Re: Please rid me of this goto

[Walter Bright via Digitalmars-d]

On 6/23/2016 10:22 AM, Andrei Alexandrescu wrote:

https://dpaste.dzfl.pl/e53acb41885a

Paste bin links are ephemeral. The code from the link:

/**
*/
int pow(int lhs, uint rhs, ref bool overflow)
{ return powImpl(lhs, rhs, overflow); }
/// ditto
long pow(long lhs, uint rhs, ref bool overflow)
{ return powImpl(lhs, rhs, overflow); }
/// ditto
uint pow(uint lhs, uint rhs, ref bool overflow)
{ return powImpl(lhs, rhs, overflow); }
/// ditto
ulong pow(ulong lhs, uint rhs, ref bool overflow)
{ return powImpl(lhs, rhs, overflow); }

// Inspiration: http://www.stepanovpapers.com/PAM.pdf
private T powImpl(T)(T b, uint e, ref bool overflow)
{
    import core.checkedint : muls, mulu;
    import std.traits : isUnsigned;
    static if (isUnsigned!T) alias mul = mulu;
    else alias mul = muls;

    if (e <= 1)
    {
        if (e == 1) return b;
        if (b == 0) overflow = true;
        return 1;
    }

    T r = b;
    assert(e > 1);
    --e;
    // Loop invariant: r * (b ^^ e) is the actual result
    outer: for (;;)
    {
        if (e % 2 != 0) goto geeba;
        for (;;)
        {
            b = mul(b, b, overflow);
            e /= 2;
            continue outer;
        geeba:
            r = mul(r, b, overflow);
            if (e == 1) return r;
        }
    }
    assert(0);
}

unittest
{
    import std.stdio;
    bool overflow;
    foreach (uint i; 0 .. 21)
    {
        assert(pow(3u, i, overflow) == 3u ^^ i);
        assert(!overflow);
    }
    assert(pow(3u, 21u, overflow) == 3u ^^ 21);
    assert(overflow);
}

void main(){}