use std::{
    fs,
    path::{Path, PathBuf},
    process,
    time::Instant,
};

use mist_parser::error::ParseError;

pub fn build() -> PathBuf {
    let start = Instant::now();

    let root = std::env::current_dir()
        .ok()
        .expect("Unable to find project root");

    let src_dir = root.join("src");
    let out_dir = root.join(".mist/src");

    build_dir(&root, &src_dir, &src_dir, &out_dir);

    let elapsed = start.elapsed();

    println!(
        "\x1b[32m\nTranspile successful\x1b[0m in \x1b[34m{:.2?}\x1b[0m",
        elapsed
    );

    root
}

fn build_dir(root: &Path, base_src: &Path, current_dir: &Path, out_dir: &Path) {
    let entries = match fs::read_dir(current_dir) {
        Ok(entries) => entries,
        Err(e) => {
            eprintln!(
                "error: failed to read directory {}\n  {}",
                current_dir.display(),
                e
            );

            process::exit(1);
        }
    };

    for entry in entries {
        let entry = match entry {
            Ok(entry) => entry,
            Err(e) => {
                eprintln!("error: failed to read directory entry\n  {}", e);

                process::exit(1);
            }
        };

        let path = entry.path();

        // recurse into nested directories
        if path.is_dir() {
            build_dir(root, base_src, &path, out_dir);
            continue;
        }

        let relative = path.strip_prefix(base_src).unwrap();

        // Handle non-mist files with a cache check
        if path.extension().and_then(|e| e.to_str()) != Some("mist") {
            let dest_path = out_dir.join(relative);

            // Create parent directories for static assets if needed
            if let Some(parent) = dest_path.parent() {
                let _ = fs::create_dir_all(parent);
            }

            if should_skip(&path, &dest_path) {
                continue;
            }

            fs::copy(&path, dest_path).expect("Failed to copy non-mist file");
            continue;
        }

        let output_path = out_dir.join(relative).with_extension("rs");

        // Cache layer: Skip if the generated .rs file is newer than the .mist source
        if should_skip(&path, &output_path) {
            continue;
        }

        // create parent directories
        if let Some(parent) = output_path.parent() {
            if let Err(e) = fs::create_dir_all(parent) {
                eprintln!(
                    "error: failed to create output directory {}\n  {}",
                    parent.display(),
                    e
                );

                process::exit(1);
            }
        }

        // read source
        let source = match fs::read_to_string(&path) {
            Ok(s) => s,
            Err(e) => {
                eprintln!("error: failed to read file {}\n  {}", path.display(), e);

                process::exit(1);
            }
        };

        let parser_result = mist_parser::parse(&source).map_err(|e| match e {
            ParseError::Ast(e) => {
                let start_pos = e.span.start_pos().line_col();

                let span = e.span.as_str();

                format!(
                    "\n{}:{}:{}\n \x1b[31mError\x1b[0m: {}\n\t{}{}\t{}",
                    path.as_os_str().display(),
                    start_pos.0,
                    start_pos.1,
                    e.error_message,
                    span,
                    if span.ends_with("\n") { "" } else { "\n" },
                    "^".repeat(span.trim().len()),
                )
            }
            ParseError::PreAst(e) => format!("{e}"),
        });

        let ast = match parser_result {
            Ok(ast) => ast,
            Err(e) => {
                eprintln!("error: parse failed in {}\n{}", path.display(), e);

                process::exit(1);
            }
        };

        // semantic::walk_ast(semantic::scope::Scope::from_top(root, &ast), &mut ast);

        let mut gc = crate::codegen::RustCodegen::new();
        let output = gc.generate(ast);

        if let Err(e) = fs::write(&output_path, output) {
            eprintln!(
                "error: failed to write output {}\n  {}",
                output_path.display(),
                e
            );

            process::exit(1);
        }
    }
}

fn should_skip(source: &Path, output: &Path) -> bool {
    if let (Ok(src_meta), Ok(out_meta)) = (fs::metadata(source), fs::metadata(output)) {
        if let (Ok(src_time), Ok(out_time)) = (src_meta.modified(), out_meta.modified()) {
            return out_time >= src_time;
        }
    }
    false
}