prettypretty 0.12.0

#[cfg(feature = "pyffi")]
use pyo3::prelude::*;

use crate::core::{convert, normalize, ColorSpace};
use crate::Float;

/// Compute Delta-E for Oklab or Oklrab.
#[allow(non_snake_case)]
pub(crate) fn delta_e_ok(coordinates1: &[Float; 3], coordinates2: &[Float; 3]) -> Float {
    let [ref L1, ref a1, ref b1] = *coordinates1;
    let [ref L2, ref a2, ref b2] = *coordinates2;

    let ΔL = L1 - L2;
    let Δa = a1 - a2;
    let Δb = b1 - b2;

    ΔL.mul_add(ΔL, Δa.mul_add(Δa, Δb * Δb)).sqrt()
}

/// Find the candidate color closest to the origin.
///
/// This function compares the origin to every candidate color, computing the
/// distance metric with the given function, and returns the index of the
/// closest candidate color—or `None` if there are no candidates.
pub(crate) fn find_closest<'c, C, F>(
    origin: &[f64; 3],
    candidates: C,
    mut compute_distance: F,
) -> Option<usize>
where
    C: IntoIterator<Item = &'c [f64; 3]>,
    F: FnMut(&[f64; 3], &[f64; 3]) -> f64,
{
    let mut min_distance = f64::INFINITY;
    let mut min_index = None;

    for (index, candidate) in candidates.into_iter().enumerate() {
        let distance = compute_distance(origin, candidate);
        if distance < min_distance {
            min_distance = distance;
            min_index = Some(index);
        }
    }

    min_index
}

// --------------------------------------------------------------------------------------------------------------------

/// Determine how a coordinate carries forward.
///
/// This function determines how to [carry
/// forward](https://www.w3.org/TR/css-color-4/#interpolation-missing) a missing
/// coordinate, i.e., a coordinate that is not-a-number, from the source color
/// space to the interpolation color space. The caller specifies the coordinate
/// by its index (from 0 to 2) and, if the coordinate carries forward, the function
/// returns the index of the forwarded coordinate.
///
/// # Panics
///
/// This function panics if the index is out of bounds.
fn carry_forward(from_space: ColorSpace, to_space: ColorSpace, index: usize) -> Option<usize> {
    use ColorSpace::*;

    if !(0..=2).contains(&index) {
        panic!("0..=2.contains({}) does not hold!", index)
    }

    match (from_space, to_space, index) {
        // Analogous components are (r,x) -- (g,y) -- (b,z) -- (L) -- (Lr) -- (C) -- (h) -- (a) -- (b)
        (
            Srgb | LinearSrgb | DisplayP3 | LinearDisplayP3 | Rec2020 | LinearRec2020 | Xyz,
            Srgb | LinearSrgb | DisplayP3 | LinearDisplayP3 | Rec2020 | LinearRec2020 | Xyz,
            _,
        ) => Some(index),
        (Oklab | Oklch, Oklab | Oklch, 0) => Some(0),
        (Oklrab | Oklrch, Oklrab | Oklrch, 0) => Some(0),
        (Oklab | Oklrab, Oklab | Oklrab, 1 | 2) => Some(index),
        (Oklch | Oklrch, Oklch | Oklrch, 1 | 2) => Some(index),
        _ => None,
    }
}

/// Convert the coordinates while carrying forward missing values.
fn prepare_coordinate_interpolation(
    from_space: ColorSpace,
    to_space: ColorSpace,
    coordinates: &[Float; 3],
) -> [Float; 3] {
    // Normalize coordinates and convert to interpolation space
    let mut intermediate = convert(from_space, to_space, &normalize(from_space, coordinates));

    // Carry forward missing components
    for (index, coordinate) in coordinates.iter().enumerate() {
        if coordinate.is_nan() {
            if let Some(index) = carry_forward(from_space, to_space, index) {
                intermediate[index] = Float::NAN;
            }
        }
    }

    intermediate
}

/// A choice of strategy for interpolating hues.
///
/// This enum is used by [`Color::interpolate`](crate::Color::interpolate).
///
/// Since hues are expressed as angles, the same perceptual hue has an infinite
/// number of representations modulo 360. Furthermore, there are two ways of
/// interpolating between two hues, clockwise and counter-clockwise. Consistent
/// with [CSS Color 4](https://www.w3.org/TR/css-color-4/#hue-interpolation),
/// the interpolation strategy selects the way based either on the distance
/// between hues, [`HueInterpolation::Shorter`] and
/// [`HueInterpolation::Longer`], or on the direction,
/// [`HueInterpolation::Increasing`] and [`HueInterpolation::Decreasing`].
#[cfg_attr(
    feature = "pyffi",
    pyclass(eq, eq_int, frozen, hash, module = "prettypretty.color")
)]
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub enum HueInterpolation {
    /// Take the shorter arc between the two hue angles.
    Shorter,
    /// Take the longer arc between the two hue angles.
    Longer,
    /// Keep increasing hue angles.
    Increasing,
    /// Keep decreasing hue angles.
    Decreasing,
}

/// Adjust the pair of hues based on interpolation strategy.
fn prepare_hue_interpolation(strategy: HueInterpolation, h1: Float, h2: Float) -> [Float; 2] {
    match strategy {
        HueInterpolation::Shorter => {
            if 180.0 < h2 - h1 {
                return [h1 + 360.0, h2];
            } else if h2 - h1 < -180.0 {
                return [h1, h2 + 360.0];
            }
        }
        HueInterpolation::Longer => {
            if (0.0..=180.0).contains(&(h2 - h1)) {
                return [h1 + 360.0, h2];
            } else if (-180.0..=0.0).contains(&(h2 - h1)) {
                return [h1, h2 + 360.0];
            }
        }
        HueInterpolation::Increasing => {
            if h2 < h1 {
                return [h1, h2 + 360.0];
            }
        }
        HueInterpolation::Decreasing => {
            if h1 < h2 {
                return [h1 + 360.0, h2];
            }
        }
    }

    [h1, h2]
}

/// Prepare coordinates for interpolation.
///
/// This function prepares a pair of coordinates for interpolation with
/// [`interpolate`] accorrding to the rules of [CSS Color
/// 4](https://www.w3.org/TR/css-color-4/#interpolation). Those rules are
/// surprisingly complex thanks to the specification's support for missing
/// components and hue interpolation strategies.
///
/// As required by the specification, this function carries missing components
/// forward when converting to the interpolation color space and then tries to
/// fill them with the other color's component. It also implements all four
/// interpolation strategies for hues, which select one of the two available
/// arcs between the two colors.
///
/// By separating preparation from actual interpolation, it becomes possible to
/// amortize the overhead of the former when generating several interpolated
/// colors, e.g., when computing a gradient.
///
/// This function normalizes coordinates. However, if both colors in the
/// interpolation color space end up with forward-carried not-a-number values
/// for the same coordinate, those not-a-number values remain.
#[must_use = "function returns new color coordinates and does not mutate original values"]
pub(crate) fn prepare_to_interpolate(
    space1: ColorSpace,
    coordinates1: &[Float; 3],
    space2: ColorSpace,
    coordinates2: &[Float; 3],
    interpolation_space: ColorSpace,
    strategy: HueInterpolation,
) -> ([Float; 3], [Float; 3]) {
    let mut coordinates1 =
        prepare_coordinate_interpolation(space1, interpolation_space, coordinates1);
    let mut coordinates2 =
        prepare_coordinate_interpolation(space2, interpolation_space, coordinates2);

    // Fill in missing components
    for index in 0..=2 {
        if coordinates1[index].is_nan() {
            // Technically, only do this if coordinates2[index] is a number.
            coordinates1[index] = coordinates2[index];
        } else if coordinates2[index].is_nan() {
            coordinates2[index] = coordinates1[index];
        }
    }

    // Adjust hue based on interpolation strategy
    if interpolation_space.is_polar() {
        [coordinates1[2], coordinates2[2]] =
            prepare_hue_interpolation(strategy, coordinates1[2], coordinates2[2])
    }

    (coordinates1, coordinates2)
}

/// Interpolate between the prepared coordinates.
///
/// This function calculates the linear interpolation for the given factor
/// between equivalent coordinates of the two colors. For the result to be
/// meaningful, the coordinates must be prepared with
/// [`prepare_to_interpolate`].
#[must_use = "function returns new color coordinates and does not mutate original values"]
pub(crate) fn interpolate(
    fraction: Float,
    coordinates1: &[Float; 3],
    coordinates2: &[Float; 3],
) -> [Float; 3] {
    [
        fraction.mul_add(coordinates2[0] - coordinates1[0], coordinates1[0]),
        fraction.mul_add(coordinates2[1] - coordinates1[1], coordinates1[1]),
        fraction.mul_add(coordinates2[2] - coordinates1[2], coordinates1[2]),
    ]
}