jj_cli/

diff_util.rs

// Copyright 2020-2022 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.

use std::borrow::Borrow;
use std::borrow::Cow;
use std::cmp::max;
use std::io;
use std::iter;
use std::mem;
use std::ops::Range;
use std::path::Path;
use std::path::PathBuf;

use bstr::BStr;
use bstr::BString;
use clap_complete::ArgValueCandidates;
use futures::StreamExt as _;
use futures::TryStreamExt as _;
use futures::executor::block_on_stream;
use futures::stream::BoxStream;
use itertools::Itertools as _;
use jj_lib::backend::BackendError;
use jj_lib::backend::BackendResult;
use jj_lib::backend::CommitId;
use jj_lib::backend::CopyRecord;
use jj_lib::backend::TreeValue;
use jj_lib::commit::Commit;
use jj_lib::config::ConfigGetError;
use jj_lib::conflicts::ConflictMarkerStyle;
use jj_lib::conflicts::ConflictMaterializeOptions;
use jj_lib::conflicts::MaterializedFileValue;
use jj_lib::conflicts::MaterializedTreeDiffEntry;
use jj_lib::conflicts::MaterializedTreeValue;
use jj_lib::conflicts::materialize_merge_result_to_bytes;
use jj_lib::conflicts::materialized_diff_stream;
use jj_lib::copies::CopiesTreeDiffEntry;
use jj_lib::copies::CopiesTreeDiffEntryPath;
use jj_lib::copies::CopyOperation;
use jj_lib::copies::CopyRecords;
use jj_lib::diff::CompareBytesExactly;
use jj_lib::diff::CompareBytesIgnoreAllWhitespace;
use jj_lib::diff::CompareBytesIgnoreWhitespaceAmount;
use jj_lib::diff::Diff;
use jj_lib::diff::DiffHunk;
use jj_lib::diff::DiffHunkKind;
use jj_lib::diff::find_line_ranges;
use jj_lib::files;
use jj_lib::files::ConflictDiffHunk;
use jj_lib::files::DiffLineHunkSide;
use jj_lib::files::DiffLineIterator;
use jj_lib::files::DiffLineNumber;
use jj_lib::matchers::Matcher;
use jj_lib::merge::Merge;
use jj_lib::merge::MergeBuilder;
use jj_lib::merge::MergedTreeValue;
use jj_lib::merged_tree::MergedTree;
use jj_lib::object_id::ObjectId as _;
use jj_lib::repo::Repo;
use jj_lib::repo_path::InvalidRepoPathError;
use jj_lib::repo_path::RepoPath;
use jj_lib::repo_path::RepoPathUiConverter;
use jj_lib::rewrite::rebase_to_dest_parent;
use jj_lib::settings::UserSettings;
use jj_lib::store::Store;
use pollster::FutureExt as _;
use thiserror::Error;
use tracing::instrument;
use unicode_width::UnicodeWidthStr as _;

use crate::command_error::CommandError;
use crate::command_error::cli_error;
use crate::commit_templater;
use crate::config::CommandNameAndArgs;
use crate::formatter::Formatter;
use crate::merge_tools;
use crate::merge_tools::DiffGenerateError;
use crate::merge_tools::DiffToolMode;
use crate::merge_tools::ExternalMergeTool;
use crate::merge_tools::generate_diff;
use crate::merge_tools::invoke_external_diff;
use crate::merge_tools::new_utf8_temp_dir;
use crate::templater::TemplateRenderer;
use crate::text_util;
use crate::ui::Ui;

#[derive(clap::Args, Clone, Debug)]
#[command(next_help_heading = "Diff Formatting Options")]
#[command(group(clap::ArgGroup::new("short-format").args(&["summary", "stat", "types", "name_only"])))]
#[command(group(clap::ArgGroup::new("long-format").args(&["git", "color_words"])))]
pub struct DiffFormatArgs {
    /// For each path, show only whether it was modified, added, or deleted
    #[arg(long, short)]
    pub summary: bool,
    /// Show a histogram of the changes
    #[arg(long)]
    pub stat: bool,
    /// For each path, show only its type before and after
    ///
    /// The diff is shown as two letters. The first letter indicates the type
    /// before and the second letter indicates the type after. '-' indicates
    /// that the path was not present, 'F' represents a regular file, `L'
    /// represents a symlink, 'C' represents a conflict, and 'G' represents a
    /// Git submodule.
    #[arg(long)]
    pub types: bool,
    /// For each path, show only its path
    ///
    /// Typically useful for shell commands like:
    ///    `jj diff -r @- --name-only | xargs perl -pi -e's/OLD/NEW/g`
    #[arg(long)]
    pub name_only: bool,
    /// Show a Git-format diff
    #[arg(long)]
    pub git: bool,
    /// Show a word-level diff with changes indicated only by color
    #[arg(long)]
    pub color_words: bool,
    /// Generate diff by external command
    ///
    /// A builtin format can also be specified as `:<name>`. For example,
    /// `--tool=:git` is equivalent to `--git`.
    #[arg(
        long,
        add = ArgValueCandidates::new(crate::complete::diff_formatters),
    )]
    pub tool: Option<String>,
    /// Number of lines of context to show
    #[arg(long)]
    context: Option<usize>,

    // Short flags are set by command to avoid future conflicts.
    /// Ignore whitespace when comparing lines.
    #[arg(long)] // short = 'w'
    ignore_all_space: bool,
    /// Ignore changes in amount of whitespace when comparing lines.
    #[arg(long, conflicts_with = "ignore_all_space")] // short = 'b'
    ignore_space_change: bool,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum DiffFormat {
    // Non-trivial parameters are boxed in order to keep the variants small
    Summary,
    Stat(Box<DiffStatOptions>),
    Types,
    NameOnly,
    Git(Box<UnifiedDiffOptions>),
    ColorWords(Box<ColorWordsDiffOptions>),
    Tool(Box<ExternalMergeTool>),
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum BuiltinFormatKind {
    Summary,
    Stat,
    Types,
    NameOnly,
    Git,
    ColorWords,
}

impl BuiltinFormatKind {
    // Alternatively, we could use or vendor one of the crates `strum`,
    // `enum-iterator`, or `variant_count` (for a check that the length of the array
    // is correct). The latter is very simple and is also a nightly feature.
    const ALL_VARIANTS: &[Self] = &[
        Self::Summary,
        Self::Stat,
        Self::Types,
        Self::NameOnly,
        Self::Git,
        Self::ColorWords,
    ];

    fn from_name(name: &str) -> Result<Self, String> {
        match name {
            "summary" => Ok(Self::Summary),
            "stat" => Ok(Self::Stat),
            "types" => Ok(Self::Types),
            "name-only" => Ok(Self::NameOnly),
            "git" => Ok(Self::Git),
            "color-words" => Ok(Self::ColorWords),
            _ => Err(format!("Invalid builtin diff format: {name}")),
        }
    }

    fn short_from_args(args: &DiffFormatArgs) -> Option<Self> {
        if args.summary {
            Some(Self::Summary)
        } else if args.stat {
            Some(Self::Stat)
        } else if args.types {
            Some(Self::Types)
        } else if args.name_only {
            Some(Self::NameOnly)
        } else {
            None
        }
    }

    fn long_from_args(args: &DiffFormatArgs) -> Option<Self> {
        if args.git {
            Some(Self::Git)
        } else if args.color_words {
            Some(Self::ColorWords)
        } else {
            None
        }
    }

    fn is_short(self) -> bool {
        match self {
            Self::Summary | Self::Stat | Self::Types | Self::NameOnly => true,
            Self::Git | Self::ColorWords => false,
        }
    }

    fn to_arg_name(self) -> &'static str {
        match self {
            Self::Summary => "summary",
            Self::Stat => "stat",
            Self::Types => "types",
            Self::NameOnly => "name-only",
            Self::Git => "git",
            Self::ColorWords => "color-words",
        }
    }

    fn to_format(
        self,
        settings: &UserSettings,
        args: &DiffFormatArgs,
    ) -> Result<DiffFormat, ConfigGetError> {
        match self {
            Self::Summary => Ok(DiffFormat::Summary),
            Self::Stat => {
                let mut options = DiffStatOptions::default();
                options.merge_args(args);
                Ok(DiffFormat::Stat(Box::new(options)))
            }
            Self::Types => Ok(DiffFormat::Types),
            Self::NameOnly => Ok(DiffFormat::NameOnly),
            Self::Git => {
                let mut options = UnifiedDiffOptions::from_settings(settings)?;
                options.merge_args(args);
                Ok(DiffFormat::Git(Box::new(options)))
            }
            Self::ColorWords => {
                let mut options = ColorWordsDiffOptions::from_settings(settings)?;
                options.merge_args(args);
                Ok(DiffFormat::ColorWords(Box::new(options)))
            }
        }
    }
}

/// Returns the list of builtin diff format names such as `:git`
pub fn all_builtin_diff_format_names() -> Vec<String> {
    BuiltinFormatKind::ALL_VARIANTS
        .iter()
        .map(|kind| format!(":{}", kind.to_arg_name()))
        .collect()
}

fn diff_formatter_tool(
    settings: &UserSettings,
    name: &str,
) -> Result<Option<ExternalMergeTool>, CommandError> {
    let maybe_tool = merge_tools::get_external_tool_config(settings, name)?;
    if let Some(tool) = &maybe_tool {
        if tool.diff_args.is_empty() {
            return Err(cli_error(format!(
                "The tool `{name}` cannot be used for diff formatting"
            )));
        };
    };
    Ok(maybe_tool)
}

/// Returns a list of requested diff formats, which will never be empty.
pub fn diff_formats_for(
    settings: &UserSettings,
    args: &DiffFormatArgs,
) -> Result<Vec<DiffFormat>, CommandError> {
    let formats = diff_formats_from_args(settings, args)?;
    if formats.iter().all(|f| f.is_none()) {
        Ok(vec![default_diff_format(settings, args)?])
    } else {
        Ok(formats.into_iter().flatten().collect())
    }
}

/// Returns a list of requested diff formats for log-like commands, which may be
/// empty.
pub fn diff_formats_for_log(
    settings: &UserSettings,
    args: &DiffFormatArgs,
    patch: bool,
) -> Result<Vec<DiffFormat>, CommandError> {
    let [short_format, mut long_format] = diff_formats_from_args(settings, args)?;
    // --patch implies default if no "long" format is specified
    if patch && long_format.is_none() {
        // TODO: maybe better to error out if the configured default isn't a
        // "long" format?
        let default_format = default_diff_format(settings, args)?;
        if short_format.as_ref() != Some(&default_format) {
            long_format = Some(default_format);
        }
    }
    Ok([short_format, long_format].into_iter().flatten().collect())
}

fn diff_formats_from_args(
    settings: &UserSettings,
    args: &DiffFormatArgs,
) -> Result<[Option<DiffFormat>; 2], CommandError> {
    let short_kind = BuiltinFormatKind::short_from_args(args);
    let long_kind = BuiltinFormatKind::long_from_args(args);
    let mut short_format = short_kind
        .map(|kind| kind.to_format(settings, args))
        .transpose()?;
    let mut long_format = long_kind
        .map(|kind| kind.to_format(settings, args))
        .transpose()?;
    if let Some(name) = &args.tool {
        let ensure_new = |old_kind: Option<BuiltinFormatKind>| match old_kind {
            Some(old) => Err(cli_error(format!(
                "--tool={name} cannot be used with --{old}",
                old = old.to_arg_name()
            ))),
            None => Ok(()),
        };
        if let Some(name) = name.strip_prefix(':') {
            let kind = BuiltinFormatKind::from_name(name).map_err(cli_error)?;
            let format = kind.to_format(settings, args)?;
            if kind.is_short() {
                ensure_new(short_kind)?;
                short_format = Some(format);
            } else {
                ensure_new(long_kind)?;
                long_format = Some(format);
            }
        } else {
            ensure_new(long_kind)?;
            let tool = diff_formatter_tool(settings, name)?
                .unwrap_or_else(|| ExternalMergeTool::with_program(name));
            long_format = Some(DiffFormat::Tool(Box::new(tool)));
        }
    }
    Ok([short_format, long_format])
}

fn default_diff_format(
    settings: &UserSettings,
    args: &DiffFormatArgs,
) -> Result<DiffFormat, CommandError> {
    let tool_args: CommandNameAndArgs = settings.get("ui.diff-formatter")?;
    if let Some(name) = tool_args.as_str().and_then(|s| s.strip_prefix(':')) {
        Ok(BuiltinFormatKind::from_name(name)
            .map_err(|err| ConfigGetError::Type {
                name: "ui.diff-formatter".to_owned(),
                error: err.into(),
                source_path: None,
            })?
            .to_format(settings, args)?)
    } else {
        let tool = if let Some(name) = tool_args.as_str() {
            diff_formatter_tool(settings, name)?
        } else {
            None
        }
        .unwrap_or_else(|| ExternalMergeTool::with_diff_args(&tool_args));
        Ok(DiffFormat::Tool(Box::new(tool)))
    }
}

#[derive(Debug, Error)]
pub enum DiffRenderError {
    #[error("Failed to generate diff")]
    DiffGenerate(#[source] DiffGenerateError),
    #[error(transparent)]
    Backend(#[from] BackendError),
    #[error("Access denied to {path}")]
    AccessDenied {
        path: String,
        source: Box<dyn std::error::Error + Send + Sync>,
    },
    #[error(transparent)]
    InvalidRepoPath(#[from] InvalidRepoPathError),
    #[error(transparent)]
    Io(#[from] io::Error),
}

/// Configuration and environment to render textual diff.
pub struct DiffRenderer<'a> {
    repo: &'a dyn Repo,
    path_converter: &'a RepoPathUiConverter,
    conflict_marker_style: ConflictMarkerStyle,
    formats: Vec<DiffFormat>,
}

impl<'a> DiffRenderer<'a> {
    pub fn new(
        repo: &'a dyn Repo,
        path_converter: &'a RepoPathUiConverter,
        conflict_marker_style: ConflictMarkerStyle,
        formats: Vec<DiffFormat>,
    ) -> Self {
        Self {
            repo,
            path_converter,
            conflict_marker_style,
            formats,
        }
    }

    /// Generates diff between `trees`.
    pub async fn show_diff(
        &self,
        ui: &Ui, // TODO: remove Ui dependency if possible
        formatter: &mut dyn Formatter,
        trees: [&MergedTree; 2],
        matcher: &dyn Matcher,
        copy_records: &CopyRecords,
        width: usize,
    ) -> Result<(), DiffRenderError> {
        formatter
            .with_label_async("diff", async |formatter| {
                self.show_diff_trees(ui, formatter, trees, matcher, copy_records, width)
                    .await
            })
            .await
    }

    async fn show_diff_trees(
        &self,
        ui: &Ui,
        formatter: &mut dyn Formatter,
        [from_tree, to_tree]: [&MergedTree; 2],
        matcher: &dyn Matcher,
        copy_records: &CopyRecords,
        width: usize,
    ) -> Result<(), DiffRenderError> {
        let store = self.repo.store();
        let path_converter = self.path_converter;
        for format in &self.formats {
            match format {
                DiffFormat::Summary => {
                    let tree_diff =
                        from_tree.diff_stream_with_copies(to_tree, matcher, copy_records);
                    show_diff_summary(formatter, tree_diff, path_converter).await?;
                }
                DiffFormat::Stat(options) => {
                    let tree_diff =
                        from_tree.diff_stream_with_copies(to_tree, matcher, copy_records);
                    let stats =
                        DiffStats::calculate(store, tree_diff, options, self.conflict_marker_style)
                            .block_on()?;
                    show_diff_stats(formatter, &stats, path_converter, width)?;
                }
                DiffFormat::Types => {
                    let tree_diff =
                        from_tree.diff_stream_with_copies(to_tree, matcher, copy_records);
                    show_types(formatter, tree_diff, path_converter).await?;
                }
                DiffFormat::NameOnly => {
                    let tree_diff =
                        from_tree.diff_stream_with_copies(to_tree, matcher, copy_records);
                    show_names(formatter, tree_diff, path_converter).await?;
                }
                DiffFormat::Git(options) => {
                    let tree_diff =
                        from_tree.diff_stream_with_copies(to_tree, matcher, copy_records);
                    show_git_diff(
                        formatter,
                        store,
                        tree_diff,
                        options,
                        self.conflict_marker_style,
                    )
                    .await?;
                }
                DiffFormat::ColorWords(options) => {
                    let tree_diff =
                        from_tree.diff_stream_with_copies(to_tree, matcher, copy_records);
                    show_color_words_diff(
                        formatter,
                        store,
                        tree_diff,
                        path_converter,
                        options,
                        self.conflict_marker_style,
                    )
                    .await?;
                }
                DiffFormat::Tool(tool) => {
                    match tool.diff_invocation_mode {
                        DiffToolMode::FileByFile => {
                            let tree_diff =
                                from_tree.diff_stream_with_copies(to_tree, matcher, copy_records);
                            show_file_by_file_diff(
                                ui,
                                formatter,
                                store,
                                tree_diff,
                                path_converter,
                                tool,
                                self.conflict_marker_style,
                            )
                            .await
                        }
                        DiffToolMode::Dir => {
                            let mut writer = formatter.raw()?;
                            generate_diff(
                                ui,
                                writer.as_mut(),
                                from_tree,
                                to_tree,
                                matcher,
                                tool,
                                self.conflict_marker_style,
                            )
                            .map_err(DiffRenderError::DiffGenerate)
                        }
                    }?;
                }
            }
        }
        Ok(())
    }

    fn show_diff_commit_descriptions(
        &self,
        formatter: &mut dyn Formatter,
        [from_description, to_description]: [&Merge<&str>; 2],
    ) -> Result<(), DiffRenderError> {
        if from_description == to_description {
            return Ok(());
        }
        const DUMMY_PATH: &str = "JJ-COMMIT-DESCRIPTION";
        let materialize_options = ConflictMaterializeOptions {
            marker_style: self.conflict_marker_style,
            marker_len: None,
        };
        for format in &self.formats {
            match format {
                // Omit diff from "short" formats. Printing dummy file path
                // wouldn't be useful.
                DiffFormat::Summary
                | DiffFormat::Stat(_)
                | DiffFormat::Types
                | DiffFormat::NameOnly => {}
                DiffFormat::Git(options) => {
                    // Git format must be parsable, so use dummy file path.
                    show_git_diff_texts(
                        formatter,
                        [DUMMY_PATH, DUMMY_PATH],
                        [from_description, to_description],
                        options,
                        &materialize_options,
                    )?;
                }
                DiffFormat::ColorWords(options) => {
                    writeln!(formatter.labeled("header"), "Modified commit description:")?;
                    show_color_words_diff_hunks(
                        formatter,
                        [from_description, to_description],
                        options,
                        &materialize_options,
                    )?;
                }
                DiffFormat::Tool(_) => {
                    // TODO: materialize commit description as file?
                }
            }
        }
        Ok(())
    }

    /// Generates diff between `from_commits` and `to_commit` based off their
    /// parents. The `from_commits` will temporarily be rebased onto the
    /// `to_commit` parents to exclude unrelated changes.
    pub async fn show_inter_diff(
        &self,
        ui: &Ui,
        formatter: &mut dyn Formatter,
        from_commits: &[Commit],
        to_commit: &Commit,
        matcher: &dyn Matcher,
        width: usize,
    ) -> Result<(), DiffRenderError> {
        let from_description = if from_commits.is_empty() {
            Merge::resolved("")
        } else {
            // TODO: use common predecessor as the base description?
            MergeBuilder::from_iter(itertools::intersperse(
                from_commits.iter().map(|c| c.description()),
                "",
            ))
            .build()
            .simplify()
        };
        let to_description = Merge::resolved(to_commit.description());
        let from_tree = rebase_to_dest_parent(self.repo, from_commits, to_commit)?;
        let to_tree = to_commit.tree_async().await?;
        let copy_records = CopyRecords::default(); // TODO
        formatter
            .with_label_async("diff", async |formatter| {
                self.show_diff_commit_descriptions(
                    formatter,
                    [&from_description, &to_description],
                )?;
                self.show_diff_trees(
                    ui,
                    formatter,
                    [&from_tree, &to_tree],
                    matcher,
                    &copy_records,
                    width,
                )
                .await
            })
            .await
    }

    /// Generates diff of the given `commit` compared to its parents.
    pub async fn show_patch(
        &self,
        ui: &Ui,
        formatter: &mut dyn Formatter,
        commit: &Commit,
        matcher: &dyn Matcher,
        width: usize,
    ) -> Result<(), DiffRenderError> {
        let from_tree = commit.parent_tree(self.repo)?;
        let to_tree = commit.tree_async().await?;
        let mut copy_records = CopyRecords::default();
        for parent_id in commit.parent_ids() {
            let records = get_copy_records(self.repo.store(), parent_id, commit.id(), matcher)?;
            copy_records.add_records(records)?;
        }
        self.show_diff(
            ui,
            formatter,
            [&from_tree, &to_tree],
            matcher,
            &copy_records,
            width,
        )
        .await
    }
}

pub fn get_copy_records<'a>(
    store: &'a Store,
    root: &CommitId,
    head: &CommitId,
    matcher: &'a dyn Matcher,
) -> BackendResult<impl Iterator<Item = BackendResult<CopyRecord>> + use<'a>> {
    // TODO: teach backend about matching path prefixes?
    let stream = store.get_copy_records(None, root, head)?;
    // TODO: test record.source as well? should be AND-ed or OR-ed?
    Ok(block_on_stream(stream).filter_ok(|record| matcher.matches(&record.target)))
}

/// How conflicts are processed and rendered in diffs.
#[derive(Clone, Copy, Debug, Default, Eq, PartialEq, serde::Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum ConflictDiffMethod {
    /// Compares materialized contents.
    #[default]
    Materialize,
    /// Compares individual pairs of left and right contents.
    Pair,
}

#[derive(Clone, Debug, Default, Eq, PartialEq)]
pub struct LineDiffOptions {
    /// How equivalence of lines is tested.
    pub compare_mode: LineCompareMode,
    // TODO: add --ignore-blank-lines, etc. which aren't mutually exclusive.
}

impl LineDiffOptions {
    fn merge_args(&mut self, args: &DiffFormatArgs) {
        self.compare_mode = if args.ignore_all_space {
            LineCompareMode::IgnoreAllSpace
        } else if args.ignore_space_change {
            LineCompareMode::IgnoreSpaceChange
        } else {
            LineCompareMode::Exact
        };
    }
}

#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub enum LineCompareMode {
    /// Compares lines literally.
    #[default]
    Exact,
    /// Compares lines ignoring any whitespace occurrences.
    IgnoreAllSpace,
    /// Compares lines ignoring changes in whitespace amount.
    IgnoreSpaceChange,
}

fn diff_by_line<'input, T: AsRef<[u8]> + ?Sized + 'input>(
    inputs: impl IntoIterator<Item = &'input T>,
    options: &LineDiffOptions,
) -> Diff<'input> {
    // TODO: If we add --ignore-blank-lines, its tokenizer will have to attach
    // blank lines to the preceding range. Maybe it can also be implemented as a
    // post-process (similar to refine_changed_regions()) that expands unchanged
    // regions across blank lines.
    match options.compare_mode {
        LineCompareMode::Exact => {
            Diff::for_tokenizer(inputs, find_line_ranges, CompareBytesExactly)
        }
        LineCompareMode::IgnoreAllSpace => {
            Diff::for_tokenizer(inputs, find_line_ranges, CompareBytesIgnoreAllWhitespace)
        }
        LineCompareMode::IgnoreSpaceChange => {
            Diff::for_tokenizer(inputs, find_line_ranges, CompareBytesIgnoreWhitespaceAmount)
        }
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct ColorWordsDiffOptions {
    /// How conflicts are processed and rendered.
    pub conflict: ConflictDiffMethod,
    /// Number of context lines to show.
    pub context: usize,
    /// How lines are tokenized and compared.
    pub line_diff: LineDiffOptions,
    /// Maximum number of removed/added word alternation to inline.
    pub max_inline_alternation: Option<usize>,
}

impl ColorWordsDiffOptions {
    pub fn from_settings(settings: &UserSettings) -> Result<Self, ConfigGetError> {
        let max_inline_alternation = {
            let name = "diff.color-words.max-inline-alternation";
            match settings.get_int(name)? {
                -1 => None, // unlimited
                n => Some(usize::try_from(n).map_err(|err| ConfigGetError::Type {
                    name: name.to_owned(),
                    error: err.into(),
                    source_path: None,
                })?),
            }
        };
        Ok(Self {
            conflict: settings.get("diff.color-words.conflict")?,
            context: settings.get("diff.color-words.context")?,
            line_diff: LineDiffOptions::default(),
            max_inline_alternation,
        })
    }

    fn merge_args(&mut self, args: &DiffFormatArgs) {
        if let Some(context) = args.context {
            self.context = context;
        }
        self.line_diff.merge_args(args);
    }
}

fn show_color_words_diff_hunks<T: AsRef<[u8]>>(
    formatter: &mut dyn Formatter,
    [lefts, rights]: [&Merge<T>; 2],
    options: &ColorWordsDiffOptions,
    materialize_options: &ConflictMaterializeOptions,
) -> io::Result<()> {
    let line_number = DiffLineNumber { left: 1, right: 1 };
    let labels = ["removed", "added"];
    if let (Some(left), Some(right)) = (lefts.as_resolved(), rights.as_resolved()) {
        let contents = [left, right].map(BStr::new);
        show_color_words_resolved_hunks(formatter, contents, line_number, labels, options)?;
        return Ok(());
    }
    match options.conflict {
        ConflictDiffMethod::Materialize => {
            let left = materialize_merge_result_to_bytes(lefts, materialize_options);
            let right = materialize_merge_result_to_bytes(rights, materialize_options);
            let contents = [&left, &right].map(BStr::new);
            show_color_words_resolved_hunks(formatter, contents, line_number, labels, options)?;
        }
        ConflictDiffMethod::Pair => {
            let lefts = files::merge(lefts);
            let rights = files::merge(rights);
            let contents = [&lefts, &rights];
            show_color_words_conflict_hunks(formatter, contents, line_number, labels, options)?;
        }
    }
    Ok(())
}

fn show_color_words_conflict_hunks(
    formatter: &mut dyn Formatter,
    [lefts, rights]: [&Merge<BString>; 2],
    mut line_number: DiffLineNumber,
    labels: [&str; 2],
    options: &ColorWordsDiffOptions,
) -> io::Result<DiffLineNumber> {
    let num_lefts = lefts.as_slice().len();
    let line_diff = diff_by_line(lefts.iter().chain(rights.iter()), &options.line_diff);
    // Matching entries shouldn't appear consecutively in diff of two inputs.
    // However, if the inputs have conflicts, there may be a hunk that can be
    // resolved, resulting [matching, resolved, matching] sequence.
    let mut contexts: Vec<[&BStr; 2]> = Vec::new();
    let mut emitted = false;

    for hunk in files::conflict_diff_hunks(line_diff.hunks(), num_lefts) {
        match hunk.kind {
            // There may be conflicts in matching hunk, but just pick one. It
            // would be too verbose to show all conflict pairs as context.
            DiffHunkKind::Matching => {
                contexts.push([hunk.lefts.first(), hunk.rights.first()]);
            }
            DiffHunkKind::Different => {
                let num_after = if emitted { options.context } else { 0 };
                let num_before = options.context;
                line_number = show_color_words_context_lines(
                    formatter,
                    &contexts,
                    line_number,
                    labels,
                    options,
                    num_after,
                    num_before,
                )?;
                contexts.clear();
                emitted = true;
                line_number = if let (Some(&left), Some(&right)) =
                    (hunk.lefts.as_resolved(), hunk.rights.as_resolved())
                {
                    let contents = [left, right];
                    show_color_words_diff_lines(formatter, contents, line_number, labels, options)?
                } else {
                    show_color_words_unresolved_hunk(
                        formatter,
                        &hunk,
                        line_number,
                        labels,
                        options,
                    )?
                }
            }
        }
    }

    let num_after = if emitted { options.context } else { 0 };
    let num_before = 0;
    show_color_words_context_lines(
        formatter,
        &contexts,
        line_number,
        labels,
        options,
        num_after,
        num_before,
    )
}

fn show_color_words_unresolved_hunk(
    formatter: &mut dyn Formatter,
    hunk: &ConflictDiffHunk,
    line_number: DiffLineNumber,
    [label1, label2]: [&str; 2],
    options: &ColorWordsDiffOptions,
) -> io::Result<DiffLineNumber> {
    let hunk_desc = if hunk.lefts.is_resolved() {
        "Created conflict"
    } else if hunk.rights.is_resolved() {
        "Resolved conflict"
    } else {
        "Modified conflict"
    };
    writeln!(formatter.labeled("hunk_header"), "<<<<<<< {hunk_desc}")?;

    // Pad with identical (negative, positive) terms. It's common that one of
    // the sides is resolved, or both sides have the same numbers of terms. If
    // both sides are conflicts, and the numbers of terms are different, the
    // choice of padding terms is arbitrary.
    let num_terms = max(hunk.lefts.as_slice().len(), hunk.rights.as_slice().len());
    let lefts = hunk.lefts.iter().enumerate();
    let rights = hunk.rights.iter().enumerate();
    let padded = iter::zip(
        lefts.chain(iter::repeat((0, hunk.lefts.first()))),
        rights.chain(iter::repeat((0, hunk.rights.first()))),
    )
    .take(num_terms);
    let mut max_line_number = line_number;
    for (i, ((left_index, &left_content), (right_index, &right_content))) in padded.enumerate() {
        let positive = i % 2 == 0;
        writeln!(
            formatter.labeled("hunk_header"),
            "{sep} left {left_name} #{left_index} to right {right_name} #{right_index}",
            sep = if positive { "+++++++" } else { "-------" },
            // these numbers should be compatible with the "tree-set" language #5307
            left_name = if left_index % 2 == 0 { "side" } else { "base" },
            left_index = left_index / 2 + 1,
            right_name = if right_index % 2 == 0 { "side" } else { "base" },
            right_index = right_index / 2 + 1,
        )?;
        let contents = [left_content, right_content];
        let labels = match positive {
            true => [label1, label2],
            false => [label2, label1],
        };
        // Individual hunk pair may be largely the same, so diff it again.
        let new_line_number =
            show_color_words_resolved_hunks(formatter, contents, line_number, labels, options)?;
        // Take max to assign unique line numbers to trailing hunks. The line
        // numbers can't be real anyway because preceding conflict hunks might
        // have been resolved.
        max_line_number.left = max(max_line_number.left, new_line_number.left);
        max_line_number.right = max(max_line_number.right, new_line_number.right);
    }

    writeln!(formatter.labeled("hunk_header"), ">>>>>>> Conflict ends")?;
    Ok(max_line_number)
}

fn show_color_words_resolved_hunks(
    formatter: &mut dyn Formatter,
    contents: [&BStr; 2],
    mut line_number: DiffLineNumber,
    labels: [&str; 2],
    options: &ColorWordsDiffOptions,
) -> io::Result<DiffLineNumber> {
    let line_diff = diff_by_line(contents, &options.line_diff);
    // Matching entries shouldn't appear consecutively in diff of two inputs.
    let mut context: Option<[&BStr; 2]> = None;
    let mut emitted = false;

    for hunk in line_diff.hunks() {
        let hunk_contents: [&BStr; 2] = hunk.contents[..].try_into().unwrap();
        match hunk.kind {
            DiffHunkKind::Matching => {
                context = Some(hunk_contents);
            }
            DiffHunkKind::Different => {
                let num_after = if emitted { options.context } else { 0 };
                let num_before = options.context;
                line_number = show_color_words_context_lines(
                    formatter,
                    context.as_slice(),
                    line_number,
                    labels,
                    options,
                    num_after,
                    num_before,
                )?;
                context = None;
                emitted = true;
                line_number = show_color_words_diff_lines(
                    formatter,
                    hunk_contents,
                    line_number,
                    labels,
                    options,
                )?;
            }
        }
    }

    let num_after = if emitted { options.context } else { 0 };
    let num_before = 0;
    show_color_words_context_lines(
        formatter,
        context.as_slice(),
        line_number,
        labels,
        options,
        num_after,
        num_before,
    )
}

/// Prints `num_after` lines, ellipsis, and `num_before` lines.
fn show_color_words_context_lines(
    formatter: &mut dyn Formatter,
    contexts: &[[&BStr; 2]],
    mut line_number: DiffLineNumber,
    labels: [&str; 2],
    options: &ColorWordsDiffOptions,
    num_after: usize,
    num_before: usize,
) -> io::Result<DiffLineNumber> {
    const SKIPPED_CONTEXT_LINE: &str = "    ...\n";
    let extract = |side: usize| -> (Vec<&[u8]>, Vec<&[u8]>, u32) {
        let mut lines = contexts
            .iter()
            .flat_map(|contents| contents[side].split_inclusive(|b| *b == b'\n'))
            .fuse();
        let after_lines = lines.by_ref().take(num_after).collect();
        let before_lines = lines.by_ref().rev().take(num_before + 1).collect();
        let num_skipped: u32 = lines.count().try_into().unwrap();
        (after_lines, before_lines, num_skipped)
    };
    let show = |formatter: &mut dyn Formatter,
                [left_lines, right_lines]: [&[&[u8]]; 2],
                mut line_number: DiffLineNumber| {
        if left_lines == right_lines {
            for line in left_lines {
                show_color_words_line_number(
                    formatter,
                    [Some(line_number.left), Some(line_number.right)],
                    labels,
                )?;
                show_color_words_inline_hunks(
                    formatter,
                    &[(DiffLineHunkSide::Both, line.as_ref())],
                    labels,
                )?;
                line_number.left += 1;
                line_number.right += 1;
            }
            Ok(line_number)
        } else {
            let left = left_lines.concat();
            let right = right_lines.concat();
            show_color_words_diff_lines(
                formatter,
                [&left, &right].map(BStr::new),
                line_number,
                labels,
                options,
            )
        }
    };

    let (left_after, mut left_before, num_left_skipped) = extract(0);
    let (right_after, mut right_before, num_right_skipped) = extract(1);
    line_number = show(formatter, [&left_after, &right_after], line_number)?;
    if num_left_skipped > 0 || num_right_skipped > 0 {
        write!(formatter, "{SKIPPED_CONTEXT_LINE}")?;
        line_number.left += num_left_skipped;
        line_number.right += num_right_skipped;
        if left_before.len() > num_before {
            left_before.pop();
            line_number.left += 1;
        }
        if right_before.len() > num_before {
            right_before.pop();
            line_number.right += 1;
        }
    }
    left_before.reverse();
    right_before.reverse();
    line_number = show(formatter, [&left_before, &right_before], line_number)?;
    Ok(line_number)
}

fn show_color_words_diff_lines(
    formatter: &mut dyn Formatter,
    contents: [&BStr; 2],
    mut line_number: DiffLineNumber,
    labels: [&str; 2],
    options: &ColorWordsDiffOptions,
) -> io::Result<DiffLineNumber> {
    let word_diff_hunks = Diff::by_word(contents).hunks().collect_vec();
    let can_inline = match options.max_inline_alternation {
        None => true,     // unlimited
        Some(0) => false, // no need to count alternation
        Some(max_num) => {
            let groups = split_diff_hunks_by_matching_newline(&word_diff_hunks);
            groups.map(count_diff_alternation).max().unwrap_or(0) <= max_num
        }
    };
    if can_inline {
        let mut diff_line_iter =
            DiffLineIterator::with_line_number(word_diff_hunks.iter(), line_number);
        for diff_line in diff_line_iter.by_ref() {
            show_color_words_line_number(
                formatter,
                [
                    diff_line
                        .has_left_content()
                        .then_some(diff_line.line_number.left),
                    diff_line
                        .has_right_content()
                        .then_some(diff_line.line_number.right),
                ],
                labels,
            )?;
            show_color_words_inline_hunks(formatter, &diff_line.hunks, labels)?;
        }
        line_number = diff_line_iter.next_line_number();
    } else {
        let [left_lines, right_lines] = unzip_diff_hunks_to_lines(&word_diff_hunks);
        let [left_label, right_label] = labels;
        for tokens in &left_lines {
            show_color_words_line_number(formatter, [Some(line_number.left), None], labels)?;
            show_color_words_single_sided_line(formatter, tokens, left_label)?;
            line_number.left += 1;
        }
        for tokens in &right_lines {
            show_color_words_line_number(formatter, [None, Some(line_number.right)], labels)?;
            show_color_words_single_sided_line(formatter, tokens, right_label)?;
            line_number.right += 1;
        }
    }
    Ok(line_number)
}

fn show_color_words_line_number(
    formatter: &mut dyn Formatter,
    [left_line_number, right_line_number]: [Option<u32>; 2],
    [left_label, right_label]: [&str; 2],
) -> io::Result<()> {
    if let Some(line_number) = left_line_number {
        formatter.with_label(left_label, |formatter| {
            write!(formatter.labeled("line_number"), "{line_number:>4}")
        })?;
        write!(formatter, " ")?;
    } else {
        write!(formatter, "     ")?;
    }
    if let Some(line_number) = right_line_number {
        formatter.with_label(right_label, |formatter| {
            write!(formatter.labeled("line_number"), "{line_number:>4}",)
        })?;
        write!(formatter, ": ")?;
    } else {
        write!(formatter, "    : ")?;
    }
    Ok(())
}

/// Prints line hunks which may contain tokens originating from both sides.
fn show_color_words_inline_hunks(
    formatter: &mut dyn Formatter,
    line_hunks: &[(DiffLineHunkSide, &BStr)],
    [left_label, right_label]: [&str; 2],
) -> io::Result<()> {
    for (side, data) in line_hunks {
        let label = match side {
            DiffLineHunkSide::Both => None,
            DiffLineHunkSide::Left => Some(left_label),
            DiffLineHunkSide::Right => Some(right_label),
        };
        if let Some(label) = label {
            formatter.with_label(label, |formatter| {
                formatter.with_label("token", |formatter| formatter.write_all(data))
            })?;
        } else {
            formatter.write_all(data)?;
        }
    }
    let (_, data) = line_hunks.last().expect("diff line must not be empty");
    if !data.ends_with(b"\n") {
        writeln!(formatter)?;
    };
    Ok(())
}

/// Prints left/right-only line tokens with the given label.
fn show_color_words_single_sided_line(
    formatter: &mut dyn Formatter,
    tokens: &[(DiffTokenType, &[u8])],
    label: &str,
) -> io::Result<()> {
    formatter.with_label(label, |formatter| show_diff_line_tokens(formatter, tokens))?;
    let (_, data) = tokens.last().expect("diff line must not be empty");
    if !data.ends_with(b"\n") {
        writeln!(formatter)?;
    };
    Ok(())
}

/// Counts number of diff-side alternation, ignoring matching hunks.
///
/// This function is meant to measure visual complexity of diff hunks. It's easy
/// to read hunks containing some removed or added words, but is getting harder
/// as more removes and adds interleaved.
///
/// For example,
/// - `[matching]` -> 0
/// - `[left]` -> 1
/// - `[left, matching, left]` -> 1
/// - `[matching, left, right, matching, right]` -> 2
/// - `[left, right, matching, right, left]` -> 3
fn count_diff_alternation(diff_hunks: &[DiffHunk]) -> usize {
    diff_hunks
        .iter()
        .filter_map(|hunk| match hunk.kind {
            DiffHunkKind::Matching => None,
            DiffHunkKind::Different => Some(&hunk.contents),
        })
        // Map non-empty diff side to index (0: left, 1: right)
        .flat_map(|contents| contents.iter().positions(|content| !content.is_empty()))
        // Omit e.g. left->(matching->)*left
        .dedup()
        .count()
}

/// Splits hunks into slices of contiguous changed lines.
fn split_diff_hunks_by_matching_newline<'a, 'b>(
    diff_hunks: &'a [DiffHunk<'b>],
) -> impl Iterator<Item = &'a [DiffHunk<'b>]> {
    diff_hunks.split_inclusive(|hunk| match hunk.kind {
        DiffHunkKind::Matching => hunk.contents.iter().all(|content| content.contains(&b'\n')),
        DiffHunkKind::Different => false,
    })
}

struct FileContent<T> {
    /// false if this file is likely text; true if it is likely binary.
    is_binary: bool,
    contents: T,
}

impl FileContent<Merge<BString>> {
    fn is_empty(&self) -> bool {
        self.contents.as_resolved().is_some_and(|c| c.is_empty())
    }
}

fn file_content_for_diff<T>(
    path: &RepoPath,
    file: &mut MaterializedFileValue,
    map_resolved: impl FnOnce(BString) -> T,
) -> BackendResult<FileContent<T>> {
    // If this is a binary file, don't show the full contents.
    // Determine whether it's binary by whether the first 8k bytes contain a null
    // character; this is the same heuristic used by git as of writing: https://github.com/git/git/blob/eea0e59ffbed6e33d171ace5be13cde9faa41639/xdiff-interface.c#L192-L198
    const PEEK_SIZE: usize = 8000;
    // TODO: currently we look at the whole file, even though for binary files we
    // only need to know the file size. To change that we'd have to extend all
    // the data backends to support getting the length.
    let contents = BString::new(file.read_all(path).block_on()?);
    let start = &contents[..PEEK_SIZE.min(contents.len())];
    Ok(FileContent {
        is_binary: start.contains(&b'\0'),
        contents: map_resolved(contents),
    })
}

fn diff_content(
    path: &RepoPath,
    value: MaterializedTreeValue,
    materialize_options: &ConflictMaterializeOptions,
) -> BackendResult<FileContent<BString>> {
    diff_content_with(
        path,
        value,
        |content| content,
        |contents| materialize_merge_result_to_bytes(&contents, materialize_options),
    )
}

fn diff_content_as_merge(
    path: &RepoPath,
    value: MaterializedTreeValue,
) -> BackendResult<FileContent<Merge<BString>>> {
    diff_content_with(path, value, Merge::resolved, |contents| contents)
}

fn diff_content_with<T>(
    path: &RepoPath,
    value: MaterializedTreeValue,
    map_resolved: impl FnOnce(BString) -> T,
    map_conflict: impl FnOnce(Merge<BString>) -> T,
) -> BackendResult<FileContent<T>> {
    match value {
        MaterializedTreeValue::Absent => Ok(FileContent {
            is_binary: false,
            contents: map_resolved(BString::default()),
        }),
        MaterializedTreeValue::AccessDenied(err) => Ok(FileContent {
            is_binary: false,
            contents: map_resolved(format!("Access denied: {err}").into()),
        }),
        MaterializedTreeValue::File(mut file) => {
            file_content_for_diff(path, &mut file, map_resolved)
        }
        MaterializedTreeValue::Symlink { id: _, target } => Ok(FileContent {
            // Unix file paths can't contain null bytes.
            is_binary: false,
            contents: map_resolved(target.into()),
        }),
        MaterializedTreeValue::GitSubmodule(id) => Ok(FileContent {
            is_binary: false,
            contents: map_resolved(format!("Git submodule checked out at {id}").into()),
        }),
        // TODO: are we sure this is never binary?
        MaterializedTreeValue::FileConflict(file) => Ok(FileContent {
            is_binary: false,
            contents: map_conflict(file.contents),
        }),
        MaterializedTreeValue::OtherConflict { id } => Ok(FileContent {
            is_binary: false,
            contents: map_resolved(id.describe().into()),
        }),
        MaterializedTreeValue::Tree(id) => {
            panic!("Unexpected tree with id {id:?} in diff at path {path:?}");
        }
    }
}

fn basic_diff_file_type(value: &MaterializedTreeValue) -> &'static str {
    match value {
        MaterializedTreeValue::Absent => {
            panic!("absent path in diff");
        }
        MaterializedTreeValue::AccessDenied(_) => "access denied",
        MaterializedTreeValue::File(file) => {
            if file.executable {
                "executable file"
            } else {
                "regular file"
            }
        }
        MaterializedTreeValue::Symlink { .. } => "symlink",
        MaterializedTreeValue::Tree(_) => "tree",
        MaterializedTreeValue::GitSubmodule(_) => "Git submodule",
        MaterializedTreeValue::FileConflict(_) | MaterializedTreeValue::OtherConflict { .. } => {
            "conflict"
        }
    }
}

pub async fn show_color_words_diff(
    formatter: &mut dyn Formatter,
    store: &Store,
    tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    path_converter: &RepoPathUiConverter,
    options: &ColorWordsDiffOptions,
    marker_style: ConflictMarkerStyle,
) -> Result<(), DiffRenderError> {
    let materialize_options = ConflictMaterializeOptions {
        marker_style,
        marker_len: None,
    };
    let empty_content = || Merge::resolved(BString::default());
    let mut diff_stream = materialized_diff_stream(store, tree_diff);
    while let Some(MaterializedTreeDiffEntry { path, values }) = diff_stream.next().await {
        let left_path = path.source();
        let right_path = path.target();
        let left_ui_path = path_converter.format_file_path(left_path);
        let right_ui_path = path_converter.format_file_path(right_path);
        let (left_value, right_value) = values?;

        match (&left_value, &right_value) {
            (MaterializedTreeValue::AccessDenied(source), _) => {
                write!(
                    formatter.labeled("access-denied"),
                    "Access denied to {left_ui_path}:"
                )?;
                writeln!(formatter, " {source}")?;
                continue;
            }
            (_, MaterializedTreeValue::AccessDenied(source)) => {
                write!(
                    formatter.labeled("access-denied"),
                    "Access denied to {right_ui_path}:"
                )?;
                writeln!(formatter, " {source}")?;
                continue;
            }
            _ => {}
        }
        if left_value.is_absent() {
            let description = basic_diff_file_type(&right_value);
            writeln!(
                formatter.labeled("header"),
                "Added {description} {right_ui_path}:"
            )?;
            let right_content = diff_content_as_merge(right_path, right_value)?;
            if right_content.is_empty() {
                writeln!(formatter.labeled("empty"), "    (empty)")?;
            } else if right_content.is_binary {
                writeln!(formatter.labeled("binary"), "    (binary)")?;
            } else {
                show_color_words_diff_hunks(
                    formatter,
                    [&empty_content(), &right_content.contents],
                    options,
                    &materialize_options,
                )?;
            }
        } else if right_value.is_present() {
            let description = match (&left_value, &right_value) {
                (MaterializedTreeValue::File(left), MaterializedTreeValue::File(right)) => {
                    if left.executable && right.executable {
                        "Modified executable file".to_string()
                    } else if left.executable {
                        "Executable file became non-executable at".to_string()
                    } else if right.executable {
                        "Non-executable file became executable at".to_string()
                    } else {
                        "Modified regular file".to_string()
                    }
                }
                (
                    MaterializedTreeValue::FileConflict(_)
                    | MaterializedTreeValue::OtherConflict { .. },
                    MaterializedTreeValue::FileConflict(_)
                    | MaterializedTreeValue::OtherConflict { .. },
                ) => "Modified conflict in".to_string(),
                (
                    MaterializedTreeValue::FileConflict(_)
                    | MaterializedTreeValue::OtherConflict { .. },
                    _,
                ) => "Resolved conflict in".to_string(),
                (
                    _,
                    MaterializedTreeValue::FileConflict(_)
                    | MaterializedTreeValue::OtherConflict { .. },
                ) => "Created conflict in".to_string(),
                (MaterializedTreeValue::Symlink { .. }, MaterializedTreeValue::Symlink { .. }) => {
                    "Symlink target changed at".to_string()
                }
                (_, _) => {
                    let left_type = basic_diff_file_type(&left_value);
                    let right_type = basic_diff_file_type(&right_value);
                    let (first, rest) = left_type.split_at(1);
                    format!(
                        "{}{} became {} at",
                        first.to_ascii_uppercase(),
                        rest,
                        right_type
                    )
                }
            };
            let left_content = diff_content_as_merge(left_path, left_value)?;
            let right_content = diff_content_as_merge(right_path, right_value)?;
            if left_path == right_path {
                writeln!(
                    formatter.labeled("header"),
                    "{description} {right_ui_path}:"
                )?;
            } else {
                writeln!(
                    formatter.labeled("header"),
                    "{description} {right_ui_path} ({left_ui_path} => {right_ui_path}):"
                )?;
            }
            if left_content.is_binary || right_content.is_binary {
                writeln!(formatter.labeled("binary"), "    (binary)")?;
            } else if left_content.contents != right_content.contents {
                show_color_words_diff_hunks(
                    formatter,
                    [&left_content.contents, &right_content.contents],
                    options,
                    &materialize_options,
                )?;
            }
        } else {
            let description = basic_diff_file_type(&left_value);
            writeln!(
                formatter.labeled("header"),
                "Removed {description} {right_ui_path}:"
            )?;
            let left_content = diff_content_as_merge(left_path, left_value)?;
            if left_content.is_empty() {
                writeln!(formatter.labeled("empty"), "    (empty)")?;
            } else if left_content.is_binary {
                writeln!(formatter.labeled("binary"), "    (binary)")?;
            } else {
                show_color_words_diff_hunks(
                    formatter,
                    [&left_content.contents, &empty_content()],
                    options,
                    &materialize_options,
                )?;
            }
        }
    }
    Ok(())
}

pub async fn show_file_by_file_diff(
    ui: &Ui,
    formatter: &mut dyn Formatter,
    store: &Store,
    tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    path_converter: &RepoPathUiConverter,
    tool: &ExternalMergeTool,
    marker_style: ConflictMarkerStyle,
) -> Result<(), DiffRenderError> {
    let materialize_options = ConflictMaterializeOptions {
        marker_style,
        marker_len: None,
    };
    let create_file = |path: &RepoPath,
                       wc_dir: &Path,
                       value: MaterializedTreeValue|
     -> Result<PathBuf, DiffRenderError> {
        let fs_path = path.to_fs_path(wc_dir)?;
        std::fs::create_dir_all(fs_path.parent().unwrap())?;
        let content = diff_content(path, value, &materialize_options)?;
        std::fs::write(&fs_path, content.contents)?;
        Ok(fs_path)
    };

    let temp_dir = new_utf8_temp_dir("jj-diff-")?;
    let left_wc_dir = temp_dir.path().join("left");
    let right_wc_dir = temp_dir.path().join("right");
    let mut diff_stream = materialized_diff_stream(store, tree_diff);
    while let Some(MaterializedTreeDiffEntry { path, values }) = diff_stream.next().await {
        let (left_value, right_value) = values?;
        let left_path = path.source();
        let right_path = path.target();
        let left_ui_path = path_converter.format_file_path(left_path);
        let right_ui_path = path_converter.format_file_path(right_path);

        match (&left_value, &right_value) {
            (_, MaterializedTreeValue::AccessDenied(source)) => {
                write!(
                    formatter.labeled("access-denied"),
                    "Access denied to {right_ui_path}:"
                )?;
                writeln!(formatter, " {source}")?;
                continue;
            }
            (MaterializedTreeValue::AccessDenied(source), _) => {
                write!(
                    formatter.labeled("access-denied"),
                    "Access denied to {left_ui_path}:"
                )?;
                writeln!(formatter, " {source}")?;
                continue;
            }
            _ => {}
        }
        let left_path = create_file(left_path, &left_wc_dir, left_value)?;
        let right_path = create_file(right_path, &right_wc_dir, right_value)?;
        let patterns = &maplit::hashmap! {
            "left" => left_path
                .strip_prefix(temp_dir.path()).expect("path should be relative to temp_dir")
                .to_str().expect("temp_dir should be valid utf-8"),
            "right" => right_path
                .strip_prefix(temp_dir.path()).expect("path should be relative to temp_dir")
                .to_str().expect("temp_dir should be valid utf-8"),
        };

        let mut writer = formatter.raw()?;
        invoke_external_diff(ui, writer.as_mut(), tool, temp_dir.path(), patterns)
            .map_err(DiffRenderError::DiffGenerate)?;
    }
    Ok::<(), DiffRenderError>(())
}

struct GitDiffPart {
    /// Octal mode string or `None` if the file is absent.
    mode: Option<&'static str>,
    hash: String,
    content: FileContent<BString>,
}

fn git_diff_part(
    path: &RepoPath,
    value: MaterializedTreeValue,
    materialize_options: &ConflictMaterializeOptions,
) -> Result<GitDiffPart, DiffRenderError> {
    const DUMMY_HASH: &str = "0000000000";
    let mode;
    let mut hash;
    let content;
    match value {
        MaterializedTreeValue::Absent => {
            return Ok(GitDiffPart {
                mode: None,
                hash: DUMMY_HASH.to_owned(),
                content: FileContent {
                    is_binary: false,
                    contents: BString::default(),
                },
            });
        }
        MaterializedTreeValue::AccessDenied(err) => {
            return Err(DiffRenderError::AccessDenied {
                path: path.as_internal_file_string().to_owned(),
                source: err,
            });
        }
        MaterializedTreeValue::File(mut file) => {
            mode = if file.executable { "100755" } else { "100644" };
            hash = file.id.hex();
            content = file_content_for_diff(path, &mut file, |content| content)?;
        }
        MaterializedTreeValue::Symlink { id, target } => {
            mode = "120000";
            hash = id.hex();
            content = FileContent {
                // Unix file paths can't contain null bytes.
                is_binary: false,
                contents: target.into(),
            };
        }
        MaterializedTreeValue::GitSubmodule(id) => {
            // TODO: What should we actually do here?
            mode = "040000";
            hash = id.hex();
            content = FileContent {
                is_binary: false,
                contents: BString::default(),
            };
        }
        MaterializedTreeValue::FileConflict(file) => {
            mode = match file.executable {
                Some(true) => "100755",
                Some(false) | None => "100644",
            };
            hash = DUMMY_HASH.to_owned();
            content = FileContent {
                is_binary: false, // TODO: are we sure this is never binary?
                contents: materialize_merge_result_to_bytes(&file.contents, materialize_options),
            };
        }
        MaterializedTreeValue::OtherConflict { id } => {
            mode = "100644";
            hash = DUMMY_HASH.to_owned();
            content = FileContent {
                is_binary: false,
                contents: id.describe().into(),
            };
        }
        MaterializedTreeValue::Tree(_) => {
            panic!("Unexpected tree in diff at path {path:?}");
        }
    }
    hash.truncate(10);
    Ok(GitDiffPart {
        mode: Some(mode),
        hash,
        content,
    })
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub struct UnifiedDiffOptions {
    /// Number of context lines to show.
    pub context: usize,
    /// How lines are tokenized and compared.
    pub line_diff: LineDiffOptions,
}

impl UnifiedDiffOptions {
    pub fn from_settings(settings: &UserSettings) -> Result<Self, ConfigGetError> {
        Ok(Self {
            context: settings.get("diff.git.context")?,
            line_diff: LineDiffOptions::default(),
        })
    }

    fn merge_args(&mut self, args: &DiffFormatArgs) {
        if let Some(context) = args.context {
            self.context = context;
        }
        self.line_diff.merge_args(args);
    }
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum DiffLineType {
    Context,
    Removed,
    Added,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum DiffTokenType {
    Matching,
    Different,
}

type DiffTokenVec<'content> = Vec<(DiffTokenType, &'content [u8])>;

struct UnifiedDiffHunk<'content> {
    left_line_range: Range<usize>,
    right_line_range: Range<usize>,
    lines: Vec<(DiffLineType, DiffTokenVec<'content>)>,
}

impl<'content> UnifiedDiffHunk<'content> {
    fn extend_context_lines(&mut self, lines: impl IntoIterator<Item = &'content [u8]>) {
        let old_len = self.lines.len();
        self.lines.extend(lines.into_iter().map(|line| {
            let tokens = vec![(DiffTokenType::Matching, line)];
            (DiffLineType::Context, tokens)
        }));
        self.left_line_range.end += self.lines.len() - old_len;
        self.right_line_range.end += self.lines.len() - old_len;
    }

    fn extend_removed_lines(&mut self, lines: impl IntoIterator<Item = DiffTokenVec<'content>>) {
        let old_len = self.lines.len();
        self.lines
            .extend(lines.into_iter().map(|line| (DiffLineType::Removed, line)));
        self.left_line_range.end += self.lines.len() - old_len;
    }

    fn extend_added_lines(&mut self, lines: impl IntoIterator<Item = DiffTokenVec<'content>>) {
        let old_len = self.lines.len();
        self.lines
            .extend(lines.into_iter().map(|line| (DiffLineType::Added, line)));
        self.right_line_range.end += self.lines.len() - old_len;
    }
}

fn unified_diff_hunks<'content>(
    contents: [&'content BStr; 2],
    options: &UnifiedDiffOptions,
) -> Vec<UnifiedDiffHunk<'content>> {
    let mut hunks = vec![];
    let mut current_hunk = UnifiedDiffHunk {
        left_line_range: 0..0,
        right_line_range: 0..0,
        lines: vec![],
    };
    let diff = diff_by_line(contents, &options.line_diff);
    let mut diff_hunks = diff.hunks().peekable();
    while let Some(hunk) = diff_hunks.next() {
        match hunk.kind {
            DiffHunkKind::Matching => {
                // Just use the right (i.e. new) content. We could count the
                // number of skipped lines separately, but the number of the
                // context lines should match the displayed content.
                let [_, right] = hunk.contents[..].try_into().unwrap();
                let mut lines = right.split_inclusive(|b| *b == b'\n').fuse();
                if !current_hunk.lines.is_empty() {
                    // The previous hunk line should be either removed/added.
                    current_hunk.extend_context_lines(lines.by_ref().take(options.context));
                }
                let before_lines = if diff_hunks.peek().is_some() {
                    lines.by_ref().rev().take(options.context).collect()
                } else {
                    vec![] // No more hunks
                };
                let num_skip_lines = lines.count();
                if num_skip_lines > 0 {
                    let left_start = current_hunk.left_line_range.end + num_skip_lines;
                    let right_start = current_hunk.right_line_range.end + num_skip_lines;
                    if !current_hunk.lines.is_empty() {
                        hunks.push(current_hunk);
                    }
                    current_hunk = UnifiedDiffHunk {
                        left_line_range: left_start..left_start,
                        right_line_range: right_start..right_start,
                        lines: vec![],
                    };
                }
                // The next hunk should be of DiffHunk::Different type if any.
                current_hunk.extend_context_lines(before_lines.into_iter().rev());
            }
            DiffHunkKind::Different => {
                let [left_lines, right_lines] =
                    unzip_diff_hunks_to_lines(Diff::by_word(hunk.contents).hunks());
                current_hunk.extend_removed_lines(left_lines);
                current_hunk.extend_added_lines(right_lines);
            }
        }
    }
    if !current_hunk.lines.is_empty() {
        hunks.push(current_hunk);
    }
    hunks
}

/// Splits `[left, right]` hunk pairs into `[left_lines, right_lines]`.
fn unzip_diff_hunks_to_lines<'content, I>(diff_hunks: I) -> [Vec<DiffTokenVec<'content>>; 2]
where
    I: IntoIterator,
    I::Item: Borrow<DiffHunk<'content>>,
{
    let mut left_lines: Vec<DiffTokenVec<'content>> = vec![];
    let mut right_lines: Vec<DiffTokenVec<'content>> = vec![];
    let mut left_tokens: DiffTokenVec<'content> = vec![];
    let mut right_tokens: DiffTokenVec<'content> = vec![];

    for hunk in diff_hunks {
        let hunk = hunk.borrow();
        match hunk.kind {
            DiffHunkKind::Matching => {
                // TODO: add support for unmatched contexts
                debug_assert!(hunk.contents.iter().all_equal());
                for token in hunk.contents[0].split_inclusive(|b| *b == b'\n') {
                    left_tokens.push((DiffTokenType::Matching, token));
                    right_tokens.push((DiffTokenType::Matching, token));
                    if token.ends_with(b"\n") {
                        left_lines.push(mem::take(&mut left_tokens));
                        right_lines.push(mem::take(&mut right_tokens));
                    }
                }
            }
            DiffHunkKind::Different => {
                let [left, right] = hunk.contents[..]
                    .try_into()
                    .expect("hunk should have exactly two inputs");
                for token in left.split_inclusive(|b| *b == b'\n') {
                    left_tokens.push((DiffTokenType::Different, token));
                    if token.ends_with(b"\n") {
                        left_lines.push(mem::take(&mut left_tokens));
                    }
                }
                for token in right.split_inclusive(|b| *b == b'\n') {
                    right_tokens.push((DiffTokenType::Different, token));
                    if token.ends_with(b"\n") {
                        right_lines.push(mem::take(&mut right_tokens));
                    }
                }
            }
        }
    }

    if !left_tokens.is_empty() {
        left_lines.push(left_tokens);
    }
    if !right_tokens.is_empty() {
        right_lines.push(right_tokens);
    }
    [left_lines, right_lines]
}

fn show_unified_diff_hunks(
    formatter: &mut dyn Formatter,
    contents: [&BStr; 2],
    options: &UnifiedDiffOptions,
) -> io::Result<()> {
    // "If the chunk size is 0, the first number is one lower than one would
    // expect." - https://www.artima.com/weblogs/viewpost.jsp?thread=164293
    //
    // The POSIX spec also states that "the ending line number of an empty range
    // shall be the number of the preceding line, or 0 if the range is at the
    // start of the file."
    // - https://pubs.opengroup.org/onlinepubs/9799919799/utilities/diff.html
    fn to_line_number(range: Range<usize>) -> usize {
        if range.is_empty() {
            range.start
        } else {
            range.start + 1
        }
    }

    for hunk in unified_diff_hunks(contents, options) {
        writeln!(
            formatter.labeled("hunk_header"),
            "@@ -{},{} +{},{} @@",
            to_line_number(hunk.left_line_range.clone()),
            hunk.left_line_range.len(),
            to_line_number(hunk.right_line_range.clone()),
            hunk.right_line_range.len()
        )?;
        for (line_type, tokens) in &hunk.lines {
            let (label, sigil) = match line_type {
                DiffLineType::Context => ("context", " "),
                DiffLineType::Removed => ("removed", "-"),
                DiffLineType::Added => ("added", "+"),
            };
            formatter.with_label(label, |formatter| {
                write!(formatter, "{sigil}")?;
                show_diff_line_tokens(formatter, tokens)
            })?;
            let (_, content) = tokens.last().expect("hunk line must not be empty");
            if !content.ends_with(b"\n") {
                write!(formatter, "\n\\ No newline at end of file\n")?;
            }
        }
    }
    Ok(())
}

fn show_diff_line_tokens(
    formatter: &mut dyn Formatter,
    tokens: &[(DiffTokenType, &[u8])],
) -> io::Result<()> {
    for (token_type, content) in tokens {
        match token_type {
            DiffTokenType::Matching => formatter.write_all(content)?,
            DiffTokenType::Different => {
                formatter.with_label("token", |formatter| formatter.write_all(content))?;
            }
        }
    }
    Ok(())
}

pub async fn show_git_diff(
    formatter: &mut dyn Formatter,
    store: &Store,
    tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    options: &UnifiedDiffOptions,
    marker_style: ConflictMarkerStyle,
) -> Result<(), DiffRenderError> {
    let materialize_options = ConflictMaterializeOptions {
        marker_style,
        marker_len: None,
    };
    let mut diff_stream = materialized_diff_stream(store, tree_diff);
    while let Some(MaterializedTreeDiffEntry { path, values }) = diff_stream.next().await {
        let left_path = path.source();
        let right_path = path.target();
        let left_path_string = left_path.as_internal_file_string();
        let right_path_string = right_path.as_internal_file_string();
        let (left_value, right_value) = values?;

        let left_part = git_diff_part(left_path, left_value, &materialize_options)?;
        let right_part = git_diff_part(right_path, right_value, &materialize_options)?;

        formatter.with_label("file_header", |formatter| {
            writeln!(
                formatter,
                "diff --git a/{left_path_string} b/{right_path_string}"
            )?;
            let left_hash = &left_part.hash;
            let right_hash = &right_part.hash;
            match (left_part.mode, right_part.mode) {
                (None, Some(right_mode)) => {
                    writeln!(formatter, "new file mode {right_mode}")?;
                    writeln!(formatter, "index {left_hash}..{right_hash}")?;
                }
                (Some(left_mode), None) => {
                    writeln!(formatter, "deleted file mode {left_mode}")?;
                    writeln!(formatter, "index {left_hash}..{right_hash}")?;
                }
                (Some(left_mode), Some(right_mode)) => {
                    if let Some(op) = path.copy_operation() {
                        let operation = match op {
                            CopyOperation::Copy => "copy",
                            CopyOperation::Rename => "rename",
                        };
                        // TODO: include similarity index?
                        writeln!(formatter, "{operation} from {left_path_string}")?;
                        writeln!(formatter, "{operation} to {right_path_string}")?;
                    }
                    if left_mode != right_mode {
                        writeln!(formatter, "old mode {left_mode}")?;
                        writeln!(formatter, "new mode {right_mode}")?;
                        if left_hash != right_hash {
                            writeln!(formatter, "index {left_hash}..{right_hash}")?;
                        }
                    } else if left_hash != right_hash {
                        writeln!(formatter, "index {left_hash}..{right_hash} {left_mode}")?;
                    }
                }
                (None, None) => panic!("either left or right part should be present"),
            }
            Ok::<(), DiffRenderError>(())
        })?;

        if left_part.content.contents == right_part.content.contents {
            continue; // no content hunks
        }

        let left_path = match left_part.mode {
            Some(_) => format!("a/{left_path_string}"),
            None => "/dev/null".to_owned(),
        };
        let right_path = match right_part.mode {
            Some(_) => format!("b/{right_path_string}"),
            None => "/dev/null".to_owned(),
        };
        if left_part.content.is_binary || right_part.content.is_binary {
            // TODO: add option to emit Git binary diff
            writeln!(
                formatter,
                "Binary files {left_path} and {right_path} differ"
            )?;
        } else {
            formatter.with_label("file_header", |formatter| {
                writeln!(formatter, "--- {left_path}")?;
                writeln!(formatter, "+++ {right_path}")?;
                io::Result::Ok(())
            })?;
            show_unified_diff_hunks(
                formatter,
                [&left_part.content.contents, &right_part.content.contents].map(BStr::new),
                options,
            )?;
        }
    }
    Ok(())
}

/// Generates diff of non-binary contents in Git format.
fn show_git_diff_texts<T: AsRef<[u8]>>(
    formatter: &mut dyn Formatter,
    [left_path, right_path]: [&str; 2],
    contents: [&Merge<T>; 2],
    options: &UnifiedDiffOptions,
    materialize_options: &ConflictMaterializeOptions,
) -> io::Result<()> {
    formatter.with_label("file_header", |formatter| {
        writeln!(formatter, "diff --git a/{left_path} b/{right_path}")?;
        writeln!(formatter, "--- {left_path}")?;
        writeln!(formatter, "+++ {right_path}")
    })?;
    let [left, right] = contents.map(|content| match content.as_resolved() {
        Some(text) => Cow::Borrowed(BStr::new(text)),
        None => Cow::Owned(materialize_merge_result_to_bytes(
            content,
            materialize_options,
        )),
    });
    show_unified_diff_hunks(formatter, [left.as_ref(), right.as_ref()], options)
}

#[instrument(skip_all)]
pub async fn show_diff_summary(
    formatter: &mut dyn Formatter,
    mut tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    path_converter: &RepoPathUiConverter,
) -> Result<(), DiffRenderError> {
    while let Some(CopiesTreeDiffEntry { path, values }) = tree_diff.next().await {
        let (before, after) = values?;
        let (label, sigil) = diff_status_label_and_char(&path, &before, &after);
        let path = if path.copy_operation().is_some() {
            path_converter.format_copied_path(path.source(), path.target())
        } else {
            path_converter.format_file_path(path.target())
        };
        writeln!(formatter.labeled(label), "{sigil} {path}")?;
    }
    Ok(())
}

pub fn diff_status_label_and_char(
    path: &CopiesTreeDiffEntryPath,
    before: &MergedTreeValue,
    after: &MergedTreeValue,
) -> (&'static str, char) {
    if let Some(op) = path.copy_operation() {
        match op {
            CopyOperation::Copy => ("copied", 'C'),
            CopyOperation::Rename => ("renamed", 'R'),
        }
    } else {
        match (before.is_present(), after.is_present()) {
            (true, true) => ("modified", 'M'),
            (false, true) => ("added", 'A'),
            (true, false) => ("removed", 'D'),
            (false, false) => panic!("values pair must differ"),
        }
    }
}

#[derive(Clone, Debug, Default, Eq, PartialEq)]
pub struct DiffStatOptions {
    /// How lines are tokenized and compared.
    pub line_diff: LineDiffOptions,
}

impl DiffStatOptions {
    fn merge_args(&mut self, args: &DiffFormatArgs) {
        self.line_diff.merge_args(args);
    }
}

#[derive(Clone, Debug)]
pub struct DiffStats {
    entries: Vec<DiffStatEntry>,
}

impl DiffStats {
    /// Calculates stats of changed lines per file.
    pub async fn calculate(
        store: &Store,
        tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
        options: &DiffStatOptions,
        marker_style: ConflictMarkerStyle,
    ) -> BackendResult<Self> {
        let materialize_options = ConflictMaterializeOptions {
            marker_style,
            marker_len: None,
        };
        let entries = materialized_diff_stream(store, tree_diff)
            .map(|MaterializedTreeDiffEntry { path, values }| {
                let (left, right) = values?;
                let left_content = diff_content(path.source(), left, &materialize_options)?;
                let right_content = diff_content(path.target(), right, &materialize_options)?;
                let stat = get_diff_stat_entry(path, [&left_content, &right_content], options);
                BackendResult::Ok(stat)
            })
            .try_collect()
            .await?;
        Ok(Self { entries })
    }

    /// List of stats per file.
    pub fn entries(&self) -> &[DiffStatEntry] {
        &self.entries
    }

    /// Total number of inserted lines.
    pub fn count_total_added(&self) -> usize {
        self.entries
            .iter()
            .filter_map(|stat| stat.added_removed.map(|(added, _)| added))
            .sum()
    }

    /// Total number of deleted lines.
    pub fn count_total_removed(&self) -> usize {
        self.entries
            .iter()
            .filter_map(|stat| stat.added_removed.map(|(_, removed)| removed))
            .sum()
    }
}

#[derive(Clone, Debug)]
pub struct DiffStatEntry {
    pub path: CopiesTreeDiffEntryPath,
    /// Lines added and removed; None for binary files.
    pub added_removed: Option<(usize, usize)>,
    /// Change in file size in bytes.
    pub bytes_delta: isize,
}

fn get_diff_stat_entry(
    path: CopiesTreeDiffEntryPath,
    contents: [&FileContent<BString>; 2],
    options: &DiffStatOptions,
) -> DiffStatEntry {
    let [left_content, right_content] = contents;
    let added_removed = if left_content.is_binary || right_content.is_binary {
        None
    } else {
        let diff = diff_by_line(
            contents.map(|content| &content.contents),
            &options.line_diff,
        );
        let mut added = 0;
        let mut removed = 0;
        for hunk in diff.hunks() {
            match hunk.kind {
                DiffHunkKind::Matching => {}
                DiffHunkKind::Different => {
                    let [left, right] = hunk.contents[..].try_into().unwrap();
                    removed += left.split_inclusive(|b| *b == b'\n').count();
                    added += right.split_inclusive(|b| *b == b'\n').count();
                }
            }
        }
        Some((added, removed))
    };

    DiffStatEntry {
        path,
        added_removed,
        bytes_delta: right_content.contents.len() as isize - left_content.contents.len() as isize,
    }
}

pub fn show_diff_stats(
    formatter: &mut dyn Formatter,
    stats: &DiffStats,
    path_converter: &RepoPathUiConverter,
    display_width: usize,
) -> io::Result<()> {
    let ui_paths = stats
        .entries()
        .iter()
        .map(|stat| {
            if stat.path.copy_operation().is_some() {
                path_converter.format_copied_path(stat.path.source(), stat.path.target())
            } else {
                path_converter.format_file_path(stat.path.target())
            }
        })
        .collect_vec();

    // Entries format like:
    //   path/to/file | 123 ++--
    // or, for binary files:
    //   path/to/file | (binary) +1234 bytes
    //
    // Depending on display widths, we can elide part of the path,
    // and the the ++-- bar will adjust its scale to fill the rest.

    // Choose how many columns to use for the path.  The right side will use the
    // rest. Start with the longest path.  The code below might shorten it.
    let mut max_path_width = ui_paths.iter().map(|s| s.width()).max().unwrap_or(0);

    // Fit to the available display width, but always assume at least a tiny bit of
    // room.
    let available_width = max(display_width.saturating_sub(" | ".len()), 8);

    // Measure the widest right side for line diffs and reduce max_path_width if
    // needed.
    let max_diffs = stats
        .entries()
        .iter()
        .filter_map(|stat| {
            let (added, removed) = stat.added_removed?;
            Some(added + removed)
        })
        .max();
    let diff_number_width = max_diffs.map_or(0, |n| n.to_string().len());
    if max_diffs.is_some() {
        let width = diff_number_width + " ".len();
        // Ensure 30% of the space for displaying the ++-- graph:
        max_path_width =
            max_path_width.min((0.7 * available_width.saturating_sub(width) as f64) as usize);
    };

    // Measure widest right side for binary diffs and reduce max_path_width if
    // needed.
    let max_bytes = stats
        .entries
        .iter()
        .filter(|stat| stat.added_removed.is_none())
        .map(|stat| stat.bytes_delta.abs())
        .max();
    if let Some(max) = max_bytes {
        let width = if max > 0 {
            format!("(binary) {max:+} bytes").len()
        } else {
            "(binary)".len()
        };
        max_path_width = max_path_width.min(available_width.saturating_sub(width));
    }

    // Now that we've chosen the path width, use the rest of the space for the ++--
    // bar.
    let max_bar_width =
        available_width.saturating_sub(max_path_width + diff_number_width + " ".len());
    let factor = match max_diffs {
        Some(max) if max > max_bar_width => max_bar_width as f64 / max as f64,
        _ => 1.0,
    };

    for (stat, ui_path) in iter::zip(stats.entries(), &ui_paths) {
        // replace start of path with ellipsis if the path is too long
        let (path, path_width) = text_util::elide_start(ui_path, "...", max_path_width);
        let path_pad_width = max_path_width - path_width;
        write!(
            formatter,
            "{path}{:path_pad_width$} | ",
            "", // pad to max_path_width
        )?;
        if let Some((added, removed)) = stat.added_removed {
            let bar_length = ((added + removed) as f64 * factor) as usize;
            // If neither adds nor removes are present, bar length should be zero.
            // If only one is present, bar length should be at least 1.
            // If both are present, bar length should be at least 2.
            //
            // Fractional space after scaling is given to whichever of adds/removes is
            // smaller, to show at least one tick for small (but nonzero) counts.
            let bar_length = bar_length.max((added > 0) as usize + (removed > 0) as usize);
            let (bar_added, bar_removed) = if added < removed {
                let len = (added as f64 * factor).ceil() as usize;
                (len, bar_length - len)
            } else {
                let len = (removed as f64 * factor).ceil() as usize;
                (bar_length - len, len)
            };
            write!(
                formatter,
                "{:>diff_number_width$}{}",
                added + removed,
                if bar_added + bar_removed > 0 { " " } else { "" },
            )?;
            write!(formatter.labeled("added"), "{}", "+".repeat(bar_added))?;
            writeln!(formatter.labeled("removed"), "{}", "-".repeat(bar_removed))?;
        } else {
            write!(formatter.labeled("binary"), "(binary)")?;
            if stat.bytes_delta != 0 {
                let label = if stat.bytes_delta < 0 {
                    "removed"
                } else {
                    "added"
                };
                write!(formatter.labeled(label), " {:+}", stat.bytes_delta)?;
                write!(formatter, " bytes")?;
            }
            writeln!(formatter)?;
        }
    }

    let total_added = stats.count_total_added();
    let total_removed = stats.count_total_removed();
    let total_files = stats.entries().len();
    writeln!(
        formatter.labeled("stat-summary"),
        "{} file{} changed, {} insertion{}(+), {} deletion{}(-)",
        total_files,
        if total_files == 1 { "" } else { "s" },
        total_added,
        if total_added == 1 { "" } else { "s" },
        total_removed,
        if total_removed == 1 { "" } else { "s" },
    )?;
    Ok(())
}

pub async fn show_types(
    formatter: &mut dyn Formatter,
    mut tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    path_converter: &RepoPathUiConverter,
) -> Result<(), DiffRenderError> {
    while let Some(CopiesTreeDiffEntry { path, values }) = tree_diff.next().await {
        let (before, after) = values?;
        writeln!(
            formatter.labeled("modified"),
            "{}{} {}",
            diff_summary_char(&before),
            diff_summary_char(&after),
            path_converter.format_copied_path(path.source(), path.target())
        )?;
    }
    Ok(())
}

fn diff_summary_char(value: &MergedTreeValue) -> char {
    match value.as_resolved() {
        Some(None) => '-',
        Some(Some(TreeValue::File { .. })) => 'F',
        Some(Some(TreeValue::Symlink(_))) => 'L',
        Some(Some(TreeValue::GitSubmodule(_))) => 'G',
        None => 'C',
        Some(Some(TreeValue::Tree(_))) | Some(Some(TreeValue::Conflict(_))) => {
            panic!("Unexpected {value:?} in diff")
        }
    }
}

pub async fn show_names(
    formatter: &mut dyn Formatter,
    mut tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    path_converter: &RepoPathUiConverter,
) -> io::Result<()> {
    while let Some(CopiesTreeDiffEntry { path, .. }) = tree_diff.next().await {
        writeln!(
            formatter,
            "{}",
            path_converter.format_file_path(path.target())
        )?;
    }
    Ok(())
}

pub async fn show_templated(
    formatter: &mut dyn Formatter,
    mut tree_diff: BoxStream<'_, CopiesTreeDiffEntry>,
    template: &TemplateRenderer<'_, commit_templater::TreeDiffEntry>,
) -> Result<(), DiffRenderError> {
    while let Some(entry) = tree_diff.next().await {
        let entry = commit_templater::TreeDiffEntry::from_backend_entry_with_copies(entry)?;
        template.format(&entry, formatter)?;
    }
    Ok(())
}