//! Serialization of a [`U320`] into a 40-byte big-endian array.
use super::U320;

impl U320 {
    /// Returns the 40-byte big-endian representation of this value.
    ///
    /// Byte `[0]` is the most significant and byte `[39]` is the least
    /// significant. Each limb is serialized via `u64::to_be_bytes`,
    /// reading from MSB (internal index 4) down to LSB (internal index 0).
    ///
    /// This is functionally identical to `Into<[u8; 40]>` but available
    /// as a `const fn` method on `&self`.
    ///
    /// # Examples
    ///
    /// ```
    /// use cnfy_uint::u320::U320;
    ///
    /// let v = U320::from_be_limbs([0, 0, 0, 0, 1]);
    /// let bytes = v.to_be_bytes();
    /// assert_eq!(bytes[39], 1);
    /// assert_eq!(bytes[0], 0);
    /// ```
    #[inline]
    pub const fn to_be_bytes(&self) -> [u8; 40] {
        let w0 = self.0[4].to_be_bytes();
        let w1 = self.0[3].to_be_bytes();
        let w2 = self.0[2].to_be_bytes();
        let w3 = self.0[1].to_be_bytes();
        let w4 = self.0[0].to_be_bytes();
        [
            w0[0], w0[1], w0[2], w0[3], w0[4], w0[5], w0[6], w0[7],
            w1[0], w1[1], w1[2], w1[3], w1[4], w1[5], w1[6], w1[7],
            w2[0], w2[1], w2[2], w2[3], w2[4], w2[5], w2[6], w2[7],
            w3[0], w3[1], w3[2], w3[3], w3[4], w3[5], w3[6], w3[7],
            w4[0], w4[1], w4[2], w4[3], w4[4], w4[5], w4[6], w4[7],
        ]
    }
}

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

    /// Round-trip: from_be_bytes then to_be_bytes returns the original.
    #[test]
    fn round_trip() {
        let mut bytes = [0u8; 40];
        bytes[0] = 0xAB;
        bytes[19] = 0xCD;
        bytes[39] = 0xEF;
        let v = U320::from_be_bytes(bytes);
        assert_eq!(v.to_be_bytes(), bytes);
    }

    /// Zero produces all-zero bytes.
    #[test]
    fn zero() {
        assert_eq!(U320::ZERO.to_be_bytes(), [0u8; 40]);
    }

    /// MAX produces all-0xFF bytes.
    #[test]
    fn max() {
        assert_eq!(U320::MAX.to_be_bytes(), [0xFF; 40]);
    }

    /// Byte 39 holds the LSB of the lowest limb.
    #[test]
    fn lsb_position() {
        let v = U320::from_be_limbs([0, 0, 0, 0, 0x42]);
        let bytes = v.to_be_bytes();
        assert_eq!(bytes[39], 0x42);
        assert_eq!(bytes[38], 0);
    }

    /// Matches the Into<[u8; 40]> implementation.
    #[test]
    fn matches_into() {
        let v = U320::from_be_limbs([0xDEADBEEF, 0x12345678, 0xCAFEBABE, 0x42, 0xFF]);
        let into_bytes: [u8; 40] = v.into();
        assert_eq!(v.to_be_bytes(), into_bytes);
    }
}
