//! Checkpoints (spec §11): a serialized state kept per bookmark so loading

//! a state costs the changes since the checkpoint rather than all of them.

//!

//! The newest checkpoint of each bookmark is pointed at by

//! `.oxy/checkpoints/<bookmark>`; older ones become unreachable and are

//! garbage collected. A checkpoint applies to a requested head set when all

//! of its heads lie in that set's closure, which the loader establishes while

//! walking from the requested heads and stopping at checkpoint members.

use super::*;

use crate::objects::Checkpoint;

impl Repo {

    fn checkpoint_pointer(&self, bookmark: &str) -> PathBuf {

        self.oxy.join("checkpoints").join(bookmark)

    }

    /// The newest checkpoint recorded for a bookmark.

    pub fn latest_checkpoint(&self, bookmark: &str) -> Result<Option<(ObjectRef, Checkpoint)>> {

        match fs::read_to_string(self.checkpoint_pointer(bookmark)) {

            Ok(h) => {

                let r = ObjectRef::from_hex(h.trim()).ok_or_else(|| RepoError::Msg("corrupt checkpoint pointer".into()))?;

                match self.store.get_typed(&r, ObjectType::Checkpoint) {

                    Ok(bytes) => Ok(Some((r, Checkpoint::decode(&bytes)?))),

                    Err(StoreError::NotFound(_)) => Ok(None),

                    Err(e) => Err(e.into()),

                }

            }

            Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(None),

            Err(e) => Err(e.into()),

        }

    }

    /// Every checkpoint pointer, for garbage collection roots.

    pub(crate) fn checkpoint_refs(&self) -> Result<Vec<ObjectRef>> {

        let dir = self.oxy.join("checkpoints");

        let mut out = Vec::new();

        if let Ok(rd) = fs::read_dir(&dir) {

            for e in rd.flatten() {

                if let Ok(h) = fs::read_to_string(e.path()) {

                    if let Some(r) = ObjectRef::from_hex(h.trim()) {

                        out.push(r);

                    }

                }

            }

        }

        Ok(out)

    }

    /// Write a checkpoint of `state` (whose heads are `heads`) for `bookmark`.

    pub(crate) fn write_checkpoint(&self, bookmark: &str, state: &State, heads: &[ChangeHash]) -> Result<(ObjectRef, usize)> {

        let payload = state.checkpoint_payload();

        let blob = self.store_blob(&payload)?;

        let cp = Checkpoint { heads: StateObject::new(heads.to_vec()).heads, count: state.len() as u64, payload: blob };

        let r = self.store.put(ObjectType::Checkpoint, &cp.encode())?;

        fs::create_dir_all(self.oxy.join("checkpoints"))?;

        write_atomic(&self.checkpoint_pointer(bookmark), r.to_hex().as_bytes())?;

        Ok((r, payload.len()))

    }

    /// Checkpoint the current bookmark's state now.

    pub fn checkpoint(&self) -> Result<(ObjectRef, usize, usize)> {

        let heads = self.bookmark_heads()?;

        let state = self.load_state_full(&heads)?;

        let (r, bytes) = self.write_checkpoint(&self.config.bookmark, &state, &heads)?;

        Ok((r, state.len(), bytes))

    }

    /// Load the state of `heads` using a checkpoint when one applies.

    pub(crate) fn load_state_checkpointed(&self, heads: &[ChangeHash]) -> Result<Option<State>> {

        let mut bookmarks: Vec<String> = vec![self.config.bookmark.clone()];

        if let Ok(rd) = fs::read_dir(self.oxy.join("checkpoints")) {

            for e in rd.flatten() {

                let name = e.file_name().to_string_lossy().to_string();

                if !bookmarks.contains(&name) {

                    bookmarks.push(name);

                }

            }

        }

        for b in bookmarks {

            let Some((_, cp)) = self.latest_checkpoint(&b)? else { continue };

            // Cheap pre-check: every checkpoint head must be stored.

            if cp.heads.iter().any(|h| !self.store.has(h)) {

                continue;

            }

            let payload = self.read_blob(&cp.payload)?;

            let base = State::from_checkpoint_payload(&payload)?;

            let cp_heads: BTreeSet<ChangeHash> = cp.heads.iter().copied().collect();

            let mut found: BTreeSet<ChangeHash> = BTreeSet::new();

            let mut delta: Vec<Change> = Vec::new();

            let mut seen: BTreeSet<ChangeHash> = BTreeSet::new();

            let mut stack: Vec<ChangeHash> = heads.to_vec();

            while let Some(h) = stack.pop() {

                if !seen.insert(h) {

                    continue;

                }

                if base.contains(&h) {

                    if cp_heads.contains(&h) {

                        found.insert(h);

                    }

                    continue;

                }

                let c = self.store.get_change(&h)?;

                stack.extend(c.deps.iter().copied());

                delta.push(c);

            }

            if found.len() != cp_heads.len() {

                continue;

            }

            let mut state = base;

            for c in delta {

                state.add_unchecked(c);

            }

            state.validate()?;

            return Ok(Some(state));

        }

        Ok(None)

    }

}