// Copyright 2023 The Jujutsu Authors//// Licensed under the Apache License, Version 2.0 (the "License");// you may not use this file except in compliance with the License.// You may obtain a copy of the License at//// https://www.apache.org/licenses/LICENSE-2.0//// Unless required by applicable law or agreed to in writing, software// distributed under the License is distributed on an "AS IS" BASIS,// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.// See the License for the specific language governing permissions and// limitations under the License. #![allow(missing_docs)] const REVERSE_HEX_CHARS: &[u8; 16] = b"zyxwvutsrqponmlk"; fn to_forward_hex_digit(b: u8) -> Option<u8> {    let value = match b {        b'k'..=b'z' => b'z' - b,        b'K'..=b'Z' => b'Z' - b,        _ => return None,    };    if value < 10 {        Some(b'0' + value)    } else {        Some(b'a' + value - 10)    }} pub fn to_forward_hex(reverse_hex: &str) -> Option<String> {    reverse_hex        .bytes()        .map(|b| to_forward_hex_digit(b).map(char::from))        .collect()} /// Encodes `data` as hex string using `z-k` "digits".pub fn encode_reverse_hex(data: &[u8]) -> String {    let chars = REVERSE_HEX_CHARS;    let encoded = data        .iter()        .flat_map(|b| [chars[usize::from(b >> 4)], chars[usize::from(b & 0xf)]])        .collect();    String::from_utf8(encoded).unwrap()} /// Calculates common prefix length of two byte sequences. The length/// to be returned is a number of hexadecimal digits.pub fn common_hex_len(bytes_a: &[u8], bytes_b: &[u8]) -> usize {    std::iter::zip(bytes_a, bytes_b)        .enumerate()        .find_map(|(i, (a, b))| match a ^ b {            0 => None,            d if d & 0xf0 == 0 => Some(i * 2 + 1),            _ => Some(i * 2),        })        .unwrap_or_else(|| bytes_a.len().min(bytes_b.len()) * 2)} #[cfg(test)]mod tests {    use super::*;     #[test]    fn test_reverse_hex() {        // Empty string        assert_eq!(encode_reverse_hex(b""), "".to_string());        assert_eq!(to_forward_hex(""), Some("".to_string()));         // Single digit        assert_eq!(to_forward_hex("z"), Some("0".to_string()));         // All digits        assert_eq!(            encode_reverse_hex(b"\x01\x23\x45\x67\x89\xab\xcd\xef"),            "zyxwvutsrqponmlk".to_string()        );        assert_eq!(            to_forward_hex("zyxwvutsrqponmlkPONMLK"),            Some("0123456789abcdefabcdef".to_string())        );         // Invalid digit        assert_eq!(to_forward_hex("j"), None);    }}