//! The "version 2" disk representation of the dirstate

//!

//! # File format

//!

//! The file starts with a fixed-sized header, whose layout is defined by the

//! `Header` struct. Its `root` field contains the slice (offset and length) to

//! the nodes representing the files and directories at the root of the

//! repository. Each node is also fixed-size, defined by the `Node` struct.

//! Nodes in turn contain slices to variable-size paths, and to their own child

//! nodes (if any) for nested files and directories.

use crate::dirstate::parsers::clear_ambiguous_mtime;

use crate::dirstate::parsers::Timestamp;

use crate::dirstate_tree::dirstate_map::{self, DirstateMap};

use crate::dirstate_tree::path_with_basename::WithBasename;

use crate::errors::HgError;

use crate::utils::hg_path::HgPath;

use crate::DirstateEntry;

use crate::DirstateError;

use crate::DirstateParents;

use bytes_cast::unaligned::{I32Be, U32Be, U64Be};

use bytes_cast::BytesCast;

use std::borrow::Cow;

use std::convert::{TryFrom, TryInto};

/// This a redundant sanity check more than an actual "magic number" since

/// `.hg/requires` already governs which format should be used.

#[derive(BytesCast)]

#[repr(C)]

struct Header {

    marker: [u8; V2_FORMAT_MARKER.len()],

    /// `dirstatemap.parents()` in `mercurial/dirstate.py` relies on this

    /// `parents` field being at this offset, immediately after `marker`.

    parents: DirstateParents,

    root: ChildNodes,

    nodes_with_entry_count: Size,

    nodes_with_copy_source_count: Size,

}

#[derive(BytesCast)]

#[repr(C)]

struct Node {

    full_path: PathSlice,

    /// In bytes from `self.full_path.start`

    base_name_start: Size,

    copy_source: OptPathSlice,

    entry: OptEntry,

    children: ChildNodes,

    tracked_descendants_count: Size,

}

/// Either nothing if `state == b'\0'`, or a dirstate entry like in the v1

/// format

#[derive(BytesCast)]

#[repr(C)]

struct OptEntry {

    state: u8,

    mode: I32Be,

    mtime: I32Be,

    size: I32Be,

}

/// Counted in bytes from the start of the file

///

/// NOTE: If we decide to never support `.hg/dirstate` files larger than 4 GiB

/// we could save space by using `U32Be` instead.

type Offset = U64Be;

/// Counted in number of items

///

/// NOTE: not supporting directories with more than 4 billion direct children,

/// or filenames more than 4 GiB.

type Size = U32Be;

/// Location of consecutive, fixed-size items.

///

/// An item can be a single byte for paths, or a struct with

/// `derive(BytesCast)`.

#[derive(BytesCast, Copy, Clone)]

#[repr(C)]

struct Slice {

    start: Offset,

    len: Size,

}

/// A contiguous sequence of `len` times `Node`, representing the child nodes

/// of either some other node or of the repository root.

///

/// Always sorted by ascending `full_path`, to allow binary search.

/// Since nodes with the same parent nodes also have the same parent path,

/// only the `base_name`s need to be compared during binary search.

type ChildNodes = Slice;

/// A `HgPath` of `len` bytes

type PathSlice = Slice;

/// Either nothing if `start == 0`, or a `HgPath` of `len` bytes

type OptPathSlice = Slice;

/// Make sure that size-affecting changes are made knowingly

fn _static_assert_size_of() {

    let _ = std::mem::transmute::<Header, [u8; 72]>;

    let _ = std::mem::transmute::<Node, [u8; 57]>;

}

pub(super) fn read<'on_disk>(

    on_disk: &'on_disk [u8],

) -> Result<(DirstateMap<'on_disk>, Option<DirstateParents>), DirstateError> {

    if on_disk.is_empty() {

        return Ok((DirstateMap::empty(on_disk), None));

    }

    let (header, _) = Header::from_bytes(on_disk)

        .map_err(|_| HgError::corrupted("truncated dirstate-v2"))?;

    let Header {

        marker,

        parents,

        root,

        nodes_with_entry_count,

        nodes_with_copy_source_count,

    } = header;

    if marker != V2_FORMAT_MARKER {

        return Err(HgError::corrupted("missing dirstated-v2 marker").into());

    }

    let dirstate_map = DirstateMap {

        on_disk,

        root: read_nodes(on_disk, *root)?,

        nodes_with_entry_count: nodes_with_entry_count.get(),

        nodes_with_copy_source_count: nodes_with_copy_source_count.get(),

    };

    let parents = Some(parents.clone());

    Ok((dirstate_map, parents))

}

impl Node {

    pub(super) fn path<'on_disk>(

        &self,

        on_disk: &'on_disk [u8],

    ) -> Result<dirstate_map::NodeKey<'on_disk>, HgError> {

        let full_path = read_hg_path(on_disk, self.full_path)?;

        let base_name_start = usize::try_from(self.base_name_start.get())

            // u32 -> usize, could only panic on a 16-bit CPU

            .expect("dirstate-v2 base_name_start out of bounds");

        if base_name_start < full_path.len() {

            Ok(WithBasename::from_raw_parts(full_path, base_name_start))

        } else {

            Err(HgError::corrupted(

                "dirstate-v2 base_name_start out of bounds",

            ))

        }

    }

    pub(super) fn copy_source<'on_disk>(

        &self,

        on_disk: &'on_disk [u8],

    ) -> Result<Option<Cow<'on_disk, HgPath>>, HgError> {

        Ok(if self.copy_source.start.get() != 0 {

            Some(read_hg_path(on_disk, self.copy_source)?)

        } else {

            None

        })

    }

    pub(super) fn entry(&self) -> Result<Option<DirstateEntry>, HgError> {

        Ok(if self.entry.state != b'\0' {

            Some(DirstateEntry {

                state: self.entry.state.try_into()?,

                mode: self.entry.mode.get(),

                mtime: self.entry.mtime.get(),

                size: self.entry.size.get(),

            })

        } else {

            None

        })

    }

    pub(super) fn to_in_memory_node<'on_disk>(

        &self,

        on_disk: &'on_disk [u8],

    ) -> Result<dirstate_map::Node<'on_disk>, HgError> {

        Ok(dirstate_map::Node {

            children: read_nodes(on_disk, self.children)?,

            copy_source: self.copy_source(on_disk)?,

            entry: self.entry()?,

            tracked_descendants_count: self.tracked_descendants_count.get(),

        })

    }

}

fn read_nodes(

    on_disk: &[u8],

    slice: ChildNodes,

) -> Result<dirstate_map::ChildNodes, HgError> {

    read_slice::<Node>(on_disk, slice)?

        .iter()

        .map(|node| {

            Ok((node.path(on_disk)?, node.to_in_memory_node(on_disk)?))

        })

        .collect()

}

fn read_hg_path(on_disk: &[u8], slice: Slice) -> Result<Cow<HgPath>, HgError> {

    let bytes = read_slice::<u8>(on_disk, slice)?;

    Ok(Cow::Borrowed(HgPath::new(bytes)))

}

fn read_slice<T>(on_disk: &[u8], slice: Slice) -> Result<&[T], HgError>

where

    T: BytesCast,

{

    // Either `usize::MAX` would result in "out of bounds" error since a single

    // `&[u8]` cannot occupy the entire addess space.

    let start = usize::try_from(slice.start.get()).unwrap_or(std::usize::MAX);

    let len = usize::try_from(slice.len.get()).unwrap_or(std::usize::MAX);

    on_disk

        .get(start..)

        .and_then(|bytes| T::slice_from_bytes(bytes, len).ok())

        .map(|(slice, _rest)| slice)

        .ok_or_else(|| {

            HgError::corrupted("dirstate v2 slice is out of bounds")

        })

}

pub(super) fn write(

    dirstate_map: &mut DirstateMap,

    parents: DirstateParents,

    now: Timestamp,

) -> Result<Vec<u8>, DirstateError> {

    // TODO: how do we want to handle this in 2038?

    let now: i32 = now.0.try_into().expect("time overflow");

    let header_len = std::mem::size_of::<Header>();

    // This ignores the space for paths, and for nodes without an entry.

    // TODO: better estimate? Skip the `Vec` and write to a file directly?

    let size_guess = header_len

        + std::mem::size_of::<Node>()

            * dirstate_map.nodes_with_entry_count as usize;

    let mut out = Vec::with_capacity(size_guess);

    // Keep space for the header. We’ll fill it out at the end when we know the

    // actual offset for the root nodes.

    out.resize(header_len, 0_u8);

    let root = write_nodes(&mut dirstate_map.root, now, &mut out)?;

    let header = Header {

        marker: *V2_FORMAT_MARKER,

        parents: parents,

        root,

        nodes_with_entry_count: dirstate_map.nodes_with_entry_count.into(),

        nodes_with_copy_source_count: dirstate_map

            .nodes_with_copy_source_count

            .into(),

    };

    out[..header_len].copy_from_slice(header.as_bytes());

    Ok(out)

}

/// Serialize the dirstate to the `v2` format after clearing ambigous `mtime`s.

fn write_nodes(

    nodes: &mut dirstate_map::ChildNodes,

    now: i32,

    out: &mut Vec<u8>,

) -> Result<ChildNodes, DirstateError> {

    // `dirstate_map::ChildNodes` is a `HashMap` with undefined iteration

    // order. Sort to enable binary search in the written file.

    let nodes = dirstate_map::Node::sorted(nodes);

    // First accumulate serialized nodes in a `Vec`

    let mut on_disk_nodes = Vec::with_capacity(nodes.len());

    for (full_path, node) in nodes {

        on_disk_nodes.push(Node {

            children: write_nodes(&mut node.children, now, out)?,

            tracked_descendants_count: node.tracked_descendants_count.into(),

            full_path: write_slice::<u8>(

                full_path.full_path().as_bytes(),

                out,

            ),

            base_name_start: u32::try_from(full_path.base_name_start())

                // Could only panic for paths over 4 GiB

                .expect("dirstate-v2 offset overflow")

                .into(),

            copy_source: if let Some(source) = &node.copy_source {

                write_slice::<u8>(source.as_bytes(), out)

            } else {

                Slice {

                    start: 0.into(),

                    len: 0.into(),

                }

            },

            entry: if let Some(entry) = &mut node.entry {

                clear_ambiguous_mtime(entry, now);

                OptEntry {

                    state: entry.state.into(),

                    mode: entry.mode.into(),

                    mtime: entry.mtime.into(),

                    size: entry.size.into(),

                }

            } else {

                OptEntry {

                    state: b'\0',

                    mode: 0.into(),

                    mtime: 0.into(),

                    size: 0.into(),

                }

            },

        })

    }

    // … so we can write them contiguously

    Ok(write_slice::<Node>(&on_disk_nodes, out))

}

fn write_slice<T>(slice: &[T], out: &mut Vec<u8>) -> Slice

where

    T: BytesCast,

{

    let start = u64::try_from(out.len())

        // Could only panic on a 128-bit CPU with a dirstate over 16 EiB

        .expect("dirstate-v2 offset overflow")

        .into();

    let len = u32::try_from(slice.len())

        // Could only panic for paths over 4 GiB or nodes with over 4 billions

        // child nodes

        .expect("dirstate-v2 offset overflow")

        .into();

    out.extend(slice.as_bytes());

    Slice { start, len }

}