//! Everything that reads from a git repository lives here.
//!
//! Functions take a `git2::Repository` and return plain Rust structs, so the
//! HTML code never has to know about libgit2. `git2::Repository` can't be
//! shared between threads, so the web handlers open a fresh one per request
//! (that's cheap: it just reads a few files).

use git2::{DiffFindOptions, DiffOptions, ObjectType, Oid, Repository, Sort};
use std::path::Path;

use crate::util;

/// Errors from this module. `NotFound` becomes a 404 page, `Git` a 500.
#[derive(Debug)]
pub enum Error {
    NotFound(String),
    Git(git2::Error),
}

impl From<git2::Error> for Error {
    fn from(e: git2::Error) -> Self {
        if e.code() == git2::ErrorCode::NotFound {
            Error::NotFound(e.message().to_string())
        } else {
            Error::Git(e)
        }
    }
}

pub type Result<T> = std::result::Result<T, Error>;

/// Open a bare (`foo.git`) or normal (`foo/.git`) repository at exactly this
/// path. `NO_SEARCH` stops libgit2 from walking up into parent directories.
pub fn open(path: &Path) -> Result<Repository> {
    let flags = git2::RepositoryOpenFlags::NO_SEARCH;
    Ok(Repository::open_ext(path, flags, std::iter::empty::<&str>())?)
}

#[derive(Debug, Clone)]
pub struct CommitInfo {
    pub id: String,
    pub short_id: String,
    pub summary: String,
    pub message: String,
    pub author_name: String,
    pub author_email: String,
    pub time: i64,
    pub offset_minutes: i32,
    pub parents: Vec<String>,
}

impl CommitInfo {
    fn from_commit(c: &git2::Commit) -> CommitInfo {
        let id = c.id().to_string();
        let author = c.author();
        CommitInfo {
            short_id: id[..10.min(id.len())].to_string(),
            id,
            summary: String::from_utf8_lossy(c.summary_bytes().unwrap_or(b"")).into_owned(),
            message: String::from_utf8_lossy(c.message_bytes()).into_owned(),
            author_name: String::from_utf8_lossy(author.name_bytes()).into_owned(),
            author_email: String::from_utf8_lossy(author.email_bytes()).into_owned(),
            time: c.time().seconds(),
            offset_minutes: c.time().offset_minutes(),
            parents: c.parent_ids().map(|p| p.to_string()).collect(),
        }
    }

    pub fn age(&self) -> String {
        util::age(util::now_unix() - self.time)
    }

    pub fn date(&self) -> String {
        util::format_time(self.time, self.offset_minutes)
    }
}

/// A branch or tag and the commit it points to.
#[derive(Debug, Clone)]
pub struct RefInfo {
    pub name: String,
    pub commit: CommitInfo,
}

/// Turn a user-supplied ref (`?h=...`) into a commit id.
///
/// `None` means "the default branch" (HEAD). We only accept branch names, tag
/// names and hex commit ids, never general git revision syntax.
pub fn resolve(repo: &Repository, rev: Option<&str>) -> Result<Oid> {
    let Some(rev) = rev.filter(|r| !r.is_empty()) else {
        return default_commit(repo);
    };
    if !util::is_safe_ref(rev) {
        return Err(Error::NotFound(format!("bad ref {rev:?}")));
    }
    for full in [format!("refs/heads/{rev}"), format!("refs/tags/{rev}")] {
        if let Ok(r) = repo.find_reference(&full) {
            return Ok(r.peel_to_commit()?.id());
        }
    }
    let is_hex = rev.len() >= 4 && rev.len() <= 40 && rev.chars().all(|c| c.is_ascii_hexdigit());
    if is_hex {
        if let Ok(obj) = repo.revparse_single(rev) {
            if let Ok(c) = obj.peel_to_commit() {
                return Ok(c.id());
            }
        }
    }
    Err(Error::NotFound(format!("no such ref {rev:?}")))
}

/// HEAD's commit, or (if HEAD points at a branch that doesn't exist yet) the
/// first branch we can find.
fn default_commit(repo: &Repository) -> Result<Oid> {
    if let Ok(head) = repo.head() {
        if let Ok(c) = head.peel_to_commit() {
            return Ok(c.id());
        }
    }
    for b in repo.branches(Some(git2::BranchType::Local))? {
        let (b, _) = b?;
        if let Ok(c) = b.get().peel_to_commit() {
            return Ok(c.id());
        }
    }
    Err(Error::NotFound("repository has no commits".into()))
}

pub fn commit(repo: &Repository, id: Oid) -> Result<CommitInfo> {
    Ok(CommitInfo::from_commit(&repo.find_commit(id)?))
}

/// Local branches, newest commit first.
pub fn branches(repo: &Repository) -> Result<Vec<RefInfo>> {
    let mut out = Vec::new();
    for b in repo.branches(Some(git2::BranchType::Local))? {
        let (b, _) = b?;
        let Ok(Some(name)) = b.name().map(|n| n.map(str::to_string)) else { continue };
        if let Ok(c) = b.get().peel_to_commit() {
            out.push(RefInfo { name, commit: CommitInfo::from_commit(&c) });
        }
    }
    out.sort_by(|a, b| b.commit.time.cmp(&a.commit.time).then(a.name.cmp(&b.name)));
    Ok(out)
}

/// Tags (lightweight and annotated), newest commit first.
pub fn tags(repo: &Repository) -> Result<Vec<RefInfo>> {
    let mut out = Vec::new();
    for r in repo.references_glob("refs/tags/*")? {
        let r = r?;
        let Ok(name) = r.shorthand().map(str::to_string) else { continue };
        if let Ok(c) = r.peel_to_commit() {
            out.push(RefInfo { name, commit: CommitInfo::from_commit(&c) });
        }
    }
    out.sort_by(|a, b| b.commit.time.cmp(&a.commit.time).then(a.name.cmp(&b.name)));
    Ok(out)
}

/// Time of the newest commit on any branch (for the "idle" column).
pub fn last_change(repo: &Repository) -> Option<i64> {
    let from_branches = branches(repo).ok()?.into_iter().map(|b| b.commit.time).max();
    let from_head = repo.head().ok().and_then(|h| h.peel_to_commit().ok()).map(|c| c.time().seconds());
    from_branches.max(from_head)
}

/// Commits reachable from `start`, newest first. Skips `skip` commits and
/// returns at most `limit`, plus a flag saying whether more exist.
pub fn log(repo: &Repository, start: Oid, skip: usize, limit: usize) -> Result<(Vec<CommitInfo>, bool)> {
    let mut walk = repo.revwalk()?;
    // Newest first, but never show a parent before its child (like `git log`).
    walk.set_sorting(Sort::TOPOLOGICAL | Sort::TIME)?;
    walk.push(start)?;
    let mut out = Vec::new();
    let mut more = false;
    for id in walk.skip(skip) {
        if out.len() == limit {
            more = true;
            break;
        }
        out.push(CommitInfo::from_commit(&repo.find_commit(id?)?));
    }
    Ok((out, more))
}

// ---------------------------------------------------------------- trees ----

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum EntryKind {
    Dir,
    File,
    Symlink,
    Submodule,
}

#[derive(Debug, Clone)]
pub struct TreeEntry {
    pub name: String,
    /// Full path from the repo root, e.g. "src/main.rs".
    pub path: String,
    pub kind: EntryKind,
    pub mode: i32,
    pub size: Option<usize>,
}

/// What lives at a path: a directory listing or a file's bytes.
pub enum PathContent {
    Dir(Vec<TreeEntry>),
    File { data: Vec<u8>, mode: i32 },
}

/// Look up `path` (already cleaned by `util::clean_repo_path`) in a commit.
pub fn path_content(repo: &Repository, commit: Oid, path: &str) -> Result<PathContent> {
    let root = repo.find_commit(commit)?.tree()?;
    if path.is_empty() {
        return Ok(PathContent::Dir(list_tree(repo, &root, "")?));
    }
    let entry =
        root.get_path(Path::new(path)).map_err(|_| Error::NotFound(format!("no such path {path:?}")))?;
    match entry.kind() {
        Some(ObjectType::Tree) => {
            let tree = repo.find_tree(entry.id())?;
            Ok(PathContent::Dir(list_tree(repo, &tree, path)?))
        }
        Some(ObjectType::Blob) => {
            let blob = repo.find_blob(entry.id())?;
            Ok(PathContent::File { data: blob.content().to_vec(), mode: entry.filemode() })
        }
        _ => Err(Error::NotFound(format!("{path:?} is a submodule"))),
    }
}

fn list_tree(repo: &Repository, tree: &git2::Tree, prefix: &str) -> Result<Vec<TreeEntry>> {
    let odb = repo.odb()?;
    let mut out: Vec<TreeEntry> = tree
        .iter()
        .map(|e| {
            let name = String::from_utf8_lossy(e.name_bytes()).into_owned();
            let kind = match (e.kind(), e.filemode()) {
                (Some(ObjectType::Tree), _) => EntryKind::Dir,
                (Some(ObjectType::Commit), _) => EntryKind::Submodule,
                (_, 0o120000) => EntryKind::Symlink,
                _ => EntryKind::File,
            };
            let size = if kind == EntryKind::File {
                odb.read_header(e.id()).ok().map(|(size, _)| size)
            } else {
                None
            };
            let path = if prefix.is_empty() { name.clone() } else { format!("{prefix}/{name}") };
            TreeEntry { name, path, kind, mode: e.filemode(), size }
        })
        .collect();
    // Directories first, then files, each alphabetically (like most git web UIs).
    out.sort_by(|a, b| (a.kind != EntryKind::Dir).cmp(&(b.kind != EntryKind::Dir)).then(a.name.cmp(&b.name)));
    Ok(out)
}

/// Find a README at the root of the commit's tree. Returns (file name, text).
pub fn readme(repo: &Repository, commit: Oid) -> Option<(String, String)> {
    let tree = repo.find_commit(commit).ok()?.tree().ok()?;
    for name in ["README.md", "readme.md", "README.markdown", "README", "README.txt"] {
        if let Some(entry) = tree.get_name(name) {
            if let Ok(blob) = repo.find_blob(entry.id()) {
                return Some((name.to_string(), String::from_utf8_lossy(blob.content()).into_owned()));
            }
        }
    }
    None
}

/// Heuristic used by git itself: a NUL byte in the first 8000 bytes means binary.
pub fn is_binary(data: &[u8]) -> bool {
    data.iter().take(8000).any(|&b| b == 0)
}

// ---------------------------------------------------------------- diffs ----

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum LineKind {
    Context,
    Add,
    Remove,
    HunkHeader,
}

#[derive(Debug, Clone)]
pub struct DiffLine {
    pub kind: LineKind,
    pub text: String,
}

#[derive(Debug, Clone)]
pub struct FileDiff {
    pub old_path: String,
    pub new_path: String,
    /// 'A'dded, 'D'eleted, 'M'odified, 'R'enamed, ...
    pub status: char,
    pub additions: usize,
    pub deletions: usize,
    pub binary: bool,
    pub lines: Vec<DiffLine>,
}

#[derive(Debug, Clone, Default)]
pub struct CommitDiff {
    pub files: Vec<FileDiff>,
    /// True when we stopped early because the diff was huge.
    pub truncated: bool,
}

/// Lines of diff we'll render before giving up (keeps huge commits fast).
const MAX_DIFF_LINES: usize = 20_000;

/// Diff a commit against its first parent (or the empty tree for a root commit).
pub fn commit_diff(repo: &Repository, id: Oid) -> Result<CommitDiff> {
    let commit = repo.find_commit(id)?;
    let new_tree = commit.tree()?;
    let old_tree = match commit.parent(0) {
        Ok(p) => Some(p.tree()?),
        Err(_) => None,
    };
    let mut opts = DiffOptions::new();
    opts.context_lines(3);
    let mut diff = repo.diff_tree_to_tree(old_tree.as_ref(), Some(&new_tree), Some(&mut opts))?;
    diff.find_similar(Some(DiffFindOptions::new().renames(true)))?;

    let mut out = CommitDiff::default();
    let mut total = 0usize;
    for idx in 0..diff.deltas().len() {
        let delta = diff.get_delta(idx).expect("index in range");
        let path_of =
            |f: git2::DiffFile| f.path().map(|p| p.to_string_lossy().into_owned()).unwrap_or_default();
        let mut file = FileDiff {
            old_path: path_of(delta.old_file()),
            new_path: path_of(delta.new_file()),
            status: status_char(delta.status()),
            additions: 0,
            deletions: 0,
            binary: delta.flags().is_binary(),
            lines: Vec::new(),
        };
        if let Some(patch) = git2::Patch::from_diff(&diff, idx)? {
            if patch.delta().flags().is_binary() {
                file.binary = true;
            }
            let (_, adds, dels) = patch.line_stats()?;
            file.additions = adds;
            file.deletions = dels;
            for h in 0..patch.num_hunks() {
                if total >= MAX_DIFF_LINES {
                    out.truncated = true;
                    break;
                }
                let (hunk, n_lines) = patch.hunk(h)?;
                file.lines.push(DiffLine { kind: LineKind::HunkHeader, text: bytes_to_line(hunk.header()) });
                for l in 0..n_lines {
                    let line = patch.line_in_hunk(h, l)?;
                    let kind = match line.origin() {
                        '+' => LineKind::Add,
                        '-' => LineKind::Remove,
                        ' ' => LineKind::Context,
                        _ => continue, // "\ No newline at end of file" markers etc.
                    };
                    file.lines.push(DiffLine { kind, text: bytes_to_line(line.content()) });
                }
                total += n_lines;
            }
        }
        out.files.push(file);
    }
    Ok(out)
}

/// One-letter status like `git log --name-status` prints.
fn status_char(d: git2::Delta) -> char {
    use git2::Delta::*;
    match d {
        Added => 'A',
        Deleted => 'D',
        Modified => 'M',
        Renamed => 'R',
        Copied => 'C',
        Typechange => 'T',
        _ => '?',
    }
}

fn bytes_to_line(b: &[u8]) -> String {
    let s = String::from_utf8_lossy(b);
    s.trim_end_matches(['\n', '\r']).to_string()
}