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extern crate image;
extern crate num_traits;

use image::{Primitive, Rgb};
use num_traits::FromPrimitive;

fn scale<T: Primitive + FromPrimitive>(start: T, end: T, f: f64) -> T {
    let start: f64 = start.to_f64().unwrap();
    let end: f64 = end.to_f64().unwrap();
    let delta: f64 = end - start;
    let res: f64 = start + delta * f;
    FromPrimitive::from_f64(res).unwrap()
}

pub fn scale_rgb<T: Primitive + FromPrimitive>(
    start: &Rgb<T>,
    end: &Rgb<T>,
    f: f64,
) -> Result<Rgb<T>, &'static str> {
    if f.is_nan() || f < 0.0_f64 || f > 1.0_f64 {
        if start == end {
            Ok(Rgb([start[0], start[1], start[2]]))
        } else {
            Err("f is outside [0; 1]")
        }
    } else {
        Ok(Rgb([
            scale(start[0], end[0], f),
            scale(start[1], end[1], f),
            scale(start[2], end[2], f),
        ]))
    }
}

#[test]
fn test_scaling_rgb_u8() {
    let white: Rgb<u8> = Rgb([255, 255, 255]);
    let black: Rgb<u8> = Rgb([0, 0, 0]);
    let orange: Rgb<u8> = Rgb([255, 127, 0]);
    let turquoise: Rgb<u8> = Rgb([0, 127, 255]);

    /* invalid values for f */
    let r = scale_rgb(&white, &black, -0.5);
    assert!(r.is_err());
    let r = scale_rgb(&white, &black, 1.5);
    assert!(r.is_err());
    let r = scale_rgb(&white, &black, std::f64::INFINITY);
    assert!(r.is_err());
    let r = scale_rgb(&white, &black, std::f64::NAN);
    assert!(r.is_err());

    /* Accept if it's the same color */
    let r = scale_rgb(&white, &white, -0.5);
    assert!(r.is_ok());
    assert!(r.unwrap() == white);
    let r = scale_rgb(&white, &white, 1.5);
    assert!(r.is_ok());
    assert!(r.unwrap() == white);
    let r = scale_rgb(&white, &white, std::f64::INFINITY);
    assert!(r.is_ok());
    assert!(r.unwrap() == white);
    let r = scale_rgb(&white, &white, std::f64::NAN);
    assert!(r.is_ok());
    assert!(r.unwrap() == white);

    let r = scale_rgb(&white, &black, 0.5);
    assert!(r.is_ok());
    let e: Rgb<u8> = Rgb([127, 127, 127]);
    let r = r.unwrap();
    assert!(r == e);

    let r = scale_rgb(&black, &white, 0.5);
    assert!(r.is_ok());
    let e: Rgb<u8> = Rgb([127, 127, 127]);
    let r = r.unwrap();
    assert!(r == e);

    let r = scale_rgb(&orange, &orange, 0.5);
    assert!(r.is_ok());
    let r = r.unwrap();
    assert!(r == orange);

    let r = scale_rgb(&orange, &orange, 0.0);
    assert!(r.is_ok());
    let r = r.unwrap();
    assert!(r == orange);

    let r = scale_rgb(&orange, &orange, 1.0);
    assert!(r.is_ok());
    let r = r.unwrap();
    assert!(r == orange);

    let r = scale_rgb(&orange, &turquoise, 0.3);
    assert!(r.is_ok());
    let e: Rgb<u8> = Rgb([178, 127, 76]);
    let r = r.unwrap();
    assert!(r == e);

    let r = scale_rgb(&orange, &turquoise, 0.8);
    assert!(r.is_ok());
    let e: Rgb<u8> = Rgb([51, 127, 204]);
    let r = r.unwrap();
    assert!(r == e);

    let r = scale_rgb(&turquoise, &orange, 0.3);
    assert!(r.is_ok());
    let e: Rgb<u8> = Rgb([76, 127, 178]);
    let r = r.unwrap();
    assert!(r == e);

    let r = scale_rgb(&turquoise, &orange, 0.8);
    assert!(r.is_ok());
    let e: Rgb<u8> = Rgb([204, 127, 51]);
    let r = r.unwrap();
    assert!(r == e);
}