git-index 0.3.0

use crate::{decode::header, extension, extension::Signature, util::from_be_u32};

pub const SIGNATURE: Signature = *b"EOIE";
pub const SIZE: usize = 4 /* offset to extensions */ + git_hash::Kind::Sha1.len_in_bytes();
pub const SIZE_WITH_HEADER: usize = crate::extension::MIN_SIZE + SIZE;

pub fn decode(data: &[u8], object_hash: git_hash::Kind) -> Option<usize> {
    let hash_len = object_hash.len_in_bytes();
    if data.len() < SIZE_WITH_HEADER + hash_len {
        return None;
    }

    let start_of_eoie = data.len() - SIZE_WITH_HEADER - hash_len;
    let ext_data = &data[start_of_eoie..data.len() - hash_len];

    let (signature, ext_size, ext_data) = extension::decode::header(ext_data);
    if signature != SIGNATURE || ext_size as usize != SIZE {
        return None;
    }

    let (offset, checksum) = ext_data.split_at(4);
    let offset = from_be_u32(offset) as usize;
    if offset < header::SIZE || offset > start_of_eoie || checksum.len() != git_hash::Kind::Sha1.len_in_bytes() {
        return None;
    }

    let mut hasher = git_features::hash::hasher(git_hash::Kind::Sha1);
    let mut last_chunk = None;
    for (signature, chunk) in extension::Iter::new(&data[offset..data.len() - SIZE_WITH_HEADER - hash_len]) {
        hasher.update(&signature);
        hasher.update(&(chunk.len() as u32).to_be_bytes());
        last_chunk = Some(chunk);
    }

    if hasher.digest() != checksum {
        return None;
    }
    // The last-to-this chunk ends where ours starts
    if last_chunk
        .map(|s| s.as_ptr_range().end != (&data[start_of_eoie]) as *const _)
        .unwrap_or(true)
    {
        return None;
    }

    Some(offset)
}