Documentation/technical/api-oid-array.txt

oid-array API
==============

The oid-array API provides storage and manipulation of sets of object
identifiers. The emphasis is on storage and processing efficiency,
making them suitable for large lists. Note that the ordering of items is
not preserved over some operations.

Data Structures
---------------

`struct oid_array`::

	A single array of object IDs. This should be initialized by
	assignment from `OID_ARRAY_INIT`.  The `oid` member contains
	the actual data. The `nr` member contains the number of items in
	the set.  The `alloc` and `sorted` members are used internally,
	and should not be needed by API callers.

Functions
---------

`oid_array_append`::
	Add an item to the set. The object ID will be placed at the end of
	the array (but note that some operations below may lose this
	ordering).

`oid_array_lookup`::
	Perform a binary search of the array for a specific object ID.
	If found, returns the offset (in number of elements) of the
	object ID. If not found, returns a negative integer. If the array
	is not sorted, this function has the side effect of sorting it.

`oid_array_clear`::
	Free all memory associated with the array and return it to the
	initial, empty state.

`oid_array_for_each`::
	Iterate over each element of the list, executing the callback
	function for each one. Does not sort the list, so any custom
	hash order is retained. If the callback returns a non-zero
	value, the iteration ends immediately and the callback's
	return is propagated; otherwise, 0 is returned.

`oid_array_for_each_unique`::
	Iterate over each unique element of the list in sorted order,
	but otherwise behave like `oid_array_for_each`. If the array
	is not sorted, this function has the side effect of sorting
	it.

`oid_array_filter`::
	Apply the callback function `want` to each entry in the array,
	retaining only the entries for which the function returns true.
	Preserve the order of the entries that are retained.

Examples
--------

-----------------------------------------
int print_callback(const struct object_id *oid,
		    void *data)
{
	printf("%s\n", oid_to_hex(oid));
	return 0; /* always continue */
}

void some_func(void)
{
	struct sha1_array hashes = OID_ARRAY_INIT;
	struct object_id oid;

	/* Read objects into our set */
	while (read_object_from_stdin(oid.hash))
		oid_array_append(&hashes, &oid);

	/* Check if some objects are in our set */
	while (read_object_from_stdin(oid.hash)) {
		if (oid_array_lookup(&hashes, &oid) >= 0)
			printf("it's in there!\n");

	/*
	 * Print the unique set of objects. We could also have
	 * avoided adding duplicate objects in the first place,
	 * but we would end up re-sorting the array repeatedly.
	 * Instead, this will sort once and then skip duplicates
	 * in linear time.
	 */
	oid_array_for_each_unique(&hashes, print_callback, NULL);
}
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