Data commit

Task/Permutations/M4/permutations.m4

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+divert(-1)
+
+# 1-based indexing of a string's characters.
+define(`get',`substr(`$1',decr(`$2'),1)')
+define(`set',`substr(`$1',0,decr(`$2'))`'$3`'substr(`$1',`$2')')
+define(`swap',
+`pushdef(`_u',`get(`$1',`$2')')`'dnl
+pushdef(`_v',`get(`$1',`$3')')`'dnl
+set(set(`$1',`$2',_v),`$3',_u)`'dnl
+popdef(`_u',`_v')')
+
+# $1-fold repetition of $2.
+define(`repeat',`ifelse($1,0,`',`$2`'$0(decr($1),`$2')')')
+
+#
+# Heap's algorithm. Algorithm 2 in Robert Sedgewick, 1977. Permutation
+# generation methods. ACM Comput. Surv. 9, 2 (June 1977), 137-164.
+#
+# This implementation permutes the characters in a string of length no
+# more than 9. On longer strings, it may strain the resources of a
+# very old implementation of m4.
+#
+define(`permutations',
+    `ifelse($2,`',`$1
+$0(`$1',repeat(len(`$1'),1),2)',
+        `ifelse(eval(($3) <= len(`$1')),1,
+             `ifelse(eval(get($2,$3) < $3),1,
+                       `swap(`$1',_$0($2,$3),$3)
+$0(swap(`$1',_$0($2,$3),$3),set($2,$3,incr(get($2,$3))),2)',
+                       `$0(`$1',set($2,$3,1),incr($3))')')')')
+define(`_permutations',`eval((($2) % 2) + ((1 - (($2) % 2)) * get($1,$2)))')
+
+divert`'dnl
+permutations(`123')
+permutations(`abcd')