vivac 0.6.0

//! `Anchor` — a stable identity for the tree's state, and what changed since.
//!
//! The model does not depend on git: it needs two primitives and nothing
//! else. The implementations are `Git` and `Null`, and **`Null` defines the
//! floor of the product**, it is not filler: with no version control the
//! tree, the stack, the vivacs and `why` all keep working whole. The only
//! thing lost is precision by change, and the `brief` swaps in plain age
//! rather than inventing precision it does not have.
//!
//! **`snapshot` spawns no subprocess.** `push` creates a vivac and a vivac
//! needs an anchor, so this falls on the write path, whose budget is 5 ms;
//! starting `git` on Windows costs between 15 and 30. `.git/HEAD` is read
//! and the reference resolved by hand. `changed_since` does shell out to
//! git, because it only runs on reads.

use std::path::{Path, PathBuf};

/// Identity of the tree state at a moment. Empty means "there is none",
/// which is a legitimate state and not an error.
#[derive(Debug, Clone, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize)]
pub struct AnchorRef {
    pub kind: String,
    pub id: String,
}

impl AnchorRef {
    pub fn is_empty_tree(&self) -> bool {
        self.id.is_empty()
    }

    /// Short prefix, the way git does it.
    pub fn short(&self) -> &str {
        let n = self.id.len().min(7);
        &self.id[..n]
    }
}

#[derive(Debug, Clone)]
pub struct Change {
    pub file_path: String,
    pub times: usize,
}

pub trait Anchor {
    fn snapshot(&self) -> AnchorRef;
    fn changed_since(&self, r: &AnchorRef) -> Vec<Change>;
}

/// No version control.
///
/// `MODEL.md` §8 proposed a merkle of the working set. **Not in v0.1**: the
/// working set has no bound, and hashing it sits on the write path, which
/// has a 5 ms budget. The performance pillar sets a ceiling a feature must
/// respect in order to exist, and this one did not. It stays an anchor with
/// no identity, which is exactly the degradation `BRIEF-SPEC.md` §6 already
/// specifies.
pub struct Null;

impl Anchor for Null {
    fn snapshot(&self) -> AnchorRef {
        AnchorRef {
            kind: "null".into(),
            id: String::new(),
        }
    }

    fn changed_since(&self, _r: &AnchorRef) -> Vec<Change> {
        vec![]
    }
}

pub struct Git {
    root: PathBuf,
    gitdir: PathBuf,
}

/// Where `.git` lives relative to a starting directory: the outcome of the
/// upward walk, and nothing about the repository's live state. A `.git`
/// does not move once a process starts, so this is safe to cache; a `HEAD`
/// does, every commit, which is why nothing past this point is.
#[derive(Clone)]
struct Location {
    root: PathBuf,
    gitdir: PathBuf,
}

type LocationCache = std::sync::Mutex<std::collections::HashMap<PathBuf, Option<Location>>>;

/// Caches `locate`'s walk by the directory it started from, for the life of
/// the process. A long-lived server calls `detect` on the same root many
/// times over a session -- every write used to, before `t192` -- and the
/// walk up the filesystem is the same answer every time; only what `HEAD`
/// holds is allowed to change underneath it.
fn location_cache() -> &'static LocationCache {
    static CACHE: std::sync::OnceLock<LocationCache> = std::sync::OnceLock::new();
    CACHE.get_or_init(Default::default)
}

fn locate_cached(root: &Path) -> Option<Location> {
    let mut cache = location_cache().lock().unwrap_or_else(|e| e.into_inner());
    if let Some(hit) = cache.get(root) {
        return hit.clone();
    }
    let found = locate(root);
    cache.insert(root.to_path_buf(), found.clone());
    found
}

/// The upward walk itself, isolated from the cache around it so the cache
/// stays a thin wrapper over a pure function.
fn locate(root: &Path) -> Option<Location> {
    let mut d = root.to_path_buf();
    loop {
        let g = d.join(".git");
        if g.is_dir() {
            return Some(Location { root: d, gitdir: g });
        }
        if g.is_file() {
            // Worktree or submodule: .git is a file holding `gitdir: <path>`.
            let t = std::fs::read_to_string(&g).ok()?;
            let p = t.trim().strip_prefix("gitdir:")?.trim();
            let abs = if Path::new(p).is_absolute() {
                PathBuf::from(p)
            } else {
                d.join(p)
            };
            return Some(Location {
                root: d,
                gitdir: abs,
            });
        }
        if !d.pop() {
            return None;
        }
    }
}

/// Picks an implementation by looking for a usable `.git` from `root`.
pub fn detect(root: &Path) -> Box<dyn Anchor> {
    match Git::new(root) {
        Some(g) => Box::new(g),
        None => Box::new(Null),
    }
}

impl Git {
    fn new(root: &Path) -> Option<Git> {
        locate_cached(root).map(|l| Git {
            root: l.root,
            gitdir: l.gitdir,
        })
    }

    /// Resolves `.git/HEAD` spawning nothing. Three cases: a direct sha
    /// (detached HEAD), a reference with its file, and a packed reference.
    fn head(&self) -> Option<String> {
        let h = std::fs::read_to_string(self.gitdir.join("HEAD")).ok()?;
        let h = h.trim();
        let Some(refname) = h.strip_prefix("ref:").map(str::trim) else {
            return is_sha(h).then(|| h.to_string());
        };
        if let Ok(s) = std::fs::read_to_string(self.gitdir.join(refname)) {
            let s = s.trim().to_string();
            if is_sha(&s) {
                return Some(s);
            }
        }
        let packed = std::fs::read_to_string(self.gitdir.join("packed-refs")).ok()?;
        packed.lines().find_map(|l| {
            let (sha, name) = l.split_once(' ')?;
            (name.trim() == refname && is_sha(sha)).then(|| sha.to_string())
        })
    }

    fn git(&self, args: &[&str]) -> Option<String> {
        let s = std::process::Command::new("git")
            .arg("-C")
            .arg(&self.root)
            .args(args)
            .output()
            .ok()?;
        s.status
            .success()
            .then(|| String::from_utf8_lossy(&s.stdout).into_owned())
    }
}

fn is_sha(s: &str) -> bool {
    s.len() >= 7 && s.len() <= 64 && s.bytes().all(|b| b.is_ascii_hexdigit())
}

impl Anchor for Git {
    fn snapshot(&self) -> AnchorRef {
        AnchorRef {
            kind: "git".into(),
            id: self.head().unwrap_or_default(),
        }
    }

    fn changed_since(&self, r: &AnchorRef) -> Vec<Change> {
        if r.is_empty_tree() || r.kind != "git" {
            return vec![];
        }
        let mut count: std::collections::BTreeMap<String, usize> = Default::default();
        // Commits since the anchor. If the sha is gone --rebase, deleted
        // branch-- git fails and this returns empty: better to say nothing
        // than to say a false number.
        if let Some(out) = self.git(&[
            "log",
            "--format=",
            "--name-only",
            &format!("{}..HEAD", r.id),
        ]) {
            for l in out.lines().map(str::trim).filter(|l| !l.is_empty()) {
                *count.entry(l.to_string()).or_default() += 1;
            }
        }
        // And whatever is uncommitted, which counts as a change.
        //
        // `-uall` is not decoration: without it git collapses a whole
        // untracked directory into one line -- `src/` -- and the file that
        // nobody has claimed yet, which is the case `reconcile` exists for,
        // never appears. Ignored paths stay ignored, so `target/` does not
        // come flooding in.
        if let Some(out) = self.git(&["status", "--porcelain", "-uall"]) {
            for l in out.lines() {
                if let Some(file_path) = l.get(3..) {
                    let file_path = file_path.rsplit(" -> ").next().unwrap_or(file_path).trim();
                    if !file_path.is_empty() {
                        *count
                            .entry(file_path.trim_matches('"').to_string())
                            .or_default() += 1;
                    }
                }
            }
        }
        let mut v: Vec<Change> = count
            .into_iter()
            .map(|(file_path, times)| Change { file_path, times })
            .collect();
        // Most-touched first; ties broken by path. Deterministic.
        v.sort_by(|a, b| {
            b.times
                .cmp(&a.times)
                .then_with(|| a.file_path.cmp(&b.file_path))
        });
        v
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn null_invents_no_precision() {
        let n = Null;
        assert!(n.snapshot().is_empty_tree());
        assert!(n.changed_since(&AnchorRef::default()).is_empty());
    }

    #[test]
    fn head_is_read_without_spawning_git() {
        // This very repository serves as the substrate.
        let g = Git::new(Path::new(".")).expect("vivac/ is a git repo");
        let s = g.snapshot();
        assert_eq!(s.kind, "git");
        assert!(is_sha(&s.id), "HEAD did not resolve: {:?}", s.id);
        assert_eq!(s.short().len(), 7);
    }

    #[test]
    fn an_anchor_from_another_world_gives_no_changes() {
        let g = Git::new(Path::new(".")).unwrap();
        let bogus = AnchorRef {
            kind: "git".into(),
            id: "0000000000000000000000000000000000000000".into(),
        };
        assert!(g
            .changed_since(&bogus)
            .iter()
            .all(|c| !c.file_path.is_empty()));
    }

    /// A long-lived process -- the MCP server, the web server -- calls
    /// `detect` many times against the same root over a session in which
    /// the agent keeps committing. Caching the walk that locates `.git`
    /// must never turn into caching the commit it finds there: a second
    /// `detect` on the same root, after `HEAD` moved, still has to read the
    /// commit that is there now.
    #[test]
    fn a_cached_location_still_reads_the_head_a_later_commit_left() {
        let root = std::env::temp_dir().join(format!(
            "vivac-anchor-live-{}-{}",
            std::process::id(),
            crate::id::ulid()
        ));
        let git_dir = root.join(".git");
        std::fs::create_dir_all(&git_dir).unwrap();
        let first = "a".repeat(40);
        std::fs::write(git_dir.join("HEAD"), &first).unwrap();

        // First call: walks up from `root` and, from here on, caches where
        // `.git` was found.
        let before = detect(&root).snapshot();
        assert_eq!(before.id, first, "the first read did not see the commit");

        // A commit happens during the session.
        let second = "b".repeat(40);
        std::fs::write(git_dir.join("HEAD"), &second).unwrap();

        // Second call: hits the cached location, but the commit it reports
        // has to be the one that is there right now, not the one cached
        // alongside the walk.
        let after = detect(&root).snapshot();
        assert_eq!(
            after.id, second,
            "the cached walk froze the commit instead of just the location"
        );

        std::fs::remove_dir_all(&root).ok();
    }
}