#include "git-compat-util.h"

#include "diffcore.h"

/*

 * Idea here is very simple.

 *

 * Almost all data we are interested in are text, but sometimes we have

 * to deal with binary data.  So we cut them into chunks delimited by

 * LF byte, or 64-byte sequence, whichever comes first, and hash them.

 *

 * For those chunks, if the source buffer has more instances of it

 * than the destination buffer, that means the difference are the

 * number of bytes not copied from source to destination.  If the

 * counts are the same, everything was copied from source to

 * destination.  If the destination has more, everything was copied,

 * and destination added more.

 *

 * We are doing an approximation so we do not really have to waste

 * memory by actually storing the sequence.  We just hash them into

 * somewhere around 2^16 hashbuckets and count the occurrences.

 */

/* Wild guess at the initial hash size */

#define INITIAL_HASH_SIZE 9

/* We leave more room in smaller hash but do not let it

 * grow to have unused hole too much.

 */

#define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))

/* A prime rather carefully chosen between 2^16..2^17, so that

 * HASHBASE < INITIAL_FREE(17).  We want to keep the maximum hashtable

 * size under the current 2<<17 maximum, which can hold this many

 * different values before overflowing to hashtable of size 2<<18.

 */

#define HASHBASE 107927

struct spanhash {

  unsigned int hashval;

  unsigned int cnt;

};

struct spanhash_top {

  int alloc_log2;

  int free;

  struct spanhash data[FLEX_ARRAY];

};

static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)

{

  struct spanhash_top *new_spanhash;

  int i;

  int osz = 1 << orig->alloc_log2;

  int sz = osz << 1;

  new_spanhash = xmalloc(st_add(sizeof(*orig),

           st_mult(sizeof(struct spanhash), sz)));

  new_spanhash->alloc_log2 = orig->alloc_log2 + 1;

  new_spanhash->free = INITIAL_FREE(new_spanhash->alloc_log2);

  memset(new_spanhash->data, 0, sizeof(struct spanhash) * sz);

  for (i = 0; i < osz; i++) {

    struct spanhash *o = &(orig->data[i]);

    int bucket;

    if (!o->cnt)

      continue;

    bucket = o->hashval & (sz - 1);

    while (1) {

      struct spanhash *h = &(new_spanhash->data[bucket++]);

      if (!h->cnt) {

        h->hashval = o->hashval;

        h->cnt = o->cnt;

        new_spanhash->free--;

        break;

      }

      if (sz <= bucket)

        bucket = 0;

    }

  }

  free(orig);

  return new_spanhash;

}

static struct spanhash_top *add_spanhash(struct spanhash_top *top,

           unsigned int hashval, int cnt)

{

  int bucket, lim;

  struct spanhash *h;

  lim = (1 << top->alloc_log2);

  bucket = hashval & (lim - 1);

  while (1) {

    h = &(top->data[bucket++]);

    if (!h->cnt) {

      h->hashval = hashval;

      h->cnt = cnt;

      top->free--;

      if (top->free < 0)

        return spanhash_rehash(top);

      return top;

    }

    if (h->hashval == hashval) {

      h->cnt += cnt;

      return top;

    }

    if (lim <= bucket)

      bucket = 0;

  }

}

static int spanhash_cmp(const void *a_, const void *b_)

{

  const struct spanhash *a = a_;

  const struct spanhash *b = b_;

  /* A count of zero compares at the end.. */

  if (!a->cnt)

    return !b->cnt ? 0 : 1;

  if (!b->cnt)

    return -1;

  return a->hashval < b->hashval ? -1 :

    a->hashval > b->hashval ? 1 : 0;

}

static struct spanhash_top *hash_chars(struct repository *r,

               struct diff_filespec *one)

{

  int i, n;

  unsigned int accum1, accum2, hashval;

  struct spanhash_top *hash;

  unsigned char *buf = one->data;

  unsigned int sz = one->size;

  int is_text = !diff_filespec_is_binary(r, one);

  i = INITIAL_HASH_SIZE;

  hash = xmalloc(st_add(sizeof(*hash),

            st_mult(sizeof(struct spanhash), (size_t)1 << i)));

  hash->alloc_log2 = i;

  hash->free = INITIAL_FREE(i);

  memset(hash->data, 0, sizeof(struct spanhash) * ((size_t)1 << i));

  n = 0;

  accum1 = accum2 = 0;

  while (sz) {

    unsigned int c = *buf++;

    unsigned int old_1 = accum1;

    sz--;

    /* Ignore CR in CRLF sequence if text */

    if (is_text && c == '\r' && sz && *buf == '\n')

      continue;

    accum1 = (accum1 << 7) ^ (accum2 >> 25);

    accum2 = (accum2 << 7) ^ (old_1 >> 25);

    accum1 += c;

    if (++n < 64 && c != '\n')

      continue;

    hashval = (accum1 + accum2 * 0x61) % HASHBASE;

    hash = add_spanhash(hash, hashval, n);

    n = 0;

    accum1 = accum2 = 0;

  }

  if (n > 0) {

    hashval = (accum1 + accum2 * 0x61) % HASHBASE;

    hash = add_spanhash(hash, hashval, n);

  }

  QSORT(hash->data, (size_t)1ul << hash->alloc_log2, spanhash_cmp);

  return hash;

}

int diffcore_count_changes(struct repository *r,

         struct diff_filespec *src,

         struct diff_filespec *dst,

         void **src_count_p,

         void **dst_count_p,

         unsigned long *src_copied,

         unsigned long *literal_added)

{

  struct spanhash *s, *d;

  struct spanhash_top *src_count, *dst_count;

  unsigned long sc, la;

  src_count = dst_count = NULL;

  if (src_count_p)

    src_count = *src_count_p;

  if (!src_count) {

    src_count = hash_chars(r, src);

    if (src_count_p)

      *src_count_p = src_count;

  }

  if (dst_count_p)

    dst_count = *dst_count_p;

  if (!dst_count) {

    dst_count = hash_chars(r, dst);

    if (dst_count_p)

      *dst_count_p = dst_count;

  }

  sc = la = 0;

  s = src_count->data;

  d = dst_count->data;

  for (;;) {

    unsigned dst_cnt, src_cnt;

    if (!s->cnt)

      break; /* we checked all in src */

    while (d->cnt) {

      if (d->hashval >= s->hashval)

        break;

      la += d->cnt;

      d++;

    }

    src_cnt = s->cnt;

    dst_cnt = 0;

    if (d->cnt && d->hashval == s->hashval) {

      dst_cnt = d->cnt;

      d++;

    }

    if (src_cnt < dst_cnt) {

      la += dst_cnt - src_cnt;

      sc += src_cnt;

    }

    else

      sc += dst_cnt;

    s++;

  }

  while (d->cnt) {

    la += d->cnt;

    d++;

  }

  if (!src_count_p)

    free(src_count);

  if (!dst_count_p)

    free(dst_count);

  *src_copied = sc;

  *literal_added = la;

  return 0;

}