use std::{convert::TryInto, path::Path}; use crate::data; /// Instantiationimpl data::File {    /// Try opening a data file at the given `path`.    ///    /// The `object_hash` is a way to read (and write) the same file format with different hashes, as the hash kind    /// isn't stored within the file format itself.    pub fn at(path: impl AsRef<Path>, object_hash: git_hash::Kind) -> Result<data::File, data::header::decode::Error> {        Self::at_inner(path.as_ref(), object_hash)    }     fn at_inner(path: &Path, object_hash: git_hash::Kind) -> Result<data::File, data::header::decode::Error> {        use crate::data::header::N32_SIZE;        let hash_len = object_hash.len_in_bytes();         let data = crate::mmap::read_only(path).map_err(|e| data::header::decode::Error::Io {            source: e,            path: path.to_owned(),        })?;        let pack_len = data.len();        if pack_len < N32_SIZE * 3 + hash_len {            return Err(data::header::decode::Error::Corrupt(format!(                "Pack data of size {pack_len} is too small for even an empty pack with shortest hash"            )));        }        let (kind, num_objects) =            data::header::decode(&data[..12].try_into().expect("enough data after previous check"))?;        Ok(data::File {            data,            path: path.to_owned(),            id: git_features::hash::crc32(path.as_os_str().to_string_lossy().as_bytes()),            version: kind,            num_objects,            hash_len,            object_hash,        })    }}