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//! Construction of a [`U320`] from a 40-byte big-endian array.
use super::U320;
impl U320 {
/// Creates a new [`U320`] from a 40-byte array in big-endian order.
///
/// Bytes `[0..8]` become the most significant limb, bytes `[8..16]`
/// become the next, and so on down to bytes `[32..40]` which become
/// the least significant limb. The result is stored in little-endian
/// internal layout.
///
/// # Examples
///
/// ```
/// use cnfy_uint::u320::U320;
///
/// let mut bytes = [0u8; 40];
/// bytes[39] = 1;
/// assert_eq!(U320::from_be_bytes(bytes), U320::from_be_limbs([0, 0, 0, 0, 1]));
/// ```
#[inline]
pub const fn from_be_bytes(bytes: [u8; 40]) -> Self {
let w0 = u64::from_be_bytes([
bytes[0], bytes[1], bytes[2], bytes[3], bytes[4], bytes[5], bytes[6], bytes[7],
]);
let w1 = u64::from_be_bytes([
bytes[8], bytes[9], bytes[10], bytes[11], bytes[12], bytes[13], bytes[14], bytes[15],
]);
let w2 = u64::from_be_bytes([
bytes[16], bytes[17], bytes[18], bytes[19], bytes[20], bytes[21], bytes[22], bytes[23],
]);
let w3 = u64::from_be_bytes([
bytes[24], bytes[25], bytes[26], bytes[27], bytes[28], bytes[29], bytes[30], bytes[31],
]);
let w4 = u64::from_be_bytes([
bytes[32], bytes[33], bytes[34], bytes[35], bytes[36], bytes[37], bytes[38], bytes[39],
]);
Self([w4, w3, w2, w1, w0])
}
}
#[cfg(test)]
mod ai_tests {
use super::*;
/// All zeros produces the zero value.
#[test]
fn zero() {
assert_eq!(
U320::from_be_bytes([0; 40]),
U320::ZERO,
);
}
/// All ones produces the maximum value.
#[test]
fn max() {
assert_eq!(
U320::from_be_bytes([0xFF; 40]),
U320::MAX,
);
}
/// Byte 39 (least significant) maps to the low bit of limb w4.
#[test]
fn single_lsb() {
let mut bytes = [0u8; 40];
bytes[39] = 1;
assert_eq!(
U320::from_be_bytes(bytes),
U320::from_be_limbs([0, 0, 0, 0, 1]),
);
}
/// Byte 0 (most significant) maps to the high bit of limb w0.
#[test]
fn single_msb() {
let mut bytes = [0u8; 40];
bytes[0] = 0x80;
assert_eq!(
U320::from_be_bytes(bytes),
U320::from_be_limbs([0x8000000000000000, 0, 0, 0, 0]),
);
}
/// Boundary between w3 and w4 at bytes 31/32.
#[test]
fn limb_boundary_w3_w4() {
let mut bytes = [0u8; 40];
bytes[31] = 1;
bytes[32] = 1;
assert_eq!(
U320::from_be_bytes(bytes),
U320::from_be_limbs([0, 0, 0, 1, 0x0100000000000000]),
);
}
/// Boundary between w0 and w1 at bytes 7/8.
#[test]
fn limb_boundary_w0_w1() {
let mut bytes = [0u8; 40];
bytes[7] = 0xFF;
bytes[8] = 0x01;
assert_eq!(
U320::from_be_bytes(bytes),
U320::from_be_limbs([0xFF, 0x0100000000000000, 0, 0, 0]),
);
}
}