diff options
author | erdgeist@erdgeist.org <erdgeist@bauklotz.fritz.box> | 2019-07-04 23:26:09 +0200 |
---|---|---|
committer | erdgeist@erdgeist.org <erdgeist@bauklotz.fritz.box> | 2019-07-04 23:26:09 +0200 |
commit | f02dfce6e6c34b3d8a7b8a0e784b506178e331fa (patch) | |
tree | 45556e6104242d4702689760433d7321ae74ec17 /pack.c |
stripdown of version 0.9
Diffstat (limited to 'pack.c')
-rw-r--r-- | pack.c | 140 |
1 files changed, 140 insertions, 0 deletions
@@ -0,0 +1,140 @@ | |||
1 | /* | ||
2 | Copyright (C) 2010 Perens LLC <bruce@perens.com> | ||
3 | |||
4 | All rights reserved. | ||
5 | |||
6 | This program is free software; you can redistribute it and/or modify | ||
7 | it under the terms of the GNU Lesser General Public License version 2.1, as | ||
8 | published by the Free Software Foundation. This program is | ||
9 | distributed in the hope that it will be useful, but WITHOUT ANY | ||
10 | WARRANTY; without even the implied warranty of MERCHANTABILITY or | ||
11 | FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public | ||
12 | License for more details. | ||
13 | |||
14 | You should have received a copy of the GNU Lesser General Public License | ||
15 | along with this program; if not, see <http://www.gnu.org/licenses/>. | ||
16 | */ | ||
17 | |||
18 | #include "defines.h" | ||
19 | #include "quantise.h" | ||
20 | #include <stdio.h> | ||
21 | |||
22 | /* Compile-time constants */ | ||
23 | /* Size of unsigned char in bits. Assumes 8 bits-per-char. */ | ||
24 | static const unsigned int WordSize = 8; | ||
25 | |||
26 | /* Mask to pick the bit component out of bitIndex. */ | ||
27 | static const unsigned int IndexMask = 0x7; | ||
28 | |||
29 | /* Used to pick the word component out of bitIndex. */ | ||
30 | static const unsigned int ShiftRight = 3; | ||
31 | |||
32 | /** Pack a bit field into a bit string, encoding the field in Gray code. | ||
33 | * | ||
34 | * The output is an array of unsigned char data. The fields are efficiently | ||
35 | * packed into the bit string. The Gray coding is a naive attempt to reduce | ||
36 | * the effect of single-bit errors, we expect to do a better job as the | ||
37 | * codec develops. | ||
38 | * | ||
39 | * This code would be simpler if it just set one bit at a time in the string, | ||
40 | * but would hit the same cache line more often. I'm not sure the complexity | ||
41 | * gains us anything here. | ||
42 | * | ||
43 | * Although field is currently of int type rather than unsigned for | ||
44 | * compatibility with the rest of the code, indices are always expected to | ||
45 | * be >= 0. | ||
46 | */ | ||
47 | void | ||
48 | pack( | ||
49 | unsigned char * bitArray, /* The output bit string. */ | ||
50 | unsigned int * bitIndex, /* Index into the string in BITS, not bytes.*/ | ||
51 | int field, /* The bit field to be packed. */ | ||
52 | unsigned int fieldWidth/* Width of the field in BITS, not bytes. */ | ||
53 | ) | ||
54 | { | ||
55 | pack_natural_or_gray(bitArray, bitIndex, field, fieldWidth, 1); | ||
56 | } | ||
57 | |||
58 | void | ||
59 | pack_natural_or_gray( | ||
60 | unsigned char * bitArray, /* The output bit string. */ | ||
61 | unsigned int * bitIndex, /* Index into the string in BITS, not bytes.*/ | ||
62 | int field, /* The bit field to be packed. */ | ||
63 | unsigned int fieldWidth,/* Width of the field in BITS, not bytes. */ | ||
64 | unsigned int gray /* non-zero for gray coding */ | ||
65 | ) | ||
66 | { | ||
67 | if (gray) { | ||
68 | /* Convert the field to Gray code */ | ||
69 | field = (field >> 1) ^ field; | ||
70 | } | ||
71 | |||
72 | do { | ||
73 | unsigned int bI = *bitIndex; | ||
74 | unsigned int bitsLeft = WordSize - (bI & IndexMask); | ||
75 | unsigned int sliceWidth = | ||
76 | bitsLeft < fieldWidth ? bitsLeft : fieldWidth; | ||
77 | unsigned int wordIndex = bI >> ShiftRight; | ||
78 | |||
79 | bitArray[wordIndex] |= | ||
80 | ((unsigned char)((field >> (fieldWidth - sliceWidth)) | ||
81 | << (bitsLeft - sliceWidth))); | ||
82 | |||
83 | *bitIndex = bI + sliceWidth; | ||
84 | fieldWidth -= sliceWidth; | ||
85 | } while ( fieldWidth != 0 ); | ||
86 | } | ||
87 | |||
88 | /** Unpack a field from a bit string, converting from Gray code to binary. | ||
89 | * | ||
90 | */ | ||
91 | int | ||
92 | unpack( | ||
93 | const unsigned char * bitArray, /* The input bit string. */ | ||
94 | unsigned int * bitIndex, /* Index into the string in BITS, not bytes.*/ | ||
95 | unsigned int fieldWidth/* Width of the field in BITS, not bytes. */ | ||
96 | ) | ||
97 | { | ||
98 | return unpack_natural_or_gray(bitArray, bitIndex, fieldWidth, 1); | ||
99 | } | ||
100 | |||
101 | /** Unpack a field from a bit string, to binary, optionally using | ||
102 | * natural or Gray code. | ||
103 | * | ||
104 | */ | ||
105 | int | ||
106 | unpack_natural_or_gray( | ||
107 | const unsigned char * bitArray, /* The input bit string. */ | ||
108 | unsigned int * bitIndex, /* Index into the string in BITS, not bytes.*/ | ||
109 | unsigned int fieldWidth,/* Width of the field in BITS, not bytes. */ | ||
110 | unsigned int gray /* non-zero for Gray coding */ | ||
111 | ) | ||
112 | { | ||
113 | unsigned int field = 0; | ||
114 | unsigned int t; | ||
115 | |||
116 | do { | ||
117 | unsigned int bI = *bitIndex; | ||
118 | unsigned int bitsLeft = WordSize - (bI & IndexMask); | ||
119 | unsigned int sliceWidth = | ||
120 | bitsLeft < fieldWidth ? bitsLeft : fieldWidth; | ||
121 | |||
122 | field |= (((bitArray[bI >> ShiftRight] >> (bitsLeft - sliceWidth)) & ((1 << sliceWidth) - 1)) << (fieldWidth - sliceWidth)); | ||
123 | |||
124 | *bitIndex = bI + sliceWidth; | ||
125 | fieldWidth -= sliceWidth; | ||
126 | } while ( fieldWidth != 0 ); | ||
127 | |||
128 | if (gray) { | ||
129 | /* Convert from Gray code to binary. Works for maximum 8-bit fields. */ | ||
130 | t = field ^ (field >> 8); | ||
131 | t ^= (t >> 4); | ||
132 | t ^= (t >> 2); | ||
133 | t ^= (t >> 1); | ||
134 | } | ||
135 | else { | ||
136 | t = field; | ||
137 | } | ||
138 | |||
139 | return t; | ||
140 | } | ||