#include "cache.h"
#include "sha1-lookup.h"

static uint32_t take2(const unsigned char *sha1)
{
        return ((sha1[0] << 8) | sha1[1]);
}

/*
 * Conventional binary search loop looks like this:
 *
 *      do {
 *              int 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 the 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 update hi with
 *      it.
 *
 *    - if it is strictly lower than the target, we update lo to be
 *      one slot after it, because we allow lo to be at the target.
 *
 * 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.
 */
/*
 * The table should contain "nr" elements.
 * The sha1 of element i (between 0 and nr - 1) should be returned
 * by "fn(i, table)".
 */
int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
             sha1_access_fn fn)
{
        size_t hi = nr;
        size_t lo = 0;
        size_t mi = 0;

        if (!nr)
                return -1;

        if (nr != 1) {
                size_t lov, hiv, miv, ofs;

                for (ofs = 0; ofs < 18; ofs += 2) {
                        lov = take2(fn(0, table) + ofs);
                        hiv = take2(fn(nr - 1, table) + ofs);
                        miv = take2(sha1 + ofs);
                        if (miv < lov)
                                return -1;
                        if (hiv < miv)
                                return -1 - nr;
                        if (lov != hiv) {
                                /*
                                 * At this point miv could be equal
                                 * to hiv (but sha1 could still be higher);
                                 * the invariant of (mi < hi) should be
                                 * kept.
                                 */
                                mi = (nr - 1) * (miv - lov) / (hiv - lov);
                                if (lo <= mi && mi < hi)
                                        break;
                                die("BUG: assertion failed in binary search");
                        }
                }
                if (18 <= ofs)
                        die("cannot happen -- lo and hi are identical");
        }

        do {
                int cmp;
                cmp = hashcmp(fn(mi, table), sha1);
                if (!cmp)
                        return mi;
                if (cmp > 0)
                        hi = mi;
                else
                        lo = mi + 1;
                mi = (hi + lo) / 2;
        } while (lo < hi);
        return -lo-1;
}

/*
 * 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 = table;
        const unsigned char *hi_key, *lo_key;
        unsigned ofs_0;
        static int debug_lookup = -1;

        if (debug_lookup < 0)
                debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");

        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.
                         */
                        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;

                if (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);
                }
                if (!(lo <= mi && mi < hi))
                        die("assertion failure lo %u mi %u hi %u %s",
                            lo, mi, hi, sha1_to_hex(key));

                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 -lo-1;
}