// Package mount provides a Datastore that has other Datastores
// mounted at various key prefixes and is threadsafe
package mount

import (
	"container/heap"
	"errors"
	"fmt"
	"sort"
	"strings"
	"sync"

	ds "github.com/ipfs/go-datastore"
	"github.com/ipfs/go-datastore/query"
)

var (
	ErrNoMount = errors.New("no datastore mounted for this key")
)

type Mount struct {
	Prefix    ds.Key
	Datastore ds.Datastore
}

func New(mounts []Mount) *Datastore {
	// make a copy so we're sure it doesn't mutate
	m := make([]Mount, len(mounts))
	for i, v := range mounts {
		m[i] = v
	}
	sort.Slice(m, func(i, j int) bool { return m[i].Prefix.String() > m[j].Prefix.String() })
	return &Datastore{mounts: m}
}

type Datastore struct {
	mounts []Mount
}

var _ ds.Datastore = (*Datastore)(nil)

func (d *Datastore) lookup(key ds.Key) (ds.Datastore, ds.Key, ds.Key) {
	for _, m := range d.mounts {
		if m.Prefix.Equal(key) || m.Prefix.IsAncestorOf(key) {
			s := strings.TrimPrefix(key.String(), m.Prefix.String())
			k := ds.NewKey(s)
			return m.Datastore, m.Prefix, k
		}
	}
	return nil, ds.NewKey("/"), key
}

type queryResults struct {
	mount   ds.Key
	results query.Results
	next    query.Result
}

func (qr *queryResults) advance() bool {
	if qr.results == nil {
		return false
	}

	qr.next = query.Result{}
	r, more := qr.results.NextSync()
	if !more {
		err := qr.results.Close()
		qr.results = nil
		if err != nil {
			// One more result, the error.
			qr.next = query.Result{Error: err}
			return true
		}
		return false
	}

	r.Key = qr.mount.Child(ds.RawKey(r.Key)).String()
	qr.next = r
	return true
}

type querySet struct {
	query query.Query
	heads []*queryResults
}

func (h *querySet) Len() int {
	return len(h.heads)
}

func (h *querySet) Less(i, j int) bool {
	return query.Less(h.query.Orders, h.heads[i].next.Entry, h.heads[j].next.Entry)
}

func (h *querySet) Swap(i, j int) {
	h.heads[i], h.heads[j] = h.heads[j], h.heads[i]
}

func (h *querySet) Push(x interface{}) {
	h.heads = append(h.heads, x.(*queryResults))
}

func (h *querySet) Pop() interface{} {
	i := len(h.heads) - 1
	last := h.heads[i]
	h.heads[i] = nil
	h.heads = h.heads[:i]
	return last
}

func (h *querySet) close() error {
	var errs []error
	for _, qr := range h.heads {
		err := qr.results.Close()
		if err != nil {
			errs = append(errs, err)
		}
	}
	h.heads = nil
	if len(errs) > 0 {
		return errs[0]
	}
	return nil
}

func (h *querySet) addResults(mount ds.Key, results query.Results) {
	r := &queryResults{
		results: results,
		mount:   mount,
	}
	if r.advance() {
		heap.Push(h, r)
	}
}

func (h *querySet) next() (query.Result, bool) {
	if len(h.heads) == 0 {
		return query.Result{}, false
	}
	head := h.heads[0]
	next := head.next

	for head.advance() {
		if head.next.Error == nil {
			for _, f := range h.query.Filters {
				if !f.Filter(head.next.Entry) {
					continue
				}
			}
			// can limit and offset be implemented here?
		}
		heap.Fix(h, 0)
		return next, true
	}
	heap.Remove(h, 0)
	return next, true
}

// lookupAll returns all mounts that might contain keys that are descendant of <key>
//
// Matching: /ao/e
//
// /          B /ao/e
// /a/        not matching
// /ao/       B /e
// /ao/e/     A /
// /ao/e/uh/  A /
// /aoe/      not matching
func (d *Datastore) lookupAll(key ds.Key) (dst []ds.Datastore, mountpoint, rest []ds.Key) {
	for _, m := range d.mounts {
		p := m.Prefix.String()
		if len(p) > 1 {
			p = p + "/"
		}

		if strings.HasPrefix(p, key.String()) {
			dst = append(dst, m.Datastore)
			mountpoint = append(mountpoint, m.Prefix)
			rest = append(rest, ds.NewKey("/"))
		} else if strings.HasPrefix(key.String(), p) {
			r := strings.TrimPrefix(key.String(), m.Prefix.String())

			dst = append(dst, m.Datastore)
			mountpoint = append(mountpoint, m.Prefix)
			rest = append(rest, ds.NewKey(r))
		}
	}
	return dst, mountpoint, rest
}

func (d *Datastore) Put(key ds.Key, value []byte) error {
	cds, _, k := d.lookup(key)
	if cds == nil {
		return ErrNoMount
	}
	return cds.Put(k, value)
}

func (d *Datastore) Get(key ds.Key) (value []byte, err error) {
	cds, _, k := d.lookup(key)
	if cds == nil {
		return nil, ds.ErrNotFound
	}
	return cds.Get(k)
}

func (d *Datastore) Has(key ds.Key) (exists bool, err error) {
	cds, _, k := d.lookup(key)
	if cds == nil {
		return false, nil
	}
	return cds.Has(k)
}

func (d *Datastore) GetSize(key ds.Key) (size int, err error) {
	cds, _, k := d.lookup(key)
	if cds == nil {
		return -1, ds.ErrNotFound
	}
	return cds.GetSize(k)
}

func (d *Datastore) Delete(key ds.Key) error {
	cds, _, k := d.lookup(key)
	if cds == nil {
		return ds.ErrNotFound
	}
	return cds.Delete(k)
}

func (d *Datastore) Query(q query.Query) (query.Results, error) {
	prefix := ds.NewKey(q.Prefix)
	dses, mounts, rests := d.lookupAll(prefix)

	// offset needs to be applied after the results are aggregated
	offset := q.Offset
	q.Offset = 0

	queries := &querySet{
		query: q,
		heads: make([]*queryResults, 0, len(dses)),
	}

	for i := range dses {
		mount := mounts[i]
		dstore := dses[i]
		rest := rests[i]

		qi := q
		qi.Prefix = rest.String()
		results, err := dstore.Query(qi)

		if err != nil {
			_ = queries.close()
			return nil, err
		}
		queries.addResults(mount, results)
	}

	qr := query.ResultsFromIterator(q, query.Iterator{
		Next:  queries.next,
		Close: queries.close,
	})

	return query.NaiveLimit(query.NaiveOffset(qr, offset), q.Limit), nil
}

func (d *Datastore) Close() error {
	for _, d := range d.mounts {
		err := d.Datastore.Close()
		if err != nil {
			return err
		}
	}
	return nil
}

// DiskUsage returns the sum of DiskUsages for the mounted datastores.
// Non PersistentDatastores will not be accounted.
func (d *Datastore) DiskUsage() (uint64, error) {
	var duTotal uint64 = 0
	for _, d := range d.mounts {
		du, err := ds.DiskUsage(d.Datastore)
		duTotal += du
		if err != nil {
			return duTotal, err
		}
	}
	return duTotal, nil
}

type mountBatch struct {
	mounts map[string]ds.Batch
	lk     sync.Mutex

	d *Datastore
}

func (d *Datastore) Batch() (ds.Batch, error) {
	return &mountBatch{
		mounts: make(map[string]ds.Batch),
		d:      d,
	}, nil
}

func (mt *mountBatch) lookupBatch(key ds.Key) (ds.Batch, ds.Key, error) {
	mt.lk.Lock()
	defer mt.lk.Unlock()

	child, loc, rest := mt.d.lookup(key)
	t, ok := mt.mounts[loc.String()]
	if !ok {
		bds, ok := child.(ds.Batching)
		if !ok {
			return nil, ds.NewKey(""), ds.ErrBatchUnsupported
		}
		var err error
		t, err = bds.Batch()
		if err != nil {
			return nil, ds.NewKey(""), err
		}
		mt.mounts[loc.String()] = t
	}
	return t, rest, nil
}

func (mt *mountBatch) Put(key ds.Key, val []byte) error {
	t, rest, err := mt.lookupBatch(key)
	if err != nil {
		return err
	}

	return t.Put(rest, val)
}

func (mt *mountBatch) Delete(key ds.Key) error {
	t, rest, err := mt.lookupBatch(key)
	if err != nil {
		return err
	}

	return t.Delete(rest)
}

func (mt *mountBatch) Commit() error {
	mt.lk.Lock()
	defer mt.lk.Unlock()

	for _, t := range mt.mounts {
		err := t.Commit()
		if err != nil {
			return err
		}
	}
	return nil
}

func (d *Datastore) Check() error {
	for _, m := range d.mounts {
		if c, ok := m.Datastore.(ds.CheckedDatastore); ok {
			if err := c.Check(); err != nil {
				return fmt.Errorf("checking datastore at %s: %s", m.Prefix.String(), err.Error())
			}
		}
	}
	return nil
}

func (d *Datastore) Scrub() error {
	for _, m := range d.mounts {
		if c, ok := m.Datastore.(ds.ScrubbedDatastore); ok {
			if err := c.Scrub(); err != nil {
				return fmt.Errorf("scrubbing datastore at %s: %s", m.Prefix.String(), err.Error())
			}
		}
	}
	return nil
}

func (d *Datastore) CollectGarbage() error {
	for _, m := range d.mounts {
		if c, ok := m.Datastore.(ds.GCDatastore); ok {
			if err := c.CollectGarbage(); err != nil {
				return fmt.Errorf("gc on datastore at %s: %s", m.Prefix.String(), err.Error())
			}
		}
	}
	return nil
}