using System;

// using System.Math;

public class Program

{

    static double FromLinear(double cLinear)

    {

        if (cLinear <= 0.0031308)

            return 12.92*cLinear;

        return 1.055*Math.Pow(cLinear,1/2.4)-0.055;

    }

    static double FromGamma(double cGamma)

    {

        if (cGamma < 0.04045)

            return cGamma/12.92;

        return Math.Pow((cGamma+0.055)/1.055, 2.4);

    }

    static int ToInt(double v)

    {

        return (int)(Math.Round(v*255.0));

    }

    // true alpha blend color

    // scaled to 16 bits

    static int TrueC(int c, int a)

    {       

        var mult =  FromGamma(c/255.0) * (a/255.0);

        return (int)(Math.Clamp(Math.Round(mult*65535),0,65535));       

    }

    static void Test(int c, int a, int bits)

    {

        // see if lin interp from corners correct

        int mask = (1<<bits)-1;

        int cL = (c>>bits)<<bits;

        int cF = c & mask;

        int aL = (a>>bits)<<bits;

        int aF = a & mask;

        var c00 = TrueC(cL,aL);

        var c01 = TrueC(cL,aL+mask);

        var c10 = TrueC(cL+mask,aL);

        var c11 = TrueC(cL+mask,aL+mask);

#if false

        var bX = 1-(double)cF/mask; // blend vals

        var bY = 1-(double)aF/mask;

        var v1 = c00 * bX + c10 * (1-bX);

        var v2 = c01 * bX + c11 * (1-bX);

        var v3 = (int)(Math.Round(v1 * bY + v2 * (1-bY)));

#else

        var bX = mask-cF; // blend vals, times mask

        var bY = mask-aF;

        var v1 = c00 * bX + c10 * (mask-bX);

        var v2 = c01 * bX + c11 * (mask-bX);

        var vA = v1 * bY + v2 * (mask-bY);

        var v3 = (vA+mask*mask)/(mask*mask) - 192; // round, additional to get from 4 to 5 bit tbl  

#endif

        var v4 = TrueC(c,a);

        var err = Math.Abs(v4-v3);

        if (err>200)

            Console.WriteLine($"{c} {a} => {v3-v4}  {v3} , {v4}");

    }

    public static void Main()

    {

        int bits = 5;

        for (int a = 0; a <= 255; ++a)

            for (int c = 0; c <= 255; ++c)

            {

                double cd = c/255.0;

                double ad = a/255.0;

                double cg = FromGamma(cd);

                double c2 = FromLinear(cg);

                if (ToInt(c2) != ToInt(cd) || ToInt(cd) != c)

                    Console.WriteLine($"C err {c}" );

                Test(c,a,bits);         

            }

        Console.WriteLine("Done");

    }

}