mem-pool: move reusable parts of memory pool into its own file

This moves the reusable parts of the memory pool logic used by
fast-import.c into its own file for use by other components.

Signed-off-by: Jameson Miller <jamill@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mem-pool.c

@@ -0,0 +1,55 @@
+/*
+ * Memory Pool implementation logic.
+ */
+
+#include "cache.h"
+#include "mem-pool.h"
+
+static struct mp_block *mem_pool_alloc_block(struct mem_pool *mem_pool, size_t block_alloc)
+{
+	struct mp_block *p;
+
+	mem_pool->pool_alloc += sizeof(struct mp_block) + block_alloc;
+	p = xmalloc(st_add(sizeof(struct mp_block), block_alloc));
+	p->next_block = mem_pool->mp_block;
+	p->next_free = (char *)p->space;
+	p->end = p->next_free + block_alloc;
+	mem_pool->mp_block = p;
+
+	return p;
+}
+
+void *mem_pool_alloc(struct mem_pool *mem_pool, size_t len)
+{
+	struct mp_block *p;
+	void *r;
+
+	/* round up to a 'uintmax_t' alignment */
+	if (len & (sizeof(uintmax_t) - 1))
+		len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
+
+	for (p = mem_pool->mp_block; p; p = p->next_block)
+		if (p->end - p->next_free >= len)
+			break;
+
+	if (!p) {
+		if (len >= (mem_pool->block_alloc / 2)) {
+			mem_pool->pool_alloc += len;
+			return xmalloc(len);
+		}
+
+		p = mem_pool_alloc_block(mem_pool, mem_pool->block_alloc);
+	}
+
+	r = p->next_free;
+	p->next_free += len;
+	return r;
+}
+
+void *mem_pool_calloc(struct mem_pool *mem_pool, size_t count, size_t size)
+{
+	size_t len = st_mult(count, size);
+	void *r = mem_pool_alloc(mem_pool, len);
+	memset(r, 0, len);
+	return r;
+}