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|
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 {object_kind} cannot be larger than {} in hex, but {hex_len} was requested", object_kind.len_in_hex())]
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()])
}
.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(),
}
}
}
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