1 files changed, 140 insertions, 0 deletions

diff --git a/pack.c b/pack.c
new file mode 100644
index 0000000..1c07230
--- /dev/null
+++ b/pack.c
@@ -0,0 +1,140 @@
+/*
+  Copyright (C) 2010 Perens LLC <bruce@perens.com>
+
+  All rights reserved.
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser General Public License version 2.1, as
+  published by the Free Software Foundation.  This program is
+  distributed in the hope that it will be useful, but WITHOUT ANY
+  WARRANTY; without even the implied warranty of MERCHANTABILITY or
+  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+  License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with this program; if not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "defines.h"
+#include "quantise.h"
+#include <stdio.h>
+
+/* Compile-time constants */
+/* Size of unsigned char in bits. Assumes 8 bits-per-char. */
+static const unsigned int       WordSize = 8;
+
+/* Mask to pick the bit component out of bitIndex. */
+static const unsigned int       IndexMask = 0x7;
+
+/* Used to pick the word component out of bitIndex. */
+static const unsigned int       ShiftRight = 3;
+
+/** Pack a bit field into a bit string, encoding the field in Gray code.
+ *
+ * The output is an array of unsigned char data. The fields are efficiently
+ * packed into the bit string. The Gray coding is a naive attempt to reduce
+ * the effect of single-bit errors, we expect to do a better job as the
+ * codec develops.
+ *
+ * This code would be simpler if it just set one bit at a time in the string,
+ * but would hit the same cache line more often. I'm not sure the complexity
+ * gains us anything here.
+ *
+ * Although field is currently of int type rather than unsigned for
+ * compatibility with the rest of the code, indices are always expected to
+ * be >= 0.
+ */
+void
+pack(
+ unsigned char *        bitArray, /* The output bit string. */
+ unsigned int *         bitIndex, /* Index into the string in BITS, not bytes.*/
+ int                    field,    /* The bit field to be packed. */
+ unsigned int           fieldWidth/* Width of the field in BITS, not bytes. */
+ )
+{
+    pack_natural_or_gray(bitArray, bitIndex, field, fieldWidth, 1);
+}
+
+void
+pack_natural_or_gray(
+ unsigned char *        bitArray,  /* The output bit string. */
+ unsigned int *         bitIndex,  /* Index into the string in BITS, not bytes.*/
+ int                    field,     /* The bit field to be packed. */
+ unsigned int           fieldWidth,/* Width of the field in BITS, not bytes. */
+ unsigned int           gray       /* non-zero for gray coding */
+ )
+{
+  if (gray) {
+    /* Convert the field to Gray code */
+    field = (field >> 1) ^ field;
+  }
+
+  do {
+    unsigned int        bI = *bitIndex;
+    unsigned int        bitsLeft = WordSize - (bI & IndexMask);
+    unsigned int        sliceWidth =
+                         bitsLeft < fieldWidth ? bitsLeft : fieldWidth;
+    unsigned int        wordIndex = bI >> ShiftRight;
+
+    bitArray[wordIndex] |=
+     ((unsigned char)((field >> (fieldWidth - sliceWidth))
+     << (bitsLeft - sliceWidth)));
+
+    *bitIndex = bI + sliceWidth;
+    fieldWidth -= sliceWidth;
+  } while ( fieldWidth != 0 );
+}
+
+/** Unpack a field from a bit string, converting from Gray code to binary.
+ *
+ */
+int
+unpack(
+ const unsigned char *  bitArray, /* The input bit string. */
+ unsigned int *         bitIndex, /* Index into the string in BITS, not bytes.*/
+ unsigned int           fieldWidth/* Width of the field in BITS, not bytes. */
+ )
+{
+    return unpack_natural_or_gray(bitArray, bitIndex, fieldWidth, 1);
+}
+
+/** Unpack a field from a bit string, to binary, optionally using
+ * natural or Gray code.
+ *
+ */
+int
+unpack_natural_or_gray(
+ const unsigned char *  bitArray,  /* The input bit string. */
+ unsigned int *         bitIndex,  /* Index into the string in BITS, not bytes.*/
+ unsigned int           fieldWidth,/* Width of the field in BITS, not bytes. */
+ unsigned int           gray       /* non-zero for Gray coding */
+ )
+{
+  unsigned int  field = 0;
+  unsigned int  t;
+
+  do {
+    unsigned int        bI = *bitIndex;
+    unsigned int        bitsLeft = WordSize - (bI & IndexMask);
+    unsigned int        sliceWidth =
+                         bitsLeft < fieldWidth ? bitsLeft : fieldWidth;
+
+    field |= (((bitArray[bI >> ShiftRight] >> (bitsLeft - sliceWidth)) & ((1 << sliceWidth) - 1)) << (fieldWidth - sliceWidth));
+
+    *bitIndex = bI + sliceWidth;
+    fieldWidth -= sliceWidth;
+  } while ( fieldWidth != 0 );
+
+  if (gray) {
+    /* Convert from Gray code to binary. Works for maximum 8-bit fields. */
+    t = field ^ (field >> 8);
+    t ^= (t >> 4);
+    t ^= (t >> 2);
+    t ^= (t >> 1);
+  }
+  else {
+    t = field;
+  }
+
+  return t;
+}