Terrarum/src/net/torvald/util/Float16.kt

package net.torvald.util

import kotlin.experimental.or

/**
 * https://stackoverflow.com/questions/6162651/half-precision-floating-point-in-java#6162687
 *
 * Created by minjaesong on 2017-04-21.
 */
typealias Float16Bits = Short

class Float16() {

    var bits = 0.toShort()
        private set

    constructor(fval: Float) : this() {
        fromFloat(fval)
    }

    fun toFloat() = Float16.toFloat(bits)
    fun fromFloat(fval: Float) {
        bits = Float16.fromFloat(fval)
    }


    operator fun times(other: Float) = fromFloat(this.toFloat() * other)
    operator fun times(other: Float16) = fromFloat(this.toFloat() * other.toFloat())

    operator fun div(other: Float) = fromFloat(this.toFloat() / other)
    operator fun div(other: Float16) = fromFloat(this.toFloat() / other.toFloat())

    // operators are stripped: you don't calculate from FP16; this is only for storing values //
    
    companion object {
        fun toFloat(hbits: Short): Float {
            val hbits = hbits.toInt().and(0xFFFF)

            var mant = hbits and 0x03ff            // 10 bits mantissa
            var exp = hbits and 0x7c00            // 5 bits exponent
            if (exp == 0x7c00)
            // NaN/Inf
                exp = 0x3fc00                    // -> NaN/Inf
            else if (exp != 0)
            // normalized value
            {
                exp += 0x1c000                   // exp - 15 + 127
                if (mant == 0 && exp > 0x1c400)
                // smooth transition
                    return java.lang.Float.intBitsToFloat(hbits and 0x8000 shl 16 or (exp shl 13) or 0x3ff)
            }
            else if (mant != 0)
            // && exp==0 -> subnormal
            {
                exp = 0x1c400                    // make it normal
                do {
                    mant = mant shl 1                   // mantissa * 2
                    exp -= 0x400                 // decrease exp by 1
                } while (mant and 0x400 == 0) // while not normal
                mant = mant and 0x3ff                    // discard subnormal bit
            }                                     // else +/-0 -> +/-0
            return java.lang.Float.intBitsToFloat(// combine all parts
                    hbits and 0x8000 shl 16 or (exp or mant shl 13))         // value << ( 23 - 10 )
        }

        fun fromFloat(fval: Float): Short {
            val fbits = java.lang.Float.floatToIntBits(fval)
            val sign = fbits.ushr(16).and(0x8000).toShort() // sign only
            var `val` = (fbits and 0x7fffffff) + 0x1000 // rounded value

            if (`val` >= 0x47800000)
            // might be or become NaN/Inf
            {                                     // avoid Inf due to rounding
                if (fbits and 0x7fffffff >= 0x47800000) {                                 // is or must become NaN/Inf
                    if (`val` < 0x7f800000)
                    // was value but too large
                        return sign or 0x7c00 // make it +/-Inf
                    return sign or 0x7c00 or // remains +/-Inf or NaN
                            (fbits and 0x007fffff).ushr(13).toShort() // keep NaN (and Inf) bits
                }
                return sign or 0x7bff.toShort() // unrounded not quite Inf
            }
            if (`val` >= 0x38800000)
            // remains normalized value
                return sign or (`val` - 0x38000000).ushr(13).toShort() // exp - 127 + 15
            if (`val` < 0x33000000)
            // too small for subnormal
                return sign // becomes +/-0
            `val` = (fbits and 0x7fffffff).ushr(23)  // tmp exp for subnormal calc

            return sign or ((fbits and 0x7fffff or 0x800000) // add subnormal bit
                            + 0x800000.ushr(`val` - 102)     // round depending on cut off
                           ).ushr(126 - `val`)   // div by 2^(1-(exp-127+15)) and >> 13 | exp=0
                    .toShort()
        }
    }
}