Fix "~n@*"-directives in clojure.pprint/cl-format. Fixes #CLJ-1134.

This solves two issues as specified by #CLJ-1134. Issue #1 is solved by doing a
relative jump forward within `absolute-reposition` in cl_format.clj, line 114 by
switching `(- (:pos navigator) position)` with `(- position (:pos navigator))`.

Issue #2 is handled by changing the default `n`-parameter to `*` depending on
whether the `@`-prefix is placed or not. If it is placed, then `n` defaults to
0, otherwise it defaults to 1.

In addition, new tests have been appended to `test_cl_format.clj` to ensure the
correctness of this patch. The tests have been tested on the Common Lisp
implementation GNU CLISP 2.49, which presumably handle the `~n@*`
correctly. This patch and GNU CLISP returns the same output for each format
call, sans case for printed symbols; Common Lisp has case-insensitive symbols,
whereas Clojure has not.

Signed-off-by: Stuart Halloway <stu@cognitect.com>

src/clj/clojure/pprint/cl_format.clj

@@ -111,7 +111,7 @@ http://www.lispworks.com/documentation/HyperSpec/Body/22_c.htm
 
 (defn- absolute-reposition [navigator position]
   (if (>= position (:pos navigator))
-    (relative-reposition navigator (- (:pos navigator) position))
+    (relative-reposition navigator (- position (:pos navigator)))
     (struct arg-navigator (:seq navigator) (drop position (:seq navigator)) position)))
 
 (defn- relative-reposition [navigator position]
@@ -1471,14 +1471,15 @@ not a pretty writer (which keeps track of columns), this function always outputs
      #(absolute-tabulation %1 %2 %3)))
 
   (\* 
-   [ :n [1 Integer] ] 
+   [ :n [nil Integer] ] 
    #{ :colon :at } {}
-   (fn [params navigator offsets]
-     (let [n (:n params)]
-       (if (:at params)
-         (absolute-reposition navigator n)
-         (relative-reposition navigator (if (:colon params) (- n) n)))
-       )))
+   (if (:at params)
+     (fn [params navigator offsets]
+       (let [n (or (:n params) 0)] ; ~@* has a default n = 0
+         (absolute-reposition navigator n)))
+     (fn [params navigator offsets]
+       (let [n (or (:n params) 1)] ; whereas ~* and ~:* have a default n = 1
+         (relative-reposition navigator (if (:colon params) (- n) n))))))
 
   (\? 
    [ ] 

test/clojure/test_clojure/pprint/test_cl_format.clj

@@ -823,3 +823,21 @@ but it was called with an argument of type short-float.\n")
 |    -2/3 |    | panda |            dog |
 "
   )
+
+(simple-tests *-at-tests
+  (format nil "~*~c defaults to ~D, so ~~@* goes ~A to the ~@*~A arg."
+          'first \n 0 'back)
+  "n defaults to 0, so ~@* goes back to the first arg."
+  (format nil "~~n@* is an ~1@*~A ~0@*~A rather than a ~2@*~A ~0@*~A."
+             'goto 'absolute 'relative)
+  "~n@* is an absolute goto rather than a relative goto."
+  (format nil "We will see no numbers: ~6@*~S"
+             0 1 2 3 4 5 :see?) "We will see no numbers: :see?"
+  (format nil "~4@*~D ~3@*~D ~2@*~D ~1@*~D ~0@*~D"
+             0 1 2 3 4) "4 3 2 1 0"
+  (format nil "~{~A a ~~{ ~3@*~A, the ~1@*~A is ~A to the ~4@*~A of ~A~}[...]"
+            '("Within" goto relative construct list arguments))
+  "Within a ~{ construct, the goto is relative to the list of arguments[...]"
+  (format nil "~{~2@*~S ~1@*~S ~4@*~S ~3@*~S ~S~}"
+          '(:a :b :c :d :e)) ":c :b :e :d :e"
+          )