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use smallvec::SmallVec;
use std::borrow::Cow;

use crate::{immutable::object, BStr};

mod decode;

///
pub mod iter;

/// A git commit parsed using [`from_bytes()`][Commit::from_bytes()].
///
/// A commit encapsulates information about a point in time at which the state of the repository is recorded, usually after a
/// change which is documented in the commit `message`.
#[derive(PartialEq, Eq, Debug, Hash, Ord, PartialOrd, Clone)]
#[cfg_attr(feature = "serde1", derive(serde::Serialize, serde::Deserialize))]
pub struct Commit<'a> {
    /// HEX hash of tree object we point to. Usually 40 bytes long.
    ///
    /// Use [`tree()`][Commit::tree()] to obtain a decoded version of it.
    #[cfg_attr(feature = "serde1", serde(borrow))]
    pub tree: &'a BStr,
    /// HEX hash of each parent commit. Empty for first commit in repository.
    pub parents: SmallVec<[&'a BStr; 2]>,
    /// Who wrote this commit.
    pub author: git_actor::immutable::Signature<'a>,
    /// Who committed this commit.
    ///
    /// This may be different from the `author` in case the author couldn't write to the repository themselves and
    /// is commonly encountered with contributed commits.
    pub committer: git_actor::immutable::Signature<'a>,
    /// The name of the message encoding, otherwise [UTF-8 should be assumed](https://github.com/git/git/blob/e67fbf927dfdf13d0b21dc6ea15dc3c7ef448ea0/commit.c#L1493:L1493).
    pub encoding: Option<&'a BStr>,
    /// The commit message documenting the change.
    pub message: &'a BStr,
    /// Extra header fields, in order of them being encountered, made accessible with the iterator returned by [`extra_headers()`][Commit::extra_headers()].
    pub extra_headers: Vec<(&'a BStr, Cow<'a, BStr>)>,
}

impl<'a> Commit<'a> {
    /// Deserialize a commit from the given `data` bytes while avoiding most allocations.
    pub fn from_bytes(data: &'a [u8]) -> Result<Commit<'a>, object::decode::Error> {
        decode::commit(data)
            .map(|(_, t)| t)
            .map_err(object::decode::Error::from)
    }
    /// Return the `tree` fields hash digest.
    pub fn tree(&self) -> git_hash::ObjectId {
        git_hash::ObjectId::from_hex(self.tree).expect("prior validation of tree hash during parsing")
    }

    /// Returns an iterator of parent object ids
    pub fn parents(&self) -> impl Iterator<Item = git_hash::ObjectId> + '_ {
        self.parents
            .iter()
            .map(|hex_hash| git_hash::ObjectId::from_hex(hex_hash).expect("prior validation of hashes during parsing"))
    }

    /// Returns a convenient iterator over all extra headers.
    pub fn extra_headers(&self) -> crate::commit::ExtraHeaders<impl Iterator<Item = (&BStr, &BStr)>> {
        crate::commit::ExtraHeaders::new(self.extra_headers.iter().map(|(k, v)| (*k, v.as_ref())))
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn size_of_commit() {
        assert_eq!(
            std::mem::size_of::<Commit<'_>>(),
            216,
            "the size of an immutable commit shouldn't change unnoticed"
        );
    }
}