use crate::errors::{HgError, HgResultExt};

use bytes_cast::{unaligned, BytesCast};

use memmap2::Mmap;

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

use crate::vfs::Vfs;

const ONDISK_VERSION: u8 = 1;

pub(super) struct NodeMapDocket {

    pub data_length: usize,

    // TODO: keep here more of the data from `parse()` when we need it

}

#[derive(BytesCast)]

#[repr(C)]

struct DocketHeader {

    uid_size: u8,

    _tip_rev: unaligned::U64Be,

    data_length: unaligned::U64Be,

    _data_unused: unaligned::U64Be,

    tip_node_size: unaligned::U64Be,

}

impl NodeMapDocket {

    /// Return `Ok(None)` when the caller should proceed without a persistent

    /// nodemap:

    ///

    /// * This revlog does not have a `.n` docket file (it is not generated for

    ///   small revlogs), or

    /// * The docket has an unsupported version number (repositories created by

    ///   later hg, maybe that should be a requirement instead?), or

    /// * The docket file points to a missing (likely deleted) data file (this

    ///   can happen in a rare race condition).

    pub fn read_from_file(

        store_vfs: &Vfs,

        index_path: &Path,

    ) -> Result<Option<(Self, Mmap)>, HgError> {

        let docket_path = index_path.with_extension("n");

        let docket_bytes = if let Some(bytes) =

            store_vfs.read(&docket_path).io_not_found_as_none()?

        {

            bytes

        } else {

            return Ok(None);

        };

        let input = if let Some((&ONDISK_VERSION, rest)) =

            docket_bytes.split_first()

        {

            rest

        } else {

            return Ok(None);

        };

        /// Treat any error as a parse error

        fn parse<T, E>(result: Result<T, E>) -> Result<T, HgError> {

            result

                .map_err(|_| HgError::corrupted("nodemap docket parse error"))

        }

        let (header, rest) = parse(DocketHeader::from_bytes(input))?;

        let uid_size = header.uid_size as usize;

        // TODO: do we care about overflow for 4 GB+ nodemap files on 32-bit

        // systems?

        let tip_node_size = header.tip_node_size.get() as usize;

        let data_length = header.data_length.get() as usize;

        let (uid, rest) = parse(u8::slice_from_bytes(rest, uid_size))?;

        let (_tip_node, _rest) =

            parse(u8::slice_from_bytes(rest, tip_node_size))?;

        let uid = parse(std::str::from_utf8(uid))?;

        let docket = NodeMapDocket { data_length };

        let data_path = rawdata_path(&docket_path, uid);

        // TODO: use `vfs.read()` here when the `persistent-nodemap.mmap`

        // config is false?

        if let Some(mmap) =

            store_vfs.mmap_open(data_path).io_not_found_as_none()?

        {

            if mmap.len() >= data_length {

                Ok(Some((docket, mmap)))

            } else {

                Err(HgError::corrupted("persistent nodemap too short"))

            }

        } else {

            // Even if .hg/requires opted in, some revlogs are deemed small

            // enough to not need a persistent nodemap.

            Ok(None)

        }

    }

}

fn rawdata_path(docket_path: &Path, uid: &str) -> PathBuf {

    let docket_name = docket_path

        .file_name()

        .expect("expected a base name")

        .to_str()

        .expect("expected an ASCII file name in the store");

    let prefix = docket_name

        .strip_suffix(".n.a")

        .or_else(|| docket_name.strip_suffix(".n"))

        .expect("expected docket path in .n or .n.a");

    let name = format!("{}-{}.nd", prefix, uid);

    docket_path

        .parent()

        .expect("expected a non-root path")

        .join(name)

}