use super::funejson::Value;

const MAX_SAFE: i64 = (1i64 << 53) - 1;

/// The largest whole number whose square does not exceed `n`.
///
/// The floating-point square root is only a first guess: it is corrected with
/// exact `i128` arithmetic, so a number just below a perfect square never
/// rounds up to its root.
///
/// # Panics
/// Panics if `n` is negative or above 2^53 - 1.
pub fn integer_sqrt(n: i64) -> i64 {
    if !(0..=MAX_SAFE).contains(&n) {
        panic!("n must be a whole number from 0 to 2^53 - 1, received {}", n);
    }
    let target = n as i128;
    let mut r = (n as f64).sqrt().floor() as i128;
    while r * r > target {
        r -= 1;
    }
    while (r + 1) * (r + 1) <= target {
        r += 1;
    }
    r as i64
}

pub fn fune_vector(args: &[Value]) -> Value {
    if let Value::Float(f) = &args[0] {
        panic!("n must be a whole number from 0 to 2^53 - 1, received {}", f);
    }
    Value::Int(integer_sqrt(args[0].as_i64()))
}