//! Lux - A functional programming language with first-class effects

mod ast;
mod diagnostics;
mod interpreter;
mod lexer;
mod modules;
mod parser;
mod schema;
mod typechecker;
mod types;

use diagnostics::render;
use interpreter::Interpreter;
use parser::Parser;
use std::io::{self, Write};
use typechecker::TypeChecker;

const VERSION: &str = "0.1.0";

const HELP: &str = r#"
Lux - A functional language with first-class effects

Commands:
  :help, :h       Show this help
  :quit, :q       Exit the REPL
  :type <expr>    Show the type of an expression
  :clear          Clear the environment
  :load <file>    Load and execute a file
  :trace on/off   Enable/disable effect tracing
  :traces         Show recorded effect traces

Examples:
  > let x = 42
  > x + 1
  43

  > fn double(n: Int): Int = n * 2
  > double(21)
  42

  > Console.print("Hello, world!")
  Hello, world!

Debugging:
  > :trace on
  > Console.print("test")
  > :traces
  [   0.123ms] Console.print("test") → ()
"#;

fn main() {
    let args: Vec<String> = std::env::args().collect();

    if args.len() > 1 {
        // Run a file
        run_file(&args[1]);
    } else {
        // Start REPL
        run_repl();
    }
}

fn run_file(path: &str) {
    use modules::ModuleLoader;
    use std::path::Path;

    let file_path = Path::new(path);
    let source = match std::fs::read_to_string(file_path) {
        Ok(s) => s,
        Err(e) => {
            eprintln!("Error reading file '{}': {}", path, e);
            std::process::exit(1);
        }
    };

    // Set up module loader with the file's directory as a search path
    let mut loader = ModuleLoader::new();
    if let Some(parent) = file_path.parent() {
        loader.add_search_path(parent.to_path_buf());
    }

    // Load and parse the program (including any imports)
    let program = match loader.load_source(&source, Some(file_path)) {
        Ok(p) => p,
        Err(e) => {
            eprintln!("Module error: {}", e);
            std::process::exit(1);
        }
    };

    let mut checker = TypeChecker::new();
    if let Err(errors) = checker.check_program_with_modules(&program, &loader) {
        for error in errors {
            let diagnostic = error.to_diagnostic();
            eprint!("{}", render(&diagnostic, &source, Some(path)));
        }
        std::process::exit(1);
    }

    let mut interp = Interpreter::new();
    match interp.run_with_modules(&program, &loader) {
        Ok(value) => {
            if !matches!(value, interpreter::Value::Unit) {
                println!("{}", value);
            }
        }
        Err(e) => {
            let diagnostic = e.to_diagnostic();
            eprint!("{}", render(&diagnostic, &source, Some(path)));
            std::process::exit(1);
        }
    }
}

fn run_repl() {
    println!("Lux v{}", VERSION);
    println!("Type :help for help, :quit to exit\n");

    let mut interp = Interpreter::new();
    let mut checker = TypeChecker::new();
    let mut buffer = String::new();
    let mut continuation = false;

    loop {
        // Print prompt
        let prompt = if continuation { "... " } else { "lux> " };
        print!("{}", prompt);
        io::stdout().flush().unwrap();

        // Read input
        let mut line = String::new();
        match io::stdin().read_line(&mut line) {
            Ok(0) => break, // EOF
            Ok(_) => {}
            Err(e) => {
                eprintln!("Error reading input: {}", e);
                continue;
            }
        }

        let line = line.trim_end();

        // Handle commands
        if !continuation && line.starts_with(':') {
            handle_command(line, &mut interp, &mut checker);
            continue;
        }

        // Accumulate input
        buffer.push_str(line);
        buffer.push('\n');

        // Check for continuation (simple heuristic: unbalanced braces)
        let open_braces = buffer.chars().filter(|c| *c == '{').count();
        let close_braces = buffer.chars().filter(|c| *c == '}').count();
        let open_parens = buffer.chars().filter(|c| *c == '(').count();
        let close_parens = buffer.chars().filter(|c| *c == ')').count();

        if open_braces > close_braces || open_parens > close_parens {
            continuation = true;
            continue;
        }

        continuation = false;
        let input = std::mem::take(&mut buffer);

        if input.trim().is_empty() {
            continue;
        }

        eval_input(&input, &mut interp, &mut checker);
    }

    println!("\nGoodbye!");
}

fn handle_command(line: &str, interp: &mut Interpreter, checker: &mut TypeChecker) {
    let parts: Vec<&str> = line.splitn(2, ' ').collect();
    let cmd = parts[0];
    let arg = parts.get(1).map(|s| s.trim());

    match cmd {
        ":help" | ":h" => {
            println!("{}", HELP);
        }
        ":quit" | ":q" => {
            println!("Goodbye!");
            std::process::exit(0);
        }
        ":type" | ":t" => {
            if let Some(expr_str) = arg {
                show_type(expr_str, checker);
            } else {
                println!("Usage: :type <expression>");
            }
        }
        ":clear" => {
            *interp = Interpreter::new();
            *checker = TypeChecker::new();
            println!("Environment cleared.");
        }
        ":load" | ":l" => {
            if let Some(path) = arg {
                load_file(path, interp, checker);
            } else {
                println!("Usage: :load <filename>");
            }
        }
        ":trace" => match arg {
            Some("on") => {
                interp.enable_tracing();
                println!("Effect tracing enabled.");
            }
            Some("off") => {
                interp.trace_effects = false;
                println!("Effect tracing disabled.");
            }
            _ => {
                println!("Usage: :trace on|off");
            }
        },
        ":traces" => {
            if interp.get_traces().is_empty() {
                println!("No effect traces recorded. Use :trace on to enable tracing.");
            } else {
                interp.print_traces();
            }
        }
        _ => {
            println!("Unknown command: {}", cmd);
            println!("Type :help for help");
        }
    }
}

fn show_type(expr_str: &str, checker: &mut TypeChecker) {
    // Wrap expression in a let to parse it
    let wrapped = format!("let _expr_ = {}", expr_str);

    match Parser::parse_source(&wrapped) {
        Ok(program) => {
            if let Err(errors) = checker.check_program(&program) {
                for error in errors {
                    println!("Type error: {}", error);
                }
            } else {
                println!("(type checking passed)");
            }
        }
        Err(e) => {
            println!("Parse error: {}", e);
        }
    }
}

fn load_file(path: &str, interp: &mut Interpreter, checker: &mut TypeChecker) {
    let source = match std::fs::read_to_string(path) {
        Ok(s) => s,
        Err(e) => {
            println!("Error reading file '{}': {}", path, e);
            return;
        }
    };

    let program = match Parser::parse_source(&source) {
        Ok(p) => p,
        Err(e) => {
            println!("Parse error: {}", e);
            return;
        }
    };

    if let Err(errors) = checker.check_program(&program) {
        for error in errors {
            println!("Type error: {}", error);
        }
        return;
    }

    match interp.run(&program) {
        Ok(_) => println!("Loaded '{}'", path),
        Err(e) => println!("Runtime error: {}", e),
    }
}

fn eval_input(input: &str, interp: &mut Interpreter, checker: &mut TypeChecker) {
    // Try to parse as a program (declarations)
    match Parser::parse_source(input) {
        Ok(program) => {
            // Type check
            if let Err(errors) = checker.check_program(&program) {
                for error in errors {
                    println!("Type error: {}", error);
                }
                return;
            }

            // Execute
            match interp.run(&program) {
                Ok(value) => {
                    if !matches!(value, interpreter::Value::Unit) {
                        println!("{}", value);
                    }
                }
                Err(e) => {
                    println!("Runtime error: {}", e);
                }
            }
        }
        Err(parse_err) => {
            // Try wrapping as an expression
            let wrapped = format!("let _result_ = {}", input.trim());
            match Parser::parse_source(&wrapped) {
                Ok(program) => {
                    if let Err(errors) = checker.check_program(&program) {
                        for error in errors {
                            println!("Type error: {}", error);
                        }
                        return;
                    }

                    match interp.run(&program) {
                        Ok(value) => {
                            println!("{}", value);
                        }
                        Err(e) => {
                            println!("Runtime error: {}", e);
                        }
                    }
                }
                Err(_) => {
                    // Use original error
                    println!("Parse error: {}", parse_err);
                }
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn eval(source: &str) -> Result<String, String> {
        let program = Parser::parse_source(source).map_err(|e| e.to_string())?;

        let mut checker = TypeChecker::new();
        checker.check_program(&program).map_err(|errors| {
            errors
                .iter()
                .map(|e| e.to_string())
                .collect::<Vec<_>>()
                .join("\n")
        })?;

        let mut interp = Interpreter::new();
        let value = interp.run(&program).map_err(|e| e.to_string())?;
        Ok(format!("{}", value))
    }

    #[test]
    fn test_arithmetic() {
        assert_eq!(eval("let x = 1 + 2").unwrap(), "3");
        assert_eq!(eval("let x = 10 - 3").unwrap(), "7");
        assert_eq!(eval("let x = 4 * 5").unwrap(), "20");
        assert_eq!(eval("let x = 15 / 3").unwrap(), "5");
    }

    #[test]
    fn test_function() {
        let source = r#"
            fn add(a: Int, b: Int): Int = a + b
            let result = add(3, 4)
        "#;
        assert_eq!(eval(source).unwrap(), "7");
    }

    #[test]
    fn test_if_expr() {
        let source = r#"
            fn max(a: Int, b: Int): Int = if a > b then a else b
            let result = max(5, 3)
        "#;
        assert_eq!(eval(source).unwrap(), "5");
    }

    #[test]
    fn test_recursion() {
        let source = r#"
            fn factorial(n: Int): Int = if n <= 1 then 1 else n * factorial(n - 1)
            let result = factorial(5)
        "#;
        assert_eq!(eval(source).unwrap(), "120");
    }

    #[test]
    fn test_lambda() {
        let source = r#"
            let double = fn(x: Int): Int => x * 2
            let result = double(21)
        "#;
        assert_eq!(eval(source).unwrap(), "42");
    }

    #[test]
    fn test_records() {
        let source = r#"
            let person = { name: "Alice", age: 30 }
            let result = person.age
        "#;
        assert_eq!(eval(source).unwrap(), "30");
    }

    #[test]
    fn test_lists() {
        let source = "let nums = [1, 2, 3]";
        assert_eq!(eval(source).unwrap(), "[1, 2, 3]");
    }

    #[test]
    fn test_tuples() {
        let source = "let pair = (42, \"hello\")";
        assert_eq!(eval(source).unwrap(), "(42, \"hello\")");
    }

    #[test]
    fn test_block() {
        let source = r#"
            let result = {
                let x = 10
                let y = 20
                x + y
            }
        "#;
        assert_eq!(eval(source).unwrap(), "30");
    }

    #[test]
    fn test_pipe() {
        let source = r#"
            fn double(x: Int): Int = x * 2
            fn add_one(x: Int): Int = x + 1
            let result = 5 |> double |> add_one
        "#;
        assert_eq!(eval(source).unwrap(), "11");
    }

    // ============ Standard Library Tests ============

    // List tests
    #[test]
    fn test_list_length() {
        assert_eq!(eval("let x = List.length([1, 2, 3])").unwrap(), "3");
        assert_eq!(eval("let x = List.length([])").unwrap(), "0");
    }

    #[test]
    fn test_list_reverse() {
        assert_eq!(
            eval("let x = List.reverse([1, 2, 3])").unwrap(),
            "[3, 2, 1]"
        );
        assert_eq!(eval("let x = List.reverse([])").unwrap(), "[]");
    }

    #[test]
    fn test_list_range() {
        assert_eq!(eval("let x = List.range(0, 5)").unwrap(), "[0, 1, 2, 3, 4]");
        assert_eq!(eval("let x = List.range(3, 3)").unwrap(), "[]");
        assert_eq!(eval("let x = List.range(-2, 2)").unwrap(), "[-2, -1, 0, 1]");
    }

    #[test]
    fn test_list_head() {
        assert_eq!(eval("let x = List.head([1, 2, 3])").unwrap(), "Some(1)");
        assert_eq!(eval("let x = List.head([])").unwrap(), "None");
    }

    #[test]
    fn test_list_tail() {
        assert_eq!(
            eval("let x = List.tail([1, 2, 3])").unwrap(),
            "Some([2, 3])"
        );
        assert_eq!(eval("let x = List.tail([1])").unwrap(), "Some([])");
        assert_eq!(eval("let x = List.tail([])").unwrap(), "None");
    }

    #[test]
    fn test_list_concat() {
        assert_eq!(
            eval("let x = List.concat([1, 2], [3, 4])").unwrap(),
            "[1, 2, 3, 4]"
        );
        assert_eq!(eval("let x = List.concat([], [1])").unwrap(), "[1]");
        assert_eq!(eval("let x = List.concat([1], [])").unwrap(), "[1]");
    }

    #[test]
    fn test_list_get() {
        assert_eq!(
            eval("let x = List.get([10, 20, 30], 0)").unwrap(),
            "Some(10)"
        );
        assert_eq!(
            eval("let x = List.get([10, 20, 30], 2)").unwrap(),
            "Some(30)"
        );
        assert_eq!(eval("let x = List.get([10, 20, 30], 5)").unwrap(), "None");
        assert_eq!(eval("let x = List.get([10, 20, 30], -1)").unwrap(), "None");
    }

    #[test]
    fn test_list_map() {
        let source = r#"
            fn double(x: Int): Int = x * 2
            let result = List.map([1, 2, 3], double)
        "#;
        assert_eq!(eval(source).unwrap(), "[2, 4, 6]");
    }

    #[test]
    fn test_list_map_lambda() {
        let source = "let x = List.map([1, 2, 3], fn(x: Int): Int => x * x)";
        assert_eq!(eval(source).unwrap(), "[1, 4, 9]");
    }

    #[test]
    fn test_list_filter() {
        let source = "let x = List.filter([1, 2, 3, 4, 5], fn(x: Int): Bool => x > 2)";
        assert_eq!(eval(source).unwrap(), "[3, 4, 5]");
    }

    #[test]
    fn test_list_filter_all() {
        let source = "let x = List.filter([1, 2, 3], fn(x: Int): Bool => x > 10)";
        assert_eq!(eval(source).unwrap(), "[]");
    }

    #[test]
    fn test_list_fold() {
        let source = "let x = List.fold([1, 2, 3, 4], 0, fn(acc: Int, x: Int): Int => acc + x)";
        assert_eq!(eval(source).unwrap(), "10");
    }

    #[test]
    fn test_list_fold_product() {
        let source = "let x = List.fold([1, 2, 3, 4], 1, fn(acc: Int, x: Int): Int => acc * x)";
        assert_eq!(eval(source).unwrap(), "24");
    }

    // String tests
    #[test]
    fn test_string_length() {
        assert_eq!(eval(r#"let x = String.length("hello")"#).unwrap(), "5");
        assert_eq!(eval(r#"let x = String.length("")"#).unwrap(), "0");
    }

    #[test]
    fn test_string_split() {
        assert_eq!(
            eval(r#"let x = String.split("a,b,c", ",")"#).unwrap(),
            r#"["a", "b", "c"]"#
        );
        assert_eq!(
            eval(r#"let x = String.split("hello", ",")"#).unwrap(),
            r#"["hello"]"#
        );
    }

    #[test]
    fn test_string_join() {
        assert_eq!(
            eval(r#"let x = String.join(["a", "b", "c"], "-")"#).unwrap(),
            r#""a-b-c""#
        );
        assert_eq!(
            eval(r#"let x = String.join(["hello"], ",")"#).unwrap(),
            r#""hello""#
        );
        assert_eq!(eval(r#"let x = String.join([], ",")"#).unwrap(), r#""""#);
    }

    #[test]
    fn test_string_trim() {
        assert_eq!(
            eval(r#"let x = String.trim("  hello  ")"#).unwrap(),
            r#""hello""#
        );
        assert_eq!(
            eval(r#"let x = String.trim("hello")"#).unwrap(),
            r#""hello""#
        );
        assert_eq!(eval(r#"let x = String.trim("   ")"#).unwrap(), r#""""#);
    }

    #[test]
    fn test_string_contains() {
        assert_eq!(
            eval(r#"let x = String.contains("hello world", "world")"#).unwrap(),
            "true"
        );
        assert_eq!(
            eval(r#"let x = String.contains("hello", "xyz")"#).unwrap(),
            "false"
        );
        assert_eq!(
            eval(r#"let x = String.contains("hello", "")"#).unwrap(),
            "true"
        );
    }

    #[test]
    fn test_string_replace() {
        assert_eq!(
            eval(r#"let x = String.replace("hello", "l", "L")"#).unwrap(),
            r#""heLLo""#
        );
        assert_eq!(
            eval(r#"let x = String.replace("aaa", "a", "b")"#).unwrap(),
            r#""bbb""#
        );
    }

    #[test]
    fn test_string_chars() {
        assert_eq!(eval(r#"let x = String.chars("hi")"#).unwrap(), "['h', 'i']");
        assert_eq!(eval(r#"let x = String.chars("")"#).unwrap(), "[]");
    }

    #[test]
    fn test_string_lines() {
        // Note: Using actual newline in the string
        let source = r#"let x = String.lines("a
b
c")"#;
        assert_eq!(eval(source).unwrap(), r#"["a", "b", "c"]"#);
    }

    // Option tests
    #[test]
    fn test_option_constructors() {
        assert_eq!(eval("let x = Some(42)").unwrap(), "Some(42)");
        assert_eq!(eval("let x = None").unwrap(), "None");
    }

    #[test]
    fn test_option_is_some() {
        assert_eq!(eval("let x = Option.isSome(Some(42))").unwrap(), "true");
        assert_eq!(eval("let x = Option.isSome(None)").unwrap(), "false");
    }

    #[test]
    fn test_option_is_none() {
        assert_eq!(eval("let x = Option.isNone(None)").unwrap(), "true");
        assert_eq!(eval("let x = Option.isNone(Some(42))").unwrap(), "false");
    }

    #[test]
    fn test_option_get_or_else() {
        assert_eq!(eval("let x = Option.getOrElse(Some(42), 0)").unwrap(), "42");
        assert_eq!(eval("let x = Option.getOrElse(None, 0)").unwrap(), "0");
    }

    #[test]
    fn test_option_map() {
        let source = "let x = Option.map(Some(5), fn(x: Int): Int => x * 2)";
        assert_eq!(eval(source).unwrap(), "Some(10)");
    }

    #[test]
    fn test_option_map_none() {
        let source = "let x = Option.map(None, fn(x: Int): Int => x * 2)";
        assert_eq!(eval(source).unwrap(), "None");
    }

    #[test]
    fn test_option_flat_map() {
        let source = "let x = Option.flatMap(Some(5), fn(x: Int): Option<Int> => Some(x * 2))";
        assert_eq!(eval(source).unwrap(), "Some(10)");
    }

    #[test]
    fn test_option_flat_map_to_none() {
        let source = "let x = Option.flatMap(Some(5), fn(x: Int): Option<Int> => None)";
        assert_eq!(eval(source).unwrap(), "None");
    }

    // Result tests
    #[test]
    fn test_result_constructors() {
        assert_eq!(eval("let x = Ok(42)").unwrap(), "Ok(42)");
        assert_eq!(eval(r#"let x = Err("error")"#).unwrap(), r#"Err("error")"#);
    }

    #[test]
    fn test_result_is_ok() {
        assert_eq!(eval("let x = Result.isOk(Ok(42))").unwrap(), "true");
        assert_eq!(eval(r#"let x = Result.isOk(Err("e"))"#).unwrap(), "false");
    }

    #[test]
    fn test_result_is_err() {
        assert_eq!(eval(r#"let x = Result.isErr(Err("e"))"#).unwrap(), "true");
        assert_eq!(eval("let x = Result.isErr(Ok(42))").unwrap(), "false");
    }

    #[test]
    fn test_result_get_or_else() {
        assert_eq!(eval("let x = Result.getOrElse(Ok(42), 0)").unwrap(), "42");
        assert_eq!(
            eval(r#"let x = Result.getOrElse(Err("e"), 0)"#).unwrap(),
            "0"
        );
    }

    #[test]
    fn test_result_map() {
        let source = "let x = Result.map(Ok(5), fn(x: Int): Int => x * 2)";
        assert_eq!(eval(source).unwrap(), "Ok(10)");
    }

    #[test]
    fn test_result_map_err() {
        let source = r#"let x = Result.map(Err("e"), fn(x: Int): Int => x * 2)"#;
        assert_eq!(eval(source).unwrap(), r#"Err("e")"#);
    }

    // Utility function tests
    #[test]
    fn test_to_string() {
        assert_eq!(eval("let x = toString(42)").unwrap(), r#""42""#);
        assert_eq!(eval("let x = toString(true)").unwrap(), r#""true""#);
        assert_eq!(eval("let x = toString([1, 2])").unwrap(), r#""[1, 2]""#);
    }

    #[test]
    fn test_type_of() {
        assert_eq!(eval("let x = typeOf(42)").unwrap(), r#""Int""#);
        assert_eq!(eval("let x = typeOf(true)").unwrap(), r#""Bool""#);
        assert_eq!(eval("let x = typeOf([1, 2])").unwrap(), r#""List""#);
        assert_eq!(eval(r#"let x = typeOf("hello")"#).unwrap(), r#""String""#);
    }

    // Pipe with stdlib tests
    #[test]
    fn test_pipe_with_list() {
        assert_eq!(
            eval("let x = [1, 2, 3] |> List.reverse").unwrap(),
            "[3, 2, 1]"
        );
        assert_eq!(eval("let x = [1, 2, 3] |> List.length").unwrap(), "3");
    }

    #[test]
    fn test_pipe_with_string() {
        assert_eq!(
            eval(r#"let x = "  hello  " |> String.trim"#).unwrap(),
            r#""hello""#
        );
    }

    // Combined stdlib usage tests
    #[test]
    fn test_list_filter_even() {
        let source = r#"
            fn isEven(x: Int): Bool = x % 2 == 0
            let result = List.filter(List.range(1, 6), isEven)
        "#;
        assert_eq!(eval(source).unwrap(), "[2, 4]");
    }

    #[test]
    fn test_option_chain() {
        let source = r#"
            fn times10(x: Int): Int = x * 10
            let head = List.head([1, 2, 3])
            let mapped = Option.map(head, times10)
            let result = Option.getOrElse(mapped, 0)
        "#;
        assert_eq!(eval(source).unwrap(), "10");
    }

    #[test]
    fn test_option_chain_empty() {
        let source = r#"
            fn times10(x: Int): Int = x * 10
            let head = List.head([])
            let mapped = Option.map(head, times10)
            let result = Option.getOrElse(mapped, 0)
        "#;
        assert_eq!(eval(source).unwrap(), "0");
    }

    // ============ Behavioral Types Tests ============

    #[test]
    fn test_behavioral_pure_function() {
        // A pure function with no effects should work
        let source = r#"
            fn double(x: Int): Int is pure = x * 2
            let result = double(21)
        "#;
        assert_eq!(eval(source).unwrap(), "42");
    }

    #[test]
    fn test_behavioral_total_function() {
        // A total function should work
        let source = r#"
            fn always42(): Int is total = 42
            let result = always42()
        "#;
        assert_eq!(eval(source).unwrap(), "42");
    }

    #[test]
    fn test_behavioral_idempotent_function() {
        // An idempotent function
        let source = r#"
            fn clamp(x: Int): Int is idempotent = if x < 0 then 0 else x
            let result = clamp(clamp(-5))
        "#;
        assert_eq!(eval(source).unwrap(), "0");
    }

    #[test]
    fn test_behavioral_multiple_properties() {
        // A function with multiple properties
        let source = r#"
            fn identity(x: Int): Int is pure, is total = x
            let result = identity(100)
        "#;
        assert_eq!(eval(source).unwrap(), "100");
    }

    #[test]
    fn test_behavioral_deterministic() {
        let source = r#"
            fn square(x: Int): Int is deterministic = x * x
            let result = square(7)
        "#;
        assert_eq!(eval(source).unwrap(), "49");
    }

    #[test]
    fn test_behavioral_pure_with_effects_error() {
        // A pure function with effects should produce a type error
        let source = r#"
            fn bad(x: Int): Int with {Logger} is pure = x
        "#;
        let result = eval(source);
        assert!(result.is_err());
        assert!(result.unwrap_err().contains("pure but has effects"));
    }

    // Diagnostic rendering tests
    mod diagnostic_tests {
        use crate::diagnostics::{render_diagnostic_plain, Diagnostic, Severity};
        use crate::ast::Span;
        use crate::typechecker::TypeError;
        use crate::interpreter::RuntimeError;

        #[test]
        fn test_type_error_to_diagnostic() {
            let error = TypeError {
                message: "Type mismatch: expected Int, got String".to_string(),
                span: Span { start: 10, end: 20 },
            };
            let diag = error.to_diagnostic();

            assert_eq!(diag.severity, Severity::Error);
            assert_eq!(diag.title, "Type Mismatch");
            assert!(diag.hints.len() > 0);
        }

        #[test]
        fn test_runtime_error_to_diagnostic() {
            let error = RuntimeError {
                message: "Division by zero".to_string(),
                span: Some(Span { start: 5, end: 10 }),
            };
            let diag = error.to_diagnostic();

            assert_eq!(diag.severity, Severity::Error);
            assert_eq!(diag.title, "Division by Zero");
            assert!(diag.hints.len() > 0);
        }

        #[test]
        fn test_diagnostic_render_with_real_code() {
            let source = "fn add(a: Int, b: Int): Int = a + b\nlet result = add(1, \"two\")";
            let diag = Diagnostic {
                severity: Severity::Error,
                title: "Type Mismatch".to_string(),
                message: "Expected Int but got String".to_string(),
                span: Span { start: 56, end: 61 },
                hints: vec!["The second argument should be an Int.".to_string()],
            };

            let output = render_diagnostic_plain(&diag, source, Some("example.lux"));

            assert!(output.contains("ERROR"));
            assert!(output.contains("example.lux"));
            assert!(output.contains("Type Mismatch"));
            assert!(output.contains("\"two\""));
            assert!(output.contains("Hint:"));
        }

        #[test]
        fn test_undefined_variable_categorization() {
            let error = TypeError {
                message: "Undefined variable: foobar".to_string(),
                span: Span { start: 0, end: 6 },
            };
            let diag = error.to_diagnostic();

            assert_eq!(diag.title, "Unknown Name");
            assert!(diag.hints.iter().any(|h| h.contains("spelling")));
        }

        #[test]
        fn test_purity_violation_categorization() {
            let error = TypeError {
                message: "Function 'foo' is declared as pure but has effects: {Console}".to_string(),
                span: Span { start: 0, end: 10 },
            };
            let diag = error.to_diagnostic();

            assert_eq!(diag.title, "Purity Violation");
        }
    }
}