HellionChat/HellionChat/Util/ColourUtil.cs

using System.Buffers.Binary;
using System.Numerics;

namespace HellionChat.Util;

internal static class ColourUtil
{
    private static (byte r, byte g, byte b) RgbaToRgbComponents(uint rgba)
    {
        var r = (byte)((rgba & 0xFF000000) >> 24);
        var g = (byte)((rgba & 0xFF0000) >> 16);
        var b = (byte)((rgba & 0xFF00) >> 8);
        return (r, g, b);
    }

    internal static uint RgbaToAbgr(uint rgba) => BinaryPrimitives.ReverseEndianness(rgba);

    internal static Vector3 RgbaToVector3(uint rgba)
    {
        var (r, g, b) = RgbaToRgbComponents(rgba);
        return new Vector3((float)r / 255, (float)g / 255, (float)b / 255);
    }

    internal static uint Vector3ToRgba(Vector3 col)
    {
        return ComponentsToRgba(
            (byte)Math.Round(col.X * 255),
            (byte)Math.Round(col.Y * 255),
            (byte)Math.Round(col.Z * 255)
        );
    }

    internal static uint Vector4ToAbgr(Vector4 col)
    {
        return RgbaToAbgr(
            ComponentsToRgba(
                (byte)Math.Round(col.X * 255),
                (byte)Math.Round(col.Y * 255),
                (byte)Math.Round(col.Z * 255),
                (byte)Math.Round(col.W * 255)
            )
        );
    }

    public static unsafe uint ArgbToRgba(uint x)
    {
        var buf = (byte*)&x;
        (buf[1], buf[2], buf[3], buf[0]) = (buf[0], buf[1], buf[2], buf[3]);
        return x;
    }

    internal static uint ComponentsToRgba(byte red, byte green, byte blue, byte alpha = 0xFF) =>
        alpha | (uint)(red << 24) | (uint)(green << 16) | (uint)(blue << 8);

    internal static uint AdjustBrightness(uint abgr, float factor)
    {
        var a = (byte)((abgr & 0xFF000000) >> 24);
        var b = (byte)((abgr & 0x00FF0000) >> 16);
        var g = (byte)((abgr & 0x0000FF00) >> 8);
        var r = (byte)(abgr & 0x000000FF);

        var nr = (byte)Math.Clamp(r * factor, 0f, 255f);
        var ng = (byte)Math.Clamp(g * factor, 0f, 255f);
        var nb = (byte)Math.Clamp(b * factor, 0f, 255f);

        return ((uint)a << 24) | ((uint)nb << 16) | ((uint)ng << 8) | nr;
    }

    // Modulates the alpha byte of an ABGR color by a factor in [0, 1].
    // RGB stays intact. Used by the PM-3 hover-lerp path where each
    // frame produces a fractional alpha value but the colour itself
    // must not shift.
    internal static uint ApplyAlpha(uint abgr, float alphaFactor)
    {
        alphaFactor = Math.Clamp(alphaFactor, 0f, 1f);
        var origAlpha = (byte)((abgr >> 24) & 0xFFu);
        var newAlpha = (byte)Math.Round(origAlpha * alphaFactor);
        return (abgr & 0x00FFFFFFu) | ((uint)newAlpha << 24);
    }

    public static uint HexToRgba(string hex)
    {
        ArgumentNullException.ThrowIfNull(hex);
        var s = hex.StartsWith('#') ? hex[1..] : hex;
        if (s.Length != 6 && s.Length != 8)
            throw new FormatException(
                $"Hex colour must be 6 or 8 hex digits, got {s.Length}: '{hex}'"
            );

        if (
            !uint.TryParse(
                s,
                System.Globalization.NumberStyles.HexNumber,
                System.Globalization.CultureInfo.InvariantCulture,
                out var value
            )
        )
            throw new FormatException($"Hex colour '{hex}' is not a valid hexadecimal value");

        if (s.Length == 6)
            value = (value << 8) | 0xFFu; // RRGGBB → RRGGBBFF

        return value;
    }
}