//! Garbage collection (spec §21): pack loose objects, then delete objects

//! nothing reachable refers to.

//!

//! Roots: the current view, the views of every operation newer than

//! `core.op-retention`, reference attestations, and the policy. Operation

//! objects are always kept so the log stays walkable, but an operation older

//! than the retention window no longer keeps its states alive, so undoing

//! that far back may fail once its objects are gone.

use super::*;

use crate::store::{PackReport, PruneReport};

use std::collections::HashSet;

#[derive(Debug, Clone, Default, PartialEq, Eq)]

pub struct GcReport {

    pub packed: PackReport,

    pub pruned: PruneReport,

    pub reachable: usize,

    /// Packs folded into one because there were more than `MAX_PACKS`.

    pub merged_packs: usize,

}

impl Repo {

    /// Pack loose objects and prune unreachable ones older than the grace

    /// period (`now` ignores the grace period).

    pub fn gc(&self, now: bool) -> Result<GcReport> {

        let packed = self.store.pack_loose()?;

        let keep = self.reachable()?;

        let grace = if now { std::time::Duration::ZERO } else { std::time::Duration::from_secs(self.config.gc_grace_days * 86_400) };

        let pruned = self.store.prune(&keep, grace)?;

        let merged_packs = self.store.merge_packs()?;

        Ok(GcReport { packed, pruned, reachable: keep.len(), merged_packs })

    }

    /// Every object that must survive garbage collection.

    pub fn reachable(&self) -> Result<HashSet<ObjectRef>> {

        let mut keep: HashSet<ObjectRef> = HashSet::new();

        let mut states: Vec<ObjectRef> = Vec::new();

        let mut changes: Vec<ChangeHash> = Vec::new();

        let mut attests: Vec<ObjectRef> = Vec::new();

        let mut policies: Vec<ObjectRef> = Vec::new();

        // Operations: always kept; views within retention are roots.

        let cutoff = Hlc::now().wall_ms.saturating_sub(self.config.op_retention_days * 86_400_000);

        for (i, (r, op)) in self.op_log()?.into_iter().enumerate() {

            keep.insert(r);

            if i == 0 || op.hlc.wall_ms >= cutoff {

                states.extend(op.view.bookmarks.values().copied());

                states.extend(op.view.remotes.values().flat_map(|b| b.values().copied()));

                changes.extend(op.view.workspaces.values().copied());

                policies.extend(op.view.policy);

            }

        }

        // Checkpoints: the object, its payload blob, and the blob's chunks.

        for r in self.checkpoint_refs()? {

            if let Ok(bytes) = self.store.get_typed(&r, ObjectType::Checkpoint) {

                keep.insert(r);

                let cp = crate::objects::Checkpoint::decode(&bytes)?;

                keep.insert(cp.payload);

                if let Ok(b) = self.store.get_typed(&cp.payload, ObjectType::Blob) {

                    keep.extend(Blob::decode(&b)?.chunks);

                }

            }

        }

        // Reference attestations.

        for name in self.view()?.bookmarks.keys() {

            if let Some((r, _)) = self.latest_ref_attest(name)? {

                attests.push(r);

            }

        }

        // A historical view may name objects pruned under an earlier, shorter

        // retention; there is nothing to keep for those.

        for st in states {

            if keep.insert(st) {

                match self.store.get_state(&st) {

                    Ok(s) => changes.extend(s.heads),

                    Err(StoreError::NotFound(_)) => continue,

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

                }

            }

        }

        for p in policies {

            if keep.insert(p) {

                let bytes = match self.store.get_typed(&p, ObjectType::Policy) {

                    Ok(b) => b,

                    Err(StoreError::NotFound(_)) => continue,

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

                };

                let policy = crate::policy::Policy::decode(&bytes)?;

                for id in &policy.identities {

                    keep.insert(id.hash());

                }

                for sig in self.store.signatures(&p)? {

                    keep.insert(sig.key);

                }

            }

        }

        // Changes: their closure, identities, blobs, chunks, seals.

        let mut stack = changes;

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

            if !keep.insert(h) {

                continue;

            }

            // Beyond the shallow boundary, or pruned under an earlier

            // retention, there is nothing to keep.

            let c = match self.store.get_change(&h) {

                Ok(c) => c,

                Err(StoreError::NotFound(_)) => continue,

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

            };

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

            if !c.author.is_zero() {

                keep.insert(c.author);

            }

            for sig in self.store.signatures(&h)? {

                keep.insert(sig.key);

            }

            for e in &c.edits {

                if let Edit::Replace { old, new, .. } = e {

                    for b in [old, new].into_iter().flatten() {

                        if keep.insert(*b) {

                            if let Ok(bytes) = self.store.get_typed(b, ObjectType::Blob) {

                                keep.extend(Blob::decode(&bytes)?.chunks);

                            }

                        }

                    }

                }

            }

            attests.extend(self.store.seals_for(&h)?);

        }

        for a in attests {

            if keep.insert(a) {

                for sig in self.store.signatures(&a)? {

                    keep.insert(sig.key);

                }

            }

        }

        Ok(keep)

    }

}