log2_fp()

static FASTLED_FORCE_INLINE s24x8 fl::s24x8::log2_fp ( s24x8 x )

inlinestaticprivate

Definition at line 319 of file s24x8.h.

                                                                   {
        u32 val = static_cast<u32>(x.mValue);
        int msb = highest_bit(val);
        i32 int_part = msb - FRAC_BITS;
        i32 t;
        if (msb >= FRAC_BITS) {
            t = static_cast<i32>(
                (val >> (msb - FRAC_BITS)) - (SCALE));
        } else {
            t = static_cast<i32>(
                (val << (FRAC_BITS - msb)) - (SCALE));
        }
        // 4-term minimax coefficients for log2(1+t), t in [0,1).
        // Stored as i64 with 16 fractional bits.
        constexpr int IFRAC = 16;
        constexpr i64 c0 = 94528LL;    // 1.44179 * 2^16
        constexpr i64 c1 = -45814LL;   // -0.69907 * 2^16
        constexpr i64 c2 = 23821LL;    // 0.36348 * 2^16
        constexpr i64 c3 = -6986LL;    // -0.10660 * 2^16
        // Extend t from 8 to 16 frac bits.
        i64 t16 = static_cast<i64>(t) << (IFRAC - FRAC_BITS);
        // Horner: t * (c0 + t * (c1 + t * (c2 + t * c3)))
        i64 acc = c3;
        acc = c2 + ((acc * t16) >> IFRAC);
        acc = c1 + ((acc * t16) >> IFRAC);
        acc = c0 + ((acc * t16) >> IFRAC);
        i64 frac_part = (acc * t16) >> IFRAC;
        // Convert from 16 frac bits back to 8.
        i32 frac8 = static_cast<i32>(frac_part >> (IFRAC - FRAC_BITS));
        return from_raw(static_cast<i32>(static_cast<u32>(int_part) << FRAC_BITS) + frac8);
    }

References s24x8(), FASTLED_FORCE_INLINE, FL_NOEXCEPT, FRAC_BITS, from_raw(), highest_bit(), SCALE, fl::t, and fl::x.

Referenced by pow().

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