//! Gradient specifications.
//!
//! We provide both linear and radial gradients; and for each flavor
//! we provide two representations, a 'generic' representation that uses
//! points in the [unit square], and a 'fixed' representation that uses
//! image-space coordinates.
//!
//! The generic representations ([`LinearGradient`] and [`RadialGradient`])
//! are useful for cases such as UI, when the same gradient may be reused
//! with different shapes. The fixed representations
//! ([`FixedLinearGradient`] and [`FixedRadialGradient`]) may be better suited
//! to working with content in existing formats such as SVG. A fixed gradient
//! can be generated from a generic gradient by mapping points from the unit
//! square onto any arbitrary rectangle.
//!
//! The fixed variants are provided because they more closely match the types
//! used by many 2d graphics APIs; but you can use the generic representations
//! anywhere you can use the fixed ones, and they will be automatically
//! resolved appropriately.
//!
//! [`LinearGradient`]: struct.LinearGradient.html
//! [`RadialGradient`]: struct.RadialGradient.html
//! [`FixedLinearGradient`]: struct.FixedLinearGradient.html
//! [`FixedRadialGradient`]: struct.FixedRadialGradient.html
//! [unit square]: https://en.wikipedia.org/wiki/Unit_square

use std::borrow::Cow;

use kurbo::{Point, Rect, Size, Vec2};

use crate::{IntoBrush, RenderContext};

use crate::Color;

/// Specification of a linear gradient.
///
/// This specification is in terms of image-space coordinates. In many
/// cases, it is better to specify coordinates relative to the `Rect`
/// of the item being drawn; for these, use [`LinearGradient`] instead.
///
/// [`LinearGradient`]: struct.LinearGradient.html
#[derive(Debug, Clone)]
pub struct FixedLinearGradient {
    /// The start point (corresponding to pos 0.0).
    pub start: Point,
    /// The end point (corresponding to pos 1.0).
    pub end: Point,
    /// The stops.
    ///
    /// There must be at least two for the gradient to be valid.
    pub stops: Vec<GradientStop>,
}

/// Specification of a radial gradient in image-space.
///
/// This specification is in terms of image-space coordinates. In many
/// cases, it is better to specify coordinates relative to the `Rect`
/// of the item being drawn; for these, use [`RadialGradient`] instead.
///
/// [`RadialGradient`]: struct.RadialGradient.html
#[derive(Debug, Clone)]
pub struct FixedRadialGradient {
    /// The center.
    pub center: Point,
    /// The offset of the origin relative to the center.
    pub origin_offset: Vec2,
    /// The radius.
    ///
    /// The circle with this radius from the center corresponds to pos 1.0.
    pub radius: f64,
    /// The stops (see similar field in [`LinearGradient`](struct.LinearGradient.html)).
    pub stops: Vec<GradientStop>,
}

/// Any fixed gradient.
///
/// This is provided as a convenience, so that we can provide API that
/// accept both [`FixedLinearGradient`] and [`FixedRadialGradient`].
/// You should not construct this type dirctly; rather construct one of those
/// types, both of which impl `Into<FixedGradient>`.
///
/// [`FixedLinearGradient`]: struct.FixedLinearGradient.html
/// [`FixedRadialGradient`]: struct.FixedRadialGradient.html
#[derive(Debug, Clone)]
pub enum FixedGradient {
    /// A linear gradient.
    Linear(FixedLinearGradient),
    /// A radial gradient.
    Radial(FixedRadialGradient),
}

/// Specification of a gradient stop.
#[derive(Debug, Clone)]
pub struct GradientStop {
    /// The coordinate of the stop.
    pub pos: f32,
    /// The color at that stop.
    pub color: Color,
}

/// A flexible, ergonomic way to describe gradient stops.
pub trait GradientStops {
    fn to_vec(self) -> Vec<GradientStop>;
}

/// A description of a linear gradient in the unit rect, which can be resolved
/// to a fixed gradient.
///
/// The start and end points in the gradient are given in [`UnitPoint`] coordinates,
/// which are then resolved to image-space coordinates for any given concrete `Rect`.
///
/// When the fixed coordinates are known, use [`FixedLinearGradient`] instead.
///
/// [`UnitPoint`]: struct.UnitPoint.html
/// [`FixedLinearGradient`]: struct.FixedLinearGradient.html
#[derive(Debug, Clone)]
pub struct LinearGradient {
    start: UnitPoint,
    end: UnitPoint,
    stops: Vec<GradientStop>,
}

/// A description of a radial gradient in the unit rect, which can be resolved
/// to a fixed gradient.
///
/// The `center` and `origin` are given in [`UnitPoint`] coordinates.
///
/// The `center` parameter specifies the `center` of the circle, so that points
/// of distance radius from the `center` map to 1.0 in the gradient stops.
/// The `origin` parameter specifies the point that maps to 0.0 in the
/// gradient stops. Using the same `origin` and `center` gives a radially
/// symmetric gradient; using different points is useful for simulating
/// lighting effects. See [configuring a radial gradient][config] for a fuller
/// explanation.
///
/// By default, `origin` and `center` are both at the center (0.5, 0.5) point.
/// This can be changed during construction with the [`with_center`] and
/// [`with_origin`] builder methods.
///
/// The [`ScaleMode`] describes how the gradient is mapped to a non-square
/// rectangle; by default this will expand on the longest axis, but this can
/// be changed with the [`with_scale_mode`] builder method.
///
/// [config]: https://docs.microsoft.com/en-us/windows/win32/direct2d/direct2d-brushes-overview#configuring-a-radial-gradient
/// [`UnitPoint`]: struct.UnitPoint.html
/// [`ScaleMode`]: enum.ScaleMode.html
/// [`with_center`]: struct.RadialGradient.html#method.with_center
/// [`with_origin`]: struct.RadialGradient.html#method.with_origin
/// [`with_scale_mode`]: struct.RadialGradient.html#method.with_scale_mode
#[derive(Debug, Clone)]
pub struct RadialGradient {
    center: UnitPoint,
    origin: UnitPoint,
    radius: f64,
    stops: Vec<GradientStop>,
    scale_mode: ScaleMode,
}

/// Mappings from the unit square into a non-square rectangle.
#[derive(Debug, Clone)]
pub enum ScaleMode {
    /// The unit 1.0 is mapped to the smaller of width & height, but the mapped
    /// item may not cover the entire rectangle.
    Fit,
    /// The unit 1.0 is mapped to the larger of width & height; some
    /// values on the other axis may extend outside the target rectangle.
    Fill,
}

/// A representation of a point relative to a unit rectangle.
#[derive(Debug, Clone, Copy)]
pub struct UnitPoint {
    u: f64,
    v: f64,
}

impl GradientStops for Vec<GradientStop> {
    fn to_vec(self) -> Vec<GradientStop> {
        self
    }
}

impl<'a> GradientStops for &'a [GradientStop] {
    fn to_vec(self) -> Vec<GradientStop> {
        self.to_owned()
    }
}

// Generate equally-spaced stops.
impl<'a> GradientStops for &'a [Color] {
    fn to_vec(self) -> Vec<GradientStop> {
        if self.is_empty() {
            Vec::new()
        } else {
            let denom = (self.len() - 1).max(1) as f32;
            self.iter()
                .enumerate()
                .map(|(i, c)| GradientStop {
                    pos: (i as f32) / denom,
                    color: c.to_owned(),
                })
                .collect()
        }
    }
}

impl<'a> GradientStops for (Color, Color) {
    fn to_vec(self) -> Vec<GradientStop> {
        let stops: &[Color] = &[self.0, self.1];
        GradientStops::to_vec(stops)
    }
}

impl<'a> GradientStops for (Color, Color, Color) {
    fn to_vec(self) -> Vec<GradientStop> {
        let stops: &[Color] = &[self.0, self.1, self.2];
        GradientStops::to_vec(stops)
    }
}

impl<'a> GradientStops for (Color, Color, Color, Color) {
    fn to_vec(self) -> Vec<GradientStop> {
        let stops: &[Color] = &[self.0, self.1, self.2, self.3];
        GradientStops::to_vec(stops)
    }
}

impl<'a> GradientStops for (Color, Color, Color, Color, Color) {
    fn to_vec(self) -> Vec<GradientStop> {
        let stops: &[Color] = &[self.0, self.1, self.2, self.3, self.4];
        GradientStops::to_vec(stops)
    }
}

impl<'a> GradientStops for (Color, Color, Color, Color, Color, Color) {
    fn to_vec(self) -> Vec<GradientStop> {
        let stops: &[Color] = &[self.0, self.1, self.2, self.3, self.4, self.5];
        GradientStops::to_vec(stops)
    }
}

impl UnitPoint {
    pub const TOP_LEFT: UnitPoint = UnitPoint::new(0.0, 0.0);
    pub const TOP: UnitPoint = UnitPoint::new(0.5, 0.0);
    pub const TOP_RIGHT: UnitPoint = UnitPoint::new(1.0, 0.0);
    pub const LEFT: UnitPoint = UnitPoint::new(0.0, 0.5);
    pub const CENTER: UnitPoint = UnitPoint::new(0.5, 0.5);
    pub const RIGHT: UnitPoint = UnitPoint::new(1.0, 0.5);
    pub const BOTTOM_LEFT: UnitPoint = UnitPoint::new(0.0, 1.0);
    pub const BOTTOM: UnitPoint = UnitPoint::new(0.5, 1.0);
    pub const BOTTOM_RIGHT: UnitPoint = UnitPoint::new(1.0, 1.0);

    /// Create a new UnitPoint.
    ///
    /// The `u` and `v` coordinates describe the point, with (0.0, 0.0) being
    /// the top-left, and (1.0, 1.0) being the bottom-right.
    pub const fn new(u: f64, v: f64) -> UnitPoint {
        UnitPoint { u, v }
    }

    /// Given a rectangle, resolve the point within the rectangle.
    pub fn resolve(self, rect: Rect) -> Point {
        Point::new(
            rect.x0 + self.u * (rect.x1 - rect.x0),
            rect.y0 + self.v * (rect.y1 - rect.y0),
        )
    }
}

impl LinearGradient {
    /// Create a new linear gradient.
    ///
    /// The `start` and `end` coordinates are [`UnitPoint`] coordinates, relative
    /// to the geometry of the shape being drawn.
    ///
    /// # Examples
    ///
    /// ```
    /// use piet::{Color, RenderContext, LinearGradient, UnitPoint};
    /// use piet::kurbo::{Circle, Point};
    ///
    /// # let mut render_ctx = piet::NullRenderContext::new();
    /// let circle = Circle::new(Point::new(100.0, 100.0), 50.0);
    /// let gradient = LinearGradient::new(
    ///     UnitPoint::TOP,
    ///     UnitPoint::BOTTOM,
    ///     (Color::WHITE, Color::BLACK)
    /// );
    /// render_ctx.fill(circle, &gradient);
    /// ```
    ///
    /// [`UnitPoint`]: struct.UnitPoint.html
    pub fn new(start: UnitPoint, end: UnitPoint, stops: impl GradientStops) -> LinearGradient {
        LinearGradient {
            start,
            end,
            stops: stops.to_vec(),
        }
    }

    // maybe these should be public API? that was my original intention but I'm not
    // sure there's a clear use, so keeping them private for now.
    /// Generate a [`FixedLinearGradient`] by mapping points in the unit square
    /// onto points in `rect`.
    ///
    /// [`FixedLinearGradient`]: struct.FixedLinearGradient.html
    fn resolve(&self, rect: Rect) -> FixedLinearGradient {
        FixedLinearGradient {
            start: self.start.resolve(rect),
            end: self.end.resolve(rect),
            stops: self.stops.clone(),
        }
    }
}

impl RadialGradient {
    /// Creates a simple `RadialGradient`. This gradient has `origin` and `center`
    /// set to `(0.5, 0.5)`, and uses the `Fill` [`ScaleMode`]. These attributes can be
    /// modified with the [`with_center`], [`with_origin`],
    /// and [`with_scale_mode`] builder methods.
    ///
    /// [`ScaleMode`]: enum.ScaleMode.html
    /// [`with_center`]: struct.RadialGradient.html#method.with_center
    /// [`with_origin`]: struct.RadialGradient.html#method.with_origin
    /// [`with_scale_mode`]: struct.RadialGradient.html#method.with_scale_mode
    pub fn new(radius: f64, stops: impl GradientStops) -> Self {
        RadialGradient {
            center: UnitPoint::CENTER,
            origin: UnitPoint::CENTER,
            radius,
            stops: stops.to_vec(),
            scale_mode: ScaleMode::Fill,
        }
    }

    /// A builder-style method for changing the center of the gradient. This
    /// does not change the `origin`; in the common case, you will want to change
    /// both values.
    ///
    /// See the main [`RadialGradient`] docs for an explanation of center vs. origin.
    ///
    /// [`RadialGradient`]: struct.RadialGradient.html
    pub fn with_center(mut self, center: UnitPoint) -> Self {
        self.center = center;
        self
    }

    /// A builder-style method for changing the origin of the gradient.
    ///
    /// See the main [`RadialGradient`] docs for an explanation of center vs. origin.
    ///
    /// [`RadialGradient`]: struct.RadialGradient.html
    pub fn with_origin(mut self, origin: UnitPoint) -> Self {
        self.origin = origin;
        self
    }

    /// A builder-style method for changing the [`ScaleMode`] of the gradient.
    ///
    /// [`ScaleMode`]: enum.ScaleMode.html
    pub fn with_scale_mode(mut self, scale_mode: ScaleMode) -> Self {
        self.scale_mode = scale_mode;
        self
    }

    /// Generate a [`FixedRadialGradient`] by mapping points in the unit square
    /// onto points in `rect`.
    ///
    /// [`FixedRadialGradient`]: struct.FixedRadialGradient.html
    fn resolve(&self, rect: Rect) -> FixedRadialGradient {
        let scale_len = match self.scale_mode {
            ScaleMode::Fill => rect.width().max(rect.height()),
            ScaleMode::Fit => rect.width().min(rect.height()),
        };

        let rect = equalize_sides_preserving_center(rect, scale_len);
        let center = self.center.resolve(rect);
        let origin = self.origin.resolve(rect);
        let origin_offset = origin - center;
        let radius = self.radius * scale_len;
        FixedRadialGradient {
            center,
            origin_offset,
            radius,
            stops: self.stops.clone(),
        }
    }
}

impl From<FixedLinearGradient> for FixedGradient {
    fn from(src: FixedLinearGradient) -> FixedGradient {
        FixedGradient::Linear(src)
    }
}

impl From<FixedRadialGradient> for FixedGradient {
    fn from(src: FixedRadialGradient) -> FixedGradient {
        FixedGradient::Radial(src)
    }
}

impl<P: RenderContext> IntoBrush<P> for FixedGradient {
    fn make_brush<'a>(&'a self, piet: &mut P, _bbox: impl FnOnce() -> Rect) -> Cow<'a, P::Brush> {
        // Also, at some point we might want to be smarter about the extra clone here.
        Cow::Owned(
            piet.gradient(self.to_owned())
                .expect("error creating gradient"),
        )
    }
}

impl<P: RenderContext> IntoBrush<P> for LinearGradient {
    fn make_brush<'a>(&'a self, piet: &mut P, bbox: impl FnOnce() -> Rect) -> Cow<'a, P::Brush> {
        let rect = bbox();
        let gradient = self.resolve(rect);
        // Perhaps the make_brush method should be fallible instead of panicking.
        Cow::Owned(piet.gradient(gradient).expect("error creating gradient"))
    }
}

impl<P: RenderContext> IntoBrush<P> for RadialGradient {
    fn make_brush<'a>(&'a self, piet: &mut P, bbox: impl FnOnce() -> Rect) -> Cow<'a, P::Brush> {
        let rect = bbox();
        let gradient = self.resolve(rect);
        // Perhaps the make_brush method should be fallible instead of panicking.
        Cow::Owned(piet.gradient(gradient).expect("error creating gradient"))
    }
}

fn equalize_sides_preserving_center(rect: Rect, new_len: f64) -> Rect {
    let size = Size::new(new_len, new_len);
    let origin = rect.center() - size.to_vec2() / 2.;
    Rect::from_origin_size(origin, size)
}