typedef u16 hash_ptr_t;
struct hash_record {
- byte *string; // in original text
- hash_ptr_t next4, next3; // 0=end
+ /* the position in the original text is implicit; it is computed by locate_string() */
+ hash_ptr_t next; // 0=end
+ hash_ptr_t prev; // high bit: 0=record in array, 1=head in hash-table (i.e. value of hashf)
};
-#define HASH_SIZE ((1<<14) + 27) // prime, size of hash-table
+#define HASH_SIZE (1<<14) // size of hash-table
#define HASH_RECORDS (1<<15) // maximum number of records in hash-table, 0 is unused ==> subtract 1
-#define CHAIN_MAX_TESTS 16 // crop longer collision chains
-#define CHAIN_GOOD_MATCH 16 // we already have a good match => end
+#define CHAIN_MAX_TESTS 8 // crop longer collision chains
+#define CHAIN_GOOD_MATCH 32 // we already have a good match => end
static inline uns
hashf(byte *string)
+ /* 0..HASH_SIZE-1 */
{
- uns hash;
-#ifdef CPU_ALLOW_UNALIGNED
- hash = * (uns *) string;
-#elif CPU_LITTLE_ENDIAN
- hash = string[0] | (string[1]<<8) | (string[2]<<16) | (string[3]<<24);
-#else
- hash = string[3] | (string[2]<<8) | (string[1]<<16) | (string[0]<<24);
-#endif
- return hash;
+ return string[0] ^ (string[1]<<3) ^ (string[2]<<6);
}
-#ifdef CPU_LITTLE_ENDIAN
-#define MASK_4TH_CHAR (~(0xff << 24))
-#else
-#define MASK_4TH_CHAR (~0xff)
-#endif
-static inline uns
-find_match(uns list_id, hash_ptr_t *hash_tab, struct hash_record *hash_rec, uns hash, byte *string, byte *string_end, byte **best_ptr)
- /*
- * We heavily rely on gcc optimisations (e.g. list_id is a constant hence
- * some branches of the code will be pruned).
- *
- * hash_tab[hash] points to the head of the link-list of strings with
- * the same hash. The records are statically stored in circular array
- * hash_rec (with 1st entry unused), and the pointers are just 16-bit
- * indices. In one array, we store 2 independent link-lists: with 3 and 4
- * hashed characters.
- *
- * The data structure fulfills 2 invariants: the strings in every collision
- * chain are ordered by age, and the head of every link-list is placed
- * correctly. The records in the middle of the link-lists may be placed
- * wrong, and this is handled here by checking the string order.
- */
+static inline byte *
+locate_string(byte *string, uns record_id, uns head)
+ /* The strings are recorded into the hash-table regularly, hence there is no
+ * need to store the pointer there. */
{
- if (list_id == 3) /* Find the 1st record in the collision chain. It is correct. */
- hash &= MASK_4TH_CHAR;
- hash %= HASH_SIZE;
- if (!hash_tab[hash])
- return 0;
- struct hash_record *record = hash_rec + hash_tab[hash];
+ string += record_id - head;
+ if (record_id >= head)
+ string -= HASH_RECORDS-1;
+ return string;
+}
+static inline uns
+find_match(uns record_id, struct hash_record *hash_rec, byte *string, byte *string_end, byte **best_ptr, uns head)
+ /* hash_tab[hash] == record_id points to the head of the double-linked
+ * link-list of strings with the same hash. The records are statically
+ * stored in circular array hash_rec (with the 1st entry unused), and the
+ * pointers are just 16-bit indices. The strings in every collision chain
+ * are ordered by age. */
+{
uns count = CHAIN_MAX_TESTS;
uns best_len = 0;
- byte *last_cmp = string_end;
- hash_ptr_t *last_ptr = NULL;
- while (count-- > 0)
+ while (record_id && count-- > 0)
{
- byte *cmp = record->string;
- if (cmp >= last_cmp) /* collision chain is ordered by age */
- {
- *last_ptr = 0; /* prune */
- break;
- }
- last_cmp = cmp;
+ byte *record_string = locate_string(string, record_id, head);
+ byte *cmp = record_string;
if (cmp[0] == string[0] && cmp[2] == string[2])
+ /* implies cmp[1] == string[1] */
{
if (cmp[3] == string[3])
{
- /* implies cmp[1] == string[1], becase we hash into 14+eps bits */
cmp += 4;
if (*cmp++ == string[4] && *cmp++ == string[5]
&& *cmp++ == string[6] && *cmp++ == string[7])
while (str <= string_end && *cmp++ == *str++);
}
}
- else if (list_id == 3)
- /* hashing 3 characters implies cmp[1] == string[1]
- hashing 4 characters implies cmp[1] != string[1] */
- cmp += 4;
else
- goto next_collision;
- uns len = cmp - record->string - 1; /* cmp points 2 characters after the last match */
+ cmp += 4;
+ uns len = cmp - record_string - 1; /* cmp points 2 characters after the last match */
if (len > best_len)
{
best_len = len;
- *best_ptr = record->string;
- if (best_len >= CHAIN_GOOD_MATCH) /* optimisation */
+ *best_ptr = record_string;
+ if (best_len >= CHAIN_GOOD_MATCH) /* optimization */
break;
}
}
-next_collision:
- if (list_id == 3) /* move to the next collision */
- last_ptr = &record->next3;
- else
- last_ptr = &record->next4;
- uns next = *last_ptr;
- if (!next)
- break;
- record = hash_rec + next;
+ record_id = hash_rec[record_id].next;
}
return best_len;
}
static uns
-hash_string(byte *string, uns hash, hash_ptr_t *tab3, hash_ptr_t *tab4, struct hash_record *hash_rec, uns head)
- /*
- * Updates are performed more often than searches, so they are a bit lazy: we
- * add new records in front of the link-lists, but only delete them when they
- * are the _head_ of some link-list. This is done in constant time.
- *
- * By reusing an already allocated space in the circular array we necessarily
- * alter records that are referred in the _middle_ of some link-list.
- * However the newly inserted string is younger than anything else in the
- * table and it can be checked easily.
- */
+hash_string(hash_ptr_t *hash_tab, uns hash, struct hash_record *hash_rec, /*byte *string,*/ uns head, uns *to_delete)
+ /* We reuse hash-records stored in a circular array. First, delete the old
+ * one and then add the new one in front of the link-list. */
{
struct hash_record *rec = hash_rec + head;
- if (rec->string) /* unlink the original record if it was the _head_ of some link-list */
+ if (*to_delete) /* unlink the original record */
{
- uns h4 = hashf(rec->string);
- uns h3 = h4 & MASK_4TH_CHAR;
- h3 %= HASH_SIZE;
- h4 %= HASH_SIZE;
- if (tab3[h3] == head)
- tab3[h3] = 0;
- if (tab4[h4] == head)
- tab4[h4] = 0;
-
+ uns prev_id = rec->prev & ((1<<15)-1);
+ if (rec->prev & (1<<15)) /* was a head */
+ hash_tab[prev_id] = 0;
+ else /* thanks to the ordering, this was a tail */
+ hash_rec[prev_id].next = 0;
}
- uns h3 = (hash & MASK_4TH_CHAR) % HASH_SIZE;
- uns h4 = hash % HASH_SIZE;
- rec->string = string; /* is younger than any string inserted before */
- rec->next3 = tab3[h3];
- rec->next4 = tab4[h4];
- tab3[h3] = head; /* add the new record before the link-list */
- tab4[h4] = head;
- head++; /* circular buffer, reuse old records, 0 is unused */
- if (head >= HASH_RECORDS)
+ rec->next = hash_tab[hash];
+ rec->prev = (1<<15) | hash;
+ hash_rec[rec->next].prev = head;
+ hash_tab[hash] = head; /* add the new record before the link-list */
+
+ if (++head >= HASH_RECORDS) /* circular buffer, reuse old records, 0 is unused */
+ {
head = 1;
+ *to_delete = 1;
+ }
return head;
}
* LIZZARD_MAX_ADD. There must be at least LIZZARD_NEEDS_CHARS characters
* allocated after in. Returns the actual compressed length. */
{
- hash_ptr_t hash_tab3[HASH_SIZE], hash_tab4[HASH_SIZE];
+ hash_ptr_t hash_tab[HASH_SIZE];
struct hash_record hash_rec[HASH_RECORDS];
byte *in_start = in;
byte *in_end = in + in_len;
byte *out_start = out;
byte *copy_start = in;
uns head = 1; /* 0 in unused */
- bzero(hash_tab3, sizeof(hash_tab3)); /* init the hash-table */
- bzero(hash_tab4, sizeof(hash_tab4));
+ uns to_delete = 0;
+ bzero(hash_tab, sizeof(hash_tab)); /* init the hash-table */
while (in < in_end)
{
uns hash = hashf(in);
byte *best;
- uns len = find_match(4, hash_tab4, hash_rec, hash, in, in_end, &best);
- if (len >= 3)
- goto match;
- len = find_match(3, hash_tab3, hash_rec, hash, in, in_end, &best);
-
-match_found:
- if (len >= 3)
- goto match;
+ uns len = find_match(hash_tab[hash], hash_rec, in, in_end, &best, head);
+ if (len < 3)
#if 0 // TODO: now, our routine does not detect matches of length 2
- if (len == 2 && (in - best->string) < ((1<<10) + 1))
- goto match;
+ if (len == 2 && (in - best->string) < ((1<<10) + 1))
+ { /* pass-thru */ }
+ else
#endif
+ {
literal:
- head = hash_string(in, hash, hash_tab3, hash_tab4, hash_rec, head);
- in++; /* add a literal */
- continue;
+ head = hash_string(hash_tab, hash, hash_rec, head, &to_delete);
+ in++; /* add a literal */
+ continue;
+ }
-match:
if (in + len > in_end) /* crop EOF */
{
len = in_end - in;
if (len < 3)
- goto match_found;
+ goto literal;
}
/* Record the match. */
uns copy_len = in - copy_start;
uns is_in_copy_mode = copy_start==in_start || copy_len >= 4;
uns shift = in - best - 1;
/* Try to use a 2-byte sequence. */
+#if 0
if (len == 2)
{
if (is_in_copy_mode || !copy_len) /* cannot use with 0 copied characters, because this bit pattern is reserved for copy mode */
goto literal;
-
+ else
+ goto dump_2sequence;
+ } else
+#endif
+ if (len == 3 && is_in_copy_mode && shift >= (1<<11) && shift < (1<<11) + (1<<10))
+ {
+ /* optimisation for length-3 matches after a copy command */
+ shift -= 1<<11;
dump_2sequence:
if (copy_len > 0)
out = flush_copy_command(copy_start==in_start, out, copy_start, copy_len);
*out++ = (shift>>6) & ~3; /* shift fits into 10 bits */
*out++ = shift & 0xff;
}
- else if (len == 3 && is_in_copy_mode && shift >= (1<<11) && shift < (1<<11) + (1<<10))
- {
- /* optimisation for length-3 matches after a copy command */
- shift -= 1<<11;
- goto dump_2sequence;
- }
/* now, len >= 3 */
else if (shift < (1<<11) && len <= 8)
{
*out++ = shift & 0xff;
}
/* Update the hash-table. */
- head = hash_string(in, hash, hash_tab3, hash_tab4, hash_rec, head);
+ head = hash_string(hash_tab, hash, hash_rec, head, &to_delete);
for (uns i=1; i<len; i++)
- head = hash_string(in+i, hashf(in+i), hash_tab3, hash_tab4, hash_rec, head);
+ head = hash_string(hash_tab, hashf(in+i), hash_rec, head, &to_delete);
in += len;
copy_start = in;
}
projects / libucw.git / commitdiff