/*

 * Copyright (C) the libgit2 contributors. All rights reserved.

 *

 * This file is part of libgit2, distributed under the GNU GPL v2 with

 * a Linking Exception. For full terms see the included COPYING file.

 */

#include <stdio.h>

#include "sha1_lookup.h"

#include "common.h"

#include "oid.h"

/*

 * Conventional binary search loop looks like this:

 *

 *	unsigned lo, hi;

 *		do {

 *				unsigned mi = (lo + hi) / 2;

 *				int cmp = "entry pointed at by mi" minus "target";

 *				if (!cmp)

 *						return (mi is the wanted one)

 *				if (cmp > 0)

 *						hi = mi; "mi is larger than target"

 *				else

 *						lo = mi+1; "mi is smaller than target"

 *		} while (lo < hi);

 *

 * The invariants are:

 *

 * - When entering the loop, lo points at a slot that is never

 *	above the target (it could be at the target), hi points at a

 *	slot that is guaranteed to be above the target (it can never

 *	be at the target).

 *

 * - We find a point 'mi' between lo and hi (mi could be the same

 *	as lo, but never can be as same as hi), and check if it hits

 *	the target. There are three cases:

 *

 *	- if it is a hit, we are happy.

 *

 *	- if it is strictly higher than the target, we set it to hi,

 *		and repeat the search.

 *

 *	- if it is strictly lower than the target, we update lo to

 *		one slot after it, because we allow lo to be at the target.

 *

 *	If the loop exits, there is no matching entry.

 *

 * When choosing 'mi', we do not have to take the "middle" but

 * anywhere in between lo and hi, as long as lo <= mi < hi is

 * satisfied. When we somehow know that the distance between the

 * target and lo is much shorter than the target and hi, we could

 * pick mi that is much closer to lo than the midway.

 *

 * Now, we can take advantage of the fact that SHA-1 is a good hash

 * function, and as long as there are enough entries in the table, we

 * can expect uniform distribution. An entry that begins with for

 * example "deadbeef..." is much likely to appear much later than in

 * the midway of the table. It can reasonably be expected to be near

 * 87% (222/256) from the top of the table.

 *

 * However, we do not want to pick "mi" too precisely. If the entry at

 * the 87% in the above example turns out to be higher than the target

 * we are looking for, we would end up narrowing the search space down

 * only by 13%, instead of 50% we would get if we did a simple binary

 * search. So we would want to hedge our bets by being less aggressive.

 *

 * The table at "table" holds at least "nr" entries of "elem_size"

 * bytes each. Each entry has the SHA-1 key at "key_offset". The

 * table is sorted by the SHA-1 key of the entries. The caller wants

 * to find the entry with "key", and knows that the entry at "lo" is

 * not higher than the entry it is looking for, and that the entry at

 * "hi" is higher than the entry it is looking for.

 */

int sha1_entry_pos(const void *table,

			size_t elem_size,

			size_t key_offset,

			unsigned lo, unsigned hi, unsigned nr,

			const unsigned char *key)

{

	const unsigned char *base = (const unsigned char*)table;

	const unsigned char *hi_key, *lo_key;

	unsigned ofs_0;

	if (!nr || lo >= hi)

		return -1;

	if (nr == hi)

		hi_key = NULL;

	else

		hi_key = base + elem_size * hi + key_offset;

	lo_key = base + elem_size * lo + key_offset;

	ofs_0 = 0;

	do {

		int cmp;

		unsigned ofs, mi, range;

		unsigned lov, hiv, kyv;

		const unsigned char *mi_key;

		range = hi - lo;

		if (hi_key) {

			for (ofs = ofs_0; ofs < 20; ofs++)

				if (lo_key[ofs] != hi_key[ofs])

					break;

			ofs_0 = ofs;

			/*

			 * byte 0 thru (ofs-1) are the same between

			 * lo and hi; ofs is the first byte that is

			 * different.

			 *

			 * If ofs==20, then no bytes are different,

			 * meaning we have entries with duplicate

			 * keys. We know that we are in a solid run

			 * of this entry (because the entries are

			 * sorted, and our lo and hi are the same,

			 * there can be nothing but this single key

			 * in between). So we can stop the search.

			 * Either one of these entries is it (and

			 * we do not care which), or we do not have

			 * it.

			 *

			 * Furthermore, we know that one of our

			 * endpoints must be the edge of the run of

			 * duplicates. For example, given this

			 * sequence:

			 *

			 *     idx 0 1 2 3 4 5

			 *     key A C C C C D

			 *

			 * If we are searching for "B", we might

			 * hit the duplicate run at lo=1, hi=3

			 * (e.g., by first mi=3, then mi=0). But we

			 * can never have lo > 1, because B < C.

			 * That is, if our key is less than the

			 * run, we know that "lo" is the edge, but

			 * we can say nothing of "hi". Similarly,

			 * if our key is greater than the run, we

			 * know that "hi" is the edge, but we can

			 * say nothing of "lo".

			 *

			 * Therefore if we do not find it, we also

			 * know where it would go if it did exist:

			 * just on the far side of the edge that we

			 * know about.

			 */

			if (ofs == 20) {

				mi = lo;

				mi_key = base + elem_size * mi + key_offset;

				cmp = memcmp(mi_key, key, 20);

				if (!cmp)

					return mi;

				if (cmp < 0)

					return -1 - hi;

				else

					return -1 - lo;

			}

			hiv = hi_key[ofs_0];

			if (ofs_0 < 19)

				hiv = (hiv << 8) | hi_key[ofs_0+1];

		} else {

			hiv = 256;

			if (ofs_0 < 19)

				hiv <<= 8;

		}

		lov = lo_key[ofs_0];

		kyv = key[ofs_0];

		if (ofs_0 < 19) {

			lov = (lov << 8) | lo_key[ofs_0+1];

			kyv = (kyv << 8) | key[ofs_0+1];

		}

		assert(lov < hiv);

		if (kyv < lov)

			return -1 - lo;

		if (hiv < kyv)

			return -1 - hi;

		/*

		 * Even if we know the target is much closer to 'hi'

		 * than 'lo', if we pick too precisely and overshoot

		 * (e.g. when we know 'mi' is closer to 'hi' than to

		 * 'lo', pick 'mi' that is higher than the target), we

		 * end up narrowing the search space by a smaller

		 * amount (i.e. the distance between 'mi' and 'hi')

		 * than what we would have (i.e. about half of 'lo'

		 * and 'hi'). Hedge our bets to pick 'mi' less

		 * aggressively, i.e. make 'mi' a bit closer to the

		 * middle than we would otherwise pick.

		 */

		kyv = (kyv * 6 + lov + hiv) / 8;

		if (lov < hiv - 1) {

			if (kyv == lov)

				kyv++;

			else if (kyv == hiv)

				kyv--;

		}

		mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;

#ifdef INDEX_DEBUG_LOOKUP

		printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);

		printf("ofs %u lov %x, hiv %x, kyv %x\n",

				ofs_0, lov, hiv, kyv);

#endif

		if (!(lo <= mi && mi < hi)) {

			giterr_set(GITERR_INVALID, "assertion failure: binary search invariant is false");

			return -1;

		}

		mi_key = base + elem_size * mi + key_offset;

		cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);

		if (!cmp)

			return mi;

		if (cmp > 0) {

			hi = mi;

			hi_key = mi_key;

		} else {

			lo = mi + 1;

			lo_key = mi_key + elem_size;

		}

	} while (lo < hi);

	return -((int)lo)-1;

}

int sha1_position(const void *table,

			size_t stride,

			unsigned lo, unsigned hi,

			const unsigned char *key)

{

	const unsigned char *base = table;

	while (lo < hi) {

		unsigned mi = (lo + hi) / 2;

		int cmp = git_oid__hashcmp(base + mi * stride, key);

		if (!cmp)

			return mi;

		if (cmp > 0)

			hi = mi;

		else

			lo = mi+1;

	}

	return -((int)lo)-1;

}