/// Fixed-point arithmetic with configurable fractional bits.
///
/// All operations use i64 internally with i128 intermediates
/// for multiply to avoid overflow. Shift-and-add only.
#[derive(Debug, Clone, Copy)]
pub struct FixedPoint {
    pub frac_bits: u8,
}

impl FixedPoint {
    pub fn new(frac_bits: u8) -> Self {
        Self { frac_bits }
    }

    #[inline]
    pub fn one(&self) -> i64 {
        1i64 << self.frac_bits
    }

    #[inline]
    pub fn from_f64(&self, val: f64) -> i64 {
        (val * (1i64 << self.frac_bits) as f64).round() as i64
    }

    #[inline]
    pub fn to_f64(&self, val: i64) -> f64 {
        val as f64 / (1i64 << self.frac_bits) as f64
    }

    #[inline]
    pub fn mul(&self, a: i64, b: i64) -> i64 {
        ((a as i128 * b as i128) >> self.frac_bits) as i64
    }

    #[inline]
    pub fn div(&self, a: i64, b: i64) -> i64 {
        if b == 0 {
            return if a >= 0 { i64::MAX } else { i64::MIN };
        }
        (((a as i128) << self.frac_bits) / b as i128) as i64
    }
}

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

    #[test]
    fn mul_basic() {
        let fp = FixedPoint::new(16);
        let a = fp.from_f64(2.5);
        let b = fp.from_f64(4.0);
        let result = fp.to_f64(fp.mul(a, b));
        assert!((result - 10.0).abs() < 0.001);
    }

    #[test]
    fn div_basic() {
        let fp = FixedPoint::new(16);
        let a = fp.from_f64(10.0);
        let b = fp.from_f64(4.0);
        let result = fp.to_f64(fp.div(a, b));
        assert!((result - 2.5).abs() < 0.001);
    }
}