use std::env;
use std::fs::File;
use std::io::{BufWriter, Write};
use std::path::Path;

fn main() {
    generate_srgb_tables();
}

/// Converts an sRGB color value to a linear sRGB color value (undoes the gamma correction).
///
/// The input and the output are supposed to be in the [0, 1] range.
#[inline]
fn linearize(c: f64) -> f64 {
    if c <= (12.92 * 0.0031308) {
        c / 12.92
    } else {
        ((c + 0.055) / 1.055).powf(2.4)
    }
}

/// Converts a linear sRGB color value to a normal sRGB color value (applies the gamma correction).
///
/// The input and the output are supposed to be in the [0, 1] range.
#[inline]
fn unlinearize(c: f64) -> f64 {
    if c <= 0.0031308 {
        12.92 * c
    } else {
        1.055 * c.powf(1f64 / 2.4) - 0.055
    }
}

fn compute_table<F: Fn(f64) -> f64>(f: F) -> [u8; 256] {
    let mut table = [0; 256];

    for i in 0..=255 {
        let c = i as f64 / 255.0;
        let x = f(c);
        table[i] = (x * 255.0).round() as u8;
    }

    table
}

fn print_table<W: Write>(w: &mut W, name: &str, table: &[u8]) {
    writeln!(w, "const {}: [u8; {}] = [", name, table.len()).unwrap();

    for x in table {
        writeln!(w, "    {},", x).unwrap();
    }

    writeln!(w, "];").unwrap();
}

fn generate_srgb_tables() {
    let linearize_table = compute_table(linearize);
    let unlinearize_table = compute_table(unlinearize);

    let path = Path::new(&env::var("OUT_DIR").unwrap()).join("srgb-codegen.rs");
    let mut file = BufWriter::new(File::create(&path).unwrap());

    print_table(&mut file, "LINEARIZE", &linearize_table);
    print_table(&mut file, "UNLINEARIZE", &unlinearize_table);
}
