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string_pool.cr
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string_pool.cr
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# A string pool is a collection of strings.
# It allows a runtime to save memory by preserving strings in a pool, allowing to
# reuse an instance of a common string instead of creating a new one.
#
# ```
# require "string_pool"
#
# pool = StringPool.new
# a = "foo" + "bar"
# b = "foo" + "bar"
# a.object_id # => 136294360
# b.object_id # => 136294336
# a = pool.get(a)
# b = pool.get(b)
# a.object_id # => 136294312
# b.object_id # => 136294312
# ```
class StringPool
# Implementation uses open addressing scheme of hash table with [quadratic probing](https://en.wikipedia.org/wiki/Quadratic_probing).
# Quadratic probing, using the triangular numbers, avoids the clumping while keeping
# cache coherency in the common case.
# As long as the table size is a power of 2, the quadratic-probing method [described by "Triangular numbers mod 2^n"](https://fgiesen.wordpress.com/2015/02/22/triangular-numbers-mod-2n/)
# will explore every table element if necessary, to find a good place to insert.
# Returns the size
#
# ```
# require "string_pool"
#
# pool = StringPool.new
# pool.size # => 0
# ```
getter size : Int32
# Creates a new empty string pool.
#
# The *initial_capacity* is useful to avoid unnecessary reallocations
# of the internal buffers in case of growth. If you have an estimate
# of the maximum number of elements the pool will hold it should
# be initialized with that capacity for improved performance.
# Inputs lower than 8 are ignored.
#
# ```
# pool = StringPool.new(256)
# pool.size # => 0
# ```
def initialize(initial_capacity = 8)
@capacity = initial_capacity >= 8 ? Math.pw2ceil(initial_capacity) : 8
@hashes = Pointer(UInt64).malloc(@capacity, 0_u64)
@values = Pointer(String).malloc(@capacity, "")
@size = 0
end
# Returns `true` if the `StringPool` has no element otherwise returns `false`.
#
# ```
# require "string_pool"
#
# pool = StringPool.new
# pool.empty? # => true
# pool.get("crystal")
# pool.empty? # => false
# ```
def empty? : Bool
@size == 0
end
# Returns a `String` with the contents of the given *slice*.
#
# If a string with those contents was already present in the pool, that one is returned.
# Otherwise a new string is created, put in the pool and returned.
#
# ```
# require "string_pool"
#
# pool = StringPool.new
# ptr = Pointer.malloc(9) { |i| ('a'.ord + i).to_u8 }
# slice = Slice.new(ptr, 3)
# pool.empty? # => true
# pool.get(slice)
# pool.empty? # => false
# ```
def get(slice : Bytes) : String
get slice.to_unsafe, slice.size
end
# Returns a `String` with the contents given by the pointer *str* of size *len*.
#
# If a string with those contents was already present in the pool, that one is returned.
# Otherwise a new string is created, put in the pool and returned.
#
# ```
# require "string_pool"
#
# pool = StringPool.new
# pool.get("hey".to_unsafe, 3)
# pool.size # => 1
# ```
def get(str : UInt8*, len) : String
hash = hash(str, len)
get(hash, str, len)
end
private def get(hash : UInt64, str : UInt8*, len)
rehash if @size >= @capacity // 4 * 3
mask = (@capacity - 1).to_u64
index = hash & mask
next_probe_offset = 1_u64
while (h = @hashes[index]) != 0
if h == hash && @values[index].bytesize == len
if str.memcmp(@values[index].to_unsafe, len) == 0
return @values[index]
end
end
index = (index + next_probe_offset) & mask
next_probe_offset += 1_u64
end
@size += 1
entry = String.new(str, len)
@hashes[index] = hash
@values[index] = entry
entry
end
private def put_on_rehash(hash : UInt64, entry : String)
mask = (@capacity - 1).to_u64
index = hash & mask
next_probe_offset = 1_u64
while @hashes[index] != 0
index = (index + next_probe_offset) & mask
next_probe_offset += 1_u64
end
@hashes[index] = hash
@values[index] = entry
end
# Returns a `String` with the contents of the given `IO::Memory`.
#
# If a string with those contents was already present in the pool, that one is returned.
# Otherwise a new string is created, put in the pool and returned.
#
# ```
# require "string_pool"
#
# pool = StringPool.new
# io = IO::Memory.new "crystal"
# pool.empty? # => true
# pool.get(io)
# pool.empty? # => false
# ```
def get(str : IO::Memory) : String
get(str.buffer, str.bytesize)
end
# Returns a `String` with the contents of the given string.
#
# If a string with those contents was already present in the pool, that one is returned.
# Otherwise a new string is created, put in the pool and returned.
#
# ```
# require "string_pool"
#
# pool = StringPool.new
# string = "crystal"
# pool.empty? # => true
# pool.get(string)
# pool.empty? # => false
# ```
def get(str : String) : String
get(str.to_unsafe, str.bytesize)
end
# Rebuilds the hash based on the current hash values for each key,
# if values of key objects have changed since they were inserted.
#
# Call this method if you modified a string submitted to the pool.
def rehash : Nil
if @capacity * 2 <= 0
raise "Hash table too big"
end
old_capacity = @capacity
old_hashes = @hashes
old_values = @values
@capacity *= 2
@hashes = Pointer(UInt64).malloc(@capacity, 0_u64)
@values = Pointer(String).malloc(@capacity, "")
old_capacity.times do |i|
if old_hashes[i] != 0
put_on_rehash(old_hashes[i], old_values[i])
end
end
end
private def hash(str, len)
hasher = Crystal::Hasher.new
hasher = str.to_slice(len).hash(hasher)
# hash should be non-zero, so `or` it with high bit
hasher.result | 0x8000000000000000_u64
end
end