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//! Retroimg processing library.
//! 
//! Convert images to appear to be displayed on retro IBM hardware.
//! It can also be used to reduce the color depth of existing images
//! for use in DOS game development.
//! 
//! See the various functions in this module
//! (and the submodule [`color`])
//! for more information.
use image::imageops::{resize, FilterType};
use image::{GenericImage, ImageBuffer, Pixel, RgbImage};
use num_rational::Ratio;
use snafu::Snafu;

pub mod color;

pub use crate::color::{ColorDepth, FixedPalette};

/// This is just another name for a cubic resize.
/// 
/// Prefer using small values of `nwidth` and `nheight`.
pub fn reduce<I: 'static>(
    img: &I,
    nwidth: u32,
    nheight: u32,
) -> ImageBuffer<I::Pixel, Vec<<I::Pixel as Pixel>::Subpixel>>
where
    I: GenericImage,
{
    resize(img, nwidth, nheight, FilterType::CatmullRom)
}

/// Crop an image to the given borders.
pub fn crop(mut image: RgbImage, left: u32, top: u32, width: u32, height: u32) -> RgbImage {
    image::imageops::crop(&mut image, left, top, width, height).to_image()
}

/// This is just another name for a nearest neighbor resize.
/// 
/// Makes it look like it has nice, large pixels.
pub fn expand<I: 'static>(
    img: &I,
    nwidth: u32,
    nheight: u32,
) -> ImageBuffer<I::Pixel, Vec<<I::Pixel as Pixel>::Subpixel>>
where
    I: GenericImage,
{
    resize(img, nwidth, nheight, FilterType::Nearest)
}

/// An error returned by [`resolve_output_resolution`].
#[derive(Debug, Snafu)]
#[snafu(visibility(pub))]
pub enum ResolutionError {
    /// not enough components to resolve output resolution
    Non,
    /// 'width' or 'height' are required alongside 'pixel_ratio'
    RatioWithoutSide,
    /// 'pixel_ratio', 'width' and 'height' cannot be used together
    TooMany,
}

/// Identify the intended output resolution based on a combination
/// of the input resolution, the desired output dimensions,
/// and/or the pixel ratio.
///
/// Only the following combinations of are valid:
/// 
/// - `output_width` and `output_height`;
/// - `pixel_ratio` and `output_width`;
/// - `pixel_ratio` and `output_height`.
pub fn resolve_output_resolution(
    width: u32,
    height: u32,
    output_width: Option<u32>,
    output_height: Option<u32>,
    pixel_ratio: Option<Ratio<u32>>,
) -> Result<(u32, u32), ResolutionError> {
    match (pixel_ratio, output_width, output_height) {
        (None, None, None) => NonSnafu.fail(),
        (None, Some(w), Some(h)) => Ok((w, h)),
        (Some(r), None, Some(h)) => {
            /*
            Rule of proportions... with a twist.

            iW ----> oW

            iH ----> oH

            Without pixel scale correction (pixel ratio `r` = 1):

            oH = iH * oW / iW = oW / iR
            oW = iW * oH / iH = oH * iR

            For other pixel ratios:

            oR = iR * r

            Therefore:

            oW = oH * oR
               = oH * iR * r
               = oH * r * iW / iH

            and

            oH = oW / oR
               = oW / (iR * r)
               = oW / ( (iW / iH) * r)
               = oW * iH / (iW * r)
            */
            let w = ((r * h * width) / height).round().to_integer();
            Ok((w, h))
        }
        (Some(r), Some(w), None) => {
            let h = (Ratio::from_integer(w) * height / (r * width))
                .round()
                .to_integer();
            Ok((w, h))
        }
        (None, None, Some(h)) => {
            let ir = Ratio::new(width, height);
            let w = (Ratio::from_integer(h) * ir).round().to_integer();
            Ok((w, h))
        }
        (None, Some(w), None) => {
            let ir = Ratio::new(width, height);
            let h = (Ratio::from_integer(w) / ir).round().to_integer();
            Ok((w, h))
        }
        (Some(_r), None, None) => RatioWithoutSideSnafu.fail(),
        (Some(_r), Some(_w), Some(_h)) => TooManySnafu.fail(),
    }
}

#[cfg(test)]
mod tests {

    #[test]
    fn test_crop() {
        // create blank image
        let mut image = image::ImageBuffer::new(100, 100);

        // draw a red horizontal line
        for x in 0..100 {
            image.put_pixel(x, 50, image::Rgb([255, 0, 0]));
        }

        // draw a green vertical line
        for y in 0..100 {
            image.put_pixel(20, y, image::Rgb([0, 255, 0]));
        }

        // draw yellow in the intersection
        image.put_pixel(20, 50, image::Rgb([255, 255, 0]));

        // crop to the middle of the image
        let cropped = super::crop(image, 20, 50, 80, 50);

        // check that the cropped image is the right size
        assert_eq!(cropped.width(), 80, "unexpected width");
        assert_eq!(cropped.height(), 50, "unexpected height");

        // check that the cropped image is the right color
        for (x, y, pixel) in cropped.enumerate_pixels() {
            let expected_color = match (x, y) {
                (0, 0) => image::Rgb([255, 255, 0]),
                (0, _) => image::Rgb([0, 255, 0]),
                (_, 0) => image::Rgb([255, 0, 0]),
                _ => image::Rgb([0, 0, 0]),
            };
            assert_eq!(pixel, &expected_color);
        }
    }
}