use crate::blend::{BlendFunction, PreAlpha};
use crate::float::Float;
use crate::{Blend, ComponentWise};

/// A pair of blending equations and corresponding parameters.
///
/// The `Equations` type is similar to how blending works in OpenGL, where a
/// blend function has can be written as `e(sp * S, dp * D)`. `e` is the
/// equation (like `s + d`), `sp` and `dp` are the source and destination
/// parameters, and `S` and `D` are the source and destination colors.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct Equations {
    /// The equation for the color components.
    pub color_equation: Equation,

    /// The equation for the alpha component.
    pub alpha_equation: Equation,

    /// The parameters for the color components.
    pub color_parameters: Parameters,

    /// The parameters for the alpha component.
    pub alpha_parameters: Parameters,
}

impl Equations {
    /// Create a pair of blending equations, where all the parameters are
    /// `One`.
    pub fn from_equations(color: Equation, alpha: Equation) -> Equations {
        Equations {
            color_equation: color,
            alpha_equation: alpha,
            color_parameters: Parameters {
                source: Parameter::One,
                destination: Parameter::One,
            },
            alpha_parameters: Parameters {
                source: Parameter::One,
                destination: Parameter::One,
            },
        }
    }

    /// Create a pair of additive blending equations with the provided
    /// parameters.
    pub fn from_parameters(source: Parameter, destination: Parameter) -> Equations {
        Equations {
            color_equation: Equation::Add,
            alpha_equation: Equation::Add,
            color_parameters: Parameters {
                source,
                destination,
            },
            alpha_parameters: Parameters {
                source,
                destination,
            },
        }
    }
}

impl<C: Blend<Color = C> + ComponentWise + Clone> BlendFunction<C> for Equations
where
    C::Scalar: Float,
{
    fn apply_to(
        self,
        source: PreAlpha<C, C::Scalar>,
        destination: PreAlpha<C, C::Scalar>,
    ) -> PreAlpha<C, C::Scalar> {
        let col_src_param = self
            .color_parameters
            .source
            .apply_to(source.clone(), destination.clone());
        let col_dst_param = self
            .color_parameters
            .destination
            .apply_to(source.clone(), destination.clone());
        let alpha_src_param = self
            .alpha_parameters
            .source
            .apply_to(source.clone(), destination.clone());
        let alpha_dst_param = self
            .alpha_parameters
            .destination
            .apply_to(source.clone(), destination.clone());

        let src_color = col_src_param.mul_color(source.color.clone());
        let dst_color = col_dst_param.mul_color(destination.color.clone());
        let src_alpha = alpha_src_param.mul_constant(source.alpha);
        let dst_alpha = alpha_dst_param.mul_constant(destination.alpha);

        let color = match self.color_equation {
            Equation::Add => src_color.component_wise(&dst_color, |a, b| a + b),
            Equation::Subtract => src_color.component_wise(&dst_color, |a, b| a - b),
            Equation::ReverseSubtract => dst_color.component_wise(&src_color, |a, b| a - b),
            Equation::Min => source.color.component_wise(&destination.color, Float::min),
            Equation::Max => source.color.component_wise(&destination.color, Float::max),
        };

        let alpha = match self.alpha_equation {
            Equation::Add => src_alpha + dst_alpha,
            Equation::Subtract => src_alpha - dst_alpha,
            Equation::ReverseSubtract => dst_alpha - src_alpha,
            Equation::Min => source.alpha.min(destination.alpha),
            Equation::Max => source.alpha.max(destination.alpha),
        };

        PreAlpha { color, alpha }
    }
}

/// A blending equation.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum Equation {
    /// Add the source and destination, according to `sp * S + dp * D`.
    Add,

    /// Subtract the destination from the source, according to `sp * S - dp *
    /// D`.
    Subtract,

    /// Subtract the source from the destination, according to `dp * D - sp *
    /// S`.
    ReverseSubtract,

    /// Create a color where each component is the smallest of each of the
    /// source and destination components. A.k.a. component wise min. The
    /// parameters are ignored.
    Min,

    /// Create a color where each component is the largest of each of the
    /// source and destination components. A.k.a. component wise max. The
    /// parameters are ignored.
    Max,
}

/// A pair of source and destination parameters.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct Parameters {
    /// The source parameter.
    pub source: Parameter,

    /// The destination parameter.
    pub destination: Parameter,
}

/// A blending parameter.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum Parameter {
    /// A simple 1.
    One,

    /// A simple 0.
    Zero,

    /// The source color, or alpha.
    SourceColor,

    /// One minus the source color, or alpha.
    OneMinusSourceColor,

    /// The destination color, or alpha.
    DestinationColor,

    /// One minus the destination color, or alpha.
    OneMinusDestinationColor,

    /// The source alpha.
    SourceAlpha,

    /// One minus the source alpha.
    OneMinusSourceAlpha,

    /// The destination alpha.
    DestinationAlpha,

    /// One minus the destination alpha.
    OneMinusDestinationAlpha,
}

impl Parameter {
    fn apply_to<C, T: Float>(
        &self,
        source: PreAlpha<C, T>,
        destination: PreAlpha<C, T>,
    ) -> ParamOut<C, T>
    where
        PreAlpha<C, T>: ComponentWise<Scalar = T>,
    {
        match *self {
            Parameter::One => ParamOut::Constant(T::one()),
            Parameter::Zero => ParamOut::Constant(T::zero()),
            Parameter::SourceColor => ParamOut::Color(source),
            Parameter::OneMinusSourceColor => {
                ParamOut::Color(source.component_wise_self(|a| T::one() - a))
            }
            Parameter::DestinationColor => ParamOut::Color(destination),
            Parameter::OneMinusDestinationColor => {
                ParamOut::Color(destination.component_wise_self(|a| T::one() - a))
            }
            Parameter::SourceAlpha => ParamOut::Constant(source.alpha),
            Parameter::OneMinusSourceAlpha => ParamOut::Constant(T::one() - source.alpha),
            Parameter::DestinationAlpha => ParamOut::Constant(destination.alpha),
            Parameter::OneMinusDestinationAlpha => ParamOut::Constant(T::one() - destination.alpha),
        }
    }
}

enum ParamOut<C, T: Float> {
    Color(PreAlpha<C, T>),
    Constant(T),
}

impl<C: ComponentWise<Scalar = T>, T: Float> ParamOut<C, T> {
    fn mul_constant(self, other: T) -> T {
        match self {
            ParamOut::Color(c) => c.alpha * other,
            ParamOut::Constant(c) => c * other,
        }
    }

    fn mul_color(self, other: C) -> C {
        match self {
            ParamOut::Color(c) => other.component_wise(&c.color, |a, b| a * b),
            ParamOut::Constant(c) => other.component_wise_self(|a| a * c),
        }
    }
}