even-perfect-numbers-scanner/PerfectNumbers.Core/MutableUInt128.cs

using System;
using System.Numerics;
using System.Runtime.CompilerServices;

namespace PerfectNumbers.Core;

/// <summary>
/// Mutable helper for 128-bit arithmetic derived from the .NET runtime UInt128 implementation.
/// Portions adapted from https://github.com/dotnet/runtime/blob/v8.0.0/src/libraries/System.Private.CoreLib/src/System/UInt128.cs.
/// </summary>
internal struct MutableUInt128
{
    public ulong High;
    public ulong Low;

    public MutableUInt128(ulong value)
    {
        High = 0UL;
        Low = value;
    }

    public MutableUInt128(ulong high, ulong low)
    {
        High = high;
        Low = low;
    }

    public MutableUInt128(UInt128 value)
    {
        High = (ulong)(value >> 64);
        Low = (ulong)value;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Set(ulong value)
    {
        High = 0UL;
        Low = value;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Set(UInt128 value)
    {
        High = (ulong)(value >> 64);
        Low = (ulong)value;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Set(ulong high, ulong low)
    {
        High = high;
        Low = low;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public UInt128 ToUInt128() => ((UInt128)High << 64) | Low;

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Add(ulong value)
    {
        ulong low = Low + value;
        if (low < value)
        {
            High++;
        }

        Low = low;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Add(in MutableUInt128 value)
    {
        ulong low = Low + value.Low;
        ulong carry = low < Low ? 1UL : 0UL;
        ulong high = High + value.High + carry;

        Low = low;
        High = high;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Subtract(ulong value)
    {
        ulong low = Low;
        ulong result = low - value;
        ulong borrow = result > low ? 1UL : 0UL;

        Low = result;
        High -= borrow;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Subtract(in MutableUInt128 value)
    {
        ulong low = Low;
        ulong result = low - value.Low;
        ulong borrow = low < value.Low ? 1UL : 0UL;

        Low = result;
        High = High - value.High - borrow;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Multiply(ulong value)
    {
        ulong currentHigh = High;
        ulong currentLow = Low;

        ulong carry = Multiply64(currentLow, value, out ulong newLow);
        Multiply64(currentHigh, value, out ulong shiftedHigh);
        ulong newHigh = shiftedHigh + carry;

        Low = newLow;
        High = newHigh;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static ulong Multiply64(ulong left, ulong right, out ulong low)
    {
        const ulong Mask32 = 0xFFFF_FFFFUL;

        ulong leftLow = left & Mask32;
        ulong leftHigh = left >> 32;
        ulong rightLow = right & Mask32;
        ulong rightHigh = right >> 32;

        ulong lowLow = leftLow * rightLow;
        ulong lowHigh = leftLow * rightHigh;
        ulong highLow = leftHigh * rightLow;
        ulong highHigh = leftHigh * rightHigh;

        ulong carry = (lowLow >> 32) + (lowHigh & Mask32) + (highLow & Mask32);
        ulong high = highHigh + (lowHigh >> 32) + (highLow >> 32) + (carry >> 32);

        low = ((carry & Mask32) << 32) | (lowLow & Mask32);
        return high;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void MultiplyAdd(ulong multiplier, ulong addend)
    {
        Multiply(multiplier);
        Add(addend);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public ulong Mod(ulong modulus)
    {
        if (modulus == 0UL)
        {
            throw new DivideByZeroException();
        }

        if (High == 0UL)
        {
            return Low % modulus;
        }

        MutableUInt128 remainder = default;
        int highBitLength = 64 - BitOperations.LeadingZeroCount(High);
        if (highBitLength > 0)
        {
            ProcessWord(ref remainder, High, highBitLength, modulus);
        }

        ProcessWord(ref remainder, Low, 64, modulus);
        return remainder.Low;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private static void ProcessWord(ref MutableUInt128 remainder, ulong word, int bitCount, ulong modulus)
    {
        for (int bit = bitCount - 1; bit >= 0; bit--)
        {
            remainder.ShiftLeftOneBit();
            if (((word >> bit) & 1UL) != 0)
            {
                remainder.Add(1UL);
            }

            remainder.Reduce(modulus);
        }
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private void ShiftLeftOneBit()
    {
        High = (High << 1) | (Low >> 63);
        Low <<= 1;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    private void Reduce(ulong modulus)
    {
        if (High != 0UL || Low >= modulus)
        {
            Subtract(modulus);
            if (High != 0UL || Low >= modulus)
            {
                Subtract(modulus);
            }
        }
    }
}