log2_fp()

static FASTLED_FORCE_INLINE s12x4 fl::s12x4::log2_fp ( s12x4 x )

inlinestaticprivate

Definition at line 310 of file s12x4.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 i32 with 12 fractional bits. Max product ~2^21, fits i32 comfortably.
        constexpr int IFRAC = 12;
        constexpr i32 c0 = 5907;    // 1.44179 * 2^12
        constexpr i32 c1 = -2864;   // -0.69907 * 2^12
        constexpr i32 c2 = 1489;    // 0.36348 * 2^12
        constexpr i32 c3 = -437;    // -0.10660 * 2^12
        // Extend t from 4 to 12 frac bits.
        i32 t12 = static_cast<i32>(t) << (IFRAC - FRAC_BITS);
        // Horner: t * (c0 + t * (c1 + t * (c2 + t * c3)))
        i32 acc = c3;
        acc = c2 + ((acc * t12) >> IFRAC);
        acc = c1 + ((acc * t12) >> IFRAC);
        acc = c0 + ((acc * t12) >> IFRAC);
        i32 frac_part = (acc * t12) >> IFRAC;
        // Convert from 12 frac bits back to 4.
        i16 frac4 = static_cast<i16>(frac_part >> (IFRAC - FRAC_BITS));
        return from_raw(static_cast<i16>((static_cast<i32>(static_cast<u32>(int_part) << FRAC_BITS)) + frac4));
    }

References s12x4(), FASTLED_FORCE_INLINE, FL_NOEXCEPT, FRAC_BITS, from_raw(), highest_bit(), SCALE, fl::t, and fl::x.

Referenced by pow().

Here is the call graph for this function:

Here is the caller graph for this function: