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//! Hash functions and hash utilities
//!
//! With the `fast-sha1` feature, the [`Sha1`] hash type will use a more elaborate implementation utilizing hardware support
//! in case it is available.
//! Otherwise, a minimal yet performant implementation is used instead for a decent trade-off between compile times and run-time performance.

#[cfg(not(feature = "fast-sha1"))]
mod _impl {
    use super::Sha1Digest;

    /// A implementation of the Sha1 hash, which can be used once.
    #[derive(Default, Clone)]
    pub struct Sha1(sha1::Sha1);

    impl Sha1 {
        /// Digest the given `bytes`.
        pub fn update(&mut self, bytes: &[u8]) {
            self.0.update(bytes)
        }
        /// Finalize the hash and produce a digest.
        pub fn digest(self) -> Sha1Digest {
            self.0.digest().bytes()
        }
    }
}

/// A 20 bytes digest produced by a [`Sha1`] hash implementation.
pub type Sha1Digest = [u8; 20];

#[cfg(feature = "fast-sha1")]
mod _impl {
    use super::Sha1Digest;
    use fastsha1::Digest;

    /// A implementation of the Sha1 hash, which can be used once.
    #[derive(Default, Clone)]
    pub struct Sha1(fastsha1::Sha1);

    impl Sha1 {
        /// Digest the given `bytes`.
        pub fn update(&mut self, bytes: &[u8]) {
            self.0.update(bytes)
        }
        /// Finalize the hash and produce a digest.
        pub fn digest(self) -> Sha1Digest {
            self.0.finalize().into()
        }
    }
}

pub use _impl::Sha1;

/// Compute a CRC32 hash from the given `bytes`, returning the CRC32 hash.
///
/// When calling this function for the first time, `previous_value` should be `0`. Otherwise it
/// should be the previous return value of this function to provide a hash of multiple sequential
/// chunks of `bytes`.
pub fn crc32_update(previous_value: u32, bytes: &[u8]) -> u32 {
    crc::crc32::update(previous_value, &crc::crc32::IEEE_TABLE, bytes)
}

/// Compute a CRC32 value of the given input `bytes`.
///
/// In case multiple chunkes of `bytes` are present, one should use [`crc32_update()`] instead.
pub use crc::crc32::checksum_ieee as crc32;

/// Compute the hash of `kind` for the bytes in the file at `path`, hashing only the first `num_bytes_from_start`
/// while initializing and calling `progress`.
///
/// `num_bytes_from_start` is useful to avoid reading trailing hashes, which are never part of the hash itself.
///
/// # Note
///
/// * Only available with the `git-object` feature enabled due to usage of the [`git_hash::Kind`] enum and the
///   [`git_hash::owned::Digest`] return value.
/// * [Interrupts][crate::interrupt] are supported.
pub fn bytes_of_file(
    path: impl AsRef<std::path::Path>,
    num_bytes_from_start: usize,
    kind: git_hash::Kind,
    progress: &mut impl crate::progress::Progress,
) -> std::io::Result<git_hash::owned::Digest> {
    let mut hasher = match kind {
        git_hash::Kind::Sha1 => crate::hash::Sha1::default(),
    };
    let start = std::time::Instant::now();
    // init progress before the possibility for failure, as convenience in case people want to recover
    progress.init(Some(num_bytes_from_start), crate::progress::bytes());

    let mut file = std::fs::File::open(path)?;
    use std::io::Read;
    const BUF_SIZE: usize = u16::MAX as usize;
    let mut buf = [0u8; BUF_SIZE];
    let mut bytes_left = num_bytes_from_start;

    while bytes_left > 0 {
        let out = &mut buf[..BUF_SIZE.min(bytes_left)];
        file.read_exact(out)?;
        bytes_left -= out.len();
        progress.inc_by(out.len());
        hasher.update(out);
        if crate::interrupt::is_triggered() {
            return Err(std::io::Error::new(std::io::ErrorKind::Other, "Interrupted"));
        }
    }

    let id = git_hash::owned::Digest::new_sha1(hasher.digest());
    progress.show_throughput(start);
    Ok(id)
}