// Package gc provides garbage collection for go-dms3.
package gc

import (
	"context"
	"errors"
	"fmt"
	"strings"

	bserv "gitlab.dms3.io/dms3/go-blockservice"
	cid "gitlab.dms3.io/dms3/go-cid"
	dstore "gitlab.dms3.io/dms3/go-datastore"
	bstore "gitlab.dms3.io/dms3/go-dms3-blockstore"
	offline "gitlab.dms3.io/dms3/go-dms3-exchange-offline"
	pin "gitlab.dms3.io/dms3/go-dms3-pinner"
	ld "gitlab.dms3.io/dms3/go-ld-format"
	logging "gitlab.dms3.io/dms3/go-log"
	dag "gitlab.dms3.io/dms3/go-merkledag"
	"gitlab.dms3.io/dms3/go-verifcid"
)

var log = logging.Logger("gc")

// Result represents an incremental output from a garbage collection
// run.  It contains either an error, or the cid of a removed object.
type Result struct {
	KeyRemoved cid.Cid
	Error      error
}

// GC performs a mark and sweep garbage collection of the blocks in the blockstore
// first, it creates a 'marked' set and adds to it the following:
// - all recursively pinned blocks, plus all of their descendants (recursively)
// - bestEffortRoots, plus all of its descendants (recursively)
// - all directly pinned blocks
// - all blocks utilized internally by the pinner
//
// The routine then iterates over every block in the blockstore and
// deletes any block that is not found in the marked set.
func GC(ctx context.Context, bs bstore.GCBlockstore, dstor dstore.Datastore, pn pin.Pinner, bestEffortRoots []cid.Cid) <-chan Result {
	ctx, cancel := context.WithCancel(ctx)

	unlocker := bs.GCLock()

	bsrv := bserv.New(bs, offline.Exchange(bs))
	ds := dag.NewDAGService(bsrv)

	output := make(chan Result, 128)

	go func() {
		defer cancel()
		defer close(output)
		defer unlocker.Unlock()

		gcs, err := ColoredSet(ctx, pn, ds, bestEffortRoots, output)
		if err != nil {
			select {
			case output <- Result{Error: err}:
			case <-ctx.Done():
			}
			return
		}
		keychan, err := bs.AllKeysChan(ctx)
		if err != nil {
			select {
			case output <- Result{Error: err}:
			case <-ctx.Done():
			}
			return
		}

		errors := false
		var removed uint64

	loop:
		for ctx.Err() == nil { // select may not notice that we're "done".
			select {
			case k, ok := <-keychan:
				if !ok {
					break loop
				}
				if !gcs.Has(k) {
					err := bs.DeleteBlock(k)
					removed++
					if err != nil {
						errors = true
						select {
						case output <- Result{Error: &CannotDeleteBlockError{k, err}}:
						case <-ctx.Done():
							break loop
						}
						// continue as error is non-fatal
						continue loop
					}
					select {
					case output <- Result{KeyRemoved: k}:
					case <-ctx.Done():
						break loop
					}
				}
			case <-ctx.Done():
				break loop
			}
		}
		if errors {
			select {
			case output <- Result{Error: ErrCannotDeleteSomeBlocks}:
			case <-ctx.Done():
				return
			}
		}

		gds, ok := dstor.(dstore.GCDatastore)
		if !ok {
			return
		}

		err = gds.CollectGarbage()
		if err != nil {
			select {
			case output <- Result{Error: err}:
			case <-ctx.Done():
			}
			return
		}
	}()

	return output
}

// Descendants recursively finds all the descendants of the given roots and
// adds them to the given cid.Set, using the provided dag.GetLinks function
// to walk the tree.
func Descendants(ctx context.Context, getLinks dag.GetLinks, set *cid.Set, roots []cid.Cid) error {
	verifyGetLinks := func(ctx context.Context, c cid.Cid) ([]*ld.Link, error) {
		err := verifcid.ValidateCid(c)
		if err != nil {
			return nil, err
		}

		return getLinks(ctx, c)
	}

	verboseCidError := func(err error) error {
		if strings.Contains(err.Error(), verifcid.ErrBelowMinimumHashLength.Error()) ||
			strings.Contains(err.Error(), verifcid.ErrPossiblyInsecureHashFunction.Error()) {
			err = fmt.Errorf("\"%s\"\nPlease run 'dms3 pin verify'"+
				" to list insecure hashes. If you want to read them,"+
				" please downgrade your go-dms3 to 0.4.13\n", err)
			log.Error(err)
		}
		return err
	}

	for _, c := range roots {
		// Walk recursively walks the dag and adds the keys to the given set
		err := dag.Walk(ctx, verifyGetLinks, c, set.Visit, dag.Concurrent())

		if err != nil {
			err = verboseCidError(err)
			return err
		}
	}

	return nil
}

// ColoredSet computes the set of nodes in the graph that are pinned by the
// pins in the given pinner.
func ColoredSet(ctx context.Context, pn pin.Pinner, ng ld.NodeGetter, bestEffortRoots []cid.Cid, output chan<- Result) (*cid.Set, error) {
	// KeySet currently implemented in memory, in the future, may be bloom filter or
	// disk backed to conserve memory.
	errors := false
	gcs := cid.NewSet()
	getLinks := func(ctx context.Context, cid cid.Cid) ([]*ld.Link, error) {
		links, err := ld.GetLinks(ctx, ng, cid)
		if err != nil {
			errors = true
			select {
			case output <- Result{Error: &CannotFetchLinksError{cid, err}}:
			case <-ctx.Done():
				return nil, ctx.Err()
			}
		}
		return links, nil
	}
	rkeys, err := pn.RecursiveKeys(ctx)
	if err != nil {
		return nil, err
	}
	err = Descendants(ctx, getLinks, gcs, rkeys)
	if err != nil {
		errors = true
		select {
		case output <- Result{Error: err}:
		case <-ctx.Done():
			return nil, ctx.Err()
		}
	}

	bestEffortGetLinks := func(ctx context.Context, cid cid.Cid) ([]*ld.Link, error) {
		links, err := ld.GetLinks(ctx, ng, cid)
		if err != nil && err != ld.ErrNotFound {
			errors = true
			select {
			case output <- Result{Error: &CannotFetchLinksError{cid, err}}:
			case <-ctx.Done():
				return nil, ctx.Err()
			}
		}
		return links, nil
	}
	err = Descendants(ctx, bestEffortGetLinks, gcs, bestEffortRoots)
	if err != nil {
		errors = true
		select {
		case output <- Result{Error: err}:
		case <-ctx.Done():
			return nil, ctx.Err()
		}
	}

	dkeys, err := pn.DirectKeys(ctx)
	if err != nil {
		return nil, err
	}
	for _, k := range dkeys {
		gcs.Add(k)
	}

	ikeys, err := pn.InternalPins(ctx)
	if err != nil {
		return nil, err
	}
	err = Descendants(ctx, getLinks, gcs, ikeys)
	if err != nil {
		errors = true
		select {
		case output <- Result{Error: err}:
		case <-ctx.Done():
			return nil, ctx.Err()
		}
	}

	if errors {
		return nil, ErrCannotFetchAllLinks
	}

	return gcs, nil
}

// ErrCannotFetchAllLinks is returned as the last Result in the GC output
// channel when there was an error creating the marked set because of a
// problem when finding descendants.
var ErrCannotFetchAllLinks = errors.New("garbage collection aborted: could not retrieve some links")

// ErrCannotDeleteSomeBlocks is returned when removing blocks marked for
// deletion fails as the last Result in GC output channel.
var ErrCannotDeleteSomeBlocks = errors.New("garbage collection incomplete: could not delete some blocks")

// CannotFetchLinksError provides detailed information about which links
// could not be fetched and can appear as a Result in the GC output channel.
type CannotFetchLinksError struct {
	Key cid.Cid
	Err error
}

// Error implements the error interface for this type with a useful
// message.
func (e *CannotFetchLinksError) Error() string {
	return fmt.Sprintf("could not retrieve links for %s: %s", e.Key, e.Err)
}

// CannotDeleteBlockError provides detailed information about which
// blocks could not be deleted and can appear as a Result in the GC output
// channel.
type CannotDeleteBlockError struct {
	Key cid.Cid
	Err error
}

// Error implements the error interface for this type with a
// useful message.
func (e *CannotDeleteBlockError) Error() string {
	return fmt.Sprintf("could not remove %s: %s", e.Key, e.Err)
}