use std::{cmp::Ordering, convert::TryFrom}; use crate::{oid, ObjectId, Prefix}; /// The error returned by [Prefix::new()].#[derive(Debug, thiserror::Error)]#[allow(missing_docs)]pub enum Error {    #[error(        "The minimum hex length of a short object id is {}, got {hex_len}",        Prefix::MIN_HEX_LEN    )]    TooShort { hex_len: usize },    #[error("An object of kind {} cannot be larger than {object_kind} in hex, but {hex_len} was requested")]    TooLong { object_kind: crate::Kind, hex_len: usize },} ///pub mod from_hex {    /// The error returned by [Prefix::from_hex][super::Prefix::from_hex()].    #[derive(Debug, Eq, PartialEq, thiserror::Error)]    #[allow(missing_docs)]    pub enum Error {        #[error(            "The minimum hex length of a short object id is {}, got {hex_len}",            super::Prefix::MIN_HEX_LEN        )]        TooShort { hex_len: usize },        #[error("An id cannot be larger than {} chars in hex, but {hex_len} was requested", crate::Kind::longest().len_in_hex())]        TooLong { hex_len: usize },        #[error("Invalid character {c} at position {index}")]        Invalid { c: char, index: usize },    }} impl Prefix {    /// The smallest allowed prefix length below which chances for collisions are too high even in small repositories.    pub const MIN_HEX_LEN: usize = 4;     /// Create a new instance by taking a full `id` as input and truncating it to `hex_len`.    ///    /// For instance, with `hex_len` of 7 the resulting prefix is 3.5 bytes, or 3 bytes and 4 bits    /// wide, with all other bytes and bits set to zero.    pub fn new(id: impl AsRef<oid>, hex_len: usize) -> Result<Self, Error> {        let id = id.as_ref();        if hex_len > id.kind().len_in_hex() {            Err(Error::TooLong {                object_kind: id.kind(),                hex_len,            })        } else if hex_len < Self::MIN_HEX_LEN {            Err(Error::TooShort { hex_len })        } else {            let mut prefix = ObjectId::null(id.kind());            let b = prefix.as_mut_slice();            let copy_len = (hex_len + 1) / 2;            b[..copy_len].copy_from_slice(&id.as_bytes()[..copy_len]);            if hex_len % 2 == 1 {                b[hex_len / 2] &= 0xf0;            }             Ok(Prefix { bytes: prefix, hex_len })        }    }     /// Returns the prefix as object id.    ///    /// Note that it may be deceptive to use given that it looks like a full    /// object id, even though its post-prefix bytes/bits are set to zero.    pub fn as_oid(&self) -> &oid {        &self.bytes    }     /// Return the amount of hexadecimal characters that are set in the prefix.    ///    /// This gives the prefix a granularity of 4 bits.    pub fn hex_len(&self) -> usize {        self.hex_len    }     /// Provided with candidate id which is a full hash, determine how this prefix compares to it,    /// only looking at the prefix bytes, ignoring everything behind that.    pub fn cmp_oid(&self, candidate: &oid) -> Ordering {        let common_len = self.hex_len / 2;         self.bytes.as_bytes()[..common_len]            .cmp(&candidate.as_bytes()[..common_len])            .then(if self.hex_len % 2 == 1 {                let half_byte_idx = self.hex_len / 2;                self.bytes.as_bytes()[half_byte_idx].cmp(&(candidate.as_bytes()[half_byte_idx] & 0xf0))            } else {                Ordering::Equal            })    }     /// Create an instance from the given hexadecimal prefix `value`, e.g. `35e77c16` would yield a `Prefix` with `hex_len()` = 8.    pub fn from_hex(value: &str) -> Result<Self, from_hex::Error> {        use hex::FromHex;        let hex_len = value.len();         if hex_len > crate::Kind::longest().len_in_hex() {            return Err(from_hex::Error::TooLong { hex_len });        } else if hex_len < Self::MIN_HEX_LEN {            return Err(from_hex::Error::TooShort { hex_len });        };         let src = if value.len() % 2 == 0 {            Vec::from_hex(value)        } else {            let mut buf = [0u8; crate::Kind::longest().len_in_hex()];            buf[..value.len()].copy_from_slice(value.as_bytes());            buf[value.len()] = b'0';            Vec::from_hex(&buf[..value.len() + 1])        }        .map_err(|e| match e {            hex::FromHexError::InvalidHexCharacter { c, index } => from_hex::Error::Invalid { c, index },            hex::FromHexError::OddLength | hex::FromHexError::InvalidStringLength => panic!("This is already checked"),        })?;         let mut bytes = ObjectId::null(crate::Kind::from_hex_len(value.len()).expect("hex-len is already checked"));        let dst = bytes.as_mut_slice();        let copy_len = src.len();        dst[..copy_len].copy_from_slice(&src);         Ok(Prefix { bytes, hex_len })    }} /// Create an instance from the given hexadecimal prefix, e.g. `35e77c16` would yield a `Prefix`/// with `hex_len()` = 8.impl TryFrom<&str> for Prefix {    type Error = from_hex::Error;     fn try_from(value: &str) -> Result<Self, Self::Error> {        Prefix::from_hex(value)    }} impl std::fmt::Display for Prefix {    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {        self.bytes.to_hex_with_len(self.hex_len).fmt(f)    }} impl From<ObjectId> for Prefix {    fn from(oid: ObjectId) -> Self {        Prefix {            bytes: oid,            hex_len: oid.kind().len_in_hex(),        }    }}