//! Sample doughlang code

struct Expr {
    Atom (f64),
    Op (char, [Expr]),
}

fn execute(expr: Expr) -> f64 {
    match expr {
        ExprAtom(value) => value,
        ExprOp('*', [lhs, rhs]) => execute(lhs) * execute(rhs),
        ExprOp('/', [lhs, rhs]) => execute(lhs) / execute(rhs),
        ExprOp('%', [lhs, rhs]) => execute(lhs) % execute(rhs),
        ExprOp('+', [lhs, rhs]) => execute(lhs) + execute(rhs),
        ExprOp('-', [lhs, rhs]) => execute(lhs) - execute(rhs),
        // ExprOp('>', [lhs, rhs]) => (execute(lhs) as u64 >> execute(rhs) as u64) as f64,
        // ExprOp('<', [lhs, rhs]) => (execute(lhs) as u64 << execute(rhs) as u64) as f64,
        ExprOp('-', [lhs]) => - execute(lhs),
        other => {
            panic("Unknown operation: " + fmt(other))
        }
    }
}

/// Formats an expression to a string
fn fmt_expr(expr: Expr) -> str {
    match expr {
        ExprAtom(value) => fmt(value),
        ExprOp(operator, [lhs, rhs]) => fmt('(', fmt_expr(lhs), ' ', operator, ' ', fmt_expr(rhs), ')'),
        ExprOp(operator, [rhs]) => fmt(operator, fmt_expr(rhs)),
        _ => println("Unexpected expr: ", expr),
    }
}

/// Prints an expression
fn print_expr(expr: Expr) {
    println(fmt_expr(expr))
}

/// Parses expressions
fn parse(line: [char], power: i32) -> (Expr, [char]) {
    fn map((expr, line): (Expr, [char]), f: fn(Expr) -> Expr) -> (Expr, [char]) {
        (f(expr), line)
    }

    line = space(line);

    let (lhs, line) = match line {
        ['0'..'9', ..] => number(line),
        ['(', ..rest] => match parse(rest, Power::None) {
                (expr, [')', ..rest]) => (expr, rest),
                (expr, rest) => panic(fmt("Expected ')', got ", expr, ", ", rest)),
            },
        [op, ..rest] => parse(rest, pre_bp(op)).map(|lhs| Expr::Op(op, [lhs])),
        _ => panic("Unexpected end of input"),
    };

    while let [op, ..rest] = space(line) {
        let (before, after) = inf_bp(op);
        if before < power {
            break;
        };
        (lhs, line) = parse(rest, after).map(|rhs| Expr::Op(op, [lhs, rhs]));
    };
    (lhs, line)
}