use serde_json::{json, Value};
use std::{env, fs, path::PathBuf};

fn required_string<'a>(meta: &'a Value, key: &str, file: &str) -> &'a str {
    meta.get(key)
        .and_then(Value::as_str)
        .filter(|value| !value.trim().is_empty())
        .unwrap_or_else(|| panic!("{file}: meta.{key} must be a non-empty string"))
}

fn main() {
    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR"));
    let lang_dir = manifest_dir.join("lang");
    println!("cargo:rerun-if-changed={}", lang_dir.display());

    let mut files: Vec<PathBuf> = fs::read_dir(&lang_dir)
        .unwrap_or_else(|error| panic!("cannot read {}: {error}", lang_dir.display()))
        .filter_map(Result::ok)
        .map(|entry| entry.path())
        .filter(|path| path.extension().and_then(|ext| ext.to_str()) == Some("json"))
        .collect();
    files.sort();

    if files.is_empty() {
        panic!("no language files found in {}", lang_dir.display());
    }

    let mut assets = Vec::new();
    let mut manifest_languages = Vec::new();
    let mut has_english = false;

    for path in files {
        println!("cargo:rerun-if-changed={}", path.display());
        let filename = path.file_name().and_then(|v| v.to_str()).expect("UTF-8 language filename");
        let stem = path.file_stem().and_then(|v| v.to_str()).expect("UTF-8 language code");
        if !stem.chars().all(|ch| ch.is_ascii_alphanumeric() || ch == '-' || ch == '_') {
            panic!("{filename}: filename may only contain ASCII letters, digits, '-' and '_'");
        }

        let source = fs::read_to_string(&path)
            .unwrap_or_else(|error| panic!("cannot read {}: {error}", path.display()));
        let document: Value = serde_json::from_str(&source)
            .unwrap_or_else(|error| panic!("{filename}: invalid JSON: {error}"));
        let root_object = document.as_object().unwrap_or_else(|| panic!("{filename}: language pack root must be an object"));
        let unexpected: Vec<&str> = root_object.keys()
            .filter(|key| key.as_str() != "meta" && key.as_str() != "translations")
            .map(|key| key.as_str())
            .collect();
        if !unexpected.is_empty() {
            panic!("{filename}: unexpected top-level keys {:?}; translation keys must be inside 'translations'", unexpected);
        }
        let meta = document.get("meta").unwrap_or_else(|| panic!("{filename}: missing meta object"));
        let code = required_string(meta, "code", filename);
        let name = required_string(meta, "name", filename);
        let native_name = required_string(meta, "native_name", filename);
        let locale = required_string(meta, "locale", filename);
        if code != stem {
            panic!("{filename}: meta.code '{code}' must match filename '{stem}.json'");
        }
        if !document.get("translations").is_some_and(Value::is_object) {
            panic!("{filename}: translations must be a JSON object");
        }
        if code == "en" {
            has_english = true;
        }

        manifest_languages.push(json!({
            "code": code,
            "name": name,
            "native_name": native_name,
            "locale": locale,
            "path": format!("lang/{code}.json")
        }));
        assets.push((code.to_owned(), path));
    }

    if !has_english {
        panic!("lang/en.json is required as the default fallback language");
    }

    let manifest_json = serde_json::to_string(&json!({
        "default": "en",
        "languages": manifest_languages
    })).expect("serialize language manifest");

    let mut generated = String::new();
    generated.push_str("// @generated by build.rs - do not edit.\n");
    generated.push_str(&format!("pub const LANGUAGE_MANIFEST_JSON: &str = {:?};\n", manifest_json));
    generated.push_str("pub const LANGUAGE_ASSETS: &[(&str, &str)] = &[\n");
    for (code, path) in assets {
        generated.push_str(&format!(
            "    ({:?}, include_str!({:?})),\n",
            code,
            path.to_string_lossy()
        ));
    }
    generated.push_str("];\n");

    let out_dir = PathBuf::from(env::var("OUT_DIR").expect("OUT_DIR"));
    fs::write(out_dir.join("languages.rs"), generated).expect("write generated languages.rs");
}