//! Git bridge (spec §23), export side: each state the current bookmark
//! passed through becomes a Git commit through `git fast-import`, with the
//! state's full tree and the previous state's commit as parent. A state
//! that added exactly one change carries that change's message, author,
//! and `Change-Id`/`Oxy-Hash` trailers; every commit carries `Oxy-State`.
//!
//! Marks make re-exports incremental: `.oxy/git/<target>/git-marks` is
//! git's own marks file and `oxy-marks` maps state refs to marks.

use super::*;
use std::process::{Command, Stdio};
use std::sync::Arc;

#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct GitImportReport {
    pub commits: usize,
    pub merges: usize,
    /// Gitlink entries imported as subrepo nodes with Git pins.
    pub submodules: usize,
    pub heads: usize,
}

/// A running `git cat-file --batch`, for reading many blobs cheaply.
struct CatFile {
    child: std::process::Child,
    stdin: std::process::ChildStdin,
    stdout: std::io::BufReader<std::process::ChildStdout>,
}

impl CatFile {
    fn start(git_dir: &Path) -> Result<CatFile> {
        let mut child = Command::new("git")
            .arg("-C")
            .arg(git_dir)
            .args(["cat-file", "--batch"])
            .stdin(Stdio::piped())
            .stdout(Stdio::piped())
            .stderr(Stdio::null())
            .spawn()
            .map_err(|e| RepoError::Msg(format!("running git cat-file: {e}")))?;
        let stdin = child.stdin.take().unwrap();
        let stdout = std::io::BufReader::new(child.stdout.take().unwrap());
        Ok(CatFile {
            child,
            stdin,
            stdout,
        })
    }

    fn read(&mut self, sha: &str) -> Result<Vec<u8>> {
        use std::io::{BufRead, Read};
        writeln!(self.stdin, "{sha}")?;
        self.stdin.flush()?;
        let mut header = String::new();
        self.stdout.read_line(&mut header)?;
        let parts: Vec<&str> = header.trim_end().split(' ').collect();
        if parts.len() != 3 {
            return fail(format!("git cat-file: object {sha} not found"));
        }
        let size: usize = parts[2]
            .parse()
            .map_err(|_| RepoError::Msg("git cat-file: bad size".into()))?;
        let mut body = vec![0u8; size];
        self.stdout.read_exact(&mut body)?;
        let mut nl = [0u8; 1];
        self.stdout.read_exact(&mut nl)?;
        Ok(body)
    }
}

impl Drop for CatFile {
    fn drop(&mut self) {
        let _ = self.child.kill();
        let _ = self.child.wait();
    }
}

/// One commit as `git log` describes it.
struct GitCommit {
    sha: String,
    parents: Vec<String>,
    author_secs: u64,
    author: String,
    message: String,
}

fn list_commits(git_dir: &Path, branch: &str) -> Result<Vec<GitCommit>> {
    let out = Command::new("git")
        .arg("-C")
        .arg(git_dir)
        .args([
            "log",
            "--reverse",
            "--topo-order",
            "--format=%H%x00%P%x00%at%x00%an <%ae>%x00%B%x1e",
            branch,
        ])
        .output()
        .map_err(|e| RepoError::Msg(format!("running git log: {e}")))?;
    if !out.status.success() {
        return fail(format!(
            "git log {branch} failed: {}",
            String::from_utf8_lossy(&out.stderr).trim()
        ));
    }
    let text = String::from_utf8_lossy(&out.stdout);
    let mut commits = Vec::new();
    for record in text.split('\x1e') {
        let record = record.trim_start_matches('\n');
        if record.trim().is_empty() {
            continue;
        }
        let f: Vec<&str> = record.splitn(5, '\0').collect();
        if f.len() != 5 {
            return fail("unexpected git log record");
        }
        commits.push(GitCommit {
            sha: f[0].to_string(),
            parents: f[1].split_whitespace().map(str::to_string).collect(),
            author_secs: f[2].parse().unwrap_or(0),
            author: f[3].to_string(),
            message: f[4].trim_end().to_string(),
        });
    }
    Ok(commits)
}

/// `path → (mode, blob sha)` for a commit's tree.
fn list_tree(git_dir: &Path, sha: &str) -> Result<BTreeMap<String, (String, String)>> {
    let text = git(git_dir, &["ls-tree", "-r", "-z", sha])?;
    let mut out = BTreeMap::new();
    for entry in text.split('\0') {
        if entry.is_empty() {
            continue;
        }
        let Some((meta, path)) = entry.split_once('\t') else {
            continue;
        };
        let m: Vec<&str> = meta.split(' ').collect();
        if m.len() == 3 {
            out.insert(path.to_string(), (m[0].to_string(), m[2].to_string()));
        }
    }
    Ok(out)
}

/// The ChangeId for a Git commit: its `Change-Id` trailer if it has a
/// 32-hex one, else BLAKE3-128 of the commit hash (spec §23).
fn change_id_for(sha: &str, message: &str) -> ChangeId {
    for line in message.lines().rev() {
        if let Some(v) = line.strip_prefix("Change-Id:") {
            if let Some(id) = ChangeId::from_hex(v.trim()) {
                return id;
            }
        }
    }
    let h = blake3::hash(sha.as_bytes());
    ChangeId(h.as_bytes()[..16].try_into().unwrap())
}

/// Git commits with a known Oxygen counterpart.
struct GitKnown {
    /// Imported commit → its change.
    imported: HashMap<String, ChangeHash>,
    /// Exported commit (or a fetched tip) → the bookmark state it equals.
    exported: HashMap<String, ObjectRef>,
}

#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct GitExportReport {
    pub exported: usize,
    pub skipped: usize,
    /// Subrepo nodes with Oxygen pins, which Git cannot represent.
    pub skipped_subrepos: usize,
    pub branch: String,
    pub heads: usize,
}

fn git(dir: &Path, args: &[&str]) -> Result<String> {
    let out = Command::new("git")
        .arg("-C")
        .arg(dir)
        .args(args)
        .output()
        .map_err(|e| RepoError::Msg(format!("running git: {e}")))?;
    if !out.status.success() {
        return fail(format!(
            "git {} failed: {}",
            args.join(" "),
            String::from_utf8_lossy(&out.stderr).trim()
        ));
    }
    Ok(String::from_utf8_lossy(&out.stdout).trim().to_string())
}

impl Repo {
    fn git_marks_dir(&self, git_dir: &Path) -> Result<PathBuf> {
        let canonical = git_dir
            .canonicalize()
            .unwrap_or_else(|_| git_dir.to_path_buf());
        let key = hash_object(ObjectType::Chunk, canonical.to_string_lossy().as_bytes()).short(16);
        let dir = self.oxy.join("git").join(key);
        fs::create_dir_all(&dir)?;
        Ok(dir)
    }

    /// `state ref → Git commit sha` for bookmark states that equal a Git
    /// commit's tree (recorded by import and fetch).
    fn read_git_states(path: &Path) -> Result<BTreeMap<ObjectRef, String>> {
        let mut out = BTreeMap::new();
        if let Ok(text) = fs::read_to_string(path) {
            for line in text.lines() {
                if let Some((h, sha)) = line.split_once(' ') {
                    if let Some(h) = ObjectRef::from_hex(h) {
                        out.insert(h, sha.trim().to_string());
                    }
                }
            }
        }
        Ok(out)
    }

    fn record_git_state(&self, git_dir: &Path, state: &ObjectRef, sha: &str) -> Result<()> {
        let path = self.git_marks_dir(git_dir)?.join("git-states");
        let mut states = Self::read_git_states(&path)?;
        states.insert(*state, sha.to_string());
        let text: String = states
            .iter()
            .map(|(h, s)| format!("{} {s}\n", h.to_hex()))
            .collect();
        write_atomic(&path, text.as_bytes())
    }

    /// The bare mirror this repository keeps for the Git remote `url`.
    fn git_mirror(&self, url: &str) -> Result<PathBuf> {
        let key = hash_object(ObjectType::Chunk, url.as_bytes()).short(16);
        let dir = self.oxy.join("git").join(key).join("mirror.git");
        if !dir.join("HEAD").exists() {
            fs::create_dir_all(&dir)?;
            git(&dir, &["init", "--bare", "-q"])?;
        }
        Ok(dir)
    }

    /// Git commits already imported into the bookmark, by sha.
    fn imported_commits(&self) -> Result<HashMap<String, ChangeHash>> {
        let mut map = HashMap::new();
        for e in self.log()? {
            if e.is_working_copy {
                continue;
            }
            if let Some(sha) = e
                .change
                .message
                .lines()
                .rev()
                .find_map(|l| l.strip_prefix("Git-Commit: "))
            {
                map.insert(sha.trim().to_string(), e.hash);
            }
        }
        Ok(map)
    }

    /// Bring the mirror's `branch` up to date with the Git remote `url`.
    /// A remote without that branch yet leaves the mirror as it is.
    fn git_mirror_fetch(&self, mirror: &Path, url: &str, branch: &str) -> Result<()> {
        let spec = format!("+refs/heads/{branch}:refs/heads/{branch}");
        match git(mirror, &["fetch", "-q", url, &spec]) {
            Ok(_) => Ok(()),
            Err(e) => {
                let ls = git(mirror, &["ls-remote", "--heads", url, branch]).unwrap_or_default();
                if ls.trim().is_empty() {
                    Ok(())
                } else {
                    Err(e)
                }
            }
        }
    }

    /// Fetch `branch` of the Git repository at `url` (a path or a Git URL)
    /// through a mirror and import the commits not yet in the bookmark
    /// (spec §23). Works on a repository that already has history: the
    /// new changes join the bookmark like a pull.
    pub fn git_fetch(&self, url: &str, branch: &str) -> Result<GitImportReport> {
        let mirror = self.git_mirror(url)?;
        self.git_mirror_fetch(&mirror, url, branch)?;
        if git(
            &mirror,
            &[
                "rev-parse",
                "--verify",
                "-q",
                &format!("refs/heads/{branch}"),
            ],
        )
        .is_err()
        {
            return fail(format!("{url} has no branch {branch:?}"));
        }
        self.import_git_commits(&mirror, branch, &format!("oxy git fetch {url} {branch}"))
    }

    /// Export the bookmark to `branch` of the Git repository at `url`
    /// through the mirror and push it. The remote's tip must already have
    /// been fetched (`git_fetch`), so the push fast-forwards.
    pub fn git_push(&self, url: &str, branch: &str) -> Result<GitExportReport> {
        let mirror = self.git_mirror(url)?;
        self.git_mirror_fetch(&mirror, url, branch)?;
        if let Ok(tip) = git(
            &mirror,
            &[
                "rev-parse",
                "--verify",
                "-q",
                &format!("refs/heads/{branch}"),
            ],
        ) {
            let known = self.git_known(&mirror)?;
            if !known.imported.contains_key(&tip) && !known.exported.contains_key(&tip) {
                return fail(format!(
                    "{url} {branch} has commits not yet fetched; run `oxy git fetch {url}` first"
                ));
            }
        }
        let report = self.git_export_as(&mirror, branch)?;
        let spec = format!("refs/heads/{branch}:refs/heads/{branch}");
        git(&mirror, &["push", "-q", url, &spec])?;
        Ok(report)
    }

    fn read_oxy_marks(path: &Path) -> Result<BTreeMap<ObjectRef, u64>> {
        let mut out = BTreeMap::new();
        if let Ok(text) = fs::read_to_string(path) {
            for line in text.lines() {
                if let Some((h, m)) = line.split_once(' ') {
                    if let (Some(h), Ok(m)) = (ObjectRef::from_hex(h), m.parse()) {
                        out.insert(h, m);
                    }
                }
            }
        }
        Ok(out)
    }

    /// Export the current bookmark to the Git repository at `git_dir`,
    /// creating a bare one if nothing is there. Each state the bookmark
    /// passed through (per the operation log) becomes one commit whose tree
    /// is that state, so the Git branch is a linear history of full trees.
    pub fn git_export(&self, git_dir: &Path) -> Result<GitExportReport> {
        let bookmark = self.config.bookmark.clone();
        self.git_export_as(git_dir, &bookmark)
    }

    /// `git_export` to the Git branch `branch`. A state that came from a
    /// Git commit (`oxy git fetch`) is not re-exported: later states chain
    /// onto that commit, so a push back to the Git remote fast-forwards.
    pub fn git_export_as(&self, git_dir: &Path, branch: &str) -> Result<GitExportReport> {
        if !git_dir.join("HEAD").exists() && !git_dir.join(".git").exists() {
            fs::create_dir_all(git_dir)?;
            git(git_dir, &["init", "--bare", "-q"])?;
        }
        let marks_dir = self.git_marks_dir(git_dir)?;
        let git_marks = marks_dir.join("git-marks");
        let oxy_marks_path = marks_dir.join("oxy-marks");
        let mut marks = Self::read_oxy_marks(&oxy_marks_path)?;
        let git_states = Self::read_git_states(&marks_dir.join("git-states"))?;
        let mut next_mark = marks.values().copied().max().unwrap_or(0) + 1;

        let bookmark = self.config.bookmark.clone();
        let mut stream: Vec<u8> = Vec::new();
        let mut report = GitExportReport {
            branch: branch.to_string(),
            heads: self.bookmark_heads()?.len(),
            ..Default::default()
        };

        // States along the bookmark, oldest first, skipping unchanged and pruned ones.
        let mut ops = self.op_log()?;
        ops.reverse();
        let mut prev: Option<(ObjectRef, Vec<ChangeHash>)> = None;
        // The fast-import parent of the next commit: `:mark` or a commit sha.
        let mut last_parent: Option<String> = None;
        for (_, op) in &ops {
            let Some(state_ref) = op.view.bookmarks.get(&bookmark).copied() else {
                continue;
            };
            if prev.as_ref().is_some_and(|(p, _)| *p == state_ref) {
                continue;
            }
            let Ok(state) = self.store.get_state(&state_ref) else {
                continue;
            };
            if state.heads.is_empty() {
                prev = Some((state_ref, vec![]));
                continue;
            }
            if let Some(sha) = git_states.get(&state_ref) {
                report.skipped += 1;
                last_parent = Some(sha.clone());
                prev = Some((state_ref, state.heads.clone()));
                continue;
            }
            if let Some(m) = marks.get(&state_ref) {
                report.skipped += 1;
                last_parent = Some(format!(":{m}"));
                prev = Some((state_ref, state.heads.clone()));
                continue;
            }
            let prev_heads = prev.as_ref().map(|(_, h)| h.clone()).unwrap_or_default();
            let added = self
                .negotiate(&prev_heads, &state.heads)
                .unwrap_or_default();
            let loaded = self.load_state(&state.heads)?;
            let snapshot = loaded.materialize();
            let (author, secs, mut message) = match added.as_slice() {
                [h] => {
                    let c = self.store.get_change(h)?;
                    let mut m = c.message.clone();
                    if !m.ends_with('\n') {
                        m.push('\n');
                    }
                    m.push_str(&format!(
                        "\nChange-Id: {}\nOxy-Hash: {}\n",
                        c.change_id.to_hex(),
                        h.to_hex()
                    ));
                    (self.git_author(&c), c.hlc.wall_ms / 1000, m)
                }
                [] => (
                    "oxy <oxy@localhost>".to_string(),
                    op.hlc.wall_ms / 1000,
                    format!("oxy: {}\n\n", op.command),
                ),
                many => {
                    let mut m = format!("oxy: {} ({} changes)\n\n", op.command, many.len());
                    for h in many {
                        let c = self.store.get_change(h)?;
                        m.push_str(&format!(
                            "- {} {}\n",
                            c.change_id.short(8),
                            c.message.lines().next().unwrap_or("")
                        ));
                    }
                    m.push_str(&format!(
                        "\nOxy-Changes: {}\n",
                        many.iter()
                            .map(|h| h.to_hex())
                            .collect::<Vec<_>>()
                            .join(" ")
                    ));
                    ("oxy <oxy@localhost>".to_string(), op.hlc.wall_ms / 1000, m)
                }
            };
            message.push_str(&format!("Oxy-State: {}\n", state_ref.to_hex()));
            if snapshot.is_conflicted() {
                message.push_str("Oxy-Conflicted: true\n");
            }
            let mark = next_mark;
            next_mark += 1;
            let parents: Vec<String> = last_parent.clone().into_iter().collect();
            self.write_commit(
                &mut stream,
                "refs/oxy/export",
                mark,
                &author,
                secs,
                &message,
                &parents,
                &snapshot,
            )?;
            marks.insert(state_ref, mark);
            last_parent = Some(format!(":{mark}"));
            prev = Some((state_ref, state.heads.clone()));
            report.exported += 1;
        }
        if let Some(p) = &last_parent {
            stream.extend_from_slice(format!("reset refs/heads/{branch}\nfrom {p}\n\n").as_bytes());
        }
        // Tags: a tag whose state was exported as a bookmark state points at
        // that commit; otherwise its state gets a commit of its own.
        for t in self.tags()? {
            let target = match (git_states.get(&t.state), marks.get(&t.state)) {
                (Some(sha), _) => sha.clone(),
                (None, Some(m)) => format!(":{m}"),
                (None, None) => {
                    let loaded = self.load_state(&t.heads)?;
                    let snapshot = loaded.materialize();
                    let mark = next_mark;
                    next_mark += 1;
                    let message = format!(
                        "oxy: tag {}\n\n{}Oxy-State: {}\n",
                        t.name,
                        if t.message.is_empty() {
                            String::new()
                        } else {
                            format!("{}\n\n", t.message)
                        },
                        t.state.to_hex()
                    );
                    self.write_commit(
                        &mut stream,
                        "refs/oxy/export",
                        mark,
                        "oxy <oxy@localhost>",
                        Hlc::now().wall_ms / 1000,
                        &message,
                        &[],
                        &snapshot,
                    )?;
                    marks.insert(t.state, mark);
                    report.exported += 1;
                    format!(":{mark}")
                }
            };
            stream.extend_from_slice(
                format!("reset refs/tags/{}\nfrom {target}\n\n", t.name).as_bytes(),
            );
        }
        stream.extend_from_slice(b"done\n");

        let mut cmd = Command::new("git");
        // `--force`: the scratch ref may jump between unrelated states.
        cmd.arg("-C")
            .arg(git_dir)
            .args(["fast-import", "--quiet", "--done", "--force"])
            .arg(format!("--export-marks={}", git_marks.display()));
        if git_marks.exists() {
            cmd.arg(format!("--import-marks={}", git_marks.display()));
        }
        let mut child = cmd
            .stdin(Stdio::piped())
            .stdout(Stdio::null())
            .stderr(Stdio::piped())
            .spawn()
            .map_err(|e| RepoError::Msg(format!("running git fast-import: {e}")))?;
        child.stdin.take().unwrap().write_all(&stream)?;
        let out = child.wait_with_output()?;
        if !out.status.success() {
            return fail(format!(
                "git fast-import failed: {}",
                String::from_utf8_lossy(&out.stderr).trim()
            ));
        }
        let text: String = marks
            .iter()
            .map(|(h, m)| format!("{} {m}\n", h.to_hex()))
            .collect();
        write_atomic(&oxy_marks_path, text.as_bytes())?;
        let _ = git(git_dir, &["update-ref", "-d", "refs/oxy/export"]);
        report.skipped_subrepos = self.git_export_skipped.replace(0);
        Ok(report)
    }

    /// Import branch `branch` of the Git repository at `git_dir` into this
    /// (empty) repository's bookmark (spec §23): one change per commit, its
    /// edits the difference between the union of its parents' states and its
    /// tree, so merge commits become resolutions. Submodules are skipped.
    pub fn import_git(&self, git_dir: &Path, branch: &str) -> Result<GitImportReport> {
        if !self.bookmark_heads()?.is_empty() {
            return fail("git import needs an empty bookmark; import into a fresh repository");
        }
        self.import_git_commits(
            git_dir,
            branch,
            &format!("oxy git import {} {branch}", git_dir.display()),
        )
    }

    /// Import the commits of `branch` that the bookmark does not have yet,
    /// then move the bookmark to the union of its heads and theirs.
    fn import_git_commits(
        &self,
        git_dir: &Path,
        branch: &str,
        command: &str,
    ) -> Result<GitImportReport> {
        let batch = self.store.begin_batch()?;
        let result = self.import_git_commits_batched(git_dir, branch, command);
        batch.finish()?;
        result
    }

    fn import_git_commits_batched(
        &self,
        git_dir: &Path,
        branch: &str,
        command: &str,
    ) -> Result<GitImportReport> {
        let commits = list_commits(git_dir, branch)?;
        let mut cat = CatFile::start(git_dir)?;
        let mut report = GitImportReport::default();
        let mut known = self.git_known(git_dir)?;
        let old_heads = self.bookmark_heads()?;
        let mut new_changes: Vec<Change> = Vec::new();
        // The state after the most recently imported commit, reused when the
        // next commit's first parent is that commit.
        let mut last: Option<(String, State)> = None;
        let mut blob_cache: HashMap<String, Vec<u8>> = HashMap::new();
        let mut cache_bytes: usize = 0;

        for gc in &commits {
            if known.imported.contains_key(&gc.sha) || known.exported.contains_key(&gc.sha) {
                continue;
            }
            // Base: union of every parent's state.
            let mut base = match (&gc.parents.first(), &last) {
                (Some(p1), Some((sha, st))) if *p1 == sha => st.clone(),
                (Some(p1), _) => {
                    let mut st = State::new();
                    self.add_git_ancestors(&mut st, git_dir, &[p1.as_str()], &known)?;
                    st
                }
                (None, _) => State::new(),
            };
            if gc.parents.len() > 1 {
                report.merges += 1;
                let mut args: Vec<String> = gc.parents[1..].to_vec();
                args.push(format!("^{}", gc.parents[0]));
                let refs: Vec<&str> = args.iter().map(String::as_str).collect();
                self.add_git_ancestors(&mut base, git_dir, &refs, &known)?;
            }
            let base_snapshot = base.materialize();

            // The commit's tree as in-memory entries.
            let mut entries: BTreeMap<String, FsEntry> = BTreeMap::new();
            let mut gitlinks: Vec<(String, String)> = Vec::new();
            for (path, (mode, sha)) in list_tree(git_dir, &gc.sha)? {
                let (kind, exec) = match mode.as_str() {
                    "100644" | "100664" => (NodeKind::File, false),
                    "100755" => (NodeKind::File, true),
                    "120000" => (NodeKind::Symlink, false),
                    "160000" => {
                        gitlinks.push((path, sha));
                        continue;
                    }
                    _ => continue,
                };
                let content = match blob_cache.get(&sha) {
                    Some(c) => c.clone(),
                    None => {
                        let c = cat.read(&sha)?;
                        if cache_bytes + c.len() > 256 * 1024 * 1024 {
                            blob_cache.clear();
                            cache_bytes = 0;
                        }
                        cache_bytes += c.len();
                        blob_cache.insert(sha.clone(), c.clone());
                        c
                    }
                };
                entries.insert(path, FsEntry::new(kind, exec, content));
            }

            // Submodules (spec §23): gitlinks become subrepo nodes with Git
            // pins, and `.gitmodules` also yields `.oxymodules`.
            if !gitlinks.is_empty() {
                report.submodules += gitlinks.len();
                let gm = entries
                    .get(".gitmodules")
                    .map(|e| {
                        crate::subrepo::Modules::from_gitmodules(&String::from_utf8_lossy(
                            &e.content,
                        ))
                    })
                    .unwrap_or_default();
                let mut modules = entries
                    .get(".oxymodules")
                    .map(|e| crate::subrepo::Modules::parse(&String::from_utf8_lossy(&e.content)))
                    .unwrap_or_default();
                for (path, sha) in &gitlinks {
                    let m = gm.entries.get(path);
                    let branch = m
                        .and_then(|m| m.bookmark.clone())
                        .unwrap_or_else(|| "HEAD".into());
                    entries.insert(
                        path.clone(),
                        FsEntry::new(
                            NodeKind::Subrepo,
                            false,
                            crate::subrepo::Pin::Git {
                                sha: sha.clone(),
                                branch,
                            }
                            .encode(),
                        ),
                    );
                    if let Some(m) = m {
                        modules.entries.insert(path.clone(), m.clone());
                    }
                }
                if !modules.entries.is_empty() {
                    entries.insert(
                        ".oxymodules".into(),
                        FsEntry::new(NodeKind::File, false, modules.to_text().into_bytes()),
                    );
                }
            }
            let mut built =
                self.build_from_entries(&base, &base_snapshot, HashMap::new(), &entries)?;
            // A file or submodule with several live blobs (branches that
            // replaced it independently) is resolved to the commit's blob:
            // Git's merge is the authority here, unlike a working copy,
            // where a candidate on disk is not yet a decision (spec §23).
            for (path, f) in &base_snapshot.files {
                let (FileContent::Opaque { blobs }, Some(e)) = (&f.content, entries.get(path))
                else {
                    continue;
                };
                if blobs.len() < 2 || f.kind != e.kind {
                    continue;
                }
                let chosen = self.store_blob(&e.content)?;
                for b in blobs {
                    built.edits.push(Edit::Replace {
                        node: f.node,
                        old: Some(*b),
                        new: Some(chosen),
                    });
                }
                built.resolves = true;
                built
                    .contributors
                    .extend(base.live_replaces(&f.node).into_iter().map(|(c, _)| c));
            }
            let mut deps: BTreeSet<ChangeHash> = built.contributors.iter().copied().collect();
            let mut message = gc.message.clone();
            if !message.ends_with('\n') {
                message.push('\n');
            }
            message.push_str(&format!(
                "\nGit-Commit: {}\nGit-Author: {}\n",
                gc.sha, gc.author
            ));
            let mut change = Change {
                change_id: change_id_for(&gc.sha, &gc.message),
                deps: vec![],
                author: ObjectRef::ZERO,
                hlc: Hlc {
                    wall_ms: gc.author_secs * 1000,
                    logical: 0,
                },
                message,
                tree_ops: built.tree_ops,
                edits: built.edits,
                kind: if built.resolves {
                    ChangeKind::Resolution
                } else {
                    ChangeKind::Normal
                },
            };
            deps.extend(referenced_changes(&change, &base));
            change.deps = deps.into_iter().collect();
            change.normalize();
            let hash = self.store.put_change(&change)?;
            new_changes.push(change.clone());
            base.add_unchecked(change);
            known.imported.insert(gc.sha.clone(), hash);
            last = Some((gc.sha.clone(), base));
            report.commits += 1;
        }

        // The bookmark: heads of the old state plus every new change.
        report.heads = old_heads.len();
        if last.is_none() {
            return Ok(report);
        }
        let mut all = self.load_state(&old_heads)?;
        for c in new_changes {
            all.add_unchecked(c);
        }
        let mut heads = all.heads();
        heads.sort();
        report.heads = heads.len();
        let (op_ref, op) = self.head_op()?;
        let mut view = op.view.clone();
        let old_state_ref = view.bookmarks.get(&self.config.bookmark).copied();
        let state_ref = self.store.put_state(&StateObject::new(heads.clone()))?;
        view.bookmarks
            .insert(self.config.bookmark.clone(), state_ref);
        self.record_op(&[op_ref], command, view)?;
        // The new state equals the tip's tree when the old one was itself a
        // Git commit's tree (fetched or exported), or there was none.
        let old_known = old_heads.is_empty()
            || old_state_ref.is_some_and(|s| known.exported.values().any(|e| *e == s));
        if let (Some(tip), true) = (commits.last(), old_known) {
            self.record_git_state(git_dir, &state_ref, &tip.sha)?;
        }
        // The working copy follows when it was clean on the old heads.
        if old_heads.is_empty() {
            self.checkout(&heads, true, &format!("{command} (checkout)"))?;
        } else {
            let snap = self.snapshot(command)?;
            let clean = snap.wc.tree_ops.is_empty() && snap.wc.edits.is_empty();
            let mut base = snap.base.maximal(&snap.wc.deps);
            base.sort();
            let mut sorted_old = old_heads.clone();
            sorted_old.sort();
            if clean && base == sorted_old {
                self.checkout(&heads, false, &format!("{command} (checkout)"))?;
            }
        }
        Ok(report)
    }

    /// Add to `st` the changes of every Git commit `rev-list args` lists:
    /// an imported commit's change, or the whole state an exported commit
    /// was made from (which may hold changes Git never saw).
    fn add_git_ancestors(
        &self,
        st: &mut State,
        git_dir: &Path,
        args: &[&str],
        known: &GitKnown,
    ) -> Result<()> {
        let mut full = vec!["rev-list"];
        full.extend(args);
        let out = git(git_dir, &full)?;
        for sha in out.lines() {
            if let Some(h) = known.imported.get(sha) {
                st.add_unchecked(self.store.get_change(h)?);
            } else if let Some(state_ref) = known.exported.get(sha) {
                let heads = self.store.get_state(state_ref)?.heads;
                for h in self.negotiate(&[], &heads)? {
                    st.add_unchecked(self.store.get_change(&h)?);
                }
            }
        }
        Ok(())
    }

    /// Commits this repository can relate to Oxygen history: imported
    /// ones by change, exported ones (through `git_dir`'s marks) by state.
    fn git_known(&self, git_dir: &Path) -> Result<GitKnown> {
        let imported = self.imported_commits()?;
        let marks_dir = self.git_marks_dir(git_dir)?;
        let oxy_marks = Self::read_oxy_marks(&marks_dir.join("oxy-marks"))?;
        let mut sha_of_mark: HashMap<u64, String> = HashMap::new();
        if let Ok(text) = fs::read_to_string(marks_dir.join("git-marks")) {
            for line in text.lines() {
                if let Some((m, sha)) = line.split_once(' ') {
                    if let Ok(m) = m.trim_start_matches(':').parse::<u64>() {
                        sha_of_mark.insert(m, sha.trim().to_string());
                    }
                }
            }
        }
        let mut exported = HashMap::new();
        for (state_ref, m) in oxy_marks {
            if let Some(sha) = sha_of_mark.get(&m) {
                exported.insert(sha.clone(), state_ref);
            }
        }
        for (state_ref, sha) in Self::read_git_states(&marks_dir.join("git-states"))? {
            exported.entry(sha).or_insert(state_ref);
        }
        Ok(GitKnown { imported, exported })
    }

    fn git_author(&self, c: &Change) -> String {
        match self.store.get_identity(&c.author) {
            Ok(id) if !id.name.is_empty() => format!(
                "{} <{}>",
                id.name.replace(['<', '>', '\n'], " "),
                id.email.replace(['<', '>', '\n', ' '], "")
            ),
            _ => "oxy <oxy@localhost>".to_string(),
        }
    }

    #[allow(clippy::too_many_arguments)]
    fn write_commit(
        &self,
        out: &mut Vec<u8>,
        reference: &str,
        mark: u64,
        author: &str,
        secs: u64,
        message: &str,
        parents: &[String],
        snapshot: &Snapshot,
    ) -> Result<()> {
        out.extend_from_slice(format!("commit {reference}\nmark :{mark}\nauthor {author} {secs} +0000\ncommitter {author} {secs} +0000\ndata {}\n{message}\n", message.len()).as_bytes());
        for (i, p) in parents.iter().enumerate() {
            out.extend_from_slice(
                format!("{} {p}\n", if i == 0 { "from" } else { "merge" }).as_bytes(),
            );
        }
        out.extend_from_slice(b"deleteall\n");
        let has_gitmodules = snapshot.files.contains_key(".gitmodules");
        for (path, f) in &snapshot.files {
            if path == ".oxymodules" {
                // Git reads `.gitmodules`; derive it unless the tree has one.
                if !has_gitmodules {
                    if let FileContent::Text { bytes, .. } = &f.content {
                        let gm = crate::subrepo::Modules::parse(&String::from_utf8_lossy(bytes))
                            .to_gitmodules();
                        if !gm.is_empty() {
                            out.extend_from_slice(
                                format!("M 100644 inline .gitmodules\ndata {}\n", gm.len())
                                    .as_bytes(),
                            );
                            out.extend_from_slice(gm.as_bytes());
                            out.push(b'\n');
                        }
                    }
                }
                continue;
            }
            if f.kind == NodeKind::Subrepo {
                if let FileContent::Opaque { blobs } = &f.content {
                    if let Some(b) = blobs.first() {
                        match crate::subrepo::Pin::decode(&self.read_blob(b)?)? {
                            crate::subrepo::Pin::Git { sha, .. } => {
                                out.extend_from_slice(format!("M 160000 {sha} {path}\n").as_bytes())
                            }
                            crate::subrepo::Pin::Oxy { .. } => self
                                .git_export_skipped
                                .set(self.git_export_skipped.get() + 1),
                        }
                    }
                }
                continue;
            }
            let (mode, bytes): (&str, Vec<u8>) = match (&f.kind, &f.content) {
                (NodeKind::Dir, _) => continue,
                (NodeKind::Symlink, FileContent::Opaque { blobs }) => (
                    "120000",
                    blobs
                        .first()
                        .map(|b| self.read_blob(b))
                        .transpose()?
                        .unwrap_or_default(),
                ),
                (_, FileContent::Text { bytes, .. }) => {
                    (if f.mode.exec { "100755" } else { "100644" }, bytes.clone())
                }
                (_, FileContent::Opaque { blobs }) => (
                    if f.mode.exec { "100755" } else { "100644" },
                    blobs
                        .first()
                        .map(|b| self.read_blob(b))
                        .transpose()?
                        .unwrap_or_default(),
                ),
                (_, FileContent::Dir) => continue,
            };
            out.extend_from_slice(
                format!("M {mode} inline {path}\ndata {}\n", bytes.len()).as_bytes(),
            );
            out.extend_from_slice(&bytes);
            out.push(b'\n');
        }
        out.push(b'\n');
        Ok(())
    }
}

/// What `verify_git_import` found (spec §23: import must reproduce every tree).
#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct GitVerifyReport {
    pub commits: usize,
    pub files_compared: usize,
    /// `(commit, what differs)`; empty when every commit's tree is reproduced.
    pub mismatches: Vec<(String, String)>,
}

impl Repo {
    /// Check an import against its Git source: for every commit of
    /// `branch`, the state formed by the changes imported for it and all
    /// its ancestors must materialize to exactly that commit's tree, byte
    /// for byte, with the same executable bits, symlink targets and
    /// submodule pins. Reads only; stops listing after 50 differences.
    pub fn verify_git_import(&self, git_dir: &Path, branch: &str) -> Result<GitVerifyReport> {
        let commits = list_commits(git_dir, branch)?;
        let imported = self.imported_commits()?;
        let mut cat = CatFile::start(git_dir)?;
        let mut rep = GitVerifyReport::default();
        // Changes of each commit and all its ancestors, built up in topological order.
        let mut closure: HashMap<String, Arc<BTreeSet<ChangeHash>>> = HashMap::new();
        for c in &commits {
            rep.commits += 1;
            let mut set: BTreeSet<ChangeHash> = BTreeSet::new();
            for p in &c.parents {
                if let Some(ps) = closure.get(p) {
                    set.extend(ps.iter().copied());
                }
            }
            match imported.get(&c.sha) {
                Some(h) => {
                    set.insert(*h);
                }
                None => {
                    rep.mismatches.push((
                        c.sha.clone(),
                        "no imported change carries this commit".into(),
                    ));
                }
            }
            let set = Arc::new(set);
            closure.insert(c.sha.clone(), set.clone());
            if rep.mismatches.len() >= 50 {
                break;
            }
            let heads: Vec<ChangeHash> = set.iter().copied().collect();
            let state = self.load_state(&heads)?;
            let snap = state.materialize();
            let tree = list_tree(git_dir, &c.sha)?;
            let short = &c.sha[..12.min(c.sha.len())];
            let mut differ = |what: String| rep.mismatches.push((c.sha.clone(), what));
            if snap.is_conflicted() {
                // Name the changes behind each conflicted file, by their Git commits.
                let mut detail = Vec::new();
                for (path, f) in &snap.files {
                    if !f.is_conflicted() {
                        continue;
                    }
                    let who: Vec<String> = state
                        .conflict_regions(&f.node)
                        .contributors
                        .iter()
                        .filter_map(|h| state.get(h))
                        .map(|m| {
                            let sha = m
                                .message
                                .lines()
                                .rev()
                                .find_map(|l| l.strip_prefix("Git-Commit: "))
                                .unwrap_or("?");
                            format!(
                                "{} {:?}",
                                &sha[..12.min(sha.len())],
                                m.message.lines().next().unwrap_or("")
                            )
                        })
                        .collect();
                    detail.push(format!("{path} between {}", who.join(" and ")));
                }
                differ(format!(
                    "{short}: the state is conflicted: {}",
                    detail.join("; ")
                ));
            }
            for (path, (mode, sha)) in &tree {
                let Some(f) = snap.files.get(path) else {
                    differ(format!("{short}: {path} is missing"));
                    continue;
                };
                rep.files_compared += 1;
                match mode.as_str() {
                    "160000" => match (&f.kind, &f.content) {
                        (NodeKind::Subrepo, FileContent::Opaque { blobs }) if blobs.len() == 1 => {
                            let pin = crate::subrepo::Pin::decode(&self.read_blob(&blobs[0])?)?;
                            if !matches!(&pin, crate::subrepo::Pin::Git { sha: s, .. } if s == sha)
                            {
                                differ(format!("{short}: {path} pins another commit"));
                            }
                        }
                        (kind, content) => differ(format!(
                            "{short}: {path} is not one submodule pin: {kind:?} with {}",
                            match content {
                                FileContent::Opaque { blobs } => format!("{} pins", blobs.len()),
                                FileContent::Text { .. } => "text".into(),
                                FileContent::Dir => "a directory".into(),
                            }
                        )),
                    },
                    "120000" => {
                        let target = cat.read(sha)?;
                        match (&f.kind, &f.content) {
                            (NodeKind::Symlink, FileContent::Opaque { blobs })
                                if blobs.len() == 1 && self.read_blob(&blobs[0])? == target => {}
                            _ => differ(format!("{short}: symlink {path} differs")),
                        }
                    }
                    _ => {
                        let want = cat.read(sha)?;
                        let have = match &f.content {
                            FileContent::Text { bytes, .. } => bytes.clone(),
                            FileContent::Opaque { blobs } if blobs.len() == 1 => {
                                self.read_blob(&blobs[0])?
                            }
                            _ => {
                                differ(format!("{short}: {path} is not one file"));
                                continue;
                            }
                        };
                        if have != want {
                            differ(format!(
                                "{short}: {path} differs: {}",
                                first_difference(&have, &want)
                            ));
                        }
                        if f.mode.exec != (mode == "100755") {
                            differ(format!("{short}: {path} executable bit differs"));
                        }
                    }
                }
            }
            // Import records submodules in `.oxymodules` as well (spec §23),
            // and export leaves it out again.
            let has_submodules = tree.values().any(|(m, _)| m == "160000");
            for (path, f) in &snap.files {
                if f.kind != NodeKind::Dir
                    && !tree.contains_key(path)
                    && !(has_submodules && path == ".oxymodules")
                {
                    differ(format!("{short}: {path} is extra"));
                }
            }
            if rep.mismatches.len() >= 50 {
                break;
            }
        }
        Ok(rep)
    }
}

/// Where two versions of a file first part, for a verification report.
fn first_difference(have: &[u8], want: &[u8]) -> String {
    let (h, w): (Vec<&[u8]>, Vec<&[u8]>) = (
        have.split(|b| *b == b'\n').collect(),
        want.split(|b| *b == b'\n').collect(),
    );
    let i = h
        .iter()
        .zip(&w)
        .position(|(a, b)| a != b)
        .unwrap_or(h.len().min(w.len()));
    let show = |l: Option<&&[u8]>| {
        l.map(|l| String::from_utf8_lossy(&l[..l.len().min(60)]).into_owned())
            .unwrap_or_else(|| "(end)".into())
    };
    format!(
        "line {}: has {:?}, Git has {:?} ({} lines, Git has {})",
        i + 1,
        show(h.get(i)),
        show(w.get(i)),
        h.len(),
        w.len()
    )
}