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Improve accuracy and safety of float serialization

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Multiplying by a factor of 1/1000.f (rather than dividing by 1000.f) directly
introduces an error of 1 ULP.  With this patch, an exact comparison of a
floating point literal with the deserialized F1000 form representing it is now
guaranteed to be successful.
In addition, the maxmium and minimum safely representible floating point
numbers are now well-defined as constants.
This commit is contained in:
kwolekr 2015-08-01 01:03:51 -04:00
parent bf991bde45
commit 42cf5e972d
2 changed files with 21 additions and 17 deletions
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15
src/util/serialize.h
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@ -21,6 +21,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
#define UTIL_SERIALIZE_HEADER
#include "../irrlichttypes_bloated.h"
#include "../debug.h" // for assert
#include "config.h"
#if HAVE_ENDIAN_H
#include <endian.h>
@ -30,7 +31,16 @@ with this program; if not, write to the Free Software Foundation, Inc.,
#include <string>
#define FIXEDPOINT_FACTOR 1000.0f
#define FIXEDPOINT_INVFACTOR (1.0f/FIXEDPOINT_FACTOR)
// 0x7FFFFFFF / 1000.0f is not serializable.
// The limited float precision at this magnitude may cause the result to round
// to a greater value than can be represented by a 32 bit integer when increased
// by a factor of FIXEDPOINT_FACTOR. As a result, [F1000_MIN..F1000_MAX] does
// not represent the full range, but rather the largest safe range, of values on
// all supported architectures. Note: This definition makes assumptions on
// platform float-to-int conversion behavior.
#define F1000_MIN ((float)(s32)((-0x7FFFFFFF - 1) / FIXEDPOINT_FACTOR))
#define F1000_MAX ((float)(s32)((0x7FFFFFFF) / FIXEDPOINT_FACTOR))
#define STRING_MAX_LEN 0xFFFF
#define WIDE_STRING_MAX_LEN 0xFFFF
@ -163,7 +173,7 @@ inline s64 readS64(const u8 *data)
inline f32 readF1000(const u8 *data)
{
return (f32)readS32(data) * FIXEDPOINT_INVFACTOR;
return (f32)readS32(data) / FIXEDPOINT_FACTOR;
}
inline video::SColor readARGB8(const u8 *data)
@ -252,6 +262,7 @@ inline void writeS64(u8 *data, s64 i)
inline void writeF1000(u8 *data, f32 i)
{
assert(i >= F1000_MIN && i <= F1000_MAX);
writeS32(data, i * FIXEDPOINT_FACTOR);
}