fastlog.h

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/*
 * Copyright © 2013 Laurent de Soras <laurent.de.soras@free.fr>
 *
 * This work is free. You can redistribute it and/or modify it under the
 * terms of the Do What The Fuck You Want To Public License, Version 2,
 * as published by Sam Hocevar. See http://www.wtfpl.net/ for more details.
 */
#ifndef __pbd_fastlog_h__
#define __pbd_fastlog_h__
 
#include <math.h> /* for HUGE_VAL */
 
#include "pbd/libpbd_visibility.h"
 
static inline float fast_log2 (float val)
{
        /* don't use reinterpret_cast<> because that prevents this
         * from being used by pure C code (for example, GnomeCanvasItems)
         */
        union {float f; int i;} t;
        t.f = val;
        int* const exp_ptr =  &t.i;
        int        x       = *exp_ptr;
        const int  log_2   = ((x >> 23) & 255) - 128;
 
        x &= ~(255 << 23);
        x += 127 << 23;
 
        *exp_ptr = x;
 
        val = ((-1.0f/3) * t.f + 2) * t.f - 2.0f/3;
 
        return (val + log_2);
}
 
static inline float fast_log (const float val)
{
        return (fast_log2 (val) * 0.69314718f);
}
 
static inline float fast_log10 (const float val)
{
        return fast_log2(val) / 3.312500f;
}
 
static inline float minus_infinity(void) { return -HUGE_VAL; }
 
#endif /* __pbd_fastlog_h__ */