Add a generic "object decorator" interface, and make object refs use it

This allows you to add an arbitrary "decoration" of your choice to any
object.  It's a space- and time-efficient way to add information to
arbitrary objects, especially if most objects probably do not have the
decoration.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

decorate.c

@@ -0,0 +1,89 @@
+/*
+ * decorate.c - decorate a git object with some arbitrary
+ * data.
+ */
+#include "cache.h"
+#include "object.h"
+#include "decorate.h"
+
+static unsigned int hash_obj(struct object *obj, unsigned int n)
+{
+	unsigned int hash = *(unsigned int *)obj->sha1;
+	return hash % n;
+}
+
+static void *insert_decoration(struct decoration *n, struct object *base, void *decoration)
+{
+	int size = n->size;
+	struct object_decoration *hash = n->hash;
+	int j = hash_obj(base, size);
+
+	while (hash[j].base) {
+		if (hash[j].base == base) {
+			void *old = hash[j].decoration;
+			hash[j].decoration = decoration;
+			return old;
+		}
+		j++;
+		if (++j >= size)
+			j = 0;
+	}
+	hash[j].base = base;
+	hash[j].decoration = decoration;
+	n->nr++;
+	return NULL;
+}
+
+static void grow_decoration(struct decoration *n)
+{
+	int i;
+	int old_size = n->size;
+	struct object_decoration *old_hash;
+
+	old_size = n->size;
+	old_hash = n->hash;
+
+	n->size = (old_size + 1000) * 3 / 2;
+	n->hash = xcalloc(n->size, sizeof(struct object_decoration));
+	n->nr = 0;
+
+	for (i = 0; i < old_size; i++) {
+		struct object *base = old_hash[i].base;
+		void *decoration = old_hash[i].decoration;
+
+		if (!base)
+			continue;
+		insert_decoration(n, base, decoration);
+	}
+	free(old_hash);
+}
+
+/* Add a decoration pointer, return any old one */
+void *add_decoration(struct decoration *n, struct object *obj, void *decoration)
+{
+	int nr = n->nr + 1;
+
+	if (nr > n->size * 2 / 3)
+		grow_decoration(n);
+	return insert_decoration(n, obj, decoration);
+}
+
+/* Lookup a decoration pointer */
+void *lookup_decoration(struct decoration *n, struct object *obj)
+{
+	int j;
+
+	/* nothing to lookup */
+	if (!n->size)
+		return NULL;
+	j = hash_obj(obj, n->size);
+	for (;;) {
+		struct object_decoration *ref = n->hash + j;
+		if (ref->base == obj)
+			return ref->decoration;
+		if (!ref->base)
+			return NULL;
+		if (++j == n->size)
+			j = 0;
+	}
+}