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      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  */
     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 
     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