return *istream->data++;
}
+
+/* Needed alignment of decode_table parameter to make_huffman_decode_table().
+ *
+ * Reason: We may fill the entries with SSE instructions without worrying
+ * about dealing with the unaligned case. */
+#define DECODE_TABLE_ALIGNMENT 16
+
+/* Maximum supported symbol count for make_huffman_decode_table().
+ *
+ * Reason: In direct mapping entries, we store the symbol in 11 bits. */
+#define DECODE_TABLE_MAX_SYMBOLS 2048
+
+/* Maximum supported table bits for make_huffman_decode_table().
+ *
+ * Reason: In internal binary tree nodes, offsets are encoded in 14 bits.
+ * But the real limit is 13, because we allocate entries past the end of
+ * the direct lookup part of the table for binary tree nodes. (Note: if
+ * needed this limit could be removed by encoding the offsets relative to
+ * &decode_table[1 << table_bits].) */
+#define DECODE_TABLE_MAX_TABLE_BITS 13
+
+/* Maximum supported codeword length for make_huffman_decode_table().
+ *
+ * Reason: In direct mapping entries, we encode the codeword length in 5
+ * bits, and the top 2 bits can't both be set because that has special
+ * meaning. */
+#define DECODE_TABLE_MAX_CODEWORD_LEN 23
+
/* Reads and returns the next Huffman-encoded symbol from a bitstream. If the
* input data is exhausted, the Huffman symbol is decoded as if the missing bits
- * are all zeroes. */
+ * are all zeroes.
+ *
+ * XXX: This is mostly duplicated in lzms_huffman_decode_symbol() in
+ * lzms-decompress.c. */
static inline u16
-read_huffsym(struct input_bitstream * restrict istream,
- const u16 decode_table[restrict],
- const u8 lens[restrict],
- unsigned num_syms,
- unsigned table_bits,
- unsigned max_codeword_len)
+read_huffsym(struct input_bitstream *istream, const u16 decode_table[],
+ unsigned num_syms, unsigned table_bits, unsigned max_codeword_len)
{
+ u16 entry;
+ u16 key_bits;
bitstream_ensure_bits(istream, max_codeword_len);
- /* Use the next table_bits of the input as an index into the
- * decode_table. */
- u16 key_bits = bitstream_peek_bits(istream, table_bits);
-
- u16 sym = decode_table[key_bits];
-
- if (likely(sym < num_syms)) {
- /* Fast case: The decode table directly provided the symbol. */
- bitstream_remove_bits(istream, lens[sym]);
+ /* Index the decode table by the next table_bits bits of the input. */
+ key_bits = bitstream_peek_bits(istream, table_bits);
+ entry = decode_table[key_bits];
+ if (likely(entry < 0xC000)) {
+ /* Fast case: The decode table directly provided the
+ * symbol and codeword length. The low 11 bits are the
+ * symbol, and the high 5 bits are the codeword length. */
+ bitstream_remove_bits(istream, entry >> 11);
+ return entry & 0x7FF;
} else {
- /* Slow case: The symbol took too many bits to include directly
- * in the decode table, so search for it in a binary tree at the
- * end of the decode table. */
+ /* Slow case: The codeword for the symbol is longer than
+ * table_bits, so the symbol does not have an entry
+ * directly in the first (1 << table_bits) entries of the
+ * decode table. Traverse the appropriate binary tree
+ * bit-by-bit to decode the symbol. */
bitstream_remove_bits(istream, table_bits);
do {
- key_bits = sym + bitstream_pop_bits(istream, 1);
- } while ((sym = decode_table[key_bits]) >= num_syms);
+ key_bits = (entry & 0x3FFF) + bitstream_pop_bits(istream, 1);
+ } while ((entry = decode_table[key_bits]) >= 0xC000);
+ return entry;
}
- return sym;
}
extern int
make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
- unsigned num_bits, const u8 lengths[],
+ unsigned num_bits, const u8 lens[],
unsigned max_codeword_len);
-/* Minimum alignment for the decode_table parameter to
- * make_huffman_decode_table(). */
-#define DECODE_TABLE_ALIGNMENT 16
-
#endif /* _WIMLIB_DECOMPRESS_H */