use crate::index::{self, FAN_LEN};use byteorder::{BigEndian, ByteOrder};use git_hash::SIZE_OF_SHA1_DIGEST as SHA1_SIZE;use std::{convert::TryInto, mem::size_of}; const N32_SIZE: usize = size_of::<u32>();const N64_SIZE: usize = size_of::<u64>();const V1_HEADER_SIZE: usize = FAN_LEN * N32_SIZE;const V2_HEADER_SIZE: usize = N32_SIZE * 2 + FAN_LEN * N32_SIZE;const N32_HIGH_BIT: u32 = 1 << 31; pub(crate) type PackOffset = u64; /// Represents an entry within a pack index file, effectively mapping object [`IDs`][git_hash::ObjectId] to pack data file locations.#[derive(PartialEq, Eq, Debug, Hash, Ord, PartialOrd, Clone)]#[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]pub struct Entry {    /// The ID of the object    pub oid: git_hash::ObjectId,    /// The offset to the object's header in the pack data file    pub pack_offset: PackOffset,    /// The CRC32 hash over all bytes of the pack data entry.    ///    /// This can be useful for direct copies of pack data entries from one pack to another with insurance there was no bit rot.    /// _Note_: Only available in index version 2 or newer    pub crc32: Option<u32>,} /// Iteration and accessimpl index::File {    fn iter_v1(&self) -> impl Iterator<Item = Entry> + '_ {        match self.version {            index::Version::V1 => self.data[V1_HEADER_SIZE..]                .chunks(N32_SIZE + SHA1_SIZE)                .take(self.num_objects as usize)                .map(|c| {                    let (ofs, oid) = c.split_at(N32_SIZE);                    Entry {                        oid: git_hash::ObjectId::from_20_bytes(oid),                        pack_offset: BigEndian::read_u32(ofs) as u64,                        crc32: None,                    }                }),            _ => panic!("Cannot use iter_v1() on index of type {:?}", self.version),        }    }     fn iter_v2(&self) -> impl Iterator<Item = Entry> + '_ {        let pack64_offset = self.offset_pack_offset64_v2();        match self.version {            index::Version::V2 => izip!(                self.data[V2_HEADER_SIZE..].chunks(SHA1_SIZE),                self.data[self.offset_crc32_v2()..].chunks(N32_SIZE),                self.data[self.offset_pack_offset_v2()..].chunks(N32_SIZE)            )            .take(self.num_objects as usize)            .map(move |(oid, crc32, ofs32)| Entry {                oid: git_hash::ObjectId::from_20_bytes(oid),                pack_offset: self.pack_offset_from_offset_v2(ofs32, pack64_offset),                crc32: Some(BigEndian::read_u32(crc32)),            }),            _ => panic!("Cannot use iter_v2() on index of type {:?}", self.version),        }    }     /// Returns 20 bytes sha1 at the given index in our list of (sorted) sha1 hashes.    /// The index ranges from 0 to self.num_objects()    ///    /// # Panics    ///    /// If `index` is out of bounds.    pub fn oid_at_index(&self, index: u32) -> &git_hash::oid {        let index: usize = index            .try_into()            .expect("an architecture able to hold 32 bits of integer");        let start = match self.version {            index::Version::V2 => V2_HEADER_SIZE + index * SHA1_SIZE,            index::Version::V1 => V1_HEADER_SIZE + index * (N32_SIZE + SHA1_SIZE) + N32_SIZE,        };        git_hash::oid::try_from(&self.data[start..start + SHA1_SIZE]).expect("20 bytes SHA1 to be alright")    }     /// Returns the offset into our pack data file at which to start reading the object at `index`.    ///    /// # Panics    ///    /// If `index` is out of bounds.    pub fn pack_offset_at_index(&self, index: u32) -> PackOffset {        let index: usize = index            .try_into()            .expect("an architecture able to hold 32 bits of integer");        match self.version {            index::Version::V2 => {                let start = self.offset_pack_offset_v2() + index * N32_SIZE;                self.pack_offset_from_offset_v2(&self.data[start..start + N32_SIZE], self.offset_pack_offset64_v2())            }            index::Version::V1 => {                let start = V1_HEADER_SIZE + index * (N32_SIZE + SHA1_SIZE);                BigEndian::read_u32(&self.data[start..start + N32_SIZE]) as u64            }        }    }     /// Returns the CRC32 of the object at the given `index`.    ///    /// _Note_: These are always present for index version 2 or higher.    /// # Panics    ///    /// If `index` is out of bounds.    pub fn crc32_at_index(&self, index: u32) -> Option<u32> {        let index: usize = index            .try_into()            .expect("an architecture able to hold 32 bits of integer");        match self.version {            index::Version::V2 => {                let start = self.offset_crc32_v2() + index * N32_SIZE;                Some(BigEndian::read_u32(&self.data[start..start + N32_SIZE]))            }            index::Version::V1 => None,        }    }     /// Returns the `index` of the given SHA1 for use with the [`oid_at_index()`][index::File::oid_at_index()],    /// [`pack_offset_at_index()`][index::File::pack_offset_at_index()] or [`crc32_at_index()`][index::File::crc32_at_index()].    pub fn lookup(&self, id: impl AsRef<git_hash::oid>) -> Option<u32> {        let id = id.as_ref();        let first_byte = id.first_byte() as usize;        let mut upper_bound = self.fan[first_byte];        let mut lower_bound = if first_byte != 0 { self.fan[first_byte - 1] } else { 0 };         // Bisect using indices        // TODO: Performance of V2 could possibly be better if we would be able to do a binary search        // on 20 byte chunks directly, but doing so requires transmuting and that is not safe, even though        // it should not be if the bytes match up and the type has no destructor.        while lower_bound < upper_bound {            let mid = (lower_bound + upper_bound) / 2;            let mid_sha = self.oid_at_index(mid);             use std::cmp::Ordering::*;            match id.cmp(mid_sha) {                Less => upper_bound = mid,                Equal => return Some(mid),                Greater => lower_bound = mid + 1,            }        }        None    }     /// An iterator over all [`Entries`][Entry] of this index file.    pub fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = Entry> + 'a> {        match self.version {            index::Version::V2 => Box::new(self.iter_v2()),            index::Version::V1 => Box::new(self.iter_v1()),        }    }     /// Return a vector of ascending offsets into our respective pack data file.    ///    /// Useful to control an iteration over all pack entries in a cache-friendly way.    pub fn sorted_offsets(&self) -> Vec<PackOffset> {        let mut ofs: Vec<_> = match self.version {            index::Version::V1 => self.iter().map(|e| e.pack_offset).collect(),            index::Version::V2 => {                let mut v = Vec::with_capacity(self.num_objects as usize);                let mut ofs32 = &self.data[self.offset_pack_offset_v2()..];                let pack_offset_64 = self.offset_pack_offset64_v2();                for _ in 0..self.num_objects {                    v.push(self.pack_offset_from_offset_v2(ofs32, pack_offset_64));                    ofs32 = &ofs32[4..];                }                v            }        };        ofs.sort_unstable();        ofs    }     fn offset_crc32_v2(&self) -> usize {        V2_HEADER_SIZE + self.num_objects as usize * SHA1_SIZE    }     fn offset_pack_offset_v2(&self) -> usize {        self.offset_crc32_v2() + self.num_objects as usize * N32_SIZE    }     fn offset_pack_offset64_v2(&self) -> usize {        self.offset_pack_offset_v2() + self.num_objects as usize * N32_SIZE    }     fn pack_offset_from_offset_v2(&self, offset: &[u8], pack64_offset: usize) -> PackOffset {        debug_assert_eq!(self.version, index::Version::V2);        let ofs32 = BigEndian::read_u32(offset);        if (ofs32 & N32_HIGH_BIT) == N32_HIGH_BIT {            let from = pack64_offset + (ofs32 ^ N32_HIGH_BIT) as usize * N64_SIZE;            BigEndian::read_u64(&self.data[from..from + N64_SIZE])        } else {            ofs32 as u64        }    }}