package deploy

import (
	"context"
	"errors"
	"os"
	"path/filepath"
	"strconv"
	"time"

	"github.com/iceBear67/simplepages/api"
	"github.com/iceBear67/simplepages/internal/store"
)

const (
	// defaultBlobGrace is how long a blob must have been unreferenced before
	// its content is removed.
	//
	// It is what makes the read path safe without reference counting requests.
	// A handler resolves a digest from its snapshot and then opens it; between
	// those two instants the collector could in principle delete the file. An
	// hour is an enormous margin for a gap that is measured in microseconds,
	// and it costs only some disk that was going to be reclaimed anyway.
	defaultBlobGrace = time.Hour

	// failedRetention is how long a failed deployment's row is kept. Its
	// manifest is already gone, so this is purely so that an operator
	// investigating a broken CI job can still see that it failed and why.
	failedRetention = 24 * time.Hour
)

// Collect runs one garbage collection pass: retention first, then the content
// nothing references any more.
//
// Two passes in that order, and not one, because the first is what makes work
// for the second. Deleting a deployment drops its manifest rows, the delete
// trigger takes each blob's refcount down, and only then can a blob be seen to
// be unreferenced. The second pass will not act on those blobs in this same
// run — the trigger sets last_ref_at to now and the grace period has not
// elapsed — which is deliberate: content that just became unreachable is
// exactly the content some in-flight request is most likely to still be
// reading.
//
// A dry run reports the deployments that would be deleted and the blobs that
// are collectable right now. It cannot report the blobs the deletions would
// free, because nothing has been deleted; the number is a floor, not an
// estimate, and it is honest about being one.
func (s *Service) Collect(ctx context.Context, dryRun bool) (api.GCStats, error) {
	stats := api.GCStats{DryRun: dryRun}

	if !dryRun {
		// Abandoned uploads first, so that whatever only they referenced is
		// already unreferenced by the time the blob pass looks.
		n, err := s.DB.ExpireStaleDeployments(ctx, time.Now().Add(-staleUploadAge),
			"abandoned: no activity for "+staleUploadAge.String())
		if err != nil {
			return stats, err
		}
		if n > 0 {
			s.Log.InfoContext(ctx, "expired unfinished deployments", "count", n)
		}
	}

	projects, err := s.DB.AllProjects(ctx)
	if err != nil {
		return stats, err
	}
	for _, p := range projects {
		n, err := s.collectProject(ctx, p, dryRun)
		stats.DeploymentsDeleted += n
		if err != nil {
			return stats, err
		}
	}

	blobs, err := s.DB.UnreferencedBlobs(ctx, time.Now().Add(-s.blobGrace()), 0)
	if err != nil {
		return stats, err
	}
	for _, b := range blobs {
		if dryRun {
			stats.BlobsDeleted++
			stats.BytesFreed += b.Size
			continue
		}
		deleted, err := s.DB.DeleteBlob(ctx, b.Digest, func() error { return s.CAS.Remove(b.Digest) })
		if err != nil {
			return stats, err
		}
		// Not deleted means the blob was referenced again between the listing
		// and the delete — a new deployment naming content that was about to be
		// collected. Leaving it alone is the whole point of rechecking the
		// refcount inside the transaction.
		if deleted {
			stats.BlobsDeleted++
			stats.BytesFreed += b.Size
		}
	}
	return stats, nil
}

// collectProject applies one project's retention policy.
//
// The active deployment is not considered at all: it is excluded by the query,
// so no counting mistake here can reach it.
func (s *Service) collectProject(ctx context.Context, p *store.Project, dryRun bool) (int, error) {
	deps, err := s.DB.InactiveDeployments(ctx, p.ID)
	if err != nil {
		return 0, err
	}
	grace := time.Duration(p.RetentionGraceS) * time.Second
	now := time.Now()

	var deleted, kept int
	for _, dep := range deps {
		switch dep.State {
		case store.StateReady:
			// Newest first, so the first RetentionCount of them are the ones
			// worth keeping for a rollback.
			if kept < p.RetentionCount {
				kept++
				continue
			}
			if now.Sub(retiredAt(dep)) < grace {
				continue
			}
		case store.StateFailed:
			if now.Sub(dep.CreatedAt) < failedRetention {
				continue
			}
		case store.StateDeleting:
			// Already claimed by a sweep that did not finish. No grace applies:
			// nothing may serve a tree that is half removed.
		default:
			// pending or uploading. Someone may still be uploading to it, and
			// ExpireStaleDeployments is what decides when they are not.
			continue
		}
		if dryRun {
			deleted++
			continue
		}
		if err := s.claim(ctx, p, dep); err != nil {
			switch {
			case errors.Is(err, store.ErrNotFound):
				// Deleted by someone else between the listing and now.
			case errors.Is(err, store.ErrConflict):
				// Activated between the listing and now — a rollback landed on
				// a deployment retention had picked. Correct outcome: the claim
				// is refused and the deployment stays.
				s.Log.InfoContext(ctx, "skipped a deployment that was activated during collection",
					"project", p.Name, "deployment", dep.PublicID)
			default:
				return deleted, err
			}
			continue
		}
		if err := s.removeDeployment(ctx, dep); err != nil {
			// The row is in the deleting state and committed, so recovery or
			// the next sweep will finish it. Nothing serves it in the meantime.
			s.Log.WarnContext(ctx, "could not finish deleting a deployment",
				"project", p.Name, "deployment", dep.PublicID, "err", err)
			continue
		}
		deleted++
	}
	return deleted, nil
}

// retiredAt is when a deployment stopped being served, or when it was created
// if it never was. It is what the retention grace is measured from.
func retiredAt(dep *store.Deployment) time.Time {
	if dep.DeactivatedAt != nil {
		return *dep.DeactivatedAt
	}
	return dep.CreatedAt
}

// claim marks a deployment as being deleted, which is what makes it safe to
// start removing files.
//
// The project lock is held for exactly this statement. Activation takes the
// same lock and re-reads the row under it, so the two orderings are the only
// possible ones: either activation commits first and this fails on the
// active = 0 condition, or this commits first and activation finds a deployment
// in the deleting state and refuses it. There is no interleaving in which a
// tree is removed from underneath a deployment that has just become active.
func (s *Service) claim(ctx context.Context, p *store.Project, dep *store.Deployment) error {
	unlock, err := s.locks.lock(ctx, p.ID)
	if err != nil {
		return err
	}
	defer unlock()
	return s.DB.MarkDeploymentDeleting(ctx, dep.ID)
}

// Delete removes one deployment on request. The active one cannot be deleted:
// that is a conflict, not a permission problem, and the client is expected to
// activate something else first.
func (s *Service) Delete(ctx context.Context, p *store.Project, dep *store.Deployment) error {
	if err := s.claim(ctx, p, dep); err != nil {
		switch {
		case errors.Is(err, store.ErrNotFound):
			return api.Errorf(api.CodeNotFound, "no such deployment")
		case errors.Is(err, store.ErrConflict):
			return api.Errorf(api.CodeDeploymentActive,
				"this deployment is the one the project is serving; activate another one first")
		}
		return err
	}
	if err := s.removeDeployment(ctx, dep); err != nil {
		return err
	}
	s.Log.InfoContext(ctx, "deployment deleted", "project", p.Name, "deployment", dep.PublicID)
	return nil
}

// RemoveProjectTrees deletes what a project left on disk once its rows are
// gone. Its blobs are freed by the same cascade and collected on the next pass.
func (s *Service) RemoveProjectTrees(projectID int64) error {
	if s.Dir == "" {
		return nil
	}
	return os.RemoveAll(filepath.Join(s.Dir, strconv.FormatInt(projectID, 10)))
}

// RunCollector collects on a timer until ctx is done. A failed pass is logged
// and the next one runs as scheduled: everything the collector does is
// idempotent, so there is nothing to unwind and no reason to stop.
func (s *Service) RunCollector(ctx context.Context, every time.Duration) {
	if every <= 0 {
		return
	}
	t := time.NewTicker(every)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			stats, err := s.Collect(ctx, false)
			if err != nil {
				s.Log.WarnContext(ctx, "garbage collection did not finish", "err", err)
			}
			if stats.DeploymentsDeleted > 0 || stats.BlobsDeleted > 0 {
				s.Log.InfoContext(ctx, "collected",
					"deployments", stats.DeploymentsDeleted,
					"blobs", stats.BlobsDeleted, "bytes", stats.BytesFreed)
			}
		}
	}
}

func (s *Service) blobGrace() time.Duration {
	if s.BlobGrace == 0 {
		return defaultBlobGrace
	}
	return s.BlobGrace
}