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use crate::pack::data::File;
use git_features::progress::Progress;
use git_hash::SIZE_OF_SHA1_DIGEST as SHA1_SIZE;

/// Returned by [`File::verify_checksum()`]
#[derive(thiserror::Error, Debug)]
#[allow(missing_docs)]
pub enum Error {
    #[error("pack checksum mismatch: expected {expected}, got {actual}")]
    Mismatch {
        expected: git_hash::ObjectId,
        actual: git_hash::ObjectId,
    },
    #[error("could not read pack file")]
    Io(#[from] std::io::Error),
}

/// Checksums and verify checksums
impl File {
    /// The checksum in the trailer of this pack data file
    pub fn checksum(&self) -> git_hash::ObjectId {
        git_hash::ObjectId::from_20_bytes(&self.data[self.data.len() - SHA1_SIZE..])
    }

    /// Verifies that the checksum of the packfile over all bytes preceding it indeed matches the actual checksum,
    /// returning the actual checksum equivalent to the return value of [`checksum()`][File::checksum()] if there
    /// is no mismatch.
    ///
    /// Note that if no `progress` is desired, one can pass [`git_features::progress::Discard`].
    ///
    /// Have a look at [`index::File::verify_integrity(…)`][crate::pack::index::File::verify_integrity()] for an
    /// even more thorough integrity check.
    pub fn verify_checksum(&self, mut progress: impl Progress) -> Result<git_hash::ObjectId, Error> {
        let right_before_trailer = self.data.len() - SHA1_SIZE;
        let actual = match git_features::hash::bytes_of_file(
            &self.path,
            right_before_trailer,
            git_hash::Kind::Sha1,
            &mut progress,
        ) {
            Ok(id) => id,
            Err(_io_err) => {
                let start = std::time::Instant::now();
                let mut hasher = git_features::hash::Sha1::default();
                hasher.update(&self.data[..right_before_trailer]);
                progress.inc_by(right_before_trailer);
                progress.show_throughput(start);
                git_hash::ObjectId::new_sha1(hasher.digest())
            }
        };

        let expected = self.checksum();
        if actual == expected {
            Ok(actual)
        } else {
            Err(Error::Mismatch { actual, expected })
        }
    }
}