use std::path::{Path, PathBuf};

use gix_error::{ErrorExt, Exn, Message, ResultExt, message};

use crate::{
    File,
    file::{
        BASE_GRAPHS_LIST_CHUNK_ID, COMMIT_DATA_CHUNK_ID, COMMIT_DATA_ENTRY_SIZE_SANS_HASH,
        EXTENDED_EDGES_LIST_CHUNK_ID, FAN_LEN, HEADER_LEN, OID_FAN_CHUNK_ID, OID_LOOKUP_CHUNK_ID, SIGNATURE,
    },
};

const MIN_FILE_SIZE: usize = HEADER_LEN
    + gix_chunk::file::Index::size_for_entries(3 /*OIDF, OIDL, CDAT*/)
    + FAN_LEN * 4 /* FANOUT TABLE CHUNK OIDF */
    + gix_hash::Kind::shortest().len_in_bytes();

impl File {
    /// Try to parse the commit graph file at `path`.
    pub fn at(path: impl AsRef<Path>) -> Result<File, Exn<Message>> {
        Self::try_from(path.as_ref())
    }

    /// A lower-level constructor which constructs a new instance directly from the mapping in `data`,
    /// assuming that it originated from `path`.
    ///
    /// Note that `path` is only used for verification of the hash its basename contains, but otherwise
    /// is not of importance.
    pub fn new(data: memmap2::Mmap, path: PathBuf) -> Result<File, Exn<Message>> {
        let data_size = data.len();
        if data_size < MIN_FILE_SIZE {
            return Err(message("Commit-graph file too small even for an empty graph").raise());
        }

        let mut ofs = 0;
        if &data[ofs..ofs + SIGNATURE.len()] != SIGNATURE {
            return Err(message("Commit-graph file does not start with expected signature").raise());
        }
        ofs += SIGNATURE.len();

        match data[ofs] {
            1 => (),
            x => {
                return Err(message!("Unsupported commit-graph file version: {x}").raise());
            }
        }
        ofs += 1;

        let object_hash = gix_hash::Kind::try_from(data[ofs])
            .map_err(|v| message!("Commit-graph file uses unsupported hash version: {v}").raise())?;
        ofs += 1;

        let chunk_count = data[ofs];
        // Can assert chunk_count >= MIN_CHUNKS here, but later OIDF+OIDL+CDAT presence checks make
        // it redundant.
        ofs += 1;

        let base_graph_count = data[ofs];
        ofs += 1;

        let chunks = gix_chunk::file::Index::from_bytes(&data, ofs, u32::from(chunk_count))
            .or_raise(|| message!("Couldn't read commit-graph file with {chunk_count} chunks at offset {ofs}"))?;

        let base_graphs_list_offset = chunks
            .validated_usize_offset_by_id(BASE_GRAPHS_LIST_CHUNK_ID, |chunk_range| {
                let chunk_size = chunk_range.len();
                if chunk_size % object_hash.len_in_bytes() != 0 {
                    return Err(message!("Commit-graph chunk {BASE_GRAPHS_LIST_CHUNK_ID:?} has invalid size: {msg}",
                        msg = format!(
                            "chunk size {} is not a multiple of {}",
                            chunk_size,
                            object_hash.len_in_bytes()
                        ),
                    ).raise());
                }
                let chunk_base_graph_count: u32 = (chunk_size / object_hash.len_in_bytes())
                    .try_into()
                    .expect("base graph count to fit in 32-bits");
                if chunk_base_graph_count != u32::from(base_graph_count) {
                    return Err(message!("Commit-graph {BASE_GRAPHS_LIST_CHUNK_ID:?} chunk contains {chunk_base_graph_count} base graphs, but commit-graph file header claims {base_graph_count} base graphs").raise())
                }
                Ok(chunk_range.start)
            })
            .ok()
            .transpose()?;

        let (commit_data_offset, commit_data_count): (_, u32) = chunks
            .validated_usize_offset_by_id(COMMIT_DATA_CHUNK_ID, |chunk_range| {
                let chunk_size = chunk_range.len();

                let entry_size = object_hash.len_in_bytes() + COMMIT_DATA_ENTRY_SIZE_SANS_HASH;
                if chunk_size % entry_size != 0 {
                    return Err(message!("Commit-graph chunk {COMMIT_DATA_CHUNK_ID:?} has invalid size: chunk size {chunk_size} is not a multiple of {entry_size}").raise())
                }
                Ok((
                    chunk_range.start,
                    (chunk_size / entry_size)
                        .try_into()
                        .expect("number of commits in CDAT chunk to fit in 32 bits"),
                ))
            })??;

        let fan_offset = chunks
            .validated_usize_offset_by_id(OID_FAN_CHUNK_ID, |chunk_range| {
                let chunk_size = chunk_range.len();

                let expected_size = 4 * FAN_LEN;
                if chunk_size != expected_size {
                    return Err(message!("Commit-graph chunk {OID_FAN_CHUNK_ID:?} has invalid size: expected chunk length {expected_size}, got {chunk_size}").raise())
                }
                Ok(chunk_range.start)
            })?
            .or_raise(|| message("Error getting offset for OID fan chunk"))?;

        let (oid_lookup_offset, oid_lookup_count): (_, u32) = chunks
            .validated_usize_offset_by_id(OID_LOOKUP_CHUNK_ID, |chunk_range| {
                let chunk_size = chunk_range.len();

                if chunk_size % object_hash.len_in_bytes() != 0 {
                    return Err(message!("Commit-graph chunk {OID_LOOKUP_CHUNK_ID:?} has invalid size: chunk size {chunk_size} is not a multiple of {hash_len}", hash_len = object_hash.len_in_bytes()).raise())
                }
                Ok((
                    chunk_range.start,
                    (chunk_size / object_hash.len_in_bytes())
                        .try_into()
                        .expect("number of commits in OIDL chunk to fit in 32 bits"),
                ))
            })?
            .or_raise(|| message("Error getting offset for OID lookup chunk"))?;

        let extra_edges_list_range = chunks.usize_offset_by_id(EXTENDED_EDGES_LIST_CHUNK_ID).ok();

        let trailer = &data[chunks.highest_offset() as usize..];
        if trailer.len() != object_hash.len_in_bytes() {
            return Err(message!(
                "Expected commit-graph trailer to contain {} bytes, got {}",
                object_hash.len_in_bytes(),
                trailer.len()
            )
            .raise());
        }

        if base_graph_count > 0 && base_graphs_list_offset.is_none() {
            return Err(message!("Chunk named {BASE_GRAPHS_LIST_CHUNK_ID:?} was not found in chunk file index").into());
        }

        let (fan, _) = read_fan(&data[fan_offset..]);
        if oid_lookup_count != fan[255] {
            return Err(message!("Commit-graph {OID_FAN_CHUNK_ID:?} chunk contains {chunk1_commits} commits, but {OID_LOOKUP_CHUNK_ID:?} chunk contains {chunk2_commits} commits",
                chunk1_commits = fan[255],
                chunk2_commits = oid_lookup_count,
            ).raise());
        }
        if commit_data_count != fan[255] {
            return Err(
                message!("Commit-graph {OID_FAN_CHUNK_ID:?} chunk contains {chunk1_commits} commits, but {COMMIT_DATA_CHUNK_ID:?} chunk contains {chunk2_commits} commits",
                    chunk1_commits = fan[255],
                    chunk2_commits = commit_data_count,
                ).raise(),
            );
        }
        Ok(File {
            base_graph_count,
            base_graphs_list_offset,
            commit_data_offset,
            data,
            extra_edges_list_range,
            fan,
            oid_lookup_offset,
            path,
            hash_len: object_hash.len_in_bytes(),
            object_hash,
        })
    }
}

impl TryFrom<&Path> for File {
    type Error = Exn<Message>;

    fn try_from(path: &Path) -> Result<Self, Self::Error> {
        let data = std::fs::File::open(path)
            .and_then(|file| {
                // SAFETY: we have to take the risk of somebody changing the file underneath. Git never writes into the same file.
                #[expect(unsafe_code)]
                unsafe {
                    memmap2::MmapOptions::new().map_copy_read_only(&file)
                }
            })
            .or_raise(|| message!("Could not open commit-graph file at '{path}'", path = path.display()))?;
        Self::new(data, path.to_owned())
    }
}

// Copied from gix-odb/pack/index/init.rs
fn read_fan(d: &[u8]) -> ([u32; FAN_LEN], usize) {
    assert!(d.len() >= FAN_LEN * 4);

    let mut fan = [0; FAN_LEN];
    for (c, f) in d.chunks_exact(4).zip(fan.iter_mut()) {
        *f = u32::from_be_bytes(c.try_into().unwrap());
    }
    (fan, FAN_LEN * 4)
}