package tsdb

import (
	"fmt"
	"os"
	"path/filepath"
	"sync"
	"time"

	"github.com/coreos/etcd/pkg/fileutil"
	"github.com/fabxc/tsdb/labels"
	"github.com/go-kit/kit/log"
	"github.com/pkg/errors"
	"github.com/prometheus/client_golang/prometheus"
)

type compactor struct {
	metrics *compactorMetrics
	blocks  compactableBlocks
	logger  log.Logger

	triggerc chan struct{}
	donec    chan struct{}
}

type compactorMetrics struct {
	triggered prometheus.Counter
	ran       prometheus.Counter
	failed    prometheus.Counter
	duration  prometheus.Histogram
}

func newCompactorMetrics(i int) *compactorMetrics {
	shardLabel := prometheus.Labels{
		"shard": fmt.Sprintf("%d", i),
	}

	m := &compactorMetrics{}

	m.triggered = prometheus.NewCounter(prometheus.CounterOpts{
		Name:        "tsdb_shard_compactions_triggered_total",
		Help:        "Total number of triggered compactions for the shard.",
		ConstLabels: shardLabel,
	})
	m.ran = prometheus.NewCounter(prometheus.CounterOpts{
		Name:        "tsdb_shard_compactions_total",
		Help:        "Total number of compactions that were executed for the shard.",
		ConstLabels: shardLabel,
	})
	m.failed = prometheus.NewCounter(prometheus.CounterOpts{
		Name:        "tsdb_shard_compactions_failed_total",
		Help:        "Total number of compactions that failed for the shard.",
		ConstLabels: shardLabel,
	})
	m.duration = prometheus.NewHistogram(prometheus.HistogramOpts{
		Name:        "tsdb_shard_compaction_duration",
		Help:        "Duration of compaction runs.",
		ConstLabels: shardLabel,
	})

	return m
}

type compactableBlocks interface {
	lock() sync.Locker
	compactable() []block
	reinit(dir string) error
}

func newCompactor(i int, blocks compactableBlocks, l log.Logger) (*compactor, error) {
	c := &compactor{
		triggerc: make(chan struct{}, 1),
		donec:    make(chan struct{}),
		logger:   l,
		blocks:   blocks,
		metrics:  newCompactorMetrics(i),
	}
	go c.run()

	return c, nil
}

func (c *compactor) trigger() {
	select {
	case c.triggerc <- struct{}{}:
	default:
	}
}

func (c *compactor) run() {
	for range c.triggerc {
		c.metrics.triggered.Inc()

		// Compact as long as there are candidate blocks.
		for {
			rev := c.pick()
			var bs []block
			for _, b := range rev {
				bs = append([]block{b}, bs...)
			}

			c.logger.Log("msg", "picked for compaction", "candidates", fmt.Sprintf("%v", bs))

			if len(bs) == 0 {
				break
			}

			start := time.Now()
			err := c.compact(bs...)

			c.metrics.ran.Inc()
			c.metrics.duration.Observe(time.Since(start).Seconds())

			if err != nil {
				c.logger.Log("msg", "compaction failed", "err", err)
				c.metrics.failed.Inc()
				break
			}
		}

		// Drain channel of signals triggered during compaction.
		select {
		case <-c.triggerc:
		default:
		}
	}
	close(c.donec)
}

const (
	compactionMaxSize = 1 << 30 // 1GB
	compactionBlocks  = 2
)

func (c *compactor) pick() []block {
	bs := c.blocks.compactable()
	if len(bs) == 0 {
		return nil
	}
	if len(bs) == 1 && !bs[0].persisted() {
		return bs
	}

	for i := 0; i+1 < len(bs); i += 2 {
		tpl := bs[i : i+2]
		if compactionMatch(tpl) {
			return tpl
		}
	}
	return nil
}

func compactionMatch(blocks []block) bool {
	// TODO(fabxc): check whether combined size is below maxCompactionSize.
	// Apply maximum time range? or number of series? – might already be covered by size implicitly.

	// Blocks should be roughly equal in size.
	return true
}

func (c *compactor) Close() error {
	close(c.triggerc)
	<-c.donec
	return nil
}

func mergeStats(blocks ...block) (res BlockStats) {
	res.MinTime = blocks[0].stats().MinTime
	res.MaxTime = blocks[len(blocks)-1].stats().MaxTime

	for _, b := range blocks {
		res.SampleCount += b.stats().SampleCount
	}
	return res
}

func (c *compactor) compact(blocks ...block) error {
	tmpdir := blocks[0].dir() + ".tmp"

	// Write to temporary directory to make persistence appear atomic.
	if fileutil.Exist(tmpdir) {
		if err := os.RemoveAll(tmpdir); err != nil {
			return err
		}
	}
	if err := fileutil.CreateDirAll(tmpdir); err != nil {
		return err
	}

	chunkf, err := fileutil.LockFile(chunksFileName(tmpdir), os.O_WRONLY|os.O_CREATE, 0666)
	if err != nil {
		return errors.Wrap(err, "create chunk file")
	}
	indexf, err := fileutil.LockFile(indexFileName(tmpdir), os.O_WRONLY|os.O_CREATE, 0666)
	if err != nil {
		return errors.Wrap(err, "create index file")
	}

	indexw := newIndexWriter(indexf)
	chunkw := newSeriesWriter(chunkf, indexw)

	if err := c.write(blocks, indexw, chunkw); err != nil {
		return errors.Wrap(err, "write compaction")
	}

	if err := chunkw.Close(); err != nil {
		return errors.Wrap(err, "close chunk writer")
	}
	if err := indexw.Close(); err != nil {
		return errors.Wrap(err, "close index writer")
	}
	if err := fileutil.Fsync(chunkf.File); err != nil {
		return errors.Wrap(err, "fsync chunk file")
	}
	if err := fileutil.Fsync(indexf.File); err != nil {
		return errors.Wrap(err, "fsync index file")
	}
	if err := chunkf.Close(); err != nil {
		return errors.Wrap(err, "close chunk file")
	}
	if err := indexf.Close(); err != nil {
		return errors.Wrap(err, "close index file")
	}

	c.blocks.lock().Lock()
	defer c.blocks.lock().Unlock()

	if err := renameDir(tmpdir, blocks[0].dir()); err != nil {
		return errors.Wrap(err, "rename dir")
	}
	for _, b := range blocks[1:] {
		if err := os.RemoveAll(b.dir()); err != nil {
			return errors.Wrap(err, "delete dir")
		}
	}

	var merr MultiError

	for _, b := range blocks {
		merr.Add(errors.Wrapf(c.blocks.reinit(b.dir()), "reinit block at %q", b.dir()))
	}
	return merr.Err()
}

func (c *compactor) write(blocks []block, indexw IndexWriter, chunkw SeriesWriter) error {
	var set compactionSet
	for i, b := range blocks {
		all, err := b.index().Postings("", "")
		if err != nil {
			return err
		}
		// TODO(fabxc): find more transparent way of handling this.
		if hb, ok := b.(*HeadBlock); ok {
			all = hb.remapPostings(all)
		}
		s := newCompactionSeriesSet(b.index(), b.series(), all)

		if i == 0 {
			set = s
			continue
		}
		set, err = newCompactionMerger(set, s)
		if err != nil {
			return err
		}
	}

	// We fully rebuild the postings list index from merged series.
	var (
		postings = &memPostings{m: make(map[term][]uint32, 512)}
		values   = map[string]stringset{}
		i        = uint32(0)
		stats    = mergeStats(blocks...)
	)

	for set.Next() {
		lset, chunks := set.At()
		if err := chunkw.WriteSeries(i, lset, chunks); err != nil {
			return err
		}

		stats.ChunkCount += uint32(len(chunks))
		stats.SeriesCount++

		for _, l := range lset {
			valset, ok := values[l.Name]
			if !ok {
				valset = stringset{}
				values[l.Name] = valset
			}
			valset.set(l.Value)

			postings.add(i, term{name: l.Name, value: l.Value})
		}
		i++
	}
	if set.Err() != nil {
		return set.Err()
	}

	if err := indexw.WriteStats(stats); err != nil {
		return err
	}

	s := make([]string, 0, 256)
	for n, v := range values {
		s = s[:0]

		for x := range v {
			s = append(s, x)
		}
		if err := indexw.WriteLabelIndex([]string{n}, s); err != nil {
			return err
		}
	}

	for t := range postings.m {
		if err := indexw.WritePostings(t.name, t.value, postings.get(t)); err != nil {
			return err
		}
	}
	// Write a postings list containing all series.
	all := make([]uint32, i)
	for i := range all {
		all[i] = uint32(i)
	}
	if err := indexw.WritePostings("", "", newListPostings(all)); err != nil {
		return err
	}
	return nil
}

type compactionSet interface {
	Next() bool
	At() (labels.Labels, []ChunkMeta)
	Err() error
}

type compactionSeriesSet struct {
	p      Postings
	index  IndexReader
	series SeriesReader

	l   labels.Labels
	c   []ChunkMeta
	err error
}

func newCompactionSeriesSet(i IndexReader, s SeriesReader, p Postings) *compactionSeriesSet {
	return &compactionSeriesSet{
		index:  i,
		series: s,
		p:      p,
	}
}

func (c *compactionSeriesSet) Next() bool {
	if !c.p.Next() {
		return false
	}

	c.l, c.c, c.err = c.index.Series(c.p.At())
	if c.err != nil {
		return false
	}
	for i := range c.c {
		chk := &c.c[i]

		chk.Chunk, c.err = c.series.Chunk(chk.Ref)
		if c.err != nil {
			return false
		}
	}

	return true
}

func (c *compactionSeriesSet) Err() error {
	if c.err != nil {
		return c.err
	}
	return c.p.Err()
}

func (c *compactionSeriesSet) At() (labels.Labels, []ChunkMeta) {
	return c.l, c.c
}

type compactionMerger struct {
	a, b compactionSet

	aok, bok bool
	l        labels.Labels
	c        []ChunkMeta
}

type compactionSeries struct {
	labels labels.Labels
	chunks []ChunkMeta
}

func newCompactionMerger(a, b compactionSet) (*compactionMerger, error) {
	c := &compactionMerger{
		a: a,
		b: b,
	}
	// Initialize first elements of both sets as Next() needs
	// one element look-ahead.
	c.aok = c.a.Next()
	c.bok = c.b.Next()

	return c, c.Err()
}

func (c *compactionMerger) compare() int {
	if !c.aok {
		return 1
	}
	if !c.bok {
		return -1
	}
	a, _ := c.a.At()
	b, _ := c.b.At()
	return labels.Compare(a, b)
}

func (c *compactionMerger) Next() bool {
	if !c.aok && !c.bok || c.Err() != nil {
		return false
	}

	d := c.compare()
	// Both sets contain the current series. Chain them into a single one.
	if d > 0 {
		c.l, c.c = c.b.At()
		c.bok = c.b.Next()
	} else if d < 0 {
		c.l, c.c = c.a.At()
		c.aok = c.a.Next()
	} else {
		l, ca := c.a.At()
		_, cb := c.b.At()

		c.l = l
		c.c = append(ca, cb...)

		c.aok = c.a.Next()
		c.bok = c.b.Next()
	}
	return true
}

func (c *compactionMerger) Err() error {
	if c.a.Err() != nil {
		return c.a.Err()
	}
	return c.b.Err()
}

func (c *compactionMerger) At() (labels.Labels, []ChunkMeta) {
	return c.l, c.c
}

func renameDir(from, to string) error {
	if err := os.RemoveAll(to); err != nil {
		return err
	}
	if err := os.Rename(from, to); err != nil {
		return err
	}

	// Directory was renamed; sync parent dir to persist rename.
	pdir, err := fileutil.OpenDir(filepath.Dir(to))
	if err != nil {
		return err
	}
	if err = fileutil.Fsync(pdir); err != nil {
		return err
	}
	if err = pdir.Close(); err != nil {
		return err
	}
	return nil
}