1 /*
2 * Copyright (C) 2008 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #ifndef _DALVIK_FLOAT12_H
18 #define _DALVIK_FLOAT12_H
19
20 /* Encodes a 32-bit number in 12 bits with +/-1.5% error,
21 * though the majority (80%) are within +/-0.25%.
22 *
23 * The encoding looks like:
24 *
25 * EEEMMMMM MMMMMMMM MMMMMMMM
26 * 76543210 76543210 76543210
27 *
28 * where EEE is a base-16 exponent and MMMM is the mantissa.
29 * The output value is (MMMM * 16^EEE), or (MMMM << (EEE * 4)).
30 *
31 * TODO: do this in a less brain-dead way. I'm sure we can do
32 * it without all of these loops.
33 */
intToFloat12(unsigned int val)34 inline unsigned short intToFloat12(unsigned int val)
35 {
36 int oval = val;
37 int shift = 0;
38
39 /* Shift off the precision we don't care about.
40 * Don't round here; it biases the values too high
41 * (such that the encoded value is always greater
42 * than the actual value)
43 */
44 unsigned int pval = val;
45 while (val > 0x1ff) {
46 pval = val;
47 val >>= 1;
48 shift++;
49 }
50 if (shift > 0 && (pval & 1)) {
51 /* Round based on the last bit we shifted off.
52 */
53 val++;
54 if (val > 0x1ff) {
55 val = (val + 1) >> 1;
56 shift++;
57 }
58 }
59
60 /* Shift off enough bits to create a valid exponent.
61 * Since we care about the bits we're losing, be sure
62 * to round them.
63 */
64 while (shift % 4 != 0) {
65 val = (val + 1) >> 1;
66 shift++;
67 }
68
69 /* In the end, only round by the most-significant lost bit.
70 * This centers the values around the closest match.
71 * All of the rounding we did above guarantees that this
72 * round won't overflow past 0x1ff.
73 */
74 if (shift > 0) {
75 val = ((oval >> (shift - 1)) + 1) >> 1;
76 }
77
78 val |= (shift / 4) << 9;
79 return val;
80 }
81
float12ToInt(unsigned short f12)82 inline unsigned int float12ToInt(unsigned short f12)
83 {
84 return (f12 & 0x1ff) << ((f12 >> 9) * 4);
85 }
86
87 #if 0 // testing
88
89 #include <stdio.h>
90 int main(int argc, char *argv[])
91 {
92 if (argc != 3) {
93 fprintf(stderr, "usage: %s <min> <max>\n", argv[0]);
94 return 1;
95 }
96
97 unsigned int min = atoi(argv[1]);
98 unsigned int max = atoi(argv[2]);
99 if (min > max) {
100 int t = min;
101 max = min;
102 min = t;
103 } else if (min == max) {
104 max++;
105 }
106
107 while (min < max) {
108 unsigned int out;
109 unsigned short sf;
110
111 sf = intToFloat12(min);
112 out = float12ToInt(sf);
113 // printf("%d 0x%03x / 0x%03x %d (%d)\n", min, min, sf, out, (int)min - (int)out);
114 printf("%6.6f %d %d\n", ((float)(int)(min - out)) / (float)(int)min, min, out);
115 if (min <= 8192) {
116 min++;
117 } else if (min < 10000) {
118 min += 10;
119 } else if (min < 100000) {
120 min += 1000;
121 } else {
122 min += 10000;
123 }
124 }
125 return 0;
126 }
127
128 #endif // testing
129
130 #endif // _DALVIK_FLOAT12_H
131