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 <stdint.h>
18
19 namespace {
20
21 // Table for Seal's algorithm for Number of Trailing Zeros. Hacker's Delight
22 // online, Figure 5-18 (http://www.hackersdelight.org/revisions.pdf)
23 // The entries whose value is -1 are never referenced.
24 int NTZ_TABLE[] = {
25 32, 0, 1, 12, 2, 6, -1, 13, 3, -1, 7, -1, -1, -1, -1, 14,
26 10, 4, -1, -1, 8, -1, -1, 25, -1, -1, -1, -1, -1, 21, 27, 15,
27 31, 11, 5, -1, -1, -1, -1, -1, 9, -1, -1, 24, -1, -1, 20, 26,
28 30, -1, -1, -1, -1, 23, -1, 19, 29, -1, 22, 18, 28, 17, 16, -1
29 };
30
numberOfTrailingZeros32(int32_t i)31 int numberOfTrailingZeros32(int32_t i) {
32 int64_t i64 = i;
33 i64 = (i64 & -i64) * 0x0450FBAF;
34 uint32_t u = i64;
35 return NTZ_TABLE[(u) >> 26];
36 }
37
highestOneBit(uint64_t n)38 uint64_t highestOneBit(uint64_t n) {
39 n |= (n >> 1);
40 n |= (n >> 2);
41 n |= (n >> 4);
42 n |= (n >> 8);
43 n |= (n >> 16);
44 n |= (n >> 32);
45 return n - (n >> 1);
46 }
47
_powerOf2(uint64_t u)48 uint64_t _powerOf2(uint64_t u) {
49 uint64_t powerOf2 = highestOneBit(u);
50 return powerOf2 ? powerOf2 : 1;
51 }
52
53 // Based on Long.numberOfTrailingZeros in Long.java
numberOfTrailingZeros(uint64_t u)54 int numberOfTrailingZeros(uint64_t u) {
55 int32_t low = u;
56 return low !=0 ? numberOfTrailingZeros32(low)
57 : 32 + numberOfTrailingZeros32((int32_t) (u >> 32));
58 }
59 }
60
61 namespace webm {
62
63 // Encode the id and/or size of an EBML element bytes by setting a leading length descriptor bit:
64 //
65 // 1xxxxxxx - 1-byte values
66 // 01xxxxxx xxxxxxxx -
67 // 001xxxxx xxxxxxxx xxxxxxxx -
68 // 0001xxxx xxxxxxxx xxxxxxxx xxxxxxxx - ...
69 // 00001xxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx -
70 // 000001xx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx -
71 // 0000001x xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx -
72 // 00000001 xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx - 8-byte values
73 //
74 // This function uses the least the number of bytes possible.
encodeUnsigned(uint64_t u)75 uint64_t encodeUnsigned(uint64_t u) {
76 uint64_t powerOf2 = _powerOf2(u);
77 if (u + 1 == powerOf2 << 1)
78 powerOf2 <<= 1;
79 int shiftWidth = (7 + numberOfTrailingZeros(powerOf2)) / 7 * 7;
80 long lengthDescriptor = 1 << shiftWidth;
81 return lengthDescriptor | u;
82 }
83
84 // Like above but pads the input value with leading zeros up to the specified width. The length
85 // descriptor is calculated based on width.
encodeUnsigned(uint64_t u,int width)86 uint64_t encodeUnsigned(uint64_t u, int width) {
87 int shiftWidth = 7 * width;
88 uint64_t lengthDescriptor = 1;
89 lengthDescriptor <<= shiftWidth;
90 return lengthDescriptor | u;
91 }
92
93 // Calculate the length of an EBML coded id or size from its length descriptor.
sizeOf(uint64_t u)94 int sizeOf(uint64_t u) {
95 uint64_t powerOf2 = _powerOf2(u);
96 int unsignedLength = numberOfTrailingZeros(powerOf2) / 8 + 1;
97 return unsignedLength;
98 }
99
100 // Serialize an EBML coded id or size in big-endian order.
serializeCodedUnsigned(uint64_t u,uint8_t * bary)101 int serializeCodedUnsigned(uint64_t u, uint8_t* bary) {
102 int unsignedLength = sizeOf(u);
103 for (int i = unsignedLength - 1; i >= 0; i--) {
104 bary[i] = u & 0xff;
105 u >>= 8;
106 }
107 return unsignedLength;
108 }
109
110 }
111