dlib.coding.varint source code

1 /*
2 Copyright (c) 2015-2021 Timur Gafarov
3 
4 Boost Software License - Version 1.0 - August 17th, 2003
5 
6 Permission is hereby granted, free of charge, to any person or organization
7 obtaining a copy of the software and accompanying documentation covered by
8 this license (the "Software") to use, reproduce, display, distribute,
9 execute, and transmit the Software, and to prepare derivative works of the
10 Software, and to permit third-parties to whom the Software is furnished to
11 do so, all subject to the following:
12 
13 The copyright notices in the Software and this entire statement, including
14 the above license grant, this restriction and the following disclaimer,
15 must be included in all copies of the Software, in whole or in part, and
16 all derivative works of the Software, unless such copies or derivative
17 works are solely in the form of machine-executable object code generated by
18 a source language processor.
19 
20 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
21 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
22 FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
23 SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
24 FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
25 ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
26 DEALINGS IN THE SOFTWARE.
27 */
28 
29 /**
30  * Variable-sized integer type
31  *
32  * Copyright: Timur Gafarov 2015-2021.
33  * License: $(LINK2 boost.org/LICENSE_1_0.txt, Boost License 1.0).
34  * Authors: Timur Gafarov
35  */
36 module dlib.coding.varint;
37 
38 /**
39  * Protobuf-style variable-sized integer
40  */
41 struct Varint
42 {
43     ubyte[8] buffer;
44     size_t size;
45 
46     /// Returns bytes of an integer
47     ubyte[] bytes() return
48     {
49         return buffer[0..size];
50     }
51 }
52 
53 /**
54  * Returns size in bytes necessary to store an integer
55  */
56 size_t getVarintSize(ulong n)
57 {
58     enum ulong N1 = 128;
59     enum ulong N2 = 16384;
60     enum ulong N3 = 2097152;
61     enum ulong N4 = 268435456;
62     enum ulong N5 = 34359738368;
63     enum ulong N6 = 4398046511104;
64     enum ulong N7 = 562949953421312;
65     enum ulong N8 = 72057594037927936;
66 
67     return (
68         n < N1 ? 1
69       : n < N2 ? 2
70       : n < N3 ? 3
71       : n < N4 ? 4
72       : n < N5 ? 5
73       : n < N6 ? 6
74       : n < N7 ? 7
75       :          8
76     );
77 }
78 
79 /**
80  * Encode a fixed-size integer to Varint
81  */
82 Varint encodeVarint(ulong n)
83 {
84     Varint res;
85     res.size = getVarintSize(n);
86     ubyte* ptr = res.buffer.ptr;
87 
88     for (uint i = 0; i < res.size; i++)
89     {
90         if (i == res.size - 1)
91             *(ptr++) = n & 0xFF;
92         else
93             *(ptr++) = cast(ubyte)(n | 0x80);
94         n >>= 7;
95     }
96 
97     return res;
98 }
99 
100 /**
101  * Decode Varing to fixed-size integer
102  */
103 ulong decodeVarint(Varint vint)
104 {
105     ulong result = 0;
106     int bits = 0;
107     ubyte* ptr = vint.buffer.ptr;
108     ulong ll;
109 
110     while (*ptr & 0x80)
111     {
112         ll = *ptr;
113         result += ((ll & 0x7F) << bits);
114         ptr++;
115         bits += 7;
116     }
117 
118     ll = *ptr;
119     result += ((ll & 0x7F) << bits);
120 
121     return result;
122 }