mergesort: rename it to llist_mergesort()

Even though the function is generic enough, <anything>sort() inherits
connotations from the standard function qsort() that sorts an array.
Rename it to llist_mergesort() and describe the external interface in
its header file.

This incidentally avoids name clashes with mergesort() some platforms
declare in, and contaminate user namespace with, their <stdlib.h>.

Reported-by: Brian Gernhardt
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -429,8 +429,8 @@ static void commit_list_set_next(void *a, void *next)
 
 void commit_list_sort_by_date(struct commit_list **list)
 {
-	*list = mergesort(*list, commit_list_get_next, commit_list_set_next,
-			  commit_list_compare_by_date);
+	*list = llist_mergesort(*list, commit_list_get_next, commit_list_set_next,
+				commit_list_compare_by_date);
 }
 
 struct commit *pop_most_recent_commit(struct commit_list **list,

mergesort.c

@@ -23,10 +23,10 @@ static void *pop_item(struct mergesort_sublist *l,
 	return p;
 }
 
-void *mergesort(void *list,
-		void *(*get_next_fn)(const void *),
-		void (*set_next_fn)(void *, void *),
-		int (*compare_fn)(const void *, const void *))
+void *llist_mergesort(void *list,
+		      void *(*get_next_fn)(const void *),
+		      void (*set_next_fn)(void *, void *),
+		      int (*compare_fn)(const void *, const void *))
 {
 	unsigned long l;
 

mergesort.h

@@ -1,9 +1,17 @@
 #ifndef MERGESORT_H
 #define MERGESORT_H
 
-void *mergesort(void *list,
-		void *(*get_next_fn)(const void *),
-		void (*set_next_fn)(void *, void *),
-		int (*compare_fn)(const void *, const void *));
+/*
+ * Sort linked list in place.
+ * - get_next_fn() returns the next element given an element of a linked list.
+ * - set_next_fn() takes two elements A and B, and makes B the "next" element
+ *   of A on the list.
+ * - compare_fn() takes two elements A and B, and returns negative, 0, positive
+ *   as the same sign as "subtracting" B from A.
+ */
+void *llist_mergesort(void *list,
+		      void *(*get_next_fn)(const void *),
+		      void (*set_next_fn)(void *, void *),
+		      int (*compare_fn)(const void *, const void *));
 
 #endif

test-mergesort.c

@@ -42,7 +42,7 @@ int main(int argc, const char **argv)
 		p = line;
 	}
 
-	lines = mergesort(lines, get_next, set_next, compare_strings);
+	lines = llist_mergesort(lines, get_next, set_next, compare_strings);
 
 	while (lines) {
 		printf("%s", lines->text);