silkdot/src/color.zig

const std = @import("std");
const fmt = std.fmt;

pub const RGB = struct {
    r: u8,
    g: u8,
    b: u8,

    pub fn init(comptime string: []const u8) !RGB {
        std.debug.assert(string.len == 6 or string.len == 7);
        comptime var index: usize = 0;
        if (string[0] == '#') index += 1;
        return .{
            .r = try fmt.parseInt(u8, string[index .. index + 2], 16),
            .g = try fmt.parseInt(u8, string[index + 2 .. index + 4], 16),
            .b = try fmt.parseInt(u8, string[index + 4 .. index + 6], 16),
        };
    }
};

pub const RGBA = struct {
    r: f32,
    g: f32,
    b: f32,
    /// Alpha channel between 0.0 - 1.0
    a: f32 = 1.0,

    /// Create from string at comptime
    pub fn init(comptime string: []const u8) !RGBA {
        std.debug.assert(string.len == 8 or string.len == 9);
        comptime var index: usize = 0;
        if (string[0] == '#') index += 1;
        const r = try fmt.parseInt(u8, string[index .. index + 2], 16);
        const g = try fmt.parseInt(u8, string[index + 2 .. index + 4], 16);
        const b = try fmt.parseInt(u8, string[index + 4 .. index + 6], 16);
        const a = try fmt.parseInt(u8, string[index + 6 .. index + 8], 16);
        return .{
            .r = @as(f32, @floatFromInt(r)) / 0xff,
            .g = @as(f32, @floatFromInt(g)) / 0xff,
            .b = @as(f32, @floatFromInt(b)) / 0xff,
            .a = @as(f32, @floatFromInt(a)) / 0xff,
        };
    }

    pub fn eql(a: *const RGBA, b: *const RGBA) bool {
        return a.r == b.r and
            a.g == b.g and
            a.b == b.b and
            a.a == b.a;
    }

    pub fn blend(base: *const RGBA, add: *const RGBA) RGBA {
        var r: RGBA = undefined;
        r.a = 1 - (1 - add.a) * (1 - base.a);
        if (r.a < std.math.floatEps(f32)) {
            r.r = 0;
            r.g = 0;
            r.b = 0;
            return r;
        }
        r.r = (add.r * add.a / r.a + base.r * base.a * (1 - add.a)) / r.a;
        r.g = (add.g * add.a / r.a + base.g * base.a * (1 - add.a)) / r.a;
        r.b = (add.b * add.a / r.a + base.b * base.a * (1 - add.a)) / r.a;
        return r;
    }
};

test "RGB init from string" {
    const red = try RGB.init("#ff0000");
    try std.testing.expectEqual(0xff, red.r);
    try std.testing.expectEqual(0x00, red.g);
    try std.testing.expectEqual(0x00, red.b);

    const green = try RGB.init("00ff00");
    try std.testing.expectEqual(0x00, green.r);
    try std.testing.expectEqual(0xff, green.g);
    try std.testing.expectEqual(0x00, green.b);

    const blue = try RGB.init("#0000ff");
    try std.testing.expectEqual(0x00, blue.r);
    try std.testing.expectEqual(0x00, blue.g);
    try std.testing.expectEqual(0xff, blue.b);

    try std.testing.expectError(fmt.ParseIntError.InvalidCharacter, RGB.init("xyzxyz"));
}

test "RGBA blend" {
    const red = RGBA{ .r = 1.0, .g = 0, .b = 0, .a = 0.0 };
    const blue = RGBA{ .r = 0, .g = 0, .b = 1.0, .a = 0.0 };
    const result = red.blend(&blue);
    std.debug.print("\n{any}\n", .{result});
    const reeb = RGBA{ .r = 1.0, .g = 0, .b = 0, .a = 1.0 };
    const blueb = RGBA{ .r = 0, .g = 0, .b = 1.0, .a = 0.0 };
    const resultt = reeb.blend(&blueb);
    std.debug.print("{any}\n", .{resultt});
    const roob = RGBA{ .r = 1.0, .g = 0, .b = 0, .a = 0.5 };
    const bloob = RGBA{ .r = 0, .g = 0, .b = 1.0, .a = 0.5 };
    const resulttt = roob.blend(&bloob);
    std.debug.print("{any}\n", .{resulttt});
}