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 27f83a0..0730b24 100644 (file)
--- a/diff-delta.c
+++ b/diff-delta.c
@@ -30,19 +30,20 @@ struct index {
 
 static struct index ** delta_index(const unsigned char *buf,
                                   unsigned long bufsize,
+                                  unsigned long trg_bufsize,
                                   unsigned int *hash_shift)
 {
        unsigned long hsize;
-       unsigned int hshift, i;
+       unsigned int i, hshift, hlimit, *hash_count;
        const unsigned char *data;
        struct index *entry, **hash;
        void *mem;
 
        /* determine index hash size */
        hsize = bufsize / 4;
-       for (i = 8; (1 << i) < hsize && i < 16; i++);
+       for (i = 8; (1 << i) < hsize && i < 24; i += 2);
        hsize = 1 << i;
-       hshift = i - 8;
+       hshift = (i - 8) / 2;
        *hash_shift = hshift;
 
        /* allocate lookup index */
@@ -53,15 +54,59 @@ 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 + bufsize - 2;
        while (data > buf) {
                entry->ptr = --data;
-               i = data[0] ^ data[1] ^ (data[2] << hshift);
+               i = data[0] ^ ((data[1] ^ (data[2] << hshift)) << hshift);
                entry->next = hash[i];
                hash[i] = entry++;
+               hash_count[i]++;
        }
 
+       /*
+        * 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 < 16)
+               hlimit = 16;
+
+       /*
+        * Now make sure none of the hash buckets has more entries than
+        * we're willing to test.  Otherwise we short-circuit 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;
 }
 
@@ -85,7 +130,7 @@ 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;
 
@@ -126,8 +171,8 @@ void *diff_delta(void *from_buf, unsigned long from_size,
 
        while (data < top) {
                unsigned int moff = 0, msize = 0;
-               if (data + 2 < top) {
-                       i = data[0] ^ data[1] ^ (data[2] << hash_shift);
+               if (data + 3 <= top) {
+                       i = data[0] ^ ((data[1] ^ (data[2] << hash_shift)) << hash_shift);
                        for (entry = hash[i]; entry; entry = entry->next) {
                                const unsigned char *ref = entry->ptr;
                                const unsigned char *src = data;
@@ -138,11 +183,9 @@ void *diff_delta(void *from_buf, unsigned long from_size,
                                        ref_size = 0x10000;
                                if (ref_size <= msize)
                                        break;
-                               while (ref_size && *src++ == *ref) {
-                                       ref++;
-                                       ref_size--;
-                               }
-                               ref_size = ref - entry->ptr;
+                               if (*ref != *src)
+                                       continue;
+                               while (ref_size-- && *++src == *++ref);
                                if (msize < ref - entry->ptr) {
                                        /* this is our best match so far */
                                        msize = ref - entry->ptr;