using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace Uwaa.PNG;

/// <summary>
/// A 32-bit RGBA pixel in a <see cref="Png"/> image.
/// </summary>
[StructLayout(LayoutKind.Sequential)]
public readonly struct Pixel : IEquatable<Pixel>
{
    /// <summary>
    /// The red value for the pixel.
    /// </summary>
    public byte R { get; }

    /// <summary>
    /// The green value for the pixel.
    /// </summary>
    public byte G { get; }

    /// <summary>
    /// The blue value for the pixel.
    /// </summary>
    public byte B { get; }

    /// <summary>
    /// The alpha transparency value for the pixel.
    /// </summary>
    public byte A { get; }

    /// <summary>
    /// Create a new <see cref="Pixel"/>.
    /// </summary>
    /// <param name="r">The red value for the pixel.</param>
    /// <param name="g">The green value for the pixel.</param>
    /// <param name="b">The blue value for the pixel.</param>
    /// <param name="a">The alpha transparency value for the pixel.</param>
    public Pixel(byte r, byte g, byte b, byte a)
    {
        R = r;
        G = g;
        B = b;
        A = a;
    }

    /// <summary>
    /// Create a new <see cref="Pixel"/> which is fully opaque.
    /// </summary>
    /// <param name="r">The red value for the pixel.</param>
    /// <param name="g">The green value for the pixel.</param>
    /// <param name="b">The blue value for the pixel.</param>
    public Pixel(byte r, byte g, byte b)
    {
        R = r;
        G = g;
        B = b;
        A = 255;
    }

    /// <summary>
    /// Create a new grayscale <see cref="Pixel"/>.
    /// </summary>
    /// <param name="grayscale">The grayscale value.</param>
    public Pixel(byte grayscale)
    {
        R = grayscale;
        G = grayscale;
        B = grayscale;
        A = 255;
    }

    public override string ToString() => $"({R}, {G}, {B}, {A})";

    public override bool Equals(object? obj) => obj is Pixel pixel && Equals(pixel);

    /// <summary>
    /// Whether the pixel values are equal.
    /// </summary>
    /// <param name="other">The other pixel.</param>
    /// <returns><see langword="true"/> if all pixel values are equal otherwise <see langword="false"/>.</returns>
    public bool Equals(Pixel other)
    {
        Pixel this_ = this;
        return Unsafe.As<Pixel, uint>(ref this_) == Unsafe.As<Pixel, uint>(ref other);
    }

    public override int GetHashCode() => HashCode.Combine(R, G, B, A);

    public static bool operator ==(Pixel left, Pixel right) => left.Equals(right);

    public static bool operator !=(Pixel left, Pixel right) => !(left == right);
}