zlib 1.2.2.1

crc32.c

@@ -1,12 +1,12 @@
 /* crc32.c -- compute the CRC-32 of a data stream
- * Copyright (C) 1995-2003 Mark Adler
+ * Copyright (C) 1995-2004 Mark Adler
  * For conditions of distribution and use, see copyright notice in zlib.h
  *
  * Thanks to Rodney Brown <rbrown64@csc.com.au> for his contribution of faster
  * CRC methods: exclusive-oring 32 bits of data at a time, and pre-computing
  * tables for updating the shift register in one step with three exclusive-ors
- * instead of four steps with four exclusive-ors.  This results about a factor
- * of two increase in speed on a Power PC G4 (PPC7455) using gcc -O3.
+ * instead of four steps with four exclusive-ors.  This results in about a
+ * factor of two increase in speed on a Power PC G4 (PPC7455) using gcc -O3.
  */
 
 /* @(#) $Id$ */
@@ -72,6 +72,9 @@ local void make_crc_table OF((void));
 #ifdef MAKECRCH
    local void write_table OF((FILE *, const unsigned long FAR *));
 #endif /* MAKECRCH */
+local unsigned long gf2_matrix_times OF((unsigned long *mat,
+                                         unsigned long vec));
+local void gf2_matrix_square OF((unsigned long *square, unsigned long *mat));
 
 /*
   Generate tables for a byte-wise 32-bit CRC calculation on the polynomial:
@@ -331,3 +334,89 @@ local unsigned long crc32_big(crc, buf, len)
 }
 
 #endif /* BYFOUR */
+
+#define GF2_DIM 32      /* dimension of GF(2) vectors (length of CRC) */
+
+/* ========================================================================= */
+local unsigned long gf2_matrix_times(mat, vec)
+    unsigned long *mat;
+    unsigned long vec;
+{
+    unsigned long sum;
+
+    sum = 0;
+    while (vec) {
+        if (vec & 1)
+            sum ^= *mat;
+        vec >>= 1;
+        mat++;
+    }
+    return sum;
+}
+
+/* ========================================================================= */
+local void gf2_matrix_square(square, mat)
+    unsigned long *square;
+    unsigned long *mat;
+{
+    int n;
+
+    for (n = 0; n < GF2_DIM; n++)
+        square[n] = gf2_matrix_times(mat, mat[n]);
+}
+
+/* ========================================================================= */
+uLong ZEXPORT crc32_combine(crc1, crc2, len2)
+    uLong crc1;
+    uLong crc2;
+    uLong len2;
+{
+    int n;
+    unsigned long row;
+    unsigned long even[GF2_DIM];    /* even-power-of-two zeros operator */
+    unsigned long odd[GF2_DIM];     /* odd-power-of-two zeros operator */
+
+    /* degenerate case */
+    if (len2 == 0)
+        return crc1;
+
+    /* put operator for one zero bit in odd */
+    odd[0] = 0xedb88320L;           /* CRC-32 polynomial */
+    row = 1;
+    for (n = 1; n < GF2_DIM; n++) {
+        odd[n] = row;
+        row <<= 1;
+    }
+
+    /* put operator for two zero bits in even */
+    gf2_matrix_square(even, odd);
+
+    /* put operator for four zero bits in odd */
+    gf2_matrix_square(odd, even);
+
+    /* apply len2 zeros to crc1 (first square will put the operator for one
+       zero byte, eight zero bits, in even) */
+    do {
+        /* apply zeros operator for this bit of len2 */
+        gf2_matrix_square(even, odd);
+        if (len2 & 1)
+            crc1 = gf2_matrix_times(even, crc1);
+        len2 >>= 1;
+
+        /* if no more bits set, then done */
+        if (len2 == 0)
+            break;
+
+        /* another iteration of the loop with odd and even swapped */
+        gf2_matrix_square(odd, even);
+        if (len2 & 1)
+            crc1 = gf2_matrix_times(odd, crc1);
+        len2 >>= 1;
+
+        /* if no more bits set, then done */
+    } while (len2 != 0);
+
+    /* return combined crc */
+    crc1 ^= crc2;
+    return crc1;
+}