package noderepo

import (
	"bytes"
	"io"
	"sort"
	"sync"

	"github.com/btcsuite/btcd/claimtrie/change"
	"github.com/cockroachdb/pebble"
	"github.com/pkg/errors"
)

type Pebble struct {
	db *pebble.DB
}

type pooledMerger struct {
	pool   *sync.Pool
	buffer []byte
}

func (a *pooledMerger) MergeNewer(value []byte) error {
	if a.buffer == nil {
		a.buffer = a.pool.Get().([]byte)
	}
	a.buffer = append(a.buffer, value...)
	return nil
}
func (a *pooledMerger) MergeOlder(value []byte) error {
	if a.buffer == nil {
		a.buffer = a.pool.Get().([]byte)
	}
	n := len(a.buffer)
	if cap(a.buffer) >= len(value)+n {
		a.buffer = a.buffer[0 : len(value)+n]      // expand it
		copy(a.buffer[len(value):], a.buffer[0:n]) // could overlap
		copy(a.buffer, value)
	} else {
		existing := a.buffer
		a.buffer = make([]byte, 0, len(value)+len(existing))
		a.buffer = append(a.buffer, value...)
		a.buffer = append(a.buffer, existing...)
	}
	return nil
}

func (a *pooledMerger) Finish(_ bool) ([]byte, io.Closer, error) {
	return a.buffer, a, nil
}

func (a *pooledMerger) Close() error {
	a.pool.Put(a.buffer[:0])
	a.buffer = nil
	a.pool = nil
	return nil
}

func NewPebble(path string) (*Pebble, error) {

	mp := &sync.Pool{
		New: func() interface{} {
			return make([]byte, 0, 256)
		},
	}

	db, err := pebble.Open(path, &pebble.Options{
		Merger: &pebble.Merger{
			Merge: func(key, value []byte) (pebble.ValueMerger, error) {
				p := &pooledMerger{pool: mp}
				return p, p.MergeNewer(value)
			},
			Name: pebble.DefaultMerger.Name, // yes, it's a lie
		},
		Cache:        pebble.NewCache(32 << 20),
		BytesPerSync: 4 << 20,
	})

	repo := &Pebble{db: db}

	return repo, errors.Wrapf(err, "unable to open %s", path)
}

// AppendChanges makes an assumption that anything you pass to it is newer than what was saved before.
func (repo *Pebble) AppendChanges(changes []change.Change) error {

	batch := repo.db.NewBatch()
	defer batch.Close()

	buffer := bytes.NewBuffer(nil)

	for _, chg := range changes {
		buffer.Reset()
		err := chg.MarshalTo(buffer)
		if err != nil {
			return errors.Wrap(err, "in marshaller")
		}

		// expecting this next line to make a copy of the buffer, not hold it
		err = batch.Merge(chg.Name, buffer.Bytes(), pebble.NoSync)
		if err != nil {
			return errors.Wrap(err, "in merge")
		}
	}
	return errors.Wrap(batch.Commit(pebble.NoSync), "in commit")
}

func (repo *Pebble) LoadChanges(name []byte) ([]change.Change, error) {

	data, closer, err := repo.db.Get(name)
	if err != nil && err != pebble.ErrNotFound {
		return nil, errors.Wrapf(err, "in get %s", name) // does returning a name in an error expose too much?
	}
	if closer != nil {
		defer closer.Close()
	}

	return unmarshalChanges(name, data)
}

func unmarshalChanges(name, data []byte) ([]change.Change, error) {
	var changes []change.Change

	buffer := bytes.NewBuffer(data)
	for buffer.Len() > 0 {
		var chg change.Change
		err := chg.UnmarshalFrom(buffer)
		if err != nil {
			return nil, errors.Wrap(err, "in decode")
		}
		chg.Name = name
		changes = append(changes, chg)
	}

	// this was required for the normalization stuff:
	sort.SliceStable(changes, func(i, j int) bool {
		return changes[i].Height < changes[j].Height
	})

	return changes, nil
}

func (repo *Pebble) DropChanges(name []byte, finalHeight int32) error {
	changes, err := repo.LoadChanges(name)
	if err != nil {
		return errors.Wrapf(err, "in load changes for %s", name)
	}
	i := 0
	for ; i < len(changes); i++ {
		if changes[i].Height > finalHeight {
			break
		}
	}
	// making a performance assumption that DropChanges won't happen often:
	err = repo.db.Set(name, []byte{}, pebble.NoSync)
	if err != nil {
		return errors.Wrapf(err, "in set at %s", name)
	}
	return repo.AppendChanges(changes[:i])
}

func (repo *Pebble) IterateChildren(name []byte, f func(changes []change.Change) bool) error {
	start := make([]byte, len(name)+1) // zeros that last byte; need a constant len for stack alloc?
	copy(start, name)

	end := make([]byte, 256) // max name length is 255
	copy(end, name)
	for i := len(name); i < 256; i++ {
		end[i] = 255
	}

	prefixIterOptions := &pebble.IterOptions{
		LowerBound: start,
		UpperBound: end,
	}

	iter := repo.db.NewIter(prefixIterOptions)
	defer iter.Close()

	for iter.First(); iter.Valid(); iter.Next() {
		// NOTE! iter.Key() is ephemeral!
		changes, err := unmarshalChanges(iter.Key(), iter.Value())
		if err != nil {
			return errors.Wrapf(err, "from unmarshaller at %s", iter.Key())
		}
		if !f(changes) {
			break
		}
	}
	return nil
}

func (repo *Pebble) IterateAll(predicate func(name []byte) bool) {
	iter := repo.db.NewIter(nil)
	defer iter.Close()

	for iter.First(); iter.Valid(); iter.Next() {
		if !predicate(iter.Key()) {
			break
		}
	}
}

func (repo *Pebble) Close() error {

	err := repo.db.Flush()
	if err != nil {
		// if we fail to close are we going to try again later?
		return errors.Wrap(err, "on flush")
	}

	err = repo.db.Close()
	return errors.Wrap(err, "on close")
}

func (repo *Pebble) Flush() error {
	_, err := repo.db.AsyncFlush()
	return err
}