trezor-firmware/core/embed/rust/src/strutil.rs

pub fn hexlify(data: &[u8], buffer: &mut [u8]) {
    const HEX_LOWER: [u8; 16] = *b"0123456789abcdef";
    let mut i: usize = 0;
    for b in data.iter().take(buffer.len() / 2) {
        let hi: usize = ((b & 0xf0) >> 4).into();
        let lo: usize = (b & 0x0f).into();
        buffer[i] = HEX_LOWER[hi];
        buffer[i + 1] = HEX_LOWER[lo];
        i += 2;
    }
}

pub fn format_i64(num: i64, buffer: &mut [u8]) -> Option<&str> {
    let mut i = 0;
    let mut num = num;
    let negative = if num < 0 {
        num = -num;
        true
    } else {
        false
    };

    while num > 0 && i < buffer.len() {
        buffer[i] = b'0' + ((num % 10) as u8);
        num /= 10;
        i += 1;
    }
    match i {
        0 => Some("0"),
        _ if num > 0 => None,
        _ if negative && i == buffer.len() => None,
        _ => {
            if negative {
                buffer[i] = b'-';
                i += 1;
            }
            let result = &mut buffer[..i];
            result.reverse();
            Some(unsafe { core::str::from_utf8_unchecked(result) })
        }
    }
}