//! Besides Tauri's own build step: the two themes' page colour, which the window itself paints
//! before the page has (so a dark theme does not start white), read out of `ui/tokens.css` (the server's tokens, fetched by `scripts/fetch-tokens.sh`) into
//! `$OUT_DIR/tokens.rs`, so no colour is written in Rust. `lib.rs` includes that file.

use std::env;
use std::fs;
use std::path::Path;

/// The themes the app knows, by the name `tokens.css` selects them with (`data-theme`), and the
/// Rust names the constants get.
const THEMES: [(&str, &str); 2] = [("moria", "MORIA"), ("shire", "SHIRE")];
/// The tokens taken from each theme, by the custom property and the constant's suffix.
const TOKENS: [(&str, &str); 1] = [("--bg", "BG")];

fn main() {
    let tokens = Path::new("../ui/tokens.css");
    println!("cargo:rerun-if-changed={}", tokens.display());
    let css = fs::read_to_string(tokens)
        .unwrap_or_else(|e| panic!("{}: {e}; run scripts/fetch-tokens.sh", tokens.display()));
    let mut out = String::from(
        "// Generated by build.rs from ui/tokens.css: the colours the window paints itself.\n",
    );
    for (theme, name) in THEMES {
        let block = theme_block(&css, theme).unwrap_or_else(|| {
            panic!("ui/tokens.css has no `:root[data-theme=\"{theme}\"]` block")
        });
        for (token, suffix) in TOKENS {
            let (r, g, b) = hex_token(block, token).unwrap_or_else(|| {
                panic!(
                    "ui/tokens.css: `{token}` of the {theme} theme is not a six-digit hex colour"
                )
            });
            out.push_str(&format!(
                "pub const {name}_{suffix}: (u8, u8, u8) = (0x{r:02X}, 0x{g:02X}, 0x{b:02X});\n"
            ));
        }
    }
    let dest = Path::new(&env::var("OUT_DIR").expect("OUT_DIR")).join("tokens.rs");
    fs::write(&dest, out).expect("write tokens.rs");
    tauri_build::build()
}

/// The declarations of the theme's block: from the selector naming it (at the start of a line,
/// so a mention in a comment does not count) to the block's closing brace at the start of a
/// line, which is the file's own layout, one block per theme.
fn theme_block<'a>(css: &'a str, theme: &str) -> Option<&'a str> {
    let selector = format!("\n:root[data-theme=\"{theme}\"]");
    let at = css.find(&selector)?;
    let open = at + css[at..].find('{')?;
    let close = open + css[open..].find("\n}")?;
    Some(&css[open + 1..close])
}

/// `--token: #RRGGBB;` in the block, as bytes.
fn hex_token(block: &str, token: &str) -> Option<(u8, u8, u8)> {
    let value = block
        .lines()
        .map(str::trim)
        .find_map(|l| {
            l.strip_prefix(token)
                .and_then(|r| r.trim_start().strip_prefix(':'))
        })?
        .trim()
        .trim_end_matches(';')
        .trim();
    let hex = value.strip_prefix('#')?;
    if hex.len() != 6 {
        return None;
    }
    let byte = |i: usize| u8::from_str_radix(&hex[i..i + 2], 16).ok();
    Some((byte(0)?, byte(2)?, byte(4)?))
}