sha1-array API
==============

The sha1-array API provides storage and manipulation of sets of SHA-1
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 sha1_array`::

	A single array of SHA-1 hashes. This should be initialized by
	assignment from `SHA1_ARRAY_INIT`.  The `sha1` 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
---------

`sha1_array_append`::
	Add an item to the set. The sha1 will be placed at the end of
	the array (but note that some operations below may lose this
	ordering).

`sha1_array_lookup`::
	Perform a binary search of the array for a specific sha1.
	If found, returns the offset (in number of elements) of the
	sha1. If not found, returns a negative integer. If the array is
	not sorted, this function has the side effect of sorting it.

`sha1_array_clear`::
	Free all memory associated with the array and return it to the
	initial, empty state.

`sha1_array_for_each_unique`::
	Efficiently iterate over each unique element of the list,
	executing the callback function for each one. If the array is
	not sorted, this function has the side effect of sorting it. If
	the callback returns a non-zero value, the iteration ends
	immediately and the callback's return is propagated; otherwise,
	0 is returned.

Examples
--------

-----------------------------------------
int print_callback(const unsigned char sha1[20],
		    void *data)
{
	printf("%s\n", sha1_to_hex(sha1));
	return 0; /* always continue */
}

void some_func(void)
{
	struct sha1_array hashes = SHA1_ARRAY_INIT;
	unsigned char sha1[20];

	/* Read objects into our set */
	while (read_object_from_stdin(sha1))
		sha1_array_append(&hashes, sha1);

	/* Check if some objects are in our set */
	while (read_object_from_stdin(sha1)) {
		if (sha1_array_lookup(&hashes, sha1) >= 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.
	 */
	sha1_array_for_each_unique(&hashes, print_callback, NULL);
}
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