base64 encoding implemented in rust

base64.rs

pub fn hex2b64(s: &str) -> String {
    assert_eq!(s.len() % 2, 0);

    let bit_mask1 = 0x00FC_0000;
    let bit_mask2 = 0x0003_F000;
    let bit_mask3 = 0x0000_0FC0;
    let bit_mask4 = 0x0000_003F;

    let excess_bytes = s.len() / 2 % 3;
    let mut buf = String::new();

    for i in (0..s.len() - (excess_bytes * 2)).step_by(6) {
        let hex_substr_as_u32 = u32::from_str_radix(&s[i..i + 6], 16).unwrap();

        buf.push(single_u8_to_b64(
            ((hex_substr_as_u32 & bit_mask1) >> 18) as u8,
        ));
        buf.push(single_u8_to_b64(
            ((hex_substr_as_u32 & bit_mask2) >> 12) as u8,
        ));
        buf.push(single_u8_to_b64(
            ((hex_substr_as_u32 & bit_mask3) >> 6) as u8,
        ));
        buf.push(single_u8_to_b64((hex_substr_as_u32 & bit_mask4) as u8));
    }
    match excess_bytes {
        1 => {
            let padded_excess = u32::from_str_radix(&s[s.len() - 2..s.len()], 16).unwrap() << 16;
            buf.push(single_u8_to_b64(((padded_excess & bit_mask1) >> 18) as u8));
            buf.push(single_u8_to_b64(((padded_excess & bit_mask2) >> 12) as u8));
            buf.push('=');
            buf.push('=');
            buf
        }
        2 => {
            let padded_excess = u32::from_str_radix(&s[s.len() - 4..s.len()], 16).unwrap() << 8;
            buf.push(single_u8_to_b64(((padded_excess & bit_mask1) >> 18) as u8));
            buf.push(single_u8_to_b64(((padded_excess & bit_mask2) >> 12) as u8));
            buf.push(single_u8_to_b64(((padded_excess & bit_mask3) >> 6) as u8));
            buf.push('=');
            buf
        }
        _ => buf,
    }
}

pub fn single_u8_to_b64(i: u8) -> char {
    assert!(i < 0x40);
    match i {
        0..=25 => (b'A' + i) as char,         // lowerCase
        26..=51 => (b'a' + (i - 26)) as char, // upperCase
        52..=61 => (b'0' + (i - 52)) as char, // number
        62 => '+',
        63 => '/',
        _ => unreachable!(""),
    }
}