// Copyright 2020 The Jujutsu Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#![expect(missing_docs)]

use std::io;
use std::io::Write;
use std::iter::zip;
use std::pin::Pin;

use bstr::BStr;
use bstr::BString;
use bstr::ByteSlice as _;
use bstr::ByteVec as _;
use futures::Stream;
use futures::StreamExt as _;
use futures::future::try_join_all;
use futures::stream::BoxStream;
use futures::try_join;
use itertools::Itertools as _;
use tokio::io::AsyncRead;
use tokio::io::AsyncReadExt as _;

use crate::backend::BackendError;
use crate::backend::BackendResult;
use crate::backend::CommitId;
use crate::backend::CopyId;
use crate::backend::FileId;
use crate::backend::SymlinkId;
use crate::backend::TreeId;
use crate::backend::TreeValue;
use crate::conflict_labels::ConflictLabels;
use crate::copies::CopiesTreeDiffEntry;
use crate::copies::CopiesTreeDiffEntryPath;
use crate::diff::ContentDiff;
use crate::diff::DiffHunk;
use crate::diff::DiffHunkKind;
use crate::files;
use crate::files::MergeResult;
use crate::merge::Diff;
use crate::merge::Merge;
use crate::merge::MergedTreeValue;
use crate::merge::SameChange;
use crate::repo_path::RepoPath;
use crate::store::Store;
use crate::tree_merge::MergeOptions;

/// Minimum length of conflict markers.
pub const MIN_CONFLICT_MARKER_LEN: usize = 7;

/// If a file already contains lines which look like conflict markers of length
/// N, then the conflict markers we add will be of length (N + increment). This
/// number is chosen to make the conflict markers noticeably longer than the
/// existing markers.
const CONFLICT_MARKER_LEN_INCREMENT: usize = 4;

/// Comment for missing terminating newline in a term of a conflict.
const NO_ENDING_EOL_COMMENT: &str = "(no terminating newline)";

fn write_diff_hunks(hunks: &[DiffHunk], file: &mut dyn Write) -> io::Result<()> {
    for hunk in hunks {
        match hunk.kind {
            DiffHunkKind::Matching => {
                debug_assert!(hunk.contents.iter().all_equal());
                for line in hunk.contents[0].lines_with_terminator() {
                    file.write_all(b" ")?;
                    file.write_all(line)?;
                }
            }
            DiffHunkKind::Different => {
                for line in hunk.contents[0].lines_with_terminator() {
                    file.write_all(b"-")?;
                    file.write_all(line)?;
                }
                for line in hunk.contents[1].lines_with_terminator() {
                    file.write_all(b"+")?;
                    file.write_all(line)?;
                }
            }
        }
    }
    Ok(())
}

async fn get_file_contents(
    store: &Store,
    path: &RepoPath,
    term: Option<&FileId>,
) -> BackendResult<BString> {
    match term {
        Some(id) => {
            let mut reader = store.read_file(path, id).await?;
            let mut content = vec![];
            reader
                .read_to_end(&mut content)
                .await
                .map_err(|err| BackendError::ReadFile {
                    path: path.to_owned(),
                    id: id.clone(),
                    source: err.into(),
                })?;
            Ok(BString::new(content))
        }
        // If the conflict had removed the file on one side, we pretend that the file
        // was empty there.
        None => Ok(BString::new(vec![])),
    }
}

pub async fn extract_as_single_hunk(
    merge: &Merge<Option<FileId>>,
    store: &Store,
    path: &RepoPath,
) -> BackendResult<Merge<BString>> {
    merge
        .try_map_async(|term| get_file_contents(store, path, term.as_ref()))
        .await
}

/// A type similar to `MergedTreeValue` but with associated data to include in
/// e.g. the working copy or in a diff.
pub enum MaterializedTreeValue {
    Absent,
    AccessDenied(Box<dyn std::error::Error + Send + Sync>),
    File(MaterializedFileValue),
    Symlink {
        id: SymlinkId,
        target: String,
    },
    FileConflict(MaterializedFileConflictValue),
    OtherConflict {
        id: MergedTreeValue,
        labels: ConflictLabels,
    },
    GitSubmodule(CommitId),
    Tree(TreeId),
}

impl MaterializedTreeValue {
    pub fn is_absent(&self) -> bool {
        matches!(self, Self::Absent)
    }

    pub fn is_present(&self) -> bool {
        !self.is_absent()
    }
}

/// [`TreeValue::File`] with file content `reader`.
pub struct MaterializedFileValue {
    pub id: FileId,
    pub executable: bool,
    pub copy_id: CopyId,
    pub reader: Pin<Box<dyn AsyncRead + Send>>,
}

impl MaterializedFileValue {
    /// Reads file content until EOF. The provided `path` is used only for error
    /// reporting purpose.
    pub async fn read_all(&mut self, path: &RepoPath) -> BackendResult<Vec<u8>> {
        let mut buf = Vec::new();
        self.reader
            .read_to_end(&mut buf)
            .await
            .map_err(|err| BackendError::ReadFile {
                path: path.to_owned(),
                id: self.id.clone(),
                source: err.into(),
            })?;
        Ok(buf)
    }
}

/// Conflicted [`TreeValue::File`]s with file contents.
pub struct MaterializedFileConflictValue {
    /// File ids which preserve the shape of the tree conflict, to be used with
    /// [`Merge::update_from_simplified()`].
    pub unsimplified_ids: Merge<Option<FileId>>,
    /// Simplified file ids, in which redundant id pairs are dropped.
    pub ids: Merge<Option<FileId>>,
    /// Simplified conflict labels, matching `ids`.
    pub labels: ConflictLabels,
    /// File contents corresponding to the simplified `ids`.
    // TODO: or Vec<(FileId, Box<dyn Read>)> so that caller can stop reading
    // when null bytes found?
    pub contents: Merge<BString>,
    /// Merged executable bit. `None` if there are changes in both executable
    /// bit and file absence.
    pub executable: Option<bool>,
    /// Merged copy id. `None` if no single value could be determined.
    pub copy_id: Option<CopyId>,
}

/// Reads the data associated with a `MergedTreeValue` so it can be written to
/// e.g. the working copy or diff.
pub async fn materialize_tree_value(
    store: &Store,
    path: &RepoPath,
    value: MergedTreeValue,
    conflict_labels: &ConflictLabels,
) -> BackendResult<MaterializedTreeValue> {
    match materialize_tree_value_no_access_denied(store, path, value, conflict_labels).await {
        Err(BackendError::ReadAccessDenied { source, .. }) => {
            Ok(MaterializedTreeValue::AccessDenied(source))
        }
        result => result,
    }
}

async fn materialize_tree_value_no_access_denied(
    store: &Store,
    path: &RepoPath,
    value: MergedTreeValue,
    conflict_labels: &ConflictLabels,
) -> BackendResult<MaterializedTreeValue> {
    match value.into_resolved() {
        Ok(None) => Ok(MaterializedTreeValue::Absent),
        Ok(Some(TreeValue::File {
            id,
            executable,
            copy_id,
        })) => {
            let reader = store.read_file(path, &id).await?;
            Ok(MaterializedTreeValue::File(MaterializedFileValue {
                id,
                executable,
                copy_id,
                reader,
            }))
        }
        Ok(Some(TreeValue::Symlink(id))) => {
            let target = store.read_symlink(path, &id).await?;
            Ok(MaterializedTreeValue::Symlink { id, target })
        }
        Ok(Some(TreeValue::GitSubmodule(id))) => Ok(MaterializedTreeValue::GitSubmodule(id)),
        Ok(Some(TreeValue::Tree(id))) => Ok(MaterializedTreeValue::Tree(id)),
        Err(conflict) => {
            match try_materialize_file_conflict_value(store, path, &conflict, conflict_labels)
                .await?
            {
                Some(file) => Ok(MaterializedTreeValue::FileConflict(file)),
                None => Ok(MaterializedTreeValue::OtherConflict {
                    id: conflict,
                    labels: conflict_labels.clone(),
                }),
            }
        }
    }
}

/// Suppose `conflict` contains only files or absent entries, reads the file
/// contents.
pub async fn try_materialize_file_conflict_value(
    store: &Store,
    path: &RepoPath,
    conflict: &MergedTreeValue,
    conflict_labels: &ConflictLabels,
) -> BackendResult<Option<MaterializedFileConflictValue>> {
    let (Some(unsimplified_ids), Some(executable_bits)) =
        (conflict.to_file_merge(), conflict.to_executable_merge())
    else {
        return Ok(None);
    };
    let (labels, ids) = conflict_labels.simplify_with(&unsimplified_ids);
    let contents = extract_as_single_hunk(&ids, store, path).await?;
    let executable = resolve_file_executable(&executable_bits);
    Ok(Some(MaterializedFileConflictValue {
        unsimplified_ids,
        ids,
        labels,
        contents,
        executable,
        copy_id: Some(CopyId::placeholder()),
    }))
}

/// Resolves conflicts in file executable bit, returns the original state if the
/// file is deleted and executable bit is unchanged.
pub fn resolve_file_executable(merge: &Merge<Option<bool>>) -> Option<bool> {
    let resolved = merge.resolve_trivial(SameChange::Accept).copied()?;
    if resolved.is_some() {
        resolved
    } else {
        // If the merge is resolved to None (absent), there should be the same
        // number of Some(true) and Some(false). Pick the old state if
        // unambiguous, so the new file inherits the original executable bit.
        merge.removes().flatten().copied().all_equal_value().ok()
    }
}

/// Describes what style should be used when materializing conflicts.
#[derive(Clone, Copy, PartialEq, Eq, Debug, serde::Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum ConflictMarkerStyle {
    /// Style which shows a snapshot and a series of diffs to apply.
    Diff,
    /// Similar to "diff", but always picks the first side as the snapshot. May
    /// become the default in a future version.
    DiffExperimental,
    /// Style which shows a snapshot for each base and side.
    Snapshot,
    /// Style which replicates Git's "diff3" style to support external tools.
    Git,
}

impl ConflictMarkerStyle {
    /// Returns true if this style allows `%%%%%%%` conflict markers.
    pub fn allows_diff(&self) -> bool {
        matches!(self, Self::Diff | Self::DiffExperimental)
    }
}

/// Options for conflict materialization.
#[derive(Clone, Debug)]
pub struct ConflictMaterializeOptions {
    pub marker_style: ConflictMarkerStyle,
    pub marker_len: Option<usize>,
    pub merge: MergeOptions,
}

/// Characters which can be repeated to form a conflict marker line when
/// materializing and parsing conflicts.
#[derive(Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
enum ConflictMarkerLineChar {
    ConflictStart = b'<',
    ConflictEnd = b'>',
    Add = b'+',
    Remove = b'-',
    Diff = b'%',
    Note = b'\\',
    GitAncestor = b'|',
    GitSeparator = b'=',
}

impl ConflictMarkerLineChar {
    /// Get the ASCII byte used for this conflict marker.
    fn to_byte(self) -> u8 {
        self as u8
    }

    /// Parse a byte to see if it corresponds with any kind of conflict marker.
    fn parse_byte(byte: u8) -> Option<Self> {
        match byte {
            b'<' => Some(Self::ConflictStart),
            b'>' => Some(Self::ConflictEnd),
            b'+' => Some(Self::Add),
            b'-' => Some(Self::Remove),
            b'%' => Some(Self::Diff),
            b'\\' => Some(Self::Note),
            b'|' => Some(Self::GitAncestor),
            b'=' => Some(Self::GitSeparator),
            _ => None,
        }
    }
}

/// Represents a conflict marker line parsed from the file. Conflict marker
/// lines consist of a single ASCII character repeated for a certain length.
struct ConflictMarkerLine {
    kind: ConflictMarkerLineChar,
    len: usize,
}

/// Write a conflict marker to an output file.
fn write_conflict_marker(
    output: &mut dyn Write,
    kind: ConflictMarkerLineChar,
    len: usize,
    suffix_text: &str,
) -> io::Result<()> {
    let conflict_marker = BString::new(vec![kind.to_byte(); len]);

    if suffix_text.is_empty() {
        write!(output, "{conflict_marker}")
    } else {
        write!(output, "{conflict_marker} {suffix_text}")
    }
}

/// Parse a conflict marker from a line of a file. The conflict marker may have
/// any length (even less than MIN_CONFLICT_MARKER_LEN).
fn parse_conflict_marker_any_len(line: &[u8]) -> Option<ConflictMarkerLine> {
    let first_byte = *line.first()?;
    let kind = ConflictMarkerLineChar::parse_byte(first_byte)?;
    let len = line.iter().take_while(|&&b| b == first_byte).count();

    if let Some(next_byte) = line.get(len) {
        // If there is a character after the marker, it must be ASCII whitespace
        if !next_byte.is_ascii_whitespace() {
            return None;
        }
    }

    Some(ConflictMarkerLine { kind, len })
}

/// Parse a conflict marker, expecting it to be at least a certain length. Any
/// shorter conflict markers are ignored.
fn parse_conflict_marker(line: &[u8], expected_len: usize) -> Option<ConflictMarkerLineChar> {
    parse_conflict_marker_any_len(line)
        .filter(|marker| marker.len >= expected_len)
        .map(|marker| marker.kind)
}

/// Given a Merge of files, choose the conflict marker length to use when
/// materializing conflicts.
pub fn choose_materialized_conflict_marker_len<T: AsRef<[u8]>>(single_hunk: &Merge<T>) -> usize {
    let max_existing_marker_len = single_hunk
        .iter()
        .flat_map(|file| file.as_ref().lines_with_terminator())
        .filter_map(parse_conflict_marker_any_len)
        .map(|marker| marker.len)
        .max()
        .unwrap_or_default();

    max_existing_marker_len
        .saturating_add(CONFLICT_MARKER_LEN_INCREMENT)
        .max(MIN_CONFLICT_MARKER_LEN)
}

fn detect_eol(single_hunk: &Merge<impl AsRef<[u8]>>) -> &'static BStr {
    let use_crlf = single_hunk
        .iter()
        .filter_map(|content| {
            let content = content.as_ref();
            let newline_index = content.find_byte(b'\n')?;
            Some(newline_index > 0 && content[newline_index - 1] == b'\r')
        })
        .all_equal_value()
        .unwrap_or(false);
    if use_crlf {
        b"\r\n".into()
    } else {
        b"\n".into()
    }
}

pub fn materialize_merge_result<T: AsRef<[u8]>>(
    single_hunk: &Merge<T>,
    labels: &ConflictLabels,
    output: &mut dyn Write,
    options: &ConflictMaterializeOptions,
) -> io::Result<()> {
    let merge_result = files::merge_hunks(single_hunk, &options.merge);
    match merge_result {
        MergeResult::Resolved(content) => output.write_all(&content),
        MergeResult::Conflict(hunks) => {
            let marker_len = options
                .marker_len
                .unwrap_or_else(|| choose_materialized_conflict_marker_len(single_hunk));
            materialize_conflict_hunks(
                hunks,
                options.marker_style,
                marker_len,
                labels,
                output,
                detect_eol(single_hunk),
            )
        }
    }
}

pub fn materialize_merge_result_to_bytes<T: AsRef<[u8]>>(
    single_hunk: &Merge<T>,
    labels: &ConflictLabels,
    options: &ConflictMaterializeOptions,
) -> BString {
    let merge_result = files::merge_hunks(single_hunk, &options.merge);
    match merge_result {
        MergeResult::Resolved(content) => content,
        MergeResult::Conflict(hunks) => {
            let marker_len = options
                .marker_len
                .unwrap_or_else(|| choose_materialized_conflict_marker_len(single_hunk));
            let mut output = Vec::new();
            materialize_conflict_hunks(
                hunks,
                options.marker_style,
                marker_len,
                labels,
                &mut output,
                detect_eol(single_hunk),
            )
            .expect("writing to an in-memory buffer should never fail");
            output.into()
        }
    }
}

fn materialize_conflict_hunks(
    // We may modify the conflict hunks when materialize the ending EOL conflict, so we take the
    // ownership.
    hunks: Vec<Merge<BString>>,
    conflict_marker_style: ConflictMarkerStyle,
    conflict_marker_len: usize,
    labels: &ConflictLabels,
    output: &mut dyn Write,
    eol: &BStr,
) -> io::Result<()> {
    let num_conflicts = hunks
        .iter()
        .filter(|hunk| hunk.as_resolved().is_none())
        .count();
    let mut conflict_index = 0;
    for hunk in hunks {
        if let Some(content) = hunk.as_resolved() {
            output.write_all(content)?;
        } else {
            conflict_index += 1;
            let conflict_info = format!("conflict {conflict_index} of {num_conflicts}");

            // If any side doesn't have the ending EOL, we remove the ending EOL from the
            // conflict end marker line and "spread" the ending EOL to every side as a
            // separator, so that contents without an ending EOL won't be concatenated with
            // the conflict markers.
            let all_sides_have_ending_eol = hunk
                .iter()
                .all(|content| content.last().is_none_or(|last| *last == b'\n'));
            let mut sides = build_hunk_sides(hunk, labels);
            if !all_sides_have_ending_eol {
                for side in &mut sides {
                    side.contents.push_str(eol);
                }
            }

            match (conflict_marker_style, sides.as_slice()) {
                // 2-sided conflicts can use Git-style conflict markers
                (ConflictMarkerStyle::Git, [left, base, right]) => {
                    materialize_git_style_conflict(
                        left,
                        base,
                        right,
                        conflict_marker_len,
                        output,
                        eol,
                    )?;
                }
                _ => {
                    materialize_jj_style_conflict(
                        sides,
                        &conflict_info,
                        conflict_marker_style,
                        conflict_marker_len,
                        output,
                        eol,
                    )?;
                }
            }

            if all_sides_have_ending_eol {
                output.write_all(eol)?;
            }
        }
    }
    Ok(())
}

#[derive(Debug)]
struct HunkTerm {
    contents: BString,
    label: String,
}

fn build_hunk_sides(hunk: Merge<BString>, labels: &ConflictLabels) -> Merge<HunkTerm> {
    let (removes, adds) = hunk.into_removes_adds();
    let num_bases = removes.len();
    let removes = removes.enumerate().map(|(base_index, contents)| {
        let label = labels
            .get_remove(base_index)
            .map(|label| label.to_owned())
            .unwrap_or_else(|| {
                // The vast majority of conflicts one actually tries to resolve manually have 1
                // base.
                if num_bases == 1 {
                    "base".to_string()
                } else {
                    format!("base #{}", base_index + 1)
                }
            });
        HunkTerm { contents, label }
    });
    let adds = adds.enumerate().map(|(add_index, contents)| {
        let label = labels.get_add(add_index).map_or_else(
            || format!("side #{}", add_index + 1),
            |label| label.to_owned(),
        );
        HunkTerm { contents, label }
    });
    let mut hunk_terms = Merge::from_removes_adds(removes, adds);
    for term in &mut hunk_terms {
        // We don't add the no eol comment if the side is empty.
        if term.contents.last().is_some_and(|ch| *ch != b'\n') {
            term.label.push(' ');
            term.label.push_str(NO_ENDING_EOL_COMMENT);
        }
    }
    hunk_terms
}

fn materialize_git_style_conflict(
    left: &HunkTerm,
    base: &HunkTerm,
    right: &HunkTerm,
    conflict_marker_len: usize,
    output: &mut dyn Write,
    eol: &BStr,
) -> io::Result<()> {
    write_conflict_marker(
        output,
        ConflictMarkerLineChar::ConflictStart,
        conflict_marker_len,
        &left.label,
    )?;
    output.write_all(eol)?;
    output.write_all(&left.contents)?;

    write_conflict_marker(
        output,
        ConflictMarkerLineChar::GitAncestor,
        conflict_marker_len,
        &base.label,
    )?;
    output.write_all(eol)?;
    output.write_all(&base.contents)?;

    // VS Code doesn't seem to support any trailing text on the separator line
    write_conflict_marker(
        output,
        ConflictMarkerLineChar::GitSeparator,
        conflict_marker_len,
        "",
    )?;
    output.write_all(eol)?;

    output.write_all(&right.contents)?;
    // The caller handles the ending EOL conflict and decides whether to append the
    // ending EOL to the end of the conflict hunk, so we don't write an extra new
    // line character after the conflict end marker.
    write_conflict_marker(
        output,
        ConflictMarkerLineChar::ConflictEnd,
        conflict_marker_len,
        &right.label,
    )?;

    Ok(())
}

fn materialize_jj_style_conflict(
    hunk: Merge<HunkTerm>,
    conflict_info: &str,
    conflict_marker_style: ConflictMarkerStyle,
    conflict_marker_len: usize,
    output: &mut dyn Write,
    eol: &BStr,
) -> io::Result<()> {
    // Write a positive snapshot (side) of a conflict
    let write_side = |side: &HunkTerm, output: &mut dyn Write| {
        write_conflict_marker(
            output,
            ConflictMarkerLineChar::Add,
            conflict_marker_len,
            &side.label,
        )?;
        output.write_all(eol)?;
        output.write_all(&side.contents)
    };

    // Write a negative snapshot (base) of a conflict
    let write_base = |side: &HunkTerm, output: &mut dyn Write| {
        write_conflict_marker(
            output,
            ConflictMarkerLineChar::Remove,
            conflict_marker_len,
            &side.label,
        )?;
        output.write_all(eol)?;
        output.write_all(&side.contents)
    };

    // Write a diff from a negative term to a positive term
    let write_diff =
        |base: &HunkTerm, add: &HunkTerm, diff: &[DiffHunk], output: &mut dyn Write| {
            write_conflict_marker(
                output,
                ConflictMarkerLineChar::Diff,
                conflict_marker_len,
                &format!("diff from: {}", base.label),
            )?;
            output.write_all(eol)?;
            write_conflict_marker(
                output,
                ConflictMarkerLineChar::Note,
                conflict_marker_len,
                &format!("       to: {}", add.label),
            )?;
            output.write_all(eol)?;
            write_diff_hunks(diff, output)
        };

    write_conflict_marker(
        output,
        ConflictMarkerLineChar::ConflictStart,
        conflict_marker_len,
        conflict_info,
    )?;
    output.write_all(eol)?;
    let mut snapshot_written = false;
    // The only conflict marker style which can start with a diff is "diff".
    if conflict_marker_style != ConflictMarkerStyle::Diff {
        write_side(hunk.first(), output)?;
        snapshot_written = true;
    }
    for (base_index, left) in hunk.removes().enumerate() {
        let add_index = if snapshot_written {
            base_index + 1
        } else {
            base_index
        };

        let right1 = hunk.get_add(add_index).unwrap();

        // Write the base and side separately if the conflict marker style doesn't
        // support diffs.
        if !conflict_marker_style.allows_diff() {
            write_base(left, output)?;
            write_side(right1, output)?;
            continue;
        }

        let diff1 = ContentDiff::by_line([&left.contents, &right1.contents])
            .hunks()
            .collect_vec();
        // If we haven't written a snapshot yet, then we need to decide whether to
        // format the current side as a snapshot or a diff. We write the current side as
        // a diff unless the next side has a smaller diff compared to the current base.
        if !snapshot_written {
            let right2 = hunk.get_add(add_index + 1).unwrap();
            let diff2 = ContentDiff::by_line([&left.contents, &right2.contents])
                .hunks()
                .collect_vec();
            if diff_size(&diff2) < diff_size(&diff1) {
                // If the next positive term is a better match, emit the current positive term
                // as a snapshot and the next positive term as a diff.
                write_side(right1, output)?;
                write_diff(left, right2, &diff2, output)?;
                snapshot_written = true;
                continue;
            }
        }

        write_diff(left, right1, &diff1, output)?;
    }

    // If we still didn't emit a snapshot, the last side is the snapshot.
    if !snapshot_written {
        write_side(hunk.get_add(hunk.num_sides() - 1).unwrap(), output)?;
    }
    write_conflict_marker(
        output,
        ConflictMarkerLineChar::ConflictEnd,
        conflict_marker_len,
        &format!("{conflict_info} ends"),
    )?;
    Ok(())
}

fn diff_size(hunks: &[DiffHunk]) -> usize {
    hunks
        .iter()
        .map(|hunk| match hunk.kind {
            DiffHunkKind::Matching => 0,
            DiffHunkKind::Different => hunk.contents.iter().map(|content| content.len()).sum(),
        })
        .sum()
}

pub struct MaterializedTreeDiffEntry {
    pub path: CopiesTreeDiffEntryPath,
    pub values: BackendResult<Diff<MaterializedTreeValue>>,
}

pub fn materialized_diff_stream(
    store: &Store,
    tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    conflict_labels: Diff<&ConflictLabels>,
) -> impl Stream<Item = MaterializedTreeDiffEntry> {
    tree_diff
        .map(async |CopiesTreeDiffEntry { path, values }| match values {
            Err(err) => MaterializedTreeDiffEntry {
                path,
                values: Err(err),
            },
            Ok(values) => {
                let before_future = materialize_tree_value(
                    store,
                    path.source(),
                    values.before,
                    conflict_labels.before,
                );
                let after_future = materialize_tree_value(
                    store,
                    path.target(),
                    values.after,
                    conflict_labels.after,
                );
                let values = try_join!(before_future, after_future)
                    .map(|(before, after)| Diff { before, after });
                MaterializedTreeDiffEntry { path, values }
            }
        })
        .buffered((store.concurrency() / 2).max(1))
}

/// Parses conflict markers from a slice.
///
/// Returns `None` if there were no valid conflict markers. The caller
/// has to provide the expected number of merge sides (adds). Conflict
/// markers that are otherwise valid will be considered invalid if
/// they don't have the expected arity.
///
/// All conflict markers in the file must be at least as long as the expected
/// length. Any shorter conflict markers will be ignored.
// TODO: "parse" is not usually the opposite of "materialize", so maybe we
// should rename them to "serialize" and "deserialize"?
pub fn parse_conflict(
    input: &[u8],
    num_sides: usize,
    expected_marker_len: usize,
) -> Option<Vec<Merge<BString>>> {
    if input.is_empty() {
        return None;
    }
    let mut hunks = vec![];
    let mut pos = 0;
    let mut resolved_start = 0;
    let mut conflict_start = None;
    let mut conflict_start_len = 0;
    for line in input.lines_with_terminator() {
        match parse_conflict_marker(line, expected_marker_len) {
            Some(ConflictMarkerLineChar::ConflictStart) => {
                conflict_start = Some(pos);
                conflict_start_len = line.len();
            }
            Some(ConflictMarkerLineChar::ConflictEnd) => {
                if let Some(conflict_start_index) = conflict_start.take() {
                    let conflict_body = &input[conflict_start_index + conflict_start_len..pos];
                    let mut hunk = parse_conflict_hunk(conflict_body, expected_marker_len);
                    if hunk.num_sides() == num_sides {
                        let resolved_slice = &input[resolved_start..conflict_start_index];
                        if !resolved_slice.is_empty() {
                            hunks.push(Merge::resolved(BString::from(resolved_slice)));
                        }
                        if !line.ends_with(b"\n") {
                            // If the conflict end marker doesn't end with an EOL, the last EOL on
                            // every side performs only as a separator, and we need to do remove the
                            // last EOL to retrieve the original contents.
                            for term in &mut hunk {
                                if term.pop_if(|x| *x == b'\n').is_some() {
                                    term.pop_if(|x| *x == b'\r');
                                }
                            }
                        }
                        hunks.push(hunk);
                        resolved_start = pos + line.len();
                    }
                }
            }
            _ => {}
        }
        pos += line.len();
    }

    if hunks.is_empty() {
        None
    } else {
        if resolved_start < input.len() {
            hunks.push(Merge::resolved(BString::from(&input[resolved_start..])));
        }
        Some(hunks)
    }
}

/// This method handles parsing both JJ-style and Git-style conflict markers,
/// meaning that switching conflict marker styles won't prevent existing files
/// with other conflict marker styles from being parsed successfully. The
/// conflict marker style to use for parsing is determined based on the first
/// line of the hunk.
fn parse_conflict_hunk(input: &[u8], expected_marker_len: usize) -> Merge<BString> {
    // If the hunk starts with a conflict marker, find its first character
    let initial_conflict_marker = input
        .lines_with_terminator()
        .next()
        .and_then(|line| parse_conflict_marker(line, expected_marker_len));

    match initial_conflict_marker {
        // JJ-style conflicts must start with one of these 3 conflict marker lines
        Some(
            ConflictMarkerLineChar::Diff
            | ConflictMarkerLineChar::Remove
            | ConflictMarkerLineChar::Add,
        ) => parse_jj_style_conflict_hunk(input, expected_marker_len),
        // Git-style conflicts either must not start with a conflict marker line, or must start with
        // the "|||||||" conflict marker line (if the first side was empty)
        None | Some(ConflictMarkerLineChar::GitAncestor) => {
            parse_git_style_conflict_hunk(input, expected_marker_len)
        }
        // No other conflict markers are allowed at the start of a hunk
        Some(_) => Merge::resolved(BString::new(vec![])),
    }
}

fn parse_jj_style_conflict_hunk(input: &[u8], expected_marker_len: usize) -> Merge<BString> {
    enum State {
        Diff,
        Remove,
        Add,
        Unknown,
    }
    let mut state = State::Unknown;
    let mut removes = vec![];
    let mut adds = vec![];
    for line in input.lines_with_terminator() {
        match parse_conflict_marker(line, expected_marker_len) {
            Some(ConflictMarkerLineChar::Diff) => {
                state = State::Diff;
                removes.push(BString::new(vec![]));
                adds.push(BString::new(vec![]));
                continue;
            }
            Some(ConflictMarkerLineChar::Remove) => {
                state = State::Remove;
                removes.push(BString::new(vec![]));
                continue;
            }
            Some(ConflictMarkerLineChar::Add) => {
                state = State::Add;
                adds.push(BString::new(vec![]));
                continue;
            }
            Some(ConflictMarkerLineChar::Note) => {
                continue;
            }
            _ => {}
        }
        match state {
            State::Diff => {
                if let Some(rest) = line.strip_prefix(b"-") {
                    removes.last_mut().unwrap().extend_from_slice(rest);
                } else if let Some(rest) = line.strip_prefix(b"+") {
                    adds.last_mut().unwrap().extend_from_slice(rest);
                } else if let Some(rest) = line.strip_prefix(b" ") {
                    removes.last_mut().unwrap().extend_from_slice(rest);
                    adds.last_mut().unwrap().extend_from_slice(rest);
                } else if line == b"\n" || line == b"\r\n" {
                    // Some editors strip trailing whitespace, so " \n" might become "\n". It would
                    // be unfortunate if this prevented the conflict from being parsed, so we add
                    // the empty line to the "remove" and "add" as if there was a space in front
                    removes.last_mut().unwrap().extend_from_slice(line);
                    adds.last_mut().unwrap().extend_from_slice(line);
                } else {
                    // Doesn't look like a valid conflict
                    return Merge::resolved(BString::new(vec![]));
                }
            }
            State::Remove => {
                removes.last_mut().unwrap().extend_from_slice(line);
            }
            State::Add => {
                adds.last_mut().unwrap().extend_from_slice(line);
            }
            State::Unknown => {
                // Doesn't look like a valid conflict
                return Merge::resolved(BString::new(vec![]));
            }
        }
    }

    if adds.len() == removes.len() + 1 {
        Merge::from_removes_adds(removes, adds)
    } else {
        // Doesn't look like a valid conflict
        Merge::resolved(BString::new(vec![]))
    }
}

fn parse_git_style_conflict_hunk(input: &[u8], expected_marker_len: usize) -> Merge<BString> {
    #[derive(PartialEq, Eq)]
    enum State {
        Left,
        Base,
        Right,
    }
    let mut state = State::Left;
    let mut left = BString::new(vec![]);
    let mut base = BString::new(vec![]);
    let mut right = BString::new(vec![]);
    for line in input.lines_with_terminator() {
        match parse_conflict_marker(line, expected_marker_len) {
            Some(ConflictMarkerLineChar::GitAncestor) => {
                if state == State::Left {
                    state = State::Base;
                    continue;
                } else {
                    // Base must come after left
                    return Merge::resolved(BString::new(vec![]));
                }
            }
            Some(ConflictMarkerLineChar::GitSeparator) => {
                if state == State::Base {
                    state = State::Right;
                    continue;
                } else {
                    // Right must come after base
                    return Merge::resolved(BString::new(vec![]));
                }
            }
            _ => {}
        }
        match state {
            State::Left => left.extend_from_slice(line),
            State::Base => base.extend_from_slice(line),
            State::Right => right.extend_from_slice(line),
        }
    }

    if state == State::Right {
        Merge::from_vec(vec![left, base, right])
    } else {
        // Doesn't look like a valid conflict
        Merge::resolved(BString::new(vec![]))
    }
}

/// Parses conflict markers in `content` and returns an updated version of
/// `file_ids` with the new contents. If no (valid) conflict markers remain, a
/// single resolves `FileId` will be returned.
pub async fn update_from_content(
    file_ids: &Merge<Option<FileId>>,
    store: &Store,
    path: &RepoPath,
    content: &[u8],
    conflict_marker_len: usize,
) -> BackendResult<Merge<Option<FileId>>> {
    let simplified_file_ids = file_ids.simplify();

    let old_contents = extract_as_single_hunk(&simplified_file_ids, store, path).await?;
    let old_hunks = files::merge_hunks(&old_contents, store.merge_options());

    // Parse conflicts from the new content using the arity of the simplified
    // conflicts.
    let new_hunks = parse_conflict(
        content,
        simplified_file_ids.num_sides(),
        conflict_marker_len,
    );

    // Check if the new hunks are unchanged. This makes sure that unchanged file
    // conflicts aren't updated to partially-resolved contents.
    let unchanged = match (&old_hunks, &new_hunks) {
        (MergeResult::Resolved(old), None) => old == content,
        (MergeResult::Conflict(old), Some(new)) => old == new,
        (MergeResult::Resolved(_), Some(_)) | (MergeResult::Conflict(_), None) => false,
    };
    if unchanged {
        return Ok(file_ids.clone());
    }

    let Some(hunks) = new_hunks else {
        // Either there are no markers or they don't have the expected arity
        let file_id = store.write_file(path, &mut &content[..]).await?;
        return Ok(Merge::normal(file_id));
    };

    let mut contents = simplified_file_ids.map(|_| vec![]);
    for hunk in hunks {
        if let Some(slice) = hunk.as_resolved() {
            for content in &mut contents {
                content.extend_from_slice(slice);
            }
        } else {
            for (content, slice) in zip(&mut contents, hunk) {
                content.extend(Vec::from(slice));
            }
        }
    }

    // Now write the new files contents we found by parsing the file with conflict
    // markers.
    let new_file_ids: Vec<Option<FileId>> = try_join_all(zip(&contents, &simplified_file_ids).map(
        async |(content, file_id)| -> BackendResult<Option<FileId>> {
            if file_id.is_some() || !content.is_empty() {
                let file_id = store.write_file(path, &mut content.as_slice()).await?;
                Ok(Some(file_id))
            } else {
                // The missing side of a conflict is still represented by
                // the empty string we materialized it as
                Ok(None)
            }
        },
    ))
    .await?;

    // If the conflict was simplified, expand the conflict to the original
    // number of sides.
    let new_file_ids = if new_file_ids.len() != file_ids.iter().len() {
        file_ids
            .clone()
            .update_from_simplified(Merge::from_vec(new_file_ids))
    } else {
        Merge::from_vec(new_file_ids)
    };
    Ok(new_file_ids)
}

#[cfg(test)]
mod tests {
    #![expect(clippy::too_many_arguments)]

    use test_case::test_case;
    use test_case::test_matrix;

    use super::*;
    use crate::files::FileMergeHunkLevel;

    #[test]
    fn test_resolve_file_executable() {
        fn resolve<const N: usize>(values: [Option<bool>; N]) -> Option<bool> {
            resolve_file_executable(&Merge::from_vec(values.to_vec()))
        }

        // already resolved
        assert_eq!(resolve([None]), None);
        assert_eq!(resolve([Some(false)]), Some(false));
        assert_eq!(resolve([Some(true)]), Some(true));

        // trivially resolved
        assert_eq!(resolve([Some(true), Some(true), Some(true)]), Some(true));
        assert_eq!(resolve([Some(true), Some(false), Some(false)]), Some(true));
        assert_eq!(resolve([Some(false), Some(true), Some(false)]), Some(false));
        assert_eq!(resolve([None, None, Some(true)]), Some(true));

        // unresolvable
        assert_eq!(resolve([Some(false), Some(true), None]), None);

        // trivially resolved to absent, so pick the original state
        assert_eq!(resolve([Some(true), Some(true), None]), Some(true));
        assert_eq!(resolve([None, Some(false), Some(false)]), Some(false));
        assert_eq!(
            resolve([None, None, Some(true), Some(true), None]),
            Some(true)
        );

        // trivially resolved to absent, and the original state is ambiguous
        assert_eq!(
            resolve([Some(true), Some(true), None, Some(false), Some(false)]),
            None
        );
        assert_eq!(
            resolve([
                None,
                Some(true),
                Some(true),
                Some(false),
                Some(false),
                Some(false),
                Some(false),
            ]),
            None
        );
    }

    #[test_case(Merge::resolved("\n") => "\n"; "starts with LF")]
    #[test_case(Merge::resolved("a\r\n") => "\r\n"; "crlf")]
    #[test_case(Merge::resolved("a\n") => "\n"; "lf")]
    #[test_case(Merge::resolved("abc") => "\n"; "no eol")]
    #[test_case(Merge::from_vec(vec![
        "a",
        "a\n",
        "ab",
    ]) => "\n"; "only the second side has the LF eol")]
    #[test_case(Merge::from_vec(vec![
        "a\r\n",
        "ab",
        "a\n",
    ]) => "\n"; "both sides have different EOLs")]
    #[test_case(Merge::from_vec(vec![
        "a",
        "a\r\n",
        "ab",
    ]) => "\r\n"; "only the second side has the CRLF eol")]
    fn test_detect_eol(single_hunk: Merge<impl AsRef<[u8]>>) -> &'static str {
        detect_eol(&single_hunk).to_str().unwrap()
    }

    #[test]
    fn test_detect_eol_consistency() {
        let crlf_side = "crlf\r\n";
        let lf_side = "lf\n";
        let merges = [
            Merge::from_vec(vec![crlf_side, "base", lf_side]),
            Merge::from_vec(vec![lf_side, "base", crlf_side]),
        ];

        assert_eq!(detect_eol(&merges[0]), detect_eol(&merges[1]));
    }

    #[test_case(indoc::indoc!{b"
        <<<<<<< conflict 1 of 1
        %%%%%%% diff from base to side #1
        -aa
        +cc
        +++++++ side #2
        bb
        >>>>>>> conflict 1 of 1 ends
    "}, Merge::from_vec(vec![
        "cc\n",
        "aa\n",
        "bb\n",
    ]); "all sides end with EOL")]
    #[test_case(indoc::indoc!{b"
        <<<<<<< conflict 1 of 1
        %%%%%%% diff from base to side #1
        -aa
        +cc
        +++++++ side #2
        bb
        >>>>>>> conflict 1 of 1 ends"
    }, Merge::from_vec(vec![
        "cc",
        "aa",
        "bb",
    ]); "all sides end without EOL")]
    #[test_case(indoc::indoc!{b"
        <<<<<<< conflict 1 of 1
        %%%%%%% diff from base to side #1
        -aa
        +cc

        +++++++ side #2
        bb
        >>>>>>> conflict 1 of 1 ends"
    }, Merge::from_vec(vec![
        "cc\n",
        "aa\n",
        "bb",
    ]); "side 2 removes the ending EOL")]
    #[test_case(indoc::indoc!{b"
        <<<<<<< conflict 1 of 1
        %%%%%%% diff from base to side #1
        -aa
        +cc
        +++++++ side #2
        bb

        >>>>>>> conflict 1 of 1 ends"
    }, Merge::from_vec(vec![
        "cc",
        "aa",
        "bb\n",
    ]); "side 2 adds the ending EOL")]
    #[test_case(indoc::indoc!{b"
        <<<<<<< conflict 1 of 1
        %%%%%%% diff from base to side #1
        -aa
        -
        +cc
        +++++++ side #2
        bb
        
        >>>>>>> conflict 1 of 1 ends"
    }, Merge::from_vec(vec![
        "cc",
        "aa\n",
        "bb\n",
    ]); "side 1 removes the ending EOL")]
    #[test_case(indoc::indoc!{b"
        <<<<<<< conflict 1 of 1
        %%%%%%% diff from base to side #1
        -aa
        +cc
        +
        +++++++ side #2
        bb
        >>>>>>> conflict 1 of 1 ends"
    }, Merge::from_vec(vec![
        "cc\n",
        "aa",
        "bb",
    ]); "side 1 adds the ending EOL")]
    fn test_parse_conflict(contents: &[u8], expected_merge: Merge<&str>) {
        let actual_result = parse_conflict(contents, 2, 7).unwrap()[0]
            .clone()
            .map(|content| content.to_str().unwrap().to_owned());
        let expected_merge = expected_merge.map(|content| content.to_string());
        assert_eq!(actual_result, expected_merge);

        // Change the EOL to CRLF and test again.
        let actual_result = parse_conflict(&contents.replace(b"\n", b"\r\n"), 2, 7).unwrap()[0]
            .clone()
            .map(|content| content.to_str().unwrap().to_owned());
        let expected_merge = expected_merge.map(|content| content.replace('\n', "\r\n"));
        assert_eq!(actual_result, expected_merge);
    }

    const BASE: &str = "aa";
    const SIDE1: &str = "bb";
    const SIDE2: &str = "cc";
    const WITH_ENDING_EOL: &str = "\n";
    const WITHOUT_ENDING_EOL: &str = "";
    const GIT_STYLE: ConflictMarkerStyle = ConflictMarkerStyle::Git;
    const DIFF_STYLE: ConflictMarkerStyle = ConflictMarkerStyle::Diff;
    const DIFF_EXPERIMENTAL_STYLE: ConflictMarkerStyle = ConflictMarkerStyle::DiffExperimental;
    const SNAPSHOT_STYLE: ConflictMarkerStyle = ConflictMarkerStyle::Snapshot;
    const LF_EOL: &str = "\n";
    const CRLF_EOL: &str = "\r\n";
    fn long(original: &str) -> String {
        std::iter::repeat_n(original, 3).collect_vec().join("\n")
    }
    fn prepended(original: &str) -> String {
        format!("{original}\n{BASE}")
    }
    #[test_matrix(
        BASE,
        [WITH_ENDING_EOL, WITHOUT_ENDING_EOL],
        [SIDE1, &long(SIDE1), &prepended(SIDE1)],
        [WITH_ENDING_EOL, WITHOUT_ENDING_EOL],
        [SIDE2, &long(SIDE2), &prepended(SIDE2)],
        [WITH_ENDING_EOL, WITHOUT_ENDING_EOL],
        [GIT_STYLE, DIFF_STYLE, DIFF_EXPERIMENTAL_STYLE, SNAPSHOT_STYLE],
        [LF_EOL, CRLF_EOL]
    )]
    fn test_materialize_conflict(
        base: &str,
        base_ending_eol: &str,
        side1: &str,
        side1_ending_eol: &str,
        side2: &str,
        side2_ending_eol: &str,
        style: ConflictMarkerStyle,
        eol: &str,
    ) {
        // Add a leading EOL to suggest the correct EOL to use for materialization.
        let base = format!("\n{base}{base_ending_eol}").replace('\n', eol);
        let side1 = format!("\n{side1}{side1_ending_eol}").replace('\n', eol);
        let side2 = format!("\n{side2}{side2_ending_eol}").replace('\n', eol);
        let merge = Merge::from_vec(vec![side2.as_str(), base.as_str(), side1.as_str()]);
        let options = ConflictMaterializeOptions {
            marker_style: style,
            marker_len: None,
            merge: MergeOptions {
                hunk_level: FileMergeHunkLevel::Line,
                same_change: SameChange::Accept,
            },
        };
        let actual_contents = String::from_utf8(
            materialize_merge_result_to_bytes(&merge, &ConflictLabels::unlabeled(), &options)
                .into(),
        )
        .unwrap();
        // We expect the materialized conflict to keep the original EOL, LF or CRLF.
        for line in actual_contents.as_bytes().lines_with_terminator() {
            let line = line.as_bstr();
            if !line.ends_with(b"\n") {
                continue;
            }
            let should_end_with_crlf = eol == "\r\n";
            assert!(
                line.ends_with(b"\r\n") == should_end_with_crlf,
                "Expect all the lines with EOL to end with {eol:?}, but got {line:?} from\n{}",
                actual_contents
                    // Replace \r to ␍ and \n to ␊ for clarity in the panic message.
                    .replace('\r', "\u{240D}")
                    .replace('\n', "\u{240A}\n")
            );
        }
        let hunks = parse_conflict(actual_contents.as_bytes(), 2, 7).unwrap();
        assert!(hunks.len() >= 2);
        // The first hunk is the empty line.
        let leading_eol = hunks[0].as_resolved().unwrap();
        let mut actual_merge = hunks[1].clone();
        for content in &mut actual_merge {
            content.insert_str(0, leading_eol);
        }
        // When both sides prepend contents, we end up with 3 hunks.
        if hunks.len() == 3 {
            let new_content = hunks[2].as_resolved().unwrap();
            for content in &mut actual_merge {
                content.extend_from_slice(new_content);
            }
        }
        assert!(hunks.len() <= 3);
        let actual_merge = actual_merge.map(|content| content.to_str().unwrap().to_owned());
        let merge = merge.map(|content| content.to_string());
        assert_eq!(actual_merge, merge);
    }
}
