1 /*
2  * Copyright (C) 2014 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 #include <math.h>
18 #include <audio_utils/sample.h>
19 
20 #define SAMPLE_NEG_MAX  0xFFFF
21 #define SAMPLE_POS_MAX  0x7FFF
22 
23 #define SIGN_MASK       0x8000
24 #define SIGN_BITS       1
25 
26 #define EXPONENT_BITS   3
27 #define EXPONENT_MAX    ((1 << EXPONENT_BITS) - 1)
28 #define EXCESS          ((1 << EXPONENT_BITS) - 2)
29 
30 #define MANTISSA_BITS   12
31 #define MANTISSA_MAX    ((1 << MANTISSA_BITS) - 1)
32 #define HIDDEN_BIT      (1 << MANTISSA_BITS)
33 #define ONE_FLOAT       ((float) (1 << (MANTISSA_BITS + 1)))
34 
35 
36 #if SIGN_BITS + EXPONENT_BITS + MANTISSA_BITS != 16
37 #error SIGN_BITS, EXPONENT_BITS and MANTISSA_BITS must sum to 16
38 #endif
39 
40 sample_minifloat_t sample_from_float(float v)
41 {
42     if (isnan(v)) {
43         return 0;
44     }
45     sample_minifloat_t sign = 0;
46     if (v < 0.0f) {
47         sign = SIGN_MASK;
48         v = -v;
49     }
50     // This check could conceivably be tighter: v < constexpr float_from_sample(1).
51     // Probably only useful to be made more accurate if this is changed to
52     // manipulate the raw IEEE single precision float bit fields.
53     if (v <= 0.0f) {
54         // originally returned sign, but now seems better not to return negative 0
55         return 0;
56     }
57     if (v >= 2.0f) {
58         return SAMPLE_POS_MAX | sign;
59     }
60     int exp;
61     float r = frexpf(v, &exp);
62     if ((exp += EXCESS) > EXPONENT_MAX) {
63         return SAMPLE_POS_MAX | sign;
64     }
65     if (-exp >= MANTISSA_BITS) {
66         // originally returned sign, but now seems better not to return negative 0
67         return 0;
68     }
69     int mantissa = (int) (r * ONE_FLOAT);
70     sample_minifloat_t ret = exp > 0 ? (exp << MANTISSA_BITS) | (mantissa & ~HIDDEN_BIT) :
71             (mantissa >> (1 - exp)) & MANTISSA_MAX;
72     // ret != 0
73     return ret | sign;
74 }
75 
76 float float_from_sample(sample_minifloat_t a)
77 {
78     int mantissa = a & MANTISSA_MAX;
79     int exponent = (a >> MANTISSA_BITS) & EXPONENT_MAX;
80     float f = ldexpf((exponent > 0 ? HIDDEN_BIT | mantissa : mantissa << 1) / ONE_FLOAT,
81             exponent - EXCESS);
82     return a & SIGN_MASK ? -f : f;
83 }
84