source: trunk/src/half.cpp @ 867

Last change on this file since 867 was 867, checked in by sam, 9 years ago

core: start working on the "half" type.

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1//
2// Lol Engine
3//
4// Copyright: (c) 2010-2011 Sam Hocevar <sam@hocevar.net>
5//   This program is free software; you can redistribute it and/or
6//   modify it under the terms of the Do What The Fuck You Want To
7//   Public License, Version 2, as published by Sam Hocevar. See
8//   http://sam.zoy.org/projects/COPYING.WTFPL for more details.
9//
10
11#if defined HAVE_CONFIG_H
12#   include "config.h"
13#endif
14
15#include "core.h"
16
17using namespace std;
18
19#define S4(x)    S1(4*(x)),  S1(4*(x)+1),  S1(4*(x)+2),  S1(4*(x)+3)
20#define S16(x)   S4(4*(x)),  S4(4*(x)+1),  S4(4*(x)+2),  S4(4*(x)+3)
21#define S64(x)  S16(4*(x)), S16(4*(x)+1), S16(4*(x)+2), S16(4*(x)+3)
22#define S256(x) S64(4*(x)), S64(4*(x)+1), S64(4*(x)+2), S64(4*(x)+3)
23
24namespace lol
25{
26
27half half::makefast(float f)
28{
29    /* Lookup table-based algorithm from “Fast Half Float Conversions”
30     * by Jeroen van der Zijp, November 2008. No rounding is performed. */
31    static uint16_t const basetable[512] =
32    {
33#define S1(i) (((i) < 103) ? 0x0000: \
34               ((i) < 113) ? 0x0400 >> (113 - (i)) : \
35               ((i) < 143) ? ((i) - 112) << 10 : 0x7c00)
36        S256(0),
37#undef S1
38#define S1(i) (0x8000 | (((i) < 103) ? 0x0000 : \
39                         ((i) < 113) ? 0x0400 >> (113 - (i)): \
40                         ((i) < 143) ? ((i) - 112) << 10 : 0x7c00))
41        S256(0),
42#undef S1
43    };
44
45    static uint8_t const shifttable[512] =
46    {
47#define S1(i) (((i) < 103) ? 24 : \
48               ((i) < 113) ? 126 - (i) : \
49               ((i) < 143 || (i) == 255) ? 13 : 24)
50        S256(0), S256(0),
51#undef S1
52    };
53
54    union { float f; uint32_t x; } u = { f };
55
56    uint16_t bits = basetable[(u.x >> 23) & 0x1ff];
57    bits |= (u.x & 0x007fffff) >> shifttable[(u.x >> 23) & 0x1ff];
58    return makebits(bits);
59}
60
61half half::makeslow(float f)
62{
63    union { float f; uint32_t x; } u = { f };
64
65    uint16_t bits = (u.x >> 16) & 0x8000; /* Get the sign */
66    uint16_t m = (u.x >> 12) & 0x07ff; /* Keep one extra bit for rounding */
67    unsigned int e = (u.x >> 23) & 0xff; /* Using int is faster here */
68
69    /* If zero, or denormal, or exponent underflows too much for a denormal,
70     * return signed zero */
71    if (e < 103)
72        return makebits(bits);
73
74    /* If NaN, Inf or exponent overflow, return NaN or Inf */
75    if (e > 142)
76    {
77        bits |= 0x7c00u;
78        bits |= e == 255 && (u.x & 0x007fffffu);
79        return makebits(bits);
80    }
81
82    /* If exponent underflows but not too much, return a denormal */
83    if (e < 113)
84    {
85        m |= 0x0800u;
86        bits |= (m >> (114 - e)) + ((m >> (113 - e)) & 1);
87        return makebits(bits);
88    }
89
90    bits |= ((e - 112) << 10) | (m >> 1);
91    bits += m & 1; /* Overflows here are expected and handled */
92    return makebits(bits);
93}
94
95} /* namespace lol */
96
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