use crate::index::{self, Version, FAN_LEN, V2_SIGNATURE};use byteorder::{BigEndian, ByteOrder};use filebuffer::FileBuffer;use git_hash::SIZE_OF_SHA1_DIGEST as SHA1_SIZE;use std::{convert::TryFrom, mem::size_of, path::Path}; /// Returned by [`index::File::at()`].#[derive(thiserror::Error, Debug)]#[allow(missing_docs)]pub enum Error {    #[error("Could not open pack index file at '{path}'")]    Io {        source: std::io::Error,        path: std::path::PathBuf,    },    #[error("{message}")]    Corrupt { message: String },    #[error("Unsupported index version: {version})")]    UnsupportedVersion { version: u32 },} const N32_SIZE: usize = size_of::<u32>();const FOOTER_SIZE: usize = SHA1_SIZE * 2; /// Instantiationimpl index::File {    /// Open the pack index file at the given `path`.    pub fn at(path: impl AsRef<Path>) -> Result<index::File, Error> {        Self::try_from(path.as_ref())    }} impl TryFrom<&Path> for index::File {    type Error = Error;     fn try_from(path: &Path) -> Result<Self, Self::Error> {        let data = FileBuffer::open(&path).map_err(|e| Error::Io {            source: e,            path: path.to_owned(),        })?;        let idx_len = data.len();        if idx_len < FAN_LEN * N32_SIZE + FOOTER_SIZE {            return Err(Error::Corrupt {                message: format!("Pack index of size {} is too small for even an empty index", idx_len),            });        }        let (kind, fan, num_objects) = {            let (kind, d) = {                let (sig, d) = data.split_at(V2_SIGNATURE.len());                if sig == V2_SIGNATURE {                    (Version::V2, d)                } else {                    (Version::V1, &data[..])                }            };            let d = {                if let Version::V2 = kind {                    let (vd, dr) = d.split_at(N32_SIZE);                    let version = BigEndian::read_u32(vd);                    if version != Version::V2 as u32 {                        return Err(Error::UnsupportedVersion { version });                    }                    dr                } else {                    d                }            };            let (fan, bytes_read) = read_fan(d);            let (_, _d) = d.split_at(bytes_read);            let num_objects = fan[FAN_LEN - 1];             (kind, fan, num_objects)        };        Ok(index::File {            data,            path: path.to_owned(),            version: kind,            num_objects,            fan,        })    }} fn read_fan(d: &[u8]) -> ([u32; FAN_LEN], usize) {    let mut fan = [0; FAN_LEN];    for (c, f) in d.chunks(N32_SIZE).zip(fan.iter_mut()) {        *f = BigEndian::read_u32(c);    }    (fan, FAN_LEN * N32_SIZE)}