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-rw-r--r--object.c29
1 files changed, 28 insertions, 1 deletions
diff --git a/object.c b/object.c
index 584f7ac..57a0890 100644
--- a/object.c
+++ b/object.c
@@ -43,14 +43,32 @@ int type_from_string(const char *str)
die("invalid object type \"%s\"", str);
}
+/*
+ * Return a numerical hash value between 0 and n-1 for the object with
+ * the specified sha1. n must be a power of 2. Please note that the
+ * return value is *not* consistent across computer architectures.
+ */
static unsigned int hash_obj(const unsigned char *sha1, unsigned int n)
{
unsigned int hash;
+
+ /*
+ * Since the sha1 is essentially random, we just take the
+ * required number of bits directly from the first
+ * sizeof(unsigned int) bytes of sha1. First we have to copy
+ * the bytes into a properly aligned integer. If we cared
+ * about getting consistent results across architectures, we
+ * would have to call ntohl() here, too.
+ */
memcpy(&hash, sha1, sizeof(unsigned int));
- /* Assumes power-of-2 hash sizes in grow_object_hash */
return hash & (n - 1);
}
+/*
+ * Insert obj into the hash table hash, which has length size (which
+ * must be a power of 2). On collisions, simply overflow to the next
+ * empty bucket.
+ */
static void insert_obj_hash(struct object *obj, struct object **hash, unsigned int size)
{
unsigned int j = hash_obj(obj->sha1, size);
@@ -63,6 +81,10 @@ static void insert_obj_hash(struct object *obj, struct object **hash, unsigned i
hash[j] = obj;
}
+/*
+ * Look up the record for the given sha1 in the hash map stored in
+ * obj_hash. Return NULL if it was not found.
+ */
struct object *lookup_object(const unsigned char *sha1)
{
unsigned int i, first;
@@ -92,6 +114,11 @@ struct object *lookup_object(const unsigned char *sha1)
return obj;
}
+/*
+ * Increase the size of the hash map stored in obj_hash to the next
+ * power of 2 (but at least 32). Copy the existing values to the new
+ * hash map.
+ */
static void grow_object_hash(void)
{
int i;