Optimize the implementation of _cairo_lzw_compress.

This switches the implementation of _cairo_lzw_compress from using
cairo-hash.c to a custom implementation of the same algorithm that
takes advantage of the specific details of what is needed here.

This final version now has comparable performance to the libtiff code,
but should hopefully be easier to understand.

Add a bunch of comments explaining how the various pieces work.

1 files changed, 217 insertions, 160 deletions

diff --git a/src/cairo-lzw.c b/src/cairo-lzw.c
index 67a41bde3..1de618cc5 100644
--- a/src/cairo-lzw.c
+++ b/src/cairo-lzw.c
@@ -36,10 +36,9 @@
 
 #include "cairoint.h"
 
-static unsigned long
-_cairo_hash_bytes (const unsigned char *c, int size);
-
 typedef struct _lzw_buf {
+    cairo_status_t status;
+
     unsigned char *data;
     int data_size;
     int num_data;
@@ -52,46 +51,56 @@ typedef struct _lzw_buf {
  *
  * Initialize an lzw_buf_t to the given size in bytes.
  *
- * Returns CAIRO_STATUS_SUCCESS or CAIRO_STATUS_NO_MEMORY.
+ * To store objects into the lzw_buf_t, call _lzw_buf_store_bits and
+ * when finished, call _lzw_buf_store_pending, (which flushes out the
+ * last few bits that hadn't yet made a complete byte yet).
+ *
+ * Instead of returning failure from any functions, lzw_buf_t provides
+ * a status value that the caller can query, (and should query at
+ * least once when done with the object). The status value will be
+ * either CAIRO_STATUS_SUCCESS or CAIRO_STATUS_NO_MEMORY;
  */
-static cairo_status_t
+static void
 _lzw_buf_init (lzw_buf_t *buf, int size)
 {
     if (size == 0)
 	size = 16;
 
+    buf->status = CAIRO_STATUS_SUCCESS;
+
     buf->data = malloc (size);
     if (buf->data == NULL) {
 	buf->data_size = 0;
-	return CAIRO_STATUS_NO_MEMORY;
+	buf->status = CAIRO_STATUS_NO_MEMORY;
+	return;
     }
 
     buf->data_size = size;
     buf->num_data = 0;
     buf->pending = 0;
     buf->pending_bits = 0;
-
-    return CAIRO_STATUS_SUCCESS;
-}
-
-static void
-_lzw_buf_fini (lzw_buf_t *buf)
-{
-    assert (buf->pending_bits == 0);
-
-    free (buf->data);
-    buf->data = 0;
 }
 
+/* Increase the buffer size by doubling.
+ *
+ * Returns CAIRO_STATUS_SUCCESS or CAIRO_STATUS_NO_MEMORY
+ */
 static cairo_status_t
 _lzw_buf_grow (lzw_buf_t *buf)
 {
     int new_size = buf->data_size * 2;
     unsigned char *new_data;
 
+    if (buf->status)
+	return buf->status;
+
     new_data = realloc (buf->data, new_size);
-    if (new_data == NULL)
-	return CAIRO_STATUS_NO_MEMORY;
+    if (new_data == NULL) {
+	free (buf->data);
+	buf->data_size = 0;
+	buf->status = CAIRO_STATUS_NO_MEMORY;
+	return buf->status;
+    }
 
     buf->data = new_data;
     buf->data_size = new_size;
@@ -107,17 +116,17 @@ _lzw_buf_grow (lzw_buf_t *buf)
  * See also _lzw_buf_store_pending which must be called after the last
  * call to _lzw_buf_store_bits.
  *
- * Returns CAIRO_STATUS_SUCCESS or CAIRO_STATUS_NO_MEMORY.
+ * Sets buf->status to either CAIRO_STATUS_SUCCESS or CAIRO_STATUS_NO_MEMORY.
  */
-static cairo_status_t
+static void
 _lzw_buf_store_bits (lzw_buf_t *buf, uint16_t value, int num_bits)
 {
     cairo_status_t status;
 
     assert (value <= (1 << num_bits) - 1);
 
-    if (getenv ("CAIRO_DEBUG_LZW"))
-	printf ("%d(%d) ", value, num_bits);
+    if (buf->status)
+	return;
 
     buf->pending = (buf->pending << num_bits) | value;
     buf->pending_bits += num_bits;
@@ -126,13 +135,11 @@ _lzw_buf_store_bits (lzw_buf_t *buf, uint16_t value, int num_bits)
 	if (buf->num_data >= buf->data_size) {
 	    status = _lzw_buf_grow (buf);
 	    if (status)
-		return status;
+		return;
 	}
 	buf->data[buf->num_data++] = buf->pending >> (buf->pending_bits - 8);
 	buf->pending_bits -= 8;
     }
-
-    return CAIRO_STATUS_SUCCESS;
 }
 
 /* Store the last remaining pending bits into the buffer.
@@ -140,217 +147,267 @@ _lzw_buf_store_bits (lzw_buf_t *buf, uint16_t value, int num_bits)
  * NOTE: This function must be called after the last call to
  * _lzw_buf_store_bits.
  *
- * Returns CAIRO_STATUS_SUCCESS or CAIRO_STATUS_NO_MEMORY.
+ * Sets buf->status to either CAIRO_STATUS_SUCCESS or CAIRO_STATUS_NO_MEMORY.
  */
-static cairo_status_t
+static void
 _lzw_buf_store_pending  (lzw_buf_t *buf)
 {
     cairo_status_t status;
 
+    if (buf->status)
+	return;
+
     if (buf->pending_bits == 0)
-	return CAIRO_STATUS_SUCCESS;
+	return;
 
     assert (buf->pending_bits < 8);
 
     if (buf->num_data >= buf->data_size) {
 	status = _lzw_buf_grow (buf);
 	if (status)
-	    return status;
+	    return;
     }
 
     buf->data[buf->num_data++] = buf->pending << (8 - buf->pending_bits);
     buf->pending_bits = 0;
-
-    return CAIRO_STATUS_SUCCESS;
 }
 
-typedef struct _lzw_symbol {
-    cairo_hash_entry_t hash_entry;
+/* LZW defines a few magic code values */
+#define LZW_CODE_CLEAR_TABLE	256
+#define LZW_CODE_EOD		257
+#define LZW_CODE_FIRST		258
+
+/* We pack three separate values into a symbol as follows:
+ *
+ * 12 bits (31 down to 20):	CODE: code value used to represent this symbol
+ * 12 bits (19 down to  8):	PREV: previous code value in chain
+ *  8 bits ( 7 down to  0):	NEXT: next byte value in chain
+ */
+typedef uint32_t lzw_symbol_t;
+
+#define LZW_SYMBOL_SET(sym, prev, next)			((sym) = ((prev) << 8)|(next))
+#define LZW_SYMBOL_SET_CODE(sym, code, prev, next)	((sym) = ((code << 20)|(prev) << 8)|(next))
+#define LZW_SYMBOL_GET_CODE(sym)			(((sym) >> 20))
+#define LZW_SYMBOL_GET_PREV(sym)			(((sym) >>  8) & 0x7ff)
+#define LZW_SYMBOL_GET_BYTE(sym)			(((sym) >>  0) & 0x0ff)
 
-    /* "key" is the symbol */
-    unsigned char *data;
-    int size;
-    /* "value" is the code */
-    int value;
-} lzw_symbol_t;
+/* The PREV+NEXT fields can be seen as the key used to fetch values
+ * from the hash table, while the code is the value fetched.
+ */
+#define LZW_SYMBOL_KEY_MASK	0x000fffff
 
+/* Since code values are only stored starting with 258 we can safely
+ * use a zero value to represent free slots in the hash table. */
+#define LZW_SYMBOL_FREE		0x00000000
+
+/* These really aren't very free for modifying. First, the PostScript
+ * specification sets the 9-12 bit range. Second, the encoding of
+ * lzw_symbol_t above also relies on 2 of LZW_BITS_MAX plus one byte
+ * fitting within 32 bits.
+ *
+ * But other than that, the LZW compression scheme could function with
+ * more bits per code.
+ */
 #define LZW_BITS_MIN		9
 #define LZW_BITS_MAX		12
 #define LZW_BITS_BOUNDARY(bits)	((1<<(bits))-1)
 #define LZW_MAX_SYMBOLS		(1<<LZW_BITS_MAX)
 
-typedef struct _lzw_symbols {
-    lzw_symbol_t symbols[LZW_MAX_SYMBOLS];
-    int num_symbols;
-
-    cairo_hash_table_t *table;
-} lzw_symbols_t;
-
-static cairo_bool_t
-_lzw_symbols_equal (const void *key_a, const void *key_b)
-{
-    const lzw_symbol_t *symbol_a = key_a;
-    const lzw_symbol_t *symbol_b = key_b;
-
-    if (symbol_a->size != symbol_b->size)
-	return FALSE;
-
-    return ! memcmp (symbol_a->data, symbol_b->data, symbol_a->size);
-}
-
-static cairo_status_t
-_lzw_symbols_init (lzw_symbols_t *symbols)
-{
-    symbols->num_symbols = 0;
-
-    symbols->table = _cairo_hash_table_create (_lzw_symbols_equal);
-    if (symbols->table == NULL)
-	return CAIRO_STATUS_NO_MEMORY;
+#define LZW_SYMBOL_TABLE_SIZE	9013
+#define LZW_SYMBOL_MOD1		LZW_SYMBOL_TABLE_SIZE
+#define LZW_SYMBOL_MOD2		9011
 
-    return CAIRO_STATUS_SUCCESS;
-}
+typedef struct _lzw_symbol_table {
+    lzw_symbol_t table[LZW_SYMBOL_TABLE_SIZE];
+} lzw_symbol_table_t;
 
+/* Initialize the hash table to entirely empty */
 static void
-_lzw_symbols_fini (lzw_symbols_t *symbols)
+_lzw_symbol_table_init (lzw_symbol_table_t *table)
 {
-    int i;
-
-    for (i=0; i < symbols->num_symbols; i++)
-	_cairo_hash_table_remove (symbols->table, &symbols->symbols[i].hash_entry);
-
-    symbols->num_symbols = 0;
-
-    _cairo_hash_table_destroy (symbols->table);
+    memset (table->table, 0, LZW_SYMBOL_TABLE_SIZE * sizeof (lzw_symbol_t));
 }
 
+/* Lookup a symbol in the symbol table. The PREV and NEXT fields of
+ * symbol form the key for the lookup.
+ *
+ * If succesful, then this function returns TRUE and slot_ret will be
+ * left pointing at the result that will have the CODE field of
+ * interest.
+ *
+ * If the lookup fails, then this function returns FALSE and slot_ret
+ * will be pointing at the location in the table to which a new CODE
+ * value should be stored along with PREV and NEXT.
+ */
 static cairo_bool_t
-_lzw_symbols_has (lzw_symbols_t *symbols,
-		  lzw_symbol_t  *symbol,
-		  lzw_symbol_t **code)
+_lzw_symbol_table_lookup (lzw_symbol_table_t	 *table,
+			  lzw_symbol_t		  symbol,
+			  lzw_symbol_t		**slot_ret)
 {
-    symbol->hash_entry.hash = _cairo_hash_bytes (symbol->data, symbol->size);
-
-    return _cairo_hash_table_lookup (symbols->table,
-				     &symbol->hash_entry,
-				     (cairo_hash_entry_t **) code);
-}
-
-static cairo_status_t
-_lzw_symbols_store (lzw_symbols_t *symbols,
-		    lzw_symbol_t  *symbol)
-{
-    cairo_status_t status;
-
-    symbol->hash_entry.hash = _cairo_hash_bytes (symbol->data, symbol->size);
-
-    symbols->symbols[symbols->num_symbols] = *symbol;
+    /* The algorithm here is identical to that in cairo-hash.c. We
+     * copy it here to allow for a rather more efficient
+     * implementation due to several circumstances that do not apply
+     * to the more general case:
+     *
+     * 1) We have a known bound on the total number of symbols, so we
+     *    have a fixed-size table without any copying when growing
+     *
+     * 2) We never delete any entries, so we don't need to
+     *    support/check for DEAD entries during lookup.
+     *
+     * 3) The object fits in 32 bits so we store each object in its
+     *    entirety within the table rather than storing objects
+     *    externally and putting pointers in the table, (which here
+     *    would just double the storage requirements and have negative
+     *    impacts on memory locality).
+     */
+    int i, idx, step, hash = symbol & LZW_SYMBOL_KEY_MASK;
+    lzw_symbol_t candidate;
+
+    idx = hash % LZW_SYMBOL_MOD1;
+    step = 0;
+
+    *slot_ret = NULL;
+    for (i = 0; i < LZW_SYMBOL_TABLE_SIZE; i++)
+    {
+	candidate = table->table[idx];
+	if (candidate == LZW_SYMBOL_FREE)
+	{
+	    *slot_ret = &table->table[idx];
+	    return FALSE;
+	}
+	else /* candidate is LIVE */
+	{
+	    if ((candidate & LZW_SYMBOL_KEY_MASK) ==
+		(symbol & LZW_SYMBOL_KEY_MASK))
+	    {
+		*slot_ret = &table->table[idx];
+		return TRUE;
+	    }
+	}
 
-    status = _cairo_hash_table_insert (symbols->table, &symbols->symbols[symbols->num_symbols].hash_entry);
-    if (status)
-	return status;
+	if (step == 0) {
+	    step = hash % LZW_SYMBOL_MOD2;
+	    if (step == 0)
+		step = 1;
+	}
 
-    symbols->num_symbols++;
+	idx += step;
+	if (idx >= LZW_SYMBOL_TABLE_SIZE)
+	    idx -= LZW_SYMBOL_TABLE_SIZE;
+    }
 
-    return CAIRO_STATUS_SUCCESS;
+    return FALSE;
 }
 
-#define LZW_CODE_CLEAR_TABLE	256
-#define LZW_CODE_EOD		257
-#define LZW_CODE_FIRST		258
-
-unsigned char *
+/* Compress a bytestream using the LZW algorithm.
+ *
+ * This is an original implementation based on reading the
+ * specification of the LZWDecode filter in the PostScript Language
+ * Reference. The free parameters in the LZW algorithm are set to the
+ * values mandated by PostScript, (symbols encoded with widths from 9
+ * to 12 bits).
+ *
+ * This function returns a pointer to a newly allocated buffer holding
+ * the compressed data, or NULL if an out-of-memory situation
+ * occurs.
+ *
+ * Notice that any one of the _lzw_buf functions called here could
+ * trigger an out-of-memory condition. But lzw_buf_t uses cairo's
+ * shutdown-on-error idiom, so it's safe to continue to call into
+ * lzw_buf without having to check for errors, (until a final check at
+ * the end).
+ */
+cairo_public unsigned char *
 _cairo_lzw_compress (unsigned char *data, unsigned long *size_in_out)
 {
-    cairo_status_t status;
     int bytes_remaining = *size_in_out;
-    lzw_symbols_t symbols;
-    lzw_symbol_t symbol, *tmp, *code;
     lzw_buf_t buf;
+    lzw_symbol_table_t table;
+    lzw_symbol_t symbol, *slot;
     int code_next = LZW_CODE_FIRST;
     int code_bits = LZW_BITS_MIN;
+    int prev, next;
 
-    status = _lzw_buf_init (&buf, *size_in_out / 4);
-    if (status)
+    if (*size_in_out == 0)
 	return NULL;
 
-    status = _lzw_symbols_init (&symbols);
-    if (status) {
-	_lzw_buf_fini (&buf);
-	return NULL;
-    }
+    _lzw_buf_init (&buf, *size_in_out);
 
-    _lzw_buf_store_bits (&buf, LZW_CODE_CLEAR_TABLE, code_bits);
+    _lzw_symbol_table_init (&table);
     
-    symbol.data = data;
-    symbol.size = 2;
+    /* The LZW header is a clear table code. */
+    _lzw_buf_store_bits (&buf, LZW_CODE_CLEAR_TABLE, code_bits);
 
-    while (bytes_remaining) {
-	code = NULL;
-	while (symbol.size <= bytes_remaining &&
-	       _lzw_symbols_has (&symbols, &symbol, &tmp))
-	{
-	    code = tmp;
-	    symbol.size++;
+    while (1) {
+
+	/* Find the longest existing code in the symbol table that
+	 * matches the current input, if any. */
+	prev = *data++;
+	bytes_remaining--;
+	if (bytes_remaining) {
+	    do
+	    {
+		next = *data++;
+		bytes_remaining--;
+		LZW_SYMBOL_SET (symbol, prev, next);
+		if (_lzw_symbol_table_lookup (&table, symbol, &slot))
+		    prev = LZW_SYMBOL_GET_CODE (*slot);
+	    } while (bytes_remaining && *slot != LZW_SYMBOL_FREE);
+	    if (*slot == LZW_SYMBOL_FREE) {
+		data--;
+		bytes_remaining++;
+	    }
 	}
 
-	if (code)
-	    _lzw_buf_store_bits (&buf, code->value, code_bits);
-	else
-	    _lzw_buf_store_bits (&buf, symbol.data[0], code_bits);
+	/* Write the code into the output. This is either a byte read
+	 * directly from the input, or a code from the last successful
+	 * lookup. */
+	_lzw_buf_store_bits (&buf, prev, code_bits);
 
-	if (symbol.size == bytes_remaining + 1)
+	if (bytes_remaining == 0)
 	    break;
 
-	symbol.value = code_next++;
-	_lzw_symbols_store (&symbols, &symbol);
+	LZW_SYMBOL_SET_CODE (*slot, code_next++, prev, next);
 
-	/* XXX: This is just for compatibility testing against libtiff. */
+	/* The libtiff code detects a full table and clears it a bit
+	 * earlier than strictly necessary. Here is the code to do
+	 * that which might be useful in doing correctness comparisons
+	 * against the output of libtiff. */
+#if CLEAR_TABLE_EARLY_FOR_IDENTICAL_OUTPUT_COMPAREED_TO_LIBTIFF
 	if (code_next == LZW_BITS_BOUNDARY(LZW_BITS_MAX) - 1) {
-	    _lzw_symbols_fini (&symbols);
-	    _lzw_symbols_init (&symbols);
+	    _lzw_symbol_table_init (&table);
 	    _lzw_buf_store_bits (&buf, LZW_CODE_CLEAR_TABLE, code_bits);
 	    code_bits = LZW_BITS_MIN;
 	    code_next = LZW_CODE_FIRST;
 	}
+#endif
 
 	if (code_next > LZW_BITS_BOUNDARY(code_bits))
 	{
 	    code_bits++;
 	    if (code_bits > LZW_BITS_MAX) {
-		_lzw_symbols_fini (&symbols);
-		_lzw_symbols_init (&symbols);
-		_lzw_buf_store_bits (&buf, LZW_CODE_CLEAR_TABLE, code_bits);
+		_lzw_symbol_table_init (&table);
+		_lzw_buf_store_bits (&buf, LZW_CODE_CLEAR_TABLE, code_bits - 1);
 		code_bits = LZW_BITS_MIN;
 		code_next = LZW_CODE_FIRST;
 	    }
 	}
-
-	if (code) {
-	    symbol.data += (symbol.size - 1);
-	    bytes_remaining -= (symbol.size - 1);
-	} else {
-	    symbol.data += 1;
-	    bytes_remaining -= 1;
-	}
-	symbol.size = 2;
     }
 
+    /* The LZW footer is an end-of-data code. */
     _lzw_buf_store_bits (&buf, LZW_CODE_EOD, code_bits);
 
     _lzw_buf_store_pending (&buf);
 
-    _lzw_symbols_fini (&symbols);
+    /* See if we ever ran out of memory while writing to buf. */
+    if (buf.status == CAIRO_STATUS_NO_MEMORY) {
+	*size_in_out = 0;
+	return NULL;
+    }
+
+    assert (buf.status == CAIRO_STATUS_SUCCESS);
 
     *size_in_out = buf.num_data;
     return buf.data;
 }
-
-static unsigned long
-_cairo_hash_bytes (const unsigned char *c, int size)
-{
-    /* This is the djb2 hash. */
-    unsigned long hash = 5381;
-    while (size--)
-	hash = ((hash << 5) + hash) + *c++;
-    return hash;
-}