vector: Timsort all of the things

Drop the GLibc implementation of Merge Sort and replace it with Timsort.

The algorithm has been tuned to work on arrays of pointers (void **),
so there's no longer a need to abstract the byte-width of each element
in the array.

All the comparison callbacks now take pointers-to-elements, not
pointers-to-pointers, so there's now one less level of dereferencing.

E.g.

	 int index_cmp(const void *a, const void *b)
	 {
	-	const git_index_entry *entry_a = *(const git_index_entry **)(a);
	+	const git_index_entry *entry_a = (const git_index_entry *)(a);

The result is up to a 40% speed-up when sorting vectors. Memory usage
remains lineal.

A new `bsearch` implementation has been added, whose callback also
supplies pointer-to-elements, to uniform the Vector API again.

11 files changed, 424 insertions, 85 deletions

diff --git a/src/config.c b/src/config.c
index 730917634..815b08c55 100644
--- a/src/config.c
+++ b/src/config.c
@@ -56,8 +56,8 @@ void git_config_free(git_config *cfg)
 
 static int config_backend_cmp(const void *a, const void *b)
 {
-	const file_internal *bk_a = *(const file_internal **)(a);
-	const file_internal *bk_b = *(const file_internal **)(b);
+	const file_internal *bk_a = (const file_internal *)(a);
+	const file_internal *bk_b = (const file_internal *)(b);
 
 	return bk_b->priority - bk_a->priority;
 }
diff --git a/src/index.c b/src/index.c
index dd33db92a..353c30025 100644
--- a/src/index.c
+++ b/src/index.c
@@ -109,15 +109,15 @@ static int write_index(git_index *index, git_filebuf *file);
 int index_srch(const void *key, const void *array_member)
 {
 	const char *filename = (const char *)key;
-	const git_index_entry *entry = *(const git_index_entry **)(array_member);
+	const git_index_entry *entry = (const git_index_entry *)(array_member);
 
 	return strcmp(filename, entry->path);
 }
 
 int index_cmp(const void *a, const void *b)
 {
-	const git_index_entry *entry_a = *(const git_index_entry **)(a);
-	const git_index_entry *entry_b = *(const git_index_entry **)(b);
+	const git_index_entry *entry_a = (const git_index_entry *)(a);
+	const git_index_entry *entry_b = (const git_index_entry *)(b);
 
 	return strcmp(entry_a->path, entry_b->path);
 }
@@ -125,15 +125,15 @@ int index_cmp(const void *a, const void *b)
 int unmerged_srch(const void *key, const void *array_member)
 {
 	const char *path = (const char *) key;
-	const git_index_entry_unmerged *entry = *(const git_index_entry_unmerged **) (array_member);
+	const git_index_entry_unmerged *entry = (const git_index_entry_unmerged *)(array_member);
 
 	return strcmp(path, entry->path);
 }
 
 int unmerged_cmp(const void *a, const void *b)
 {
-	const git_index_entry_unmerged *info_a = *(const git_index_entry_unmerged **)(a);
-	const git_index_entry_unmerged *info_b = *(const git_index_entry_unmerged **)(b);
+	const git_index_entry_unmerged *info_a = (const git_index_entry_unmerged *)(a);
+	const git_index_entry_unmerged *info_b = (const git_index_entry_unmerged *)(b);
 
 	return strcmp(info_a->path, info_b->path);
 }
diff --git a/src/odb.c b/src/odb.c
index b09931660..caa4e1bef 100644
--- a/src/odb.c
+++ b/src/odb.c
@@ -223,8 +223,8 @@ static int init_fake_wstream(git_odb_stream **stream_p, git_odb_backend *backend
 
 static int backend_sort_cmp(const void *a, const void *b)
 {
-	const backend_internal *backend_a = *(const backend_internal **)(a);
-	const backend_internal *backend_b = *(const backend_internal **)(b);
+	const backend_internal *backend_a = (const backend_internal *)(a);
+	const backend_internal *backend_b = (const backend_internal *)(b);
 
 	if (backend_a->is_alternate == backend_b->is_alternate)
 		return (backend_b->priority - backend_a->priority);
diff --git a/src/odb_pack.c b/src/odb_pack.c
index 44604ef91..20b0f1e4d 100644
--- a/src/odb_pack.c
+++ b/src/odb_pack.c
@@ -699,8 +699,8 @@ static int pack_index_open(struct pack_file *p)
 
 static int packfile_sort__cb(const void *a_, const void *b_)
 {
-	struct pack_file *a = *((struct pack_file **)a_);
-	struct pack_file *b = *((struct pack_file **)b_);
+	struct pack_file *a = (struct pack_file *)a_;
+	struct pack_file *b = (struct pack_file *)b_;
 	int st;
 
 	/*
diff --git a/src/refs.c b/src/refs.c
index 84224e88e..5e0fe09b2 100644
--- a/src/refs.c
+++ b/src/refs.c
@@ -825,8 +825,8 @@ static int packed_remove_loose(git_repository *repo, git_vector *packing_list)
 
 static int packed_sort(const void *a, const void *b)
 {
-	const git_reference *ref_a = *(const git_reference **)a;
-	const git_reference *ref_b = *(const git_reference **)b;
+	const git_reference *ref_a = (const git_reference *)a;
+	const git_reference *ref_b = (const git_reference *)b;
 
 	return strcmp(ref_a->name, ref_b->name);
 }
diff --git a/src/tree.c b/src/tree.c
index 51b19f535..fff5068f8 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -40,7 +40,7 @@ static int valid_attributes(const int attributes) {
 int entry_search_cmp(const void *key, const void *array_member)
 {
 	const char *filename = (const char *)key;
-	const git_tree_entry *entry = *(const git_tree_entry **)(array_member);
+	const git_tree_entry *entry = (const git_tree_entry *)(array_member);
 
 	return strcmp(filename, entry->filename);
 }
@@ -53,8 +53,8 @@ static int valid_attributes(const int attributes) {
 
 int entry_sort_cmp(const void *a, const void *b)
 {
-	const git_tree_entry *entry_a = *(const git_tree_entry **)(a);
-	const git_tree_entry *entry_b = *(const git_tree_entry **)(b);
+	const git_tree_entry *entry_a = (const git_tree_entry *)(a);
+	const git_tree_entry *entry_b = (const git_tree_entry *)(b);
 
 	return git_futils_cmp_path(entry_a->filename, strlen(entry_a->filename),
                                   entry_a->attr & 040000,
diff --git a/src/tsort.c b/src/tsort.c
new file mode 100644
index 000000000..eda9a2fd2
--- /dev/null
+++ b/src/tsort.c
@@ -0,0 +1,380 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+/**
+ * An array-of-pointers implementation of Python's Timsort
+ * Based on code by Christopher Swenson under the MIT license
+ *
+ * Copyright (c) 2010 Christopher Swenson
+ * Copyright (c) 2011 Vicent Marti
+ */
+
+#ifndef MAX
+#	define MAX(x,y) (((x) > (y) ? (x) : (y)))
+#endif
+
+#ifndef MIN
+#	define MIN(x,y) (((x) < (y) ? (x) : (y)))
+#endif
+
+#if defined(__GNUC__)
+#	define CLZ(x) __builtin_clz(x)
+#elif defined(_MSV_VER)
+#	define CLZ(x) _CountLeadingZeros(x)
+#else
+int CLZ(int32_t x)
+{
+    int e = 31;
+
+    if (!x) return 32;
+    if (x&0xFFFF0000)   { e -=16; x >>=16; }
+    if (x&0x0000FF00)   { e -= 8; x >>= 8; }
+    if (x&0x000000F0)   { e -= 4; x >>= 4; }
+    if (x&0x0000000C)   { e -= 2; x >>= 2; }
+    if (x&0x00000002)   { e -= 1; }
+    return e;
+}
+#endif
+
+typedef int (*cmp_ptr_t)(const void *, const void *);
+
+static int binsearch(void **dst, const void *x, size_t size, cmp_ptr_t cmp)
+{
+	int l, c, r;
+	void *lx, *cx, *rx;
+
+	l = 0;
+	r = size - 1;
+	c = r >> 1;
+	lx = dst[l];
+
+	/* check for beginning conditions */
+	if (cmp(x, lx) < 0)
+		return 0;
+
+	else if (cmp(x, lx) == 0) {
+		int i = 1;
+		while (cmp(x, dst[i]) == 0)
+			i++;
+		return i;
+	}
+
+	rx = dst[r];
+	/* guaranteed not to be >= rx */
+	cx = dst[c];
+	while (1) {
+		const int val = cmp(x, cx);
+		if (val < 0) {
+			if (c - l <= 1) return c;
+			r = c;
+			rx = cx;
+		} else if (val > 0) {
+			if (r - c <= 1) return c + 1;
+			l = c;
+			lx = cx;
+		} else {
+			do {
+				cx = dst[++c];
+			} while (cmp(x, cx) == 0);
+			return c;
+		}
+		c = l + ((r - l) >> 1);
+		cx = dst[c];
+	}
+}
+
+/* Binary insertion sort, but knowing that the first "start" entries are sorted.  Used in timsort. */
+static void bisort(void **dst, size_t start, size_t size, cmp_ptr_t cmp)
+{
+	size_t i;
+
+	for (i = start; i < size; i++) {
+		int j;
+		/* If this entry is already correct, just move along */
+		if (cmp(dst[i - 1], dst[i]) <= 0)
+			continue;
+
+		/* Else we need to find the right place, shift everything over, and squeeze in */
+		void *x = dst[i];
+		int location = binsearch(dst, x, i, cmp);
+		for (j = i - 1; j >= location; j--) {
+			dst[j + 1] = dst[j];
+		}
+		dst[location] = x;
+	}
+}
+
+
+/* timsort implementation, based on timsort.txt */
+struct tsort_run {
+	ssize_t start;
+	ssize_t length;
+};
+
+struct tsort_store {
+	size_t alloc;
+	cmp_ptr_t cmp;
+	void **storage;
+};
+
+static void reverse_elements(void **dst, int start, int end)
+{
+	while (start < end) {
+		void *tmp = dst[start];
+		dst[start] = dst[end];
+		dst[end] = tmp;
+
+		start++;
+		end--;
+	}
+}
+
+static int count_run(void **dst, ssize_t start, ssize_t size, struct tsort_store *store)
+{
+	ssize_t curr = start + 2;
+
+	if (size - start == 1)
+		return 1;
+
+	if (start >= size - 2) {
+		if (store->cmp(dst[size - 2], dst[size - 1]) > 0) {
+			void *tmp = dst[size - 1];
+			dst[size - 1] = dst[size - 2];
+			dst[size - 2] = tmp;
+		}
+
+		return 2;
+	}
+
+	if (store->cmp(dst[start], dst[start + 1]) <= 0) {
+		while (curr < size - 1 && store->cmp(dst[curr - 1], dst[curr]) <= 0)
+			curr++;
+
+		return curr - start;
+	} else {
+		while (curr < size - 1 && store->cmp(dst[curr - 1], dst[curr]) > 0)
+			curr++;
+
+		/* reverse in-place */
+		reverse_elements(dst, start, curr - 1);
+		return curr - start;
+	}
+}
+
+static int compute_minrun(size_t n)
+{
+	int r = 0;
+	while (n >= 64) {
+		r |= n & 1;
+		n >>= 1;
+	}
+	return n + r;
+}
+
+static int check_invariant(struct tsort_run *stack, int stack_curr)
+{
+	if (stack_curr < 2)
+		return 1;
+
+	else if (stack_curr == 2) {
+		const int A = stack[stack_curr - 2].length;
+		const int B = stack[stack_curr - 1].length;
+		return (A > B);
+	} else {
+		const int A = stack[stack_curr - 3].length;
+		const int B = stack[stack_curr - 2].length;
+		const int C = stack[stack_curr - 1].length;
+		return !((A <= B + C) || (B <= C));
+	}
+}
+
+static int resize(struct tsort_store *store, size_t new_size)
+{
+	if (store->alloc < new_size) {
+		void **tempstore = realloc(store->storage, new_size * sizeof(void *));
+
+		/**
+		 * Do not propagate on OOM; this will abort the sort and
+		 * leave the array unsorted, but no error code will be
+		 * raised
+		 */
+		if (tempstore == NULL)
+			return -1;
+
+		store->storage = tempstore;
+		store->alloc = new_size;
+	}
+
+	return 0;
+}
+
+static void merge(void **dst, const struct tsort_run *stack, int stack_curr, struct tsort_store *store)
+{
+	const ssize_t A = stack[stack_curr - 2].length;
+	const ssize_t B = stack[stack_curr - 1].length;
+	const ssize_t curr = stack[stack_curr - 2].start;
+
+	void **storage;
+	ssize_t i, j, k;
+
+	if (resize(store, MIN(A, B)) < 0)
+		return;
+
+	storage =  store->storage;
+
+	/* left merge */
+	if (A < B) {
+		memcpy(storage, &dst[curr], A * sizeof(void *));
+		i = 0;
+		j = curr + A;
+
+		for (k = curr; k < curr + A + B; k++) {
+			if ((i < A) && (j < curr + A + B)) {
+				if (store->cmp(storage[i], dst[j]) <= 0)
+					dst[k] = storage[i++];
+				else
+					dst[k] = dst[j++];
+			} else if (i < A) {
+				dst[k] = storage[i++];
+			} else
+				dst[k] = dst[j++];
+		}
+	} else {
+		memcpy(storage, &dst[curr + A], B * sizeof(void *));
+		i = B - 1;
+		j = curr + A - 1;
+
+		for (k = curr + A + B - 1; k >= curr; k--) {
+			if ((i >= 0) && (j >= curr)) {
+				if (store->cmp(dst[j], storage[i]) > 0)
+					dst[k] = dst[j--];
+				else
+					dst[k] = storage[i--];
+			} else if (i >= 0)
+				dst[k] = storage[i--];
+			else
+				dst[k] = dst[j--];
+		}
+	}
+}
+
+static ssize_t collapse(void **dst, struct tsort_run *stack, ssize_t stack_curr, struct tsort_store *store, ssize_t size)
+{
+	ssize_t A, B, C;
+
+	while (1) {
+		/* if the stack only has one thing on it, we are done with the collapse */
+		if (stack_curr <= 1)
+			break;
+
+		/* if this is the last merge, just do it */
+		if ((stack_curr == 2) && (stack[0].length + stack[1].length == size)) {
+			merge(dst, stack, stack_curr, store);
+			stack[0].length += stack[1].length;
+			stack_curr--;
+			break;
+		}
+
+		/* check if the invariant is off for a stack of 2 elements */
+		else if ((stack_curr == 2) && (stack[0].length <= stack[1].length)) {
+			merge(dst, stack, stack_curr, store);
+			stack[0].length += stack[1].length;
+			stack_curr--;
+			break;
+		}
+		else if (stack_curr == 2)
+			break;
+
+		A = stack[stack_curr - 3].length;
+		B = stack[stack_curr - 2].length;
+		C = stack[stack_curr - 1].length;
+
+		/* check first invariant */
+		if (A <= B + C) {
+			if (A < C) {
+				merge(dst, stack, stack_curr - 1, store);
+				stack[stack_curr - 3].length += stack[stack_curr - 2].length;
+				stack[stack_curr - 2] = stack[stack_curr - 1];
+				stack_curr--;
+			} else {
+				merge(dst, stack, stack_curr, store);
+				stack[stack_curr - 2].length += stack[stack_curr - 1].length;
+				stack_curr--;
+			}
+		} else if (B <= C) {
+			merge(dst, stack, stack_curr, store);
+			stack[stack_curr - 2].length += stack[stack_curr - 1].length;
+			stack_curr--;
+		} else
+			break;
+	}
+
+	return stack_curr;
+}
+
+#define PUSH_NEXT() do {\
+	len = count_run(dst, curr, size, store);\
+	run = minrun;\
+	if (run < minrun) run = minrun;\
+	if (run > (ssize_t)size - curr) run = size - curr;\
+	if (run > len) {\
+		bisort(&dst[curr], len, run, cmp);\
+		len = run;\
+	}\
+	run_stack[stack_curr++] = (struct tsort_run) {curr, len};\
+	curr += len;\
+	if (curr == (ssize_t)size) {\
+		/* finish up */ \
+		while (stack_curr > 1) { \
+			merge(dst, run_stack, stack_curr, store); \
+			run_stack[stack_curr - 2].length += run_stack[stack_curr - 1].length; \
+			stack_curr--; \
+		} \
+		if (store->storage != NULL) {\
+			free(store->storage);\
+			store->storage = NULL;\
+		}\
+		return;\
+	}\
+}\
+while (0)
+
+
+void git__tsort(void **dst, size_t size, cmp_ptr_t cmp)
+{
+	struct tsort_store _store, *store = &_store;
+	struct tsort_run run_stack[128];
+
+	ssize_t stack_curr = 0;
+	ssize_t len, run;
+	ssize_t curr = 0;
+	ssize_t minrun;
+
+	if (size < 64) {
+		bisort(dst, 1, size, cmp);
+		return;
+	}
+
+	/* compute the minimum run length */
+	minrun = compute_minrun(size);
+
+	/* temporary storage for merges */
+	store->alloc = 0;
+	store->storage = NULL;
+	store->cmp = cmp;
+
+	PUSH_NEXT();
+	PUSH_NEXT();
+	PUSH_NEXT();
+
+	while (1) {
+		if (!check_invariant(run_stack, stack_curr)) {
+			stack_curr = collapse(dst, run_stack, stack_curr, store, size);
+			continue;
+		}
+
+		PUSH_NEXT();
+	}
+}
diff --git a/src/util.c b/src/util.c
index 03e911241..4a44f9988 100644
--- a/src/util.c
+++ b/src/util.c
@@ -331,66 +331,27 @@ uint32_t git__hash(const void *key, int len, uint32_t seed)
 }
 #endif
 
-/*
- * A merge sort implementation, simplified from the qsort implementation
- * by Mike Haertel, which is a part of the GNU C Library.
+/**
+ * A modified `bsearch` from the BSD glibc.
+ *
+ * Copyright (c) 1990 Regents of the University of California.
+ * All rights reserved.
  */
-static void msort_with_tmp(void *b, size_t n, size_t s,
-		int (*cmp)(const void *, const void *),
-		char *t)
+void **git__bsearch(const void *key, void **base, size_t nmemb, int (*compar)(const void *, const void *))
 {
-	char *tmp;
-	char *b1, *b2;
-	size_t n1, n2;
-
-	if (n <= 1)
-		return;
-
-	n1 = n / 2;
-	n2 = n - n1;
-	b1 = b;
-	b2 = (char *)b + (n1 * s);
-
-	msort_with_tmp(b1, n1, s, cmp, t);
-	msort_with_tmp(b2, n2, s, cmp, t);
-
-	tmp = t;
-
-	while (n1 > 0 && n2 > 0) {
-		if (cmp(b1, b2) <= 0) {
-			memcpy(tmp, b1, s);
-			tmp += s;
-			b1 += s;
-			--n1;
-		} else {
-			memcpy(tmp, b2, s);
-			tmp += s;
-			b2 += s;
-			--n2;
-		}
-	}
-	if (n1 > 0)
-		memcpy(tmp, b1, n1 * s);
-	memcpy(b, t, (n - n2) * s);
+        int lim, cmp;
+        void **p;
+
+        for (lim = nmemb; lim != 0; lim >>= 1) {
+                p = base + (lim >> 1);
+                cmp = (*compar)(key, *p);
+                if (cmp > 0) {  /* key > p: move right */
+                        base = p + 1;
+                        lim--;
+                } else if (cmp == 0) {
+                        return (void **)p;
+                } /* else move left */
+        }
+        return NULL;
 }
 
-int git__msort(void *b, size_t n, size_t s,
-		int (*cmp)(const void *, const void *))
-{
-	const size_t size = n * s;
-	char buf[1024];
-
-	if (size < sizeof(buf)) {
-		/* The temporary array fits on the small on-stack buffer. */
-		msort_with_tmp(b, n, s, cmp, buf);
-	} else {
-		/* It's somewhat large, so malloc it.  */
-		char *tmp = git__malloc(size);
-		if (tmp == NULL)
-			return GIT_ENOMEM;
-		msort_with_tmp(b, n, s, cmp, tmp);
-		free(tmp);
-	}
-
-	return GIT_SUCCESS;
-}
diff --git a/src/util.h b/src/util.h
index c9ca4dec0..410ebfb26 100644
--- a/src/util.h
+++ b/src/util.h
@@ -118,7 +118,8 @@ extern int git__fnmatch(const char *pattern, const char *name, int flags);
 		} \
 	} while (0)
 
-extern int git__msort(void *b, size_t n, size_t s,
-		int (*cmp)(const void *, const void *));
+extern int git__tsort(void **b, size_t n, int (*cmp)(const void *, const void *));
+extern void **git__bsearch(const void *key, void **base, size_t nmemb,
+	int (*compar)(const void *, const void *));
 
 #endif /* INCLUDE_util_h__ */
diff --git a/src/vector.c b/src/vector.c
index 2fc051f0c..0b83b8b0d 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -94,7 +94,7 @@ void git_vector_sort(git_vector *v)
 	if (v->sorted || v->_cmp == NULL)
 		return;
 
-	git__msort(v->contents, v->length, sizeof(void *), v->_cmp);
+	git__tsort(v->contents, v->length, v->_cmp);
 	v->sorted = 1;
 }
 
@@ -110,7 +110,7 @@ int git_vector_bsearch2(git_vector *v, git_vector_cmp key_lookup, const void *ke
 
 	git_vector_sort(v);
 
-	find = bsearch(key, v->contents, v->length, sizeof(void *), key_lookup);
+	find = git__bsearch(key, v->contents, v->length, key_lookup);
 	if (find != NULL)
 		return (int)(find - v->contents);
 
@@ -174,7 +174,7 @@ void git_vector_uniq(git_vector *v)
 	cmp = v->_cmp ? v->_cmp : strict_comparison;
 
 	for (i = 0, j = 1 ; j < v->length; ++j)
-		if (!cmp(v->contents + i, v->contents + j))
+		if (!cmp(v->contents[i], v->contents[j]))
 			v->contents[i] = v->contents[j];
 		else
 			v->contents[++i] = v->contents[j];
diff --git a/tests/t00-core.c b/tests/t00-core.c
index 7bcc77aeb..660f10c89 100644
--- a/tests/t00-core.c
+++ b/tests/t00-core.c
@@ -75,10 +75,7 @@ END_TEST
 
 static int test_cmp(const void *a, const void *b)
 {
-	int n1 = *(int *)a;
-	int n2 = *(int *)b;
-
-	return n1 - n2;
+	return a - b;
 }
 
 BEGIN_TEST(vector2, "remove duplicates")