/// This module is about comparison of floating point arithmetics.
/// Supported by this very informative article:
/// https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/
module gfx.math.approx;

import std.traits : isFloatingPoint;

/// Different methods to check if two floating point values are close to each other.
enum ApproxMethod
{
    /// Unit of Least Precision.
    ulp,
    /// Relative error
    rel,
    /// ULP, with absolute error
    ulpAndAbs,
    /// Relative and absolute error
    relAndAbs,
}

/// Check with method given at runtime with default parameters
/// a and b can be any data sets supported by the approx template
bool approx(T)(ApproxMethod method, in T a, in T b)
{
    final switch(method)
    {
        case ApproxMethod.ulp:
            return approxUlp(a, b);
        case ApproxMethod.rel:
            return approxRel(a, b);
        case ApproxMethod.ulpAndAbs:
            return approxUlpAndAbs(a, b);
        case ApproxMethod.relAndAbs:
            return approxRelAndAbs(a, b);
    }
}

/// Compare two data sets with ApproxMethod.ulp.
/// That is call scalarApproxUlp on each couple of the data sets.
alias approxUlp = approx!(ApproxMethod.ulp);
/// Compare two data sets with ApproxMethod.rel.
/// That is call scalarApproxRel on each couple of the data sets.
alias approxRel = approx!(ApproxMethod.rel);
/// Compare two data sets with ApproxMethod.ulpAndAbs.
/// That is call scalarApproxUlpAndAbs on each couple of the data sets.
alias approxUlpAndAbs = approx!(ApproxMethod.ulpAndAbs);
/// Compare two data sets with ApproxMethod.relAndAbs.
/// That is call scalarApproxRelAndAbs on each couple of the data sets.
alias approxRelAndAbs = approx!(ApproxMethod.relAndAbs);


/// Compute the ULP difference between two floating point numbers
/// Negative result indicates that b has higher ULP value than a.
template ulpDiff(T) if (isFloatingPoint!T)
{
    long ulpDiff(in T a, in T b)
    {
        immutable fnA = FloatNum!T(a);
        immutable fnB = FloatNum!T(b);

        return (fnA - fnB);
    }
}

/// Determines if two floating point scalars are maxUlps close to each other.
template scalarApproxUlp(T) if (isFloatingPoint!T)
{
    bool scalarApproxUlp(in T a, in T b, in ulong maxUlps=4)
    {
        import std.math : abs;

        immutable fnA = FloatNum!T(a);
        immutable fnB = FloatNum!T(b);

        if (fnA.negative != fnB.negative)
        {
            return a == b; // check for +0 / -0
        }

        return (abs(fnA - fnB) <= maxUlps);
    }
}

/// Check whether the relative error between a and b is smaller than maxEps
template scalarApproxRel(T) if (isFloatingPoint!T)
{
    bool scalarApproxRel (in T a, in T b, in T maxEps=4*T.epsilon)
    {
        import std.algorithm : max;
        import std.math : abs;
        const diff = abs(b-a);
        const absA = abs(a);
        const absB = abs(b);
        const largest = max(absA, absB);
        return diff <= maxEps*largest;
    }
}

/// Determines if two floating point scalars are maxUlps close to each other.
/// If the absolute error is less than maxAbs, the test succeeds however.
/// This is useful when comparing against zero the result of a subtraction.
template scalarApproxUlpAndAbs(T) if (isFloatingPoint!T)
{
    bool scalarApproxUlpAndAbs(in T a, in T b, in T maxAbs=0.0001, in ulong maxUlps=4)
    {
        import std.math : abs;
        if (abs(b-a) <= maxAbs) return true;
        return scalarApproxUlp(a, b, maxUlps);
    }
}

/// Check whether the relative error between a and b is smaller than maxEps.
/// If the absolute error is less than maxAbs, the test succeeds however.
/// This is useful when comparing against zero the result of a subtraction.
template scalarApproxRelAndAbs(T) if (isFloatingPoint!T)
{
    bool scalarApproxRelAndAbs(in T a, in T b, in T maxAbs=0.0001, in T maxEps=4*T.epsilon)
    {
        import std.math : abs;
        if (abs(b-a) <= maxAbs) return true;
        return scalarApproxRel(a, b, maxEps);
    }
}

/// Generic template to check approx method on different sets of data.
template approx(ApproxMethod method)
{
    import gfx.math.mat : Mat;
    import gfx.math.vec : Vec;

    /// Apply method on scalar
    bool approx(T, Args...)(in T a, in T b, Args args)
    if (isFloatingPoint!T)
    {
        static if (method == ApproxMethod.ulp)
        {
            return scalarApproxUlp(a, b, args);
        }
        else static if (method == ApproxMethod.rel)
        {
            return scalarApproxRel(a, b, args);
        }
        else static if (method == ApproxMethod.ulpAndAbs)
        {
            return scalarApproxUlpAndAbs(a, b, args);
        }
        else static if (method == ApproxMethod.relAndAbs)
        {
            return scalarApproxRelAndAbs(a, b, args);
        }
        else
        {
            static assert(false, "unimplemented");
        }
    }
    /// Apply method check on vectors
    bool approx(T, size_t N, Args...)(in T[N] v1, in T[N] v2, Args args)
    if (isFloatingPoint!T)
    {
        static foreach (i; 0 .. N) {
            if (!approx(v1[i], v2[i], args)) return false;
        }
        return true;
    }
    /// ditto
    bool approx(T, size_t N, Args...)(in Vec!(T, N) v1, in Vec!(T, N) v2, Args args)
    if (isFloatingPoint!T)
    {
        return approx(v1.data, v2.data, args);
    }
    /// Apply method check on arrays
    bool approx(T, Args...)(in T[] arr1, in T[] arr2, Args args)
    if (isFloatingPoint!T)
    {
        if (arr1.length != arr2.length) return false;
        foreach (i; 0 .. arr1.length) {
            if (!approx(arr1[i], arr2[i], args)) return false;
        }
        return true;
    }
    /// Apply method check on matrices
    bool approx(T, size_t R, size_t C, Args...)(in T[C][R] m1, in T[C][R] m2, Args args)
    if (isFloatingPoint!T)
    {
        static foreach (r; 0 .. R) {
            if (!approx(m1[r], m2[r], args)) return false;
        }
        return true;
    }
    // /// ditto
    bool approx(T, size_t R, size_t C, Args...)(in Mat!(T, R, C) m1, in Mat!(T, R, C) m2, Args args)
    if (isFloatingPoint!T)
    {
        static foreach (r; 0 .. R) {
            if (!approx(m1[r], m2[r], args)) return false;
        }
        return true;
    }
}


private:

template FloatTraits(T)
if (isFloatingPoint!T)
{
    static if (is(T == float))
    {
        alias IntType = int;
        enum exponentMask = 0x7f80_0000;
    }
    else static if (is(T == double) || (is(T == real) && T.sizeof == 8))
    {
        alias IntType = long;
        enum exponentMask = 0x7ff0_0000_0000_0000;
    }
    else static if (is(T == real) && T.sizeof > 8)
    {
        static assert(false, "FloatTraits unsupported for real.");
    }
}

union FloatNum(T)
if (is(T == float) || is(T == double) || is(T == real) && T.sizeof == 8)
{
    alias F = FloatTraits!T;

    alias FloatType = T;
    alias IntType = F.IntType;

    this (FloatType f)
    {
        this.f = f;
    }

    FloatType f;
    IntType i;

    long opBinary(string op)(FloatNum rhs) const
    if (op == "-")
    {
        return i - rhs.i;
    }

    @property bool negative() const
    {
        return i < 0;
    }

    debug @property IntType mantissa() const
    {
        enum IntType one = 1;
        return i & ((one << T.mant_dig) - one);
    }

    debug @property IntType exponent() const
    {
        return ((i & F.exponentMask) >> T.mant_dig);
    }
}


union FloatNum(T)
if (is(T == real) && T.sizeof > 8)
{
    static assert(T.sizeof == 16 || T.sizeof == 12 || T.sizeof == 10, "Unexpected real size.");

    static if (T.sizeof == 10)
    {
        struct Int
        {
            long l;
            ushort h;
        }
    }
    static if (T.sizeof == 12)
    {
        struct Int
        {
            long l;
            uint h;
        }
    }
    static if (T.sizeof == 16)
    {
        struct Int
        {
            long l;
            ulong h;
        }
    }

    real f;
    Int i;

    this (real f)
    {
        this.f = f;
    }

    long opBinary(string op)(FloatNum rhs) const
    if (op == "-")
    {
        immutable lo = i.l - rhs.i.l;
        // do not check for overflow of the hi long
        return lo;
    }

    @property bool negative() const
    {
        return f < 0;
    }
}

unittest
{
    immutable fn1 = FloatNum!real(1);
    immutable fn2 = FloatNum!real(-1);

    assert(! fn1.negative);
    assert(  fn2.negative);
}

unittest
{
    import std.math : nextUp;
    immutable real r1 = 0;
    immutable real r2 = nextUp(r1);

    immutable fn1 = FloatNum!real(r1);
    immutable fn2 = FloatNum!real(r2);

    assert(fn2 - fn1 == 1);
}