package queryrange

import (
	"context"
	"net/http"
	"time"

	"github.com/cortexproject/cortex/pkg/querier/queryrange"
	"github.com/opentracing/opentracing-go"
	otlog "github.com/opentracing/opentracing-go/log"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"github.com/weaveworks/common/httpgrpc"
	"github.com/weaveworks/common/user"

	"github.com/grafana/loki/pkg/logproto"
)

type lokiResult struct {
	req queryrange.Request
	ch  chan *packedResp
}

type packedResp struct {
	resp queryrange.Response
	err  error
}

type SplitByMetrics struct {
	splits prometheus.Histogram
}

func NewSplitByMetrics(r prometheus.Registerer) *SplitByMetrics {
	return &SplitByMetrics{
		splits: promauto.With(r).NewHistogram(prometheus.HistogramOpts{
			Namespace: "loki",
			Name:      "query_frontend_partitions",
			Help:      "Number of time-based partitions (sub-requests) per request",
			Buckets:   prometheus.ExponentialBuckets(1, 4, 5), // 1 -> 1024
		}),
	}
}

type splitByInterval struct {
	next    queryrange.Handler
	limits  Limits
	merger  queryrange.Merger
	metrics *SplitByMetrics
}

// SplitByIntervalMiddleware creates a new Middleware that splits log requests by a given interval.
func SplitByIntervalMiddleware(limits Limits, merger queryrange.Merger, metrics *SplitByMetrics) queryrange.Middleware {
	return queryrange.MiddlewareFunc(func(next queryrange.Handler) queryrange.Handler {
		return &splitByInterval{
			next:    next,
			limits:  limits,
			merger:  merger,
			metrics: metrics,
		}
	})
}

func (h *splitByInterval) Feed(ctx context.Context, input []*lokiResult) chan *lokiResult {
	ch := make(chan *lokiResult)

	go func() {
		defer close(ch)
		for _, d := range input {
			select {
			case <-ctx.Done():
				return
			case ch <- d:
				continue
			}
		}
	}()

	return ch
}

func (h *splitByInterval) Process(
	ctx context.Context,
	parallelism int,
	threshold int64,
	input []*lokiResult,
) (responses []queryrange.Response, err error) {
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	ch := h.Feed(ctx, input)

	// queries with 0 limits should not be exited early
	var unlimited bool
	if threshold == 0 {
		unlimited = true
	}

	// don't spawn unnecessary goroutines
	var p int = parallelism
	if len(input) < parallelism {
		p = len(input)
	}

	for i := 0; i < p; i++ {
		go h.loop(ctx, ch)
	}

	for _, x := range input {
		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		case data := <-x.ch:
			if data.err != nil {
				return nil, err
			}

			responses = append(responses, data.resp)

			// see if we can exit early if a limit has been reached
			if casted, ok := data.resp.(*LokiResponse); !unlimited && ok {
				threshold -= casted.Count()

				if threshold <= 0 {
					return responses, nil
				}

			}

		}

	}

	return responses, nil
}

func (h *splitByInterval) loop(ctx context.Context, ch <-chan *lokiResult) {

	for data := range ch {

		sp, ctx := opentracing.StartSpanFromContext(ctx, "interval")
		data.req.LogToSpan(sp)

		resp, err := h.next.Do(ctx, data.req)

		select {
		case <-ctx.Done():
			sp.Finish()
			return
		case data.ch <- &packedResp{resp, err}:
			sp.Finish()
		}
	}
}

func (h *splitByInterval) Do(ctx context.Context, r queryrange.Request) (queryrange.Response, error) {
	//lokiRequest := r.(*LokiRequest)

	userid, err := user.ExtractOrgID(ctx)
	if err != nil {
		return nil, httpgrpc.Errorf(http.StatusBadRequest, err.Error())
	}

	interval := h.limits.QuerySplitDuration(userid)
	// skip split by if unset
	if interval == 0 {
		return h.next.Do(ctx, r)
	}

	intervals := splitByTime(r, interval)
	h.metrics.splits.Observe(float64(len(intervals)))

	// no interval should not be processed by the frontend.
	if len(intervals) == 0 {
		return h.next.Do(ctx, r)
	}

	if sp := opentracing.SpanFromContext(ctx); sp != nil {
		sp.LogFields(otlog.Int("n_intervals", len(intervals)))

	}

	var limit int64
	switch req := r.(type) {
	case *LokiRequest:
		limit = int64(req.Limit)
		if req.Direction == logproto.BACKWARD {
			for i, j := 0, len(intervals)-1; i < j; i, j = i+1, j-1 {
				intervals[i], intervals[j] = intervals[j], intervals[i]
			}
		}
	case *LokiSeriesRequest:
		// Set this to 0 since this is not used in Series Request.
		limit = 0
	default:
		return nil, httpgrpc.Errorf(http.StatusBadRequest, "unknown request type")
	}

	input := make([]*lokiResult, 0, len(intervals))
	for _, interval := range intervals {
		input = append(input, &lokiResult{
			req: interval,
			ch:  make(chan *packedResp),
		})
	}

	resps, err := h.Process(ctx, h.limits.MaxQueryParallelism(userid), limit, input)
	if err != nil {
		return nil, err
	}
	return h.merger.MergeResponse(resps...)
}

func splitByTime(req queryrange.Request, interval time.Duration) []queryrange.Request {
	var reqs []queryrange.Request

	switch r := req.(type) {
	case *LokiRequest:
		for start := r.StartTs; start.Before(r.EndTs); start = start.Add(interval) {
			end := start.Add(interval)
			if end.After(r.EndTs) {
				end = r.EndTs
			}
			reqs = append(reqs, &LokiRequest{
				Query:     r.Query,
				Limit:     r.Limit,
				Step:      r.Step,
				Direction: r.Direction,
				Path:      r.Path,
				StartTs:   start,
				EndTs:     end,
			})
		}
		return reqs
	case *LokiSeriesRequest:
		for start := r.StartTs; start.Before(r.EndTs); start = start.Add(interval) {
			end := start.Add(interval)
			if end.After(r.EndTs) {
				end = r.EndTs
			}
			reqs = append(reqs, &LokiSeriesRequest{
				Match:   r.Match,
				Path:    r.Path,
				StartTs: start,
				EndTs:   end,
			})
		}
		return reqs
	default:
		return nil
	}
}