NejCommon.NET/Utils/RoundingNumericTypeInfoResolverFactory.cs

using System.Globalization;
using System.Numerics;
using System.Text;
using Npgsql.Internal;
using Npgsql.Internal.Converters;
using Npgsql.Internal.Postgres;

namespace NejCommon.Utils;

#pragma warning disable NPG9001 // decimalype is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
public sealed class RoundingNumericTypeInfoResolverFactory
    : PgTypeInfoResolverFactory
{
    public override IPgTypeInfoResolver CreateResolver()
        => new Resolver();

    public override IPgTypeInfoResolver? CreateArrayResolver()
        => null;

    private sealed class Resolver : IPgTypeInfoResolver
    {
        private readonly TypeInfoMappingCollection _mappings =
            CreateMappings();

        public PgTypeInfo? GetTypeInfo(
            Type? type,
            DataTypeName? dataTypeName,
            PgSerializerOptions options)
            => _mappings.Find(type, dataTypeName, options);

        private static TypeInfoMappingCollection CreateMappings()
        {
            var mappings = new TypeInfoMappingCollection();

            mappings.AddStructType<decimal>(
                "numeric",
                static (options, mapping, _) =>
                    mapping.CreateInfo(
                        options,
                        new RoundingDecimalNumericConverter()),
                isDefault: true);

            return mappings;
        }
    }
}

public sealed class RoundingDecimalNumericConverter
    : PgBufferedConverter<decimal>
{
    private const ushort SignPositive = 0x0000;
    private const ushort SignNegative = 0x4000;
    private const ushort SignNaN = 0xC000;
    private const ushort SignPositiveInfinity = 0xD000;
    private const ushort SignNegativeInfinity = 0xF000;

    private static readonly BigInteger MaxDecimalCoefficient =
        (BigInteger.One << 96) - 1;

    public override bool CanConvert(
        DataFormat format,
        out BufferRequirements bufferRequirements)
    {
        // Let Npgsql buffer the entire binary numeric value before calling
        // ReadCore/WriteCore.
        return CanConvertBufferedDefault(format, out bufferRequirements);
    }

    protected override decimal ReadCore(PgReader reader)
    {
        // PostgreSQL numeric binary format:
        //
        // int16 ndigits
        // int16 weight
        // int16 sign
        // int16 dscale
        // int16 digits[ndigits]
        //
        // Each digit is base 10000.

        var digitCount = reader.ReadInt16();
        var weight = reader.ReadInt16();
        var sign = reader.ReadUInt16();

        // IMPORTANT:
        // This is DISPLAY scale, not necessarily the number of physically
        // significant fractional digits.
        //
        // e.g. 100.000000000000000000000000000000 may have
        // dscale=30 but physically contain only the digit 100.
        _ = reader.ReadInt16();

        if (digitCount < 0)
            throw new InvalidOperationException(
                $"Invalid PostgreSQL numeric digit count: {digitCount}");

        switch (sign)
        {
            case SignNaN:
                throw new InvalidCastException(
                    "Numeric NaN cannot be represented as System.Decimal.");

            case SignPositiveInfinity:
                throw new InvalidCastException(
                    "Numeric Infinity cannot be represented as System.Decimal.");

            case SignNegativeInfinity:
                throw new InvalidCastException(
                    "Numeric -Infinity cannot be represented as System.Decimal.");

            case not (SignPositive or SignNegative):
                throw new InvalidCastException(
                    $"Unknown PostgreSQL numeric sign 0x{sign:X4}.");
        }

        if (digitCount == 0)
            return decimal.Zero;

        BigInteger coefficient = BigInteger.Zero;

        for (var i = 0; i < digitCount; i++)
        {
            var digit = reader.ReadInt16();

            if (digit is < 0 or >= 10000)
                throw new InvalidOperationException(
                    $"Invalid PostgreSQL numeric digit: {digit}");

            coefficient *= 10000;
            coefficient += digit;
        }

        return ToDecimal(
            coefficient,
            digitCount,
            weight,
            sign == SignNegative);
    }

    private static decimal ToDecimal(
        BigInteger coefficient,
        int digitCount,
        int weight,
        bool negative)
    {
        if (coefficient.IsZero)
            return decimal.Zero;

        /*
         * PostgreSQL numeric:
         *
         *   digits are base 10000
         *   weight tells us the weight of the first group
         *
         * Therefore:
         *
         * coefficient * 10^(-physicalScale)
         *
         * gives us the actual numerical value.
         */
        var physicalScale =
            checked(4 * (digitCount - 1 - weight));

        // Value has zeroes to the right, e.g. a positive exponent.
        if (physicalScale < 0)
        {
            coefficient *= BigInteger.Pow(10, -physicalScale);
            physicalScale = 0;
        }

        /*
         * Strip REAL trailing zeroes first.
         *
         * This is the important part for values such as:
         *
         *   100.000000000000000000000000000000
         *
         * We care about the value, not PostgreSQL's nominal dscale.
         */
        while (physicalScale > 0 &&
               coefficient % 10 == 0)
        {
            coefficient /= 10;
            physicalScale--;
        }

        /*
         * System.Decimal is:
         *
         *   96-bit unsigned coefficient
         *   scale 0..28
         *
         * Try the highest scale we can preserve.
         *
         * If necessary, progressively round away fractional precision.
         */
        var targetScale = Math.Min(physicalScale, 28);

        for (; targetScale >= 0; targetScale--)
        {
            var digitsToDrop = physicalScale - targetScale;

            var rounded = digitsToDrop == 0
                ? coefficient
                : RoundToEven(coefficient, digitsToDrop);

            if (rounded <= MaxDecimalCoefficient)
            {
                return CreateDecimal(
                    rounded,
                    targetScale,
                    negative);
            }
        }

        // Even integer-scale representation doesn't fit.
        // This is a REAL overflow rather than useless fractional precision.
        throw new OverflowException(
            "PostgreSQL numeric magnitude does not fit in System.Decimal.");
    }

    private static BigInteger RoundToEven(
        BigInteger value,
        int decimalDigitsToDrop)
    {
        if (decimalDigitsToDrop <= 0)
            return value;

        var divisor =
            BigInteger.Pow(10, decimalDigitsToDrop);

        var quotient =
            BigInteger.DivRem(value, divisor, out var remainder);

        var doubledRemainder = remainder * 2;
        var comparison = doubledRemainder.CompareTo(divisor);

        // MidpointRounding.ToEven
        if (comparison > 0 ||
            (comparison == 0 && !quotient.IsEven))
        {
            quotient++;
        }

        return quotient;
    }

    private static decimal CreateDecimal(
        BigInteger coefficient,
        int scale,
        bool negative)
    {
        var lo = (uint)(coefficient & uint.MaxValue);
        var mid = (uint)((coefficient >> 32) & uint.MaxValue);
        var hi = (uint)((coefficient >> 64) & uint.MaxValue);

        return new decimal(
            unchecked((int)lo),
            unchecked((int)mid),
            unchecked((int)hi),
            negative,
            checked((byte)scale));
    }

    // ----------------------------
    // Writing CLR decimal -> numeric
    // ----------------------------

    public override Size GetSize(
        SizeContext context,
        decimal value,
        ref object? writeState)
    {
        var encoded = Encode(value);

        // ndigits + weight + sign + dscale = 8 bytes
        // plus 2 bytes per base-10000 digit.
        return 8 + encoded.Digits.Length * 2;
    }

    protected override void WriteCore(
        PgWriter writer,
        decimal value)
    {
        var encoded = Encode(value);

        writer.WriteInt16(
            checked((short)encoded.Digits.Length));

        writer.WriteInt16(encoded.Weight);
        writer.WriteUInt16(encoded.Sign);
        writer.WriteInt16(encoded.Scale);

        foreach (var digit in encoded.Digits)
            writer.WriteInt16(digit);
    }

    private static EncodedNumeric Encode(decimal value)
    {
        var bits = decimal.GetBits(value);

        var negative =
            (bits[3] & int.MinValue) != 0;

        var scale =
            (bits[3] >> 16) & 0xFF;

        var coefficient =
            ((BigInteger)(uint)bits[2] << 64) |
            ((BigInteger)(uint)bits[1] << 32) |
            (uint)bits[0];

        if (coefficient.IsZero)
        {
            return new EncodedNumeric(
                Array.Empty<short>(),
                0,
                SignPositive,
                checked((short)scale));
        }

        var text =
            coefficient.ToString(
                CultureInfo.InvariantCulture);

        string integerPart;
        string fractionalPart;

        if (scale == 0)
        {
            integerPart = text;
            fractionalPart = "";
        }
        else if (text.Length <= scale)
        {
            integerPart = "0";
            fractionalPart =
                new string('0', scale - text.Length) + text;
        }
        else
        {
            integerPart = text[..^scale];
            fractionalPart = text[^scale..];
        }

        // PostgreSQL numeric groups contain 4 decimal digits each.
        var integerPadding =
            (4 - integerPart.Length % 4) % 4;

        integerPart =
            new string('0', integerPadding) +
            integerPart;

        var fractionalPadding =
            (4 - fractionalPart.Length % 4) % 4;

        fractionalPart +=
            new string('0', fractionalPadding);

        var integerGroups =
            integerPart.Length / 4;

        var all = integerPart + fractionalPart;

        var groups =
            new List<short>(all.Length / 4);

        for (var i = 0; i < all.Length; i += 4)
        {
            groups.Add(
                short.Parse(
                    all.AsSpan(i, 4),
                    CultureInfo.InvariantCulture));
        }

        var weight = integerGroups - 1;

        // PostgreSQL itself doesn't transmit unnecessary leading groups.
        while (groups.Count > 0 && groups[0] == 0)
        {
            groups.RemoveAt(0);
            weight--;
        }

        // Nor unnecessary trailing groups.
        while (groups.Count > 0 &&
               groups[^1] == 0)
        {
            groups.RemoveAt(groups.Count - 1);
        }

        if (groups.Count == 0)
            weight = 0;

        return new EncodedNumeric(
            groups.ToArray(),
            checked((short)weight),
            negative
                ? SignNegative
                : SignPositive,
            checked((short)scale));
    }

    private sealed record EncodedNumeric(
        short[] Digits,
        short Weight,
        ushort Sign,
        short Scale);
}

#pragma warning restore NPG9001 // decimalype is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.