//! Garbage collection (spec §21): 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; receipts

//! live as long as the changes of their state. 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 the one a full repack writes.

    pub merged_packs: usize,

}

impl Repo {

    /// Prune unreachable objects older than the grace period (`now`

    /// ignores it), then repack everything into one pack (spec §13.4).

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

        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)?;

        // Then everything left in one pack, compressed harder than a

        // commit's (spec §13.4), as `git gc` repacks.

        let (packed, merged_packs) = self.store.repack_all(crate::store::REPACK_ZSTD_LEVEL)?;

        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);

                }

                // A version 2 payload's graph blobs and their chunks.

                for g in self.checkpoint_graph_blobs(&cp).unwrap_or_default() {

                    keep.insert(g);

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

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

                    }

                }

            }

        }

        // Tags: their states and attestations.

        let view_now = self.view()?;

        states.extend(view_now.tags.values().copied());

        for name in view_now.tags.keys() {

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

                attests.push(r);

            }

        }

        // 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_parts(&h) {

                Ok((c, texts)) => {

                    keep.extend(texts);

                    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)?);

        }

        self.reachable_receipts(&mut keep)?;

        for a in attests {

            if keep.insert(a) {

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

                    keep.insert(sig.key);

                }

            }

        }

        Ok(keep)

    }

}