VALUE that node maps
should be updated with the one of the present insertion.  The steps
to effect the update are very similar to those of step 5, however We
construct a replacement CNODE augmented with our key/value pair as a
replacement SNODE in the SAME physical position as the one we have
found -- refer to the documentation for the function `CNODE-UPDATED`
for additional specifics on the internal details that describe this
operation.  If we successfully mutate the parent inode by completing
an atomic replacement of the old cnode with the one we constructed,
then our update has succeeded and we return the range value now
successfully mapped by KEY in order to indicate our success.
Otherwise we THROW to :RESTART.

> 10.  In some circumstances, we encounter a node on our arc whose
hash code bits have matched that of the current key of this
insertion for all of the lower order bits that have been consumed so
far, up to the current depth, but that (as opposed to step 9) does
not satisfy `CTEQUAL.` and so is NOT a candidiate for update
replacement.  Except in very vare circumstances, there will be some
depth at which the active bits of its hash code will indeed be
distinct from our own, and at that point a CNODE can be constructed
that will proprerly contain both it and an snode mapping the
key/value of the current insertion.  This means we must ENTEND the
ctrie as many layers as needed to get to that depth, inserting
CNODES and INODES at each step along the way.  Now, we will first
describe the 'edge' case where we have encounted the 'rare
circumstance.' If we perform this process and arrive at a depth
where all 32 hash code bits have been consumed and, indeed, these
two unequal keys are the result of a 'hash code collision' In order
that we preserve correct operation, we respond in this case by
chaining these key/value SNODES into a linked list of LNODES.
Therefore, they can share the same arc index and when we encounter
such a thing during future traversals, we can accomodate the
collision using simple linear search and a few basic LNODE utility
functions such as `LNODE-INSERTED` `LNODE-REMOVED` `LNODE-SEARCH`
`LNODE-LENGTH` and the list constructor `ENLIST.` Once we have
`ENLIST`ed the colliding SNODES, we create a new INODE pointing to
that list, and then attempt atomic replacement of the CNODE above
with one we extend to contain that INODE.  If we do successfully
mutate the prior CNODES parent INODE resulting in its replacement
with the CNODE we constructed, then our insert has succeeded and we
return the range value now successfully mapped by KEY in order to
indicate our success.  Otherwise we THROW to :RESTART.

> 11.  Finally, let us return to those intermediate steps, mentioned
above, to specify the means by which we perform the level-by-level
extension of a given arc to accommodate both the above case of hash
collision as well as the more common one when the reason for
extension is simply to accomodate normal growth capacity and
allocation.  In both cases, though, the extensions are performed for
the same initiating cause -- to accomodate the collision of leaf
node keys resident at lower levels of the structure.  Depending on
the similarity of two colliding hash keys, the extension process may
not be resolved with a single iteration.  In the case of full
collisiion, described above, the extension process will recur, up to
a maximum depth of `(32/W)` levels, at which point an L-NODE chain
will be created.  At each iteration, a new INODE is created,
pointing to a new CNODE containing one of our conflictung pairs.
Then, `%INSERT` is attempted on that INODE and this process recurs.
Once this cycle of insert/extend completes, each INODE/CNODE pair is
returned to the parent -- the entire newly created structure
eventually returning to the point of original conflicts whre the
extension cycle began.  If we successfully mutate the parent inode
by completing an atomic replacement of the old cnode with the one
that begins this newly built structue, then our update has succeeded
and we return the range value now successfully mapped by KEY in
order to indicate our success.  Otherwise we THROW to :RESTART


_[function]_         `CTRIE-GET  (CTRIE KEY)`

> Find the entry in CTRIE whose key is KEY and returns the
associated value and T as multiple values, or returns NIL and NIL
if there is no such entry. Entries can be added using SETF.


_[function]_         `%LOOKUP  (INODE KEY LEVEL PARENT STARTGEN)`

> The general concept of the procedure for finding a given key within
a simplified SEQUENTIAL model of a CTRIE can be summarized as
follows: If the internal node is at level `L` then the W bits of the
hashcode starting from position `W * L` are used as a logical index
into the vector of `2^W` arcs that can possibly be represented
within that node (see `FLAG` and `FLAG-VECTOR`). This logical index
is then transformed into a physical index that denotes a specific
position locating this arc relative to all other arcs currently
present in the node (see `FLAG-ARC-POSITION`.  In this way, storage
within the node need not be allocated for representation of empty
arc positions. At all times the invariant is maintained that the
number of arcs allocated within a given CNODE is equal to the
Hamming-Weight of its BITMAP -- i.e., the number of nonzero bits
present (see `CL:LOGCOUNT`). The arc at this calculated relative
position is then followed, and the process repeated until arrival at
a leaf-node or empty arc position.  Locating a given key becomes
substantially more complicated in the actual lock-free concurrent
ctrie algorithm


_[function]_         `CTRIE-DROP  (CTRIE KEY)`

> Remove KEY and it's associated value from CTRIE. Returns as multiple
values the value associated with KEY and T if there was such an entry,
otherewise NIL and NIL


_[function]_         `%REMOVE  (INODE KEY LEVEL PARENT STARTGEN)`

_[function]_         `CTRIE-MAP  (CTRIE FN &KEY ATOMIC &AUX ACCUM)`

> Applies a function two arguments, FN, to each (key . value) pair present in
CTRIE.  During the extent of CTRIE-MAP, a special variable ACCUM (initially NIL)
is available for accumulation of a result value which will be returned after
the completion of the CTRIE-MAP call.  If ATOMIC is non-NIL, the operation will
be performed on a read-only atomic snapshot of CTRIE, which guarantees a
consistent, point-in-time representation of the entries present in CTRIE


_[macro]_            `CTRIE-DO  ((KEY VALUE CTRIE &KEY ATOMIC) &BODY BODY)`

> Iterate over the contents of CTRIE in the manner of dolist. For each
(key . value) pair present in CTRIE, BODY (implicit PROGN) will be
evaluated with the symbols specified for KEY and VALUE will be bound
to the respective entry constituents.  A special variable
ACCUM (initially NIL) is available for accumulation of a result
value which will be returned after the completion of the CTRIE-DO
call.  If ATOMIC is non-NIL, the operation will be performed on a
read-only atomic snapshot of CTRIE, which guarantees a consistent,
point-in-time representation of the entries present in CTRIE.

;;; EXAMPLE: (ctrie-do (k v ctrie) ;;; (format t "~&~8S => 10S%" k v))



_[function]_         `CTRIE-MAP-KEYS  (CTRIE FN &KEY ATOMIC)`

> Applies a function one argument, FN, to each key present in CTRIE.
During the extent of CTRIE-MAP-KEYS, a special variable
ACCUM (initially NIL) is available for accumulation of a result
value which will be returned after the completion of the
CTRIE-MAP-KEYS call.  If ATOMIC is non-NIL, the operation will be
performed on a read-only atomic snapshot of CTRIE, which guarantees
a consistent, point-in-time representation of the keys present in
CTRIE


_[function]_         `CTRIE-MAP-VALUES  (CTRIE FN &KEY ATOMIC)`

> Applies a function one argument, FN, to each value present in CTRIE.
During the extent of CTRIE-MAP-VALUES, a special variable
ACCUM (initially NIL) is available for accumulation of a result
value which will be returned after the completion of the
CTRIE-MAP-VALUES call.  If ATOMIC is non-NIL, the operation will be
performed on a read-only atomic snapshot of CTRIE, which guarantees
a consistent, point-in-time representation of the values present in
CTRIE


_[function]_         `CTRIE-KEYS  (CTRIE &KEY ATOMIC)`

> Construct and return a list containing all keys present in CTRIE.
If ATOMIC is non-NIL, the operation will be performed on a read-only
atomic snapshot of CTRIE, which guarantees a consistent,
point-in-time representation of the keys present in CTRIE


_[function]_         `CTRIE-VALUES  (CTRIE &KEY ATOMIC)`

> Construct and return a list containing all values present in CTRIE.
If ATOMIC is non-NIL, the operation will be performed on a read-only
atomic snapshot of CTRIE, which guarantees a consistent,
point-in-time representation of the values present in CTRIE


_[function]_         `CTRIE-SIZE  (CTRIE &KEY ATOMIC &AUX (ACCUM 0))`

> Return the number of entries present in CTRIE.  If ATOMIC is non-NIL,
the operation will be performed on a read-only atomic snapshot of CTRIE,
which guarantees a consistent, point-in-time representation of CTRIE


_[function]_         `CTRIE-EMPTY-P  (CTRIE)`

> Return T if CTRIE contains no entries, otherwise NIL. This function is
O(1) and is much more efficient than testing for `CTRIE-SIZE` of 0


_[function]_         `CTRIE-TO-ALIST  (CTRIE &KEY ATOMIC)`

> Return an alist containing all (key . value) pairs found in CTRIE.
If ATOMIC is non-NIL, the operation will be performed on a read-only
atomic snapshot of CTRIE, which guarantees a consistent,
point-in-time representation of the entries in CTRIE


_[function]_         `CTRIE-TO-HASHTABLE  (CTRIE &KEY ATOMIC HASH-TABLE &AUX
                                         (ACCUM
                                          (OR HASH-TABLE
                                              (MAKE-HASH-TABLE :TEST
                                                               (CTRIE-TEST
                                                                CTRIE)
                                                               :HASH-FUNCTION
                                                               (CTRIE-HASH
                                                                CTRIE)
                                                               :SYNCHRONIZED
                                                               ATOMIC))))`

> Return a hash-table containing all (key . value) pairs found in CTRIE.
If ATOMIC is non-NIL, the operation will be performed on a read-only
atomic snapshot of CTRIE, which guarantees a consistent,
point-in-time representation of the entries in CTRIE. If HASH-TABLE
is specified, it will be used as the destination hash-table.
Otherwise, a new hash-table will be created with hash-function and
test identical to that of CTRIE.  It will be created as synchronized
if ATOMIC is not NIL


_[function]_         `CTRIE-PPRINT  (CTRIE &OPTIONAL (STREAM T))`

> Pretty-print a representation of CTRIE as an alist containing
all (key . value) pairs found in it.  Atomicity is not guaranteed


_[function]_         `CTRIE-FROM-ALIST  (ALIST &KEY CTRIE)`

> Return a CTRIE containing all (key . value) pairs found in ALIST.
If CTRIE is specified, it will be used as the destination ctrie.
Otherwise, a new ctrie will be created. Atomicity is not guaranteed


_[function]_         `CTRIE-FROM-HASHTABLE  (HASH-TABLE &KEY CTRIE)`

> Return a CTRIE containing all (key . value) pairs found in HASH-TABLE.
If CTRIE is specified, it will be used as the destination ctrie.
Otherwise, a new ctrie will be created with test identical to that
of HASH-TABLE. The hash-function will NOT be preserved, as that
information does not appear to be recoverable from a hash-table once
created


_[function]_         `CTRIE-MAX-DEPTH  (THING)`

> Compute the maximum length of any arc. Useful as a diagnostic


_[function]_         `CTRIE-MIN-DEPTH  (THING)`

> Compute the minimum length of any arc. Useful as a diagnostic


_[function]_         `CTRIE-SNAPSHOT  (CTRIE &KEY READ-ONLY)`

> Atomically create a clone of CTRIE that may be operated upon
independently without affecting or being affected by operations or
content of the original CTRIE.  If READ-ONLY is NIL (the default),
the new CTRIE will be a READABLE/WRITABLE 'fork' of CTRIE (see
`CTRIE-FORK`) otherwise, the clone will be READABLE only, which has
considerable performance benefits in some circumstances, as the arcs
will not require `REFRESH` and should be the preferred mode when
updates (writability) of the clone are not required


_[function]_         `CTRIE-FORK  (CTRIE)`

> Atomically create a READABLE and WRITABLE clone of CTRIE that may
be operated upon independently without affecting or being affected
by the original CTRIE.


_[macro]_            `CTRIE-LAMBDA  (&ONCE CTRIE &REST REST)`

> Pandoric Object and Inter-Lexical Communication Protocol
this macro builds the most unencumbered and widely applicable
'purist edition' Of our PLAMBDA based form.  Even as such,
a lot of care has been given to many subtle ways it has been
refined to offer the most convenient and natural tool possible.
```;;;
   ;;; (plambda (#<CLOSURE (LAMBDA (&REST ARGS)) {100929EB1B}> )
   ;;;
   ;;; DISPATCHING to FUNCTIONAL MAPPING:
   ;;;   (IF (REST ARGS)
   ;;;          (APPLY ARG (REST ARGS))
   ;;;          (FUNCALL ARG #'IDENTITY)) =>
   ;;; ------------------------------------------------------------
   ;;; INITIALIZING PLAMBDA
   ;;; ------------------------------------------------------------
   ;;;   IT => #S(CTRIE
   ;;;               :READONLY-P NIL
   ;;;               :TEST EQUAL
   ;;;               :HASH SXHASH
   ;;;               :STAMP #<CLOSURE (LAMBDA # :IN CONSTANTLY) {10092B516B}>
   ;;;               :ROOT #S(INODE
   ;;;                        :GEN #:|ctrie2196|
   ;;;                        :REF #S(REF
   ;;;                                :STAMP @2012-08-19T13:34:58.314457-04:00
   ;;;                                :VALUE #S(CNODE :BITMAP 0 :ARCS #())
   ;;;                                :PREV NIL)))
   ;;;   PLIST => (:CONTAINER #<CLOSURE (LAMBDA #) {100929EACB}> 
   ;;;             :TIMESTAMP @2012-08-19T13:34:58.314464-04:00)
   ;;;   STACK => (#<CLOSURE (LAMBDA #) {100929EACB}>)
   ;;; 
   ;;; ------------------------------------------------------------
   ;;;  #<CLOSURE (LAMBDA (&REST #:ARGS55)) {100929EACB}>
   ;;;```


_[generic-function]_ `CTRIE-LAMBDA-CTRIE  (CTRIE-LAMBDA-OBJECT)`

> Returns and (with setf) changes the ctrie of the specified ctrie-lambda-object



_[function]_         `CTRIE-LAMBDA-SPAWN  (SELF &KEY READ-ONLY)`

> Causes the atomic clone of enclosed ctrie structure and builds a new
lexical closure to operate on it.  Does not bother to reproduce fancy
(expensive) object, class, bindings, but provides almost identical
functionality.  May be used to more efficintly distribute workload
in parallel


_[macro]_            `DEFINE-CTRIE  (NAME CTRIE &REST ARGS &KEY (OBJECT T) SPEC)`

> Define a 'functional' __CTRIE-LAMBDA__ that combines all the the
capabilities of the raw data structure with behavior and semantics
one would expect of any other ordinary common-lisp function.  The
resulting symbol defined as 'name will be bound in three distinct
namespaces: the `SYMBOL-VALUE` will be bound to the LAMBDA CLOSURE
object, `SYMBOL-FUNCTION` (fdefinition) will be FBOUND to the
compiled function, and the corresponding '(SETF NAME) form will be
SETF-BOUND.  the syntax for invoking NAME is as in a LISP1; i.e., no
'funcall' is required (but still works if you prefer).
Calling `(NAME key)` returns the value mapped to key, or `NIL` just
as if by `(CTRIE-GET ctrie-name key).` Analogously when used as a
setf-able place such as by `(setf (NAME key) value)` it has the
equivalent behavior to the operation `(CTRIE-PUT ctrie-name key
value).` Use of this type of binding technique has some really
convenient effects that I've quickly started to become quite fond
of.  One such idiom, for example, `(mapcar MY-CTRIE '(key1 key2 key3
key4 ...))` returns a list containing all the mapped values
corresponding to the respective keys.  One additional feature that
I've found extremely useful is included _under the hood:_ Invoking
MY-CTRIE on an object of type FUNCTION will not search the ctrie for
an entry having that function ast its key, but will instead APPLY
that function to the actual CTRIE structure wrapped within the
closure.  Thus, `(MY-CTRIE #'identity)` will return the underlying
ctrie as just an ordinary instance of a CTRIE STRUCTURE.  
There are many other functions this is handy with, like
`(MY-CTRIE #'ctrie-size)` `(MY-CTRIE #'ctrie-to-hashtable)`
etc.  Some additional examples are provided below.

;;; (define-ctrie my-ctrie) ;;; => MY-CTRIE ;;; ;;; (describe 'my-ctrie) ;;; ;;; CL-CTRIE::MY-CTRIE ;;; [symbol] ;;;
;;; MY-CTRIE names a special variable: ;;; Value: #<CLOSURE (LAMBDA # :IN MAKE-CTRIE-LAMBDA) {100F73261B}> ;;;
;;; MY-CTRIE names a compiled function: ;;; Lambda-list: (&REST ARGS1) ;;; Derived type: FUNCTION ;;;
;;; (SETF MY-CTRIE) names a compiled function: ;;; Lambda-list: (VALUE KEY) ;;; Derived type: (FUNCTION (T T) *) ;;; ;;; ;;; (my-ctrie :HONG-KONG :FOOY) ;;; => :FOOY ;;; ;;; (my-ctrie :HONG-KONG) ;;; => :FOOY ; T ;;; ;;; (map 'list #'eval (mapcar #`(my-ctrie ,a1 ,a1) (iota 12))) ;;; => (0 1 2 3 4 5 6 7 8 9 10 11) ;;; ;;; (mapcar my-ctrie (iota 12)) ;;; => (0 1 2 3 4 5 6 7 8 9 10 11)



_[macro]_            `CTRIE-ERROR  (CONDITION &REST ARGS)`

> Signal a CTRIE related condition.


_[condition]_        `CTRIE-ERROR (ERROR)`

> Abstract superclass of CTRIE related conditions.


_[condition]_        `CTRIE-STRUCTURAL-ERROR (CTRIE-ERROR)`

> Condition designating that the CTRIE data structure
 has been determined to be invalid.


_[condition]_        `CTRIE-OPERATIONAL-ERROR (CTRIE-ERROR)`

> Condition for when an operational failure or
inconsistency has occurred.


_[function]_         `CTRIE-MODIFICATION-FAILED  (REASON &KEY OP PLACE)`

> Signal a modification failure with the appropriate attendant metadata.


_[condition]_        `CTRIE-MODIFICATION-FAILED (CTRIE-OPERATIONAL-ERROR)`

> This condition indicates an unhandled failure of an attempt to
       perform stateful modification to CTRIE.  The most common case in
       which this might occur is when such an attempt is mode on a CTRIE
       designated as READONLY-P.  In any case, this condition represents an
       exception from which processing cannot continue and requires
       interactive user intervention in order to recover.


_[condition]_        `CTRIE-OPERATION-RETRIES-EXCEEDED (CTRIE-OPERATIONAL-ERROR)`

> Condition indicating an operation has failed the
 maximum number of times specified by the special-variable
 *retries*


_[condition]_        `CTRIE-NOT-IMPLEMENTED (CTRIE-ERROR)`

> Condition designating functionality for which the
 implementation has not been written, but has not been deliberately
 excluded.


_[condition]_        `CTRIE-NOT-SUPPORTED (CTRIE-ERROR)`

> Condition designating functionality that is
deliberately not supported.


_[condition]_        `CTRIE-INVALID-DYNAMIC-CONTEXT (CTRIE-OPERATIONAL-ERROR)`

> Condition indicating an operation was attempted
 outside the dynamic extent of a valid enclosing WITH-CTRIE form


_[condition]_        `CTRIE-GENERATIONAL-MISMATCH (CTRIE-STRUCTURAL-ERROR)`

> Condition indicating an operation encountered an
 outdated or inconsistent node during its attempted traversal


_[function]_         `README  (&OPTIONAL (STREAM *STANDARD-OUTPUT*))`

> Update documentation sections of the README file. When an output stream
is specified, the results are also echoed to that stream. To inhibit
output, invoke as `(readme (make-broadcast-stream))` or use `README-QUIETLY`


_[function]_         `README-QUIETLY  ()`

> Update documentation sections of the README file, inhibiting any other
printed output.


_[function]_         `APIDOC  (&OPTIONAL (SCOPE :EXTERNAL))`

> Collect a list of strings representing the documentation for
CL-CTRIE rendered in a compact format suitable for inclusion in a
lightweight text-markup format document.  If SCOPE is specified it
must be either :EXTERNAL. corresponding to those symbols exported as
the public API, or :HOME, which designates all symbols defined
locally in package.


_[function]_         `PRINC-APIDOC  (&OPTIONAL (SCOPE :EXTERNAL))`

> Print to `*STANDARD-OUTPUT*` the documentation for CL-CTRIE rendered
in a compact format.  This is intended primarily as a convenience to
the interactive user seeking quick reference at the REPL.  If SCOPE
is specified it must be either :EXTERNAL. corresponding to those
symbols exported as the public API, or :HOME, which designates all
symbols defined locally in package.


_[function]_         `COLLECT-DOCS  (&OPTIONAL (SCOPE :EXTERNAL)
                                   (SORT #'STRING<))`

> Regenerate on-disk html documentation and collect the cached
in-memory descriptors for further processing. If SCOPE is specified
it must be either :EXTERNAL. corresponding to those symbols exported
as the public API, or :HOME, which designates all symbols defined
locally in package.  Output order may be customized by an optionally
specified SORT function.


_[macro]_            `DEFINE-DIAGRAM  (TYPE (&OPTIONAL CONTEXT) &BODY BODY)`

> Define a diagrammatic representation of TYPE, optionally specialized
for a specific CONTEXT. See {defgeneric cl-ctrie::make-diagram}.


* * * * * * *