xpress_decompress.c: consolidate XPRESS decompressor into one function
authorEric Biggers <ebiggers3@gmail.com>
Mon, 20 Jul 2015 04:36:19 +0000 (23:36 -0500)
committerEric Biggers <ebiggers3@gmail.com>
Wed, 22 Jul 2015 04:55:47 +0000 (23:55 -0500)
src/xpress_decompress.c

index 8a9f491..51e781a 100644 (file)
@@ -6,7 +6,7 @@
 
 /*
  *
- * Copyright (C) 2012, 2013 Eric Biggers
+ * Copyright (C) 2012, 2013, 2015 Eric Biggers
  *
  * This file is free software; you can redistribute it and/or modify it under
  * the terms of the GNU Lesser General Public License as published by the Free
 /* This value is chosen for fast decompression.  */
 #define XPRESS_TABLEBITS 12
 
-/* Decode the matches and literal bytes in a region of XPRESS-encoded data.  */
-static int
-xpress_decode_window(struct input_bitstream *istream, const u16 *decode_table,
-                    u8 *window, unsigned window_size)
-{
-       u8 *window_ptr = window;
-       u8 *window_end = &window[window_size];
-       unsigned sym;
-       unsigned match_len;
-       unsigned offset_high_bit;
-       unsigned match_offset;
-
-       while (window_ptr != window_end) {
-
-               sym = read_huffsym(istream, decode_table,
-                                  XPRESS_TABLEBITS, XPRESS_MAX_CODEWORD_LEN);
-               if (sym < XPRESS_NUM_CHARS) {
-                       /* Literal  */
-                       *window_ptr++ = sym;
-                       continue;
-               }
-
-               /* Match  */
-               match_len = sym & 0xf;
-               offset_high_bit = (sym >> 4) & 0xf;
-
-               bitstream_ensure_bits(istream, 16);
-
-               match_offset = (1 << offset_high_bit) |
-                               bitstream_pop_bits(istream, offset_high_bit);
-
-               if (match_len == 0xf) {
-                       match_len += bitstream_read_byte(istream);
-                       if (match_len == 0xf + 0xff)
-                               match_len = bitstream_read_u16(istream);
-               }
-               match_len += XPRESS_MIN_MATCH_LEN;
-
-               if (unlikely(match_offset > window_ptr - window))
-                       return -1;
-
-               if (unlikely(match_len > window_end - window_ptr))
-                       return -1;
-
-               lz_copy(window_ptr, match_len, match_offset, window_end,
-                       XPRESS_MIN_MATCH_LEN);
-
-               window_ptr += match_len;
-       }
-       return 0;
-}
-
 static int
 xpress_decompress(const void *compressed_data, size_t compressed_size,
                  void *uncompressed_data, size_t uncompressed_size, void *_ctx)
 {
-       const u8 *cdata = compressed_data;
-       u8 *lens_p;
+       const u8 * const in_begin = compressed_data;
+       u8 * const out_begin = uncompressed_data;
+       u8 *out_next = out_begin;
+       u8 * const out_end = out_begin + uncompressed_size;
        union {
                u16 decode_table[(1 << XPRESS_TABLEBITS) + 2 * XPRESS_NUM_SYMBOLS]
                                _aligned_attribute(DECODE_TABLE_ALIGNMENT);
                u8 lens[XPRESS_NUM_SYMBOLS];
        } u;
-       struct input_bitstream istream;
+       struct input_bitstream is;
 
-       /* XPRESS uses only one Huffman code.  It contains 512 symbols, and the
-        * code lengths of these symbols are given literally as 4-bit integers
-        * in the first 256 bytes of the compressed data.  */
+       /* Read the Huffman codeword lengths.  */
        if (compressed_size < XPRESS_NUM_SYMBOLS / 2)
                return -1;
-
-       lens_p = u.lens;
-       for (unsigned i = 0; i < XPRESS_NUM_SYMBOLS / 2; i++) {
-               *lens_p++ = cdata[i] & 0xf;
-               *lens_p++ = cdata[i] >> 4;
+       for (int i = 0; i < XPRESS_NUM_SYMBOLS / 2; i++) {
+               u.lens[2 * i + 0] = in_begin[i] & 0xf;
+               u.lens[2 * i + 1] = in_begin[i] >> 4;
        }
 
+       /* Build a decoding table for the Huffman code.  */
        if (make_huffman_decode_table(u.decode_table, XPRESS_NUM_SYMBOLS,
                                      XPRESS_TABLEBITS, u.lens,
                                      XPRESS_MAX_CODEWORD_LEN))
                return -1;
 
-       init_input_bitstream(&istream, cdata + XPRESS_NUM_SYMBOLS / 2,
+       /* Decode the matches and literals.  */
+
+       init_input_bitstream(&is, in_begin + XPRESS_NUM_SYMBOLS / 2,
                             compressed_size - XPRESS_NUM_SYMBOLS / 2);
 
-       return xpress_decode_window(&istream, u.decode_table,
-                                   uncompressed_data, uncompressed_size);
+       while (out_next != out_end) {
+               unsigned sym;
+               unsigned log2_offset;
+               u32 length;
+               u32 offset;
+
+               sym = read_huffsym(&is, u.decode_table,
+                                  XPRESS_TABLEBITS, XPRESS_MAX_CODEWORD_LEN);
+               if (sym < XPRESS_NUM_CHARS) {
+                       /* Literal  */
+                       *out_next++ = sym;
+               } else {
+                       /* Match  */
+                       length = sym & 0xf;
+                       log2_offset = (sym >> 4) & 0xf;
+
+                       bitstream_ensure_bits(&is, 16);
+
+                       offset = ((u32)1 << log2_offset) |
+                                bitstream_pop_bits(&is, log2_offset);
+
+                       if (length == 0xf) {
+                               length += bitstream_read_byte(&is);
+                               if (length == 0xf + 0xff)
+                                       length = bitstream_read_u16(&is);
+                       }
+                       length += XPRESS_MIN_MATCH_LEN;
+
+                       if (unlikely(offset > out_next - out_begin))
+                               return -1;
+
+                       if (unlikely(length > out_end - out_next))
+                               return -1;
+
+                       lz_copy(out_next, length, offset, out_end,
+                               XPRESS_MIN_MATCH_LEN);
+
+                       out_next += length;
+               }
+       }
+       return 0;
 }
 
 static int