add mergesort() for linked lists

This adds a generic bottom-up mergesort implementation for singly linked
lists.  It was inspired by Simon Tatham's webpage on the topic[1], but
not so much by his implementation -- for no good reason, really, just a
case of NIH.

[1] http://www.chiark.greenend.org.uk/~sgtatham/algorithms/listsort.html

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -180,6 +180,7 @@
 /test-index-version
 /test-line-buffer
 /test-match-trees
+/test-mergesort
 /test-mktemp
 /test-obj-pool
 /test-parse-options

Makefile

@@ -465,6 +465,7 @@ TEST_PROGRAMS_NEED_X += test-genrandom
 TEST_PROGRAMS_NEED_X += test-index-version
 TEST_PROGRAMS_NEED_X += test-line-buffer
 TEST_PROGRAMS_NEED_X += test-match-trees
+TEST_PROGRAMS_NEED_X += test-mergesort
 TEST_PROGRAMS_NEED_X += test-mktemp
 TEST_PROGRAMS_NEED_X += test-obj-pool
 TEST_PROGRAMS_NEED_X += test-parse-options
@@ -578,6 +579,7 @@ LIB_H += log-tree.h
 LIB_H += mailmap.h
 LIB_H += merge-file.h
 LIB_H += merge-recursive.h
+LIB_H += mergesort.h
 LIB_H += notes.h
 LIB_H += notes-cache.h
 LIB_H += notes-merge.h
@@ -681,6 +683,7 @@ LIB_OBJS += mailmap.o
 LIB_OBJS += match-trees.o
 LIB_OBJS += merge-file.o
 LIB_OBJS += merge-recursive.o
+LIB_OBJS += mergesort.o
 LIB_OBJS += name-hash.o
 LIB_OBJS += notes.o
 LIB_OBJS += notes-cache.o

mergesort.c

@@ -0,0 +1,73 @@
+#include "cache.h"
+#include "mergesort.h"
+
+struct mergesort_sublist {
+	void *ptr;
+	unsigned long len;
+};
+
+static void *get_nth_next(void *list, unsigned long n,
+			  void *(*get_next_fn)(const void *))
+{
+	while (n-- && list)
+		list = get_next_fn(list);
+	return list;
+}
+
+static void *pop_item(struct mergesort_sublist *l,
+		      void *(*get_next_fn)(const void *))
+{
+	void *p = l->ptr;
+	l->ptr = get_next_fn(l->ptr);
+	l->len = l->ptr ? (l->len - 1) : 0;
+	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 *))
+{
+	unsigned long l;
+
+	if (!list)
+		return NULL;
+	for (l = 1; ; l *= 2) {
+		void *curr;
+		struct mergesort_sublist p, q;
+
+		p.ptr = list;
+		q.ptr = get_nth_next(p.ptr, l, get_next_fn);
+		if (!q.ptr)
+			break;
+		p.len = q.len = l;
+
+		if (compare_fn(p.ptr, q.ptr) > 0)
+			list = curr = pop_item(&q, get_next_fn);
+		else
+			list = curr = pop_item(&p, get_next_fn);
+
+		while (p.ptr) {
+			while (p.len || q.len) {
+				void *prev = curr;
+
+				if (!p.len)
+					curr = pop_item(&q, get_next_fn);
+				else if (!q.len)
+					curr = pop_item(&p, get_next_fn);
+				else if (compare_fn(p.ptr, q.ptr) > 0)
+					curr = pop_item(&q, get_next_fn);
+				else
+					curr = pop_item(&p, get_next_fn);
+				set_next_fn(prev, curr);
+			}
+			p.ptr = q.ptr;
+			p.len = l;
+			q.ptr = get_nth_next(p.ptr, l, get_next_fn);
+			q.len = q.ptr ? l : 0;
+
+		}
+		set_next_fn(curr, NULL);
+	}
+	return list;
+}

mergesort.h

@@ -0,0 +1,9 @@
+#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 *));
+
+#endif

test-mergesort.c

@@ -0,0 +1,52 @@
+#include "cache.h"
+#include "mergesort.h"
+
+struct line {
+	char *text;
+	struct line *next;
+};
+
+static void *get_next(const void *a)
+{
+	return ((const struct line *)a)->next;
+}
+
+static void set_next(void *a, void *b)
+{
+	((struct line *)a)->next = b;
+}
+
+static int compare_strings(const void *a, const void *b)
+{
+	const struct line *x = a, *y = b;
+	return strcmp(x->text, y->text);
+}
+
+int main(int argc, const char **argv)
+{
+	struct line *line, *p = NULL, *lines = NULL;
+	struct strbuf sb = STRBUF_INIT;
+
+	for (;;) {
+		if (strbuf_getwholeline(&sb, stdin, '\n'))
+			break;
+		line = xmalloc(sizeof(struct line));
+		line->text = strbuf_detach(&sb, NULL);
+		if (p) {
+			line->next = p->next;
+			p->next = line;
+		} else {
+			line->next = NULL;
+			lines = line;
+		}
+		p = line;
+	}
+
+	lines = mergesort(lines, get_next, set_next, compare_strings);
+
+	while (lines) {
+		printf("%s", lines->text);
+		lines = lines->next;
+	}
+	return 0;
+}