diff-delta: bound hash list length to avoid O(m*n) behavior

[git.git] / diff-delta.c

diff --git a/diff-delta.c b/diff-delta.c
index 2ed5984..aaee7be 100644 (file)
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -40,17 +40,18 @@ struct index {
 
 static struct index ** delta_index(const unsigned char *buf,
                                   unsigned long bufsize,
+                                  unsigned long trg_bufsize,
                                   unsigned int *hash_shift)
 {
-       unsigned int hsize, hshift, entries, blksize, i;
+       unsigned int i, hsize, hshift, hlimit, entries, *hash_count;
        const unsigned char *data;
        struct index *entry, **hash;
        void *mem;
 
        /* determine index hash size */
-       entries = (bufsize + BLK_SIZE - 1) / BLK_SIZE;
+       entries = bufsize  / BLK_SIZE;
        hsize = entries / 4;
-       for (i = 4; (1 << i) < hsize && i < 16; i++);
+       for (i = 4; (1 << i) < hsize && i < 31; i++);
        hsize = 1 << i;
        hshift = 32 - i;
        *hash_shift = hshift;
@@ -63,20 +64,62 @@ static struct index ** delta_index(const unsigned char *buf,
        entry = mem + hsize * sizeof(*hash);
        memset(hash, 0, hsize * sizeof(*hash));
 
-       /* then populate it */
+       /* allocate an array to count hash entries */
+       hash_count = calloc(hsize, sizeof(*hash_count));
+       if (!hash_count) {
+               free(hash);
+               return NULL;
+       }
+
+       /* then populate the index */
        data = buf + entries * BLK_SIZE - BLK_SIZE;
-       blksize = bufsize - (data - buf);
        while (data >= buf) {
-               unsigned int val = adler32(0, data, blksize);
+               unsigned int val = adler32(0, data, BLK_SIZE);
                i = HASH(val, hshift);
                entry->ptr = data;
                entry->val = val;
                entry->next = hash[i];
                hash[i] = entry++;
-               blksize = BLK_SIZE;
+               hash_count[i]++;
                data -= BLK_SIZE;
        }
 
+       /*
+        * Determine a limit on the number of entries in the same hash
+        * bucket.  This guard us against patological data sets causing
+        * really bad hash distribution with most entries in the same hash
+        * bucket that would bring us to O(m*n) computing costs (m and n
+        * corresponding to reference and target buffer sizes).
+        *
+        * The more the target buffer is large, the more it is important to
+        * have small entry lists for each hash buckets.  With such a limit
+        * the cost is bounded to something more like O(m+n).
+        */
+       hlimit = (1 << 26) / trg_bufsize;
+       if (hlimit < 4*BLK_SIZE)
+               hlimit = 4*BLK_SIZE;
+
+       /*
+        * Now make sure none of the hash buckets has more entries than
+        * we're willing to test.  Otherwise we cull the entry list
+        * uniformly to still preserve a good repartition across
+        * the reference buffer.
+        */
+       for (i = 0; i < hsize; i++) {
+               if (hash_count[i] < hlimit)
+                       continue;
+               entry = hash[i];
+               do {
+                       struct index *keep = entry;
+                       int skip = hash_count[i] / hlimit / 2;
+                       do {
+                               entry = entry->next;
+                       } while(--skip && entry);
+                       keep->next = entry;
+               } while(entry);
+       }
+       free(hash_count);
+
        return hash;
 }
 
@@ -100,7 +143,7 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 
        if (!from_size || !to_size)
                return NULL;
-       hash = delta_index(from_buf, from_size, &hash_shift);
+       hash = delta_index(from_buf, from_size, to_size, &hash_shift);
        if (!hash)
                return NULL;
 
@@ -141,29 +184,27 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 
        while (data < top) {
                unsigned int moff = 0, msize = 0;
-               unsigned int blksize = MIN(top - data, BLK_SIZE);
-               unsigned int val = adler32(0, data, blksize);
-               i = HASH(val, hash_shift);
-               for (entry = hash[i]; entry; entry = entry->next) {
-                       const unsigned char *ref = entry->ptr;
-                       const unsigned char *src = data;
-                       unsigned int ref_size = ref_top - ref;
-                       if (entry->val != val)
-                               continue;
-                       if (ref_size > top - src)
-                               ref_size = top - src;
-                       while (ref_size && *src++ == *ref) {
-                               ref++;
-                               ref_size--;
-                       }
-                       ref_size = ref - entry->ptr;
-                       if (ref_size > msize) {
-                               /* this is our best match so far */
-                               moff = entry->ptr - ref_data;
-                               msize = ref_size;
-                               if (msize >= 0x10000) {
-                                       msize = 0x10000;
+               if (data + BLK_SIZE <= top) {
+                       unsigned int val = adler32(0, data, BLK_SIZE);
+                       i = HASH(val, hash_shift);
+                       for (entry = hash[i]; entry; entry = entry->next) {
+                               const unsigned char *ref = entry->ptr;
+                               const unsigned char *src = data;
+                               unsigned int ref_size = ref_top - ref;
+                               if (entry->val != val)
+                                       continue;
+                               if (ref_size > top - src)
+                                       ref_size = top - src;
+                               if (ref_size > 0x10000)
+                                       ref_size = 0x10000;
+                               if (ref_size <= msize)
                                        break;
+                               while (ref_size-- && *src++ == *ref)
+                                       ref++;
+                               if (msize < ref - entry->ptr) {
+                                       /* this is our best match so far */
+                                       msize = ref - entry->ptr;
+                                       moff = entry->ptr - ref_data;
                                }
                        }
                }