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//! Lowercase hexadecimal formatting via [`fmt::LowerHex`].
use super::U256;
use core::fmt;
/// Formats a [`U256`] as a lowercase hexadecimal string without leading
/// zeros (except for the value zero itself, which formats as `"0"`).
///
/// Supports the `#` alternate flag for a `0x` prefix, matching the
/// standard library convention for `u64` and other integer types.
///
/// # Examples
///
/// ```
/// use cnfy_uint::u256::U256;
///
/// let v = U256::from_be_limbs([0, 0, 0, 255]);
/// assert_eq!(format!("{:x}", v), "ff");
/// assert_eq!(format!("{:#x}", v), "0xff");
/// ```
impl fmt::LowerHex for U256 {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// Find the first non-zero limb from MSB (index 3) down
let first_nonzero = (0..4).rev().find(|&i| self.0[i] != 0);
match first_nonzero {
None => {
// All limbs are zero
if f.alternate() {
f.write_str("0x0")
} else {
f.write_str("0")
}
}
Some(idx) => {
if f.alternate() {
f.write_str("0x")?;
}
// First non-zero limb: no leading zeros
write!(f, "{:x}", self.0[idx])?;
// Remaining limbs: pad to 16 hex digits each (descending)
let mut i = idx;
while i > 0 {
i -= 1;
write!(f, "{:016x}", self.0[i])?;
}
Ok(())
}
}
}
}
#[cfg(test)]
mod ai_tests {
use super::*;
/// Zero formats as "0".
#[test]
fn zero() {
assert_eq!(format!("{:x}", U256::ZERO), "0");
}
/// Zero with alternate flag formats as "0x0".
#[test]
fn zero_alternate() {
assert_eq!(format!("{:#x}", U256::ZERO), "0x0");
}
/// Small value formats without leading zeros.
#[test]
fn small_value() {
let v = U256::from_be_limbs([0, 0, 0, 255]);
assert_eq!(format!("{:x}", v), "ff");
}
/// Value spanning two limbs shows full padding on lower limb.
#[test]
fn two_limbs() {
let v = U256::from_be_limbs([0, 0, 1, 0]);
assert_eq!(format!("{:x}", v), "10000000000000000");
}
/// MAX value fills all hex digits.
#[test]
fn max_value() {
let s = format!("{:x}", U256::MAX);
assert_eq!(
s,
"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"
);
assert_eq!(s.len(), 64);
}
/// Alternate flag adds 0x prefix.
#[test]
fn alternate_prefix() {
let v = U256::from_be_limbs([0, 0, 0, 0xAB]);
assert_eq!(format!("{:#x}", v), "0xab");
}
/// ONE formats as "1".
#[test]
fn one() {
assert_eq!(format!("{:x}", U256::ONE), "1");
}
/// Value in the MSB limb only.
#[test]
fn msb_limb() {
let v = U256::from_be_limbs([0xCAFE, 0, 0, 0]);
assert_eq!(
format!("{:x}", v),
"cafe000000000000000000000000000000000000000000000000"
);
}
}