theme/

color.rs

//! Color math: the oklch → sRGB → HSL converters behind the token constructors,
//! WCAG contrast for the palette tests, and small paint helpers.

use gpui::{Hsla, hsla};

/// A neutral (chroma 0) oklch tone as Hsla. Chroma 0 means r == g == b exactly,
/// so this goes straight to an achromatic Hsla (skipping the hue math avoids
/// float-noise saturation).
pub fn neutral(lightness: f32) -> Hsla {
    let [v, _, _] = oklch_to_srgb(lightness, 0.0, 0.0);
    hsla(0.0, 0.0, v, 1.0)
}

/// An exact achromatic tone from an 8-bit channel value (`grey(13)` ≡ `#0d0d0d`)
/// — for surfaces matched against reference-screenshot samples.
pub fn grey(value: u8) -> Hsla {
    hsla(0.0, 0.0, value as f32 / 255.0, 1.0)
}

/// Convert an oklch color (CSS notation: L 0..1, C, H in degrees) to gpui Hsla.
pub fn oklch(l: f32, c: f32, h_deg: f32) -> Hsla {
    let [r, g, b] = oklch_to_srgb(l, c, h_deg);
    let (h, s, l) = rgb_to_hsl(r, g, b);
    hsla(h, s, l, 1.0)
}

/// oklch → sRGB (each 0..1, clamped/gamut-clipped per channel).
/// Reference: Björn Ottosson's OKLab definition (the same matrices CSS Color 4 uses).
pub fn oklch_to_srgb(l: f32, c: f32, h_deg: f32) -> [f32; 3] {
    let [r, g, b] = oklch_to_linear(l, c, h_deg);
    [gamma_encode(r), gamma_encode(g), gamma_encode(b)]
}

/// oklch → *linear* sRGB, unclamped: a component outside 0..1 is a color the
/// display cannot make, which is what [`fit_chroma`] tests for.
fn oklch_to_linear(l: f32, c: f32, h_deg: f32) -> [f32; 3] {
    let h = h_deg.to_radians();
    let a = c * h.cos();
    let b = c * h.sin();

    // OKLab → LMS (cube roots undone)
    let l_ = l + 0.396_337_78 * a + 0.215_803_76 * b;
    let m_ = l - 0.105_561_346 * a - 0.063_854_17 * b;
    let s_ = l - 0.089_484_18 * a - 1.291_485_5 * b;
    let (l3, m3, s3) = (l_ * l_ * l_, m_ * m_ * m_, s_ * s_ * s_);

    // LMS → linear sRGB
    let r = 4.076_741_7 * l3 - 3.307_711_6 * m3 + 0.230_969_93 * s3;
    let g = -1.268_438 * l3 + 2.609_757_4 * m3 - 0.341_319_4 * s3;
    let b = -0.004_196_086_3 * l3 - 0.703_418_6 * m3 + 1.707_614_7 * s3;

    [r, g, b]
}

fn gamma_encode(x: f32) -> f32 {
    let x = x.clamp(0.0, 1.0);
    if x <= 0.003_130_8 {
        12.92 * x
    } else {
        1.055 * x.powf(1.0 / 2.4) - 0.055
    }
}

fn gamma_decode(x: f32) -> f32 {
    let x = x.clamp(0.0, 1.0);
    if x <= 0.040_449_936 {
        x / 12.92
    } else {
        ((x + 0.055) / 1.055).powf(2.4)
    }
}

/// The oklch lightness of an achromatic tone: [`neutral`] inverted.
///
/// Only greys round-trip through this. A grey's three channels are equal, and
/// the three LMS matrix rows each sum to 1, so the whole OKLab transform
/// collapses to one cube root — no inverse matrix needed.
pub fn lightness(color: Hsla) -> f32 {
    gamma_decode(color.l).cbrt()
}

/// The most chroma sRGB can hold at this lightness and hue, up to `chroma`.
///
/// Near black and near white the gamut is a needle: asking for a mid-ramp
/// chroma there produces an out-of-range component, and clamping it per channel
/// shifts the hue instead of dropping the saturation. Tailwind's neutral ramps
/// taper their chroma at both ends by hand for the same reason; here the taper
/// is whatever the gamut allows, so no ramp has to be tabulated.
fn fit_chroma(l: f32, chroma: f32, hue: f32) -> f32 {
    let fits = |c: f32| {
        oklch_to_linear(l, c, hue)
            .iter()
            .all(|x| (-1e-4..=1.0 + 1e-4).contains(x))
    };
    if fits(chroma) {
        return chroma;
    }
    let (mut lo, mut hi) = (0.0, chroma);
    for _ in 0..24 {
        let mid = 0.5 * (lo + hi);
        if fits(mid) { lo = mid } else { hi = mid }
    }
    lo
}

/// Re-emit an achromatic tone at `hue`, carrying as much `chroma` as its
/// lightness can hold. Alpha rides through untouched.
pub fn tint(color: Hsla, hue: f32, chroma: f32) -> Hsla {
    if chroma <= 0.0 {
        return color;
    }
    let l = lightness(color);
    let mut out = oklch(l, fit_chroma(l, chroma, hue), hue);
    out.a = color.a;
    out
}

/// sRGB (0..1 components) → HSL, all components 0..1 (gpui's Hsla convention).
pub fn rgb_to_hsl(r: f32, g: f32, b: f32) -> (f32, f32, f32) {
    let max = r.max(g).max(b);
    let min = r.min(g).min(b);
    let l = (max + min) / 2.0;
    let delta = max - min;
    if delta < f32::EPSILON {
        return (0.0, 0.0, l);
    }
    let s = if l > 0.5 {
        delta / (2.0 - max - min)
    } else {
        delta / (max + min)
    };
    let h = if (max - r).abs() < f32::EPSILON {
        ((g - b) / delta).rem_euclid(6.0)
    } else if (max - g).abs() < f32::EPSILON {
        (b - r) / delta + 2.0
    } else {
        (r - g) / delta + 4.0
    } / 6.0;
    (h, s, l)
}

/// HSL (gpui convention, all 0..1) → sRGB components 0..1.
pub fn hsl_to_rgb(h: f32, s: f32, l: f32) -> [f32; 3] {
    if s <= f32::EPSILON {
        return [l, l, l];
    }
    let q = if l < 0.5 {
        l * (1.0 + s)
    } else {
        l + s - l * s
    };
    let p = 2.0 * l - q;
    let hue = |mut t: f32| {
        t = t.rem_euclid(1.0);
        if t < 1.0 / 6.0 {
            p + (q - p) * 6.0 * t
        } else if t < 0.5 {
            q
        } else if t < 2.0 / 3.0 {
            p + (q - p) * (2.0 / 3.0 - t) * 6.0
        } else {
            p
        }
    };
    [hue(h + 1.0 / 3.0), hue(h), hue(h - 1.0 / 3.0)]
}

/// WCAG 2.1 relative luminance of an opaque color.
pub fn relative_luminance(color: Hsla) -> f32 {
    let lin = |c: f32| {
        if c <= 0.040_45 {
            c / 12.92
        } else {
            ((c + 0.055) / 1.055).powf(2.4)
        }
    };
    let [r, g, b] = hsl_to_rgb(color.h, color.s, color.l);
    0.2126 * lin(r) + 0.7152 * lin(g) + 0.0722 * lin(b)
}

/// WCAG 2.1 contrast ratio between two opaque colors (1.0 … 21.0).
///
/// Used by the palette tests to prove each light token reproduces the contrast
/// its dark counterpart had, rather than merely looking plausible.
pub fn contrast_ratio(a: Hsla, b: Hsla) -> f32 {
    let (la, lb) = (relative_luminance(a), relative_luminance(b));
    let (hi, lo) = if la >= lb { (la, lb) } else { (lb, la) };
    (hi + 0.05) / (lo + 0.05)
}

/// Composite `fg` (which may be translucent) over an opaque `bg`, returning the
/// opaque result — the color the eye actually receives.
pub fn flatten(fg: Hsla, bg: Hsla) -> Hsla {
    let a = fg.a.clamp(0.0, 1.0);
    let [fr, fg_, fb] = hsl_to_rgb(fg.h, fg.s, fg.l);
    let [br, bg_, bb] = hsl_to_rgb(bg.h, bg.s, bg.l);
    let (h, s, l) = rgb_to_hsl(
        fr * a + br * (1.0 - a),
        fg_ * a + bg_ * (1.0 - a),
        fb * a + bb * (1.0 - a),
    );
    hsla(h, s, l, 1.0)
}

/// Linear per-component mix of two colors (paint helper for the gradient spinner).
pub fn mix(a: Hsla, b: Hsla, t: f32) -> Hsla {
    let t = t.clamp(0.0, 1.0);
    let lerp = |x: f32, y: f32| x + (y - x) * t;
    // Mix through hue naively — both spinner endpoints sit close enough on the
    // wheel that shortest-arc handling isn't needed for our palette.
    hsla(
        lerp(a.h, b.h),
        lerp(a.s, b.s),
        lerp(a.l, b.l),
        lerp(a.a, b.a),
    )
}