package fitness

import (
	"sort"
	"time"

	"github.com/tegwick/fluid-core/internal/contract"
)

// Standard metric names the measurer derives from telemetry.
//
// These are the metrics the Blueprint's worked example turns on, and having
// them named in one place stops a hypothesis and an evaluation from measuring
// subtly different things under the same word.
const (
	MetricRequestsPerTask = "requests_per_completed_task"
	MetricP95LatencyMS    = "p95_latency_ms"
	MetricErrorRate       = "error_rate"
	MetricSuccessRate     = "success_rate"
)

// Measurer derives metric observations from raw telemetry.
type Measurer struct {
	// ChainGap bounds one completed task when the consumer supplies no chain
	// id, matching the topology analyzer's grouping.
	ChainGap time.Duration
}

// NewMeasurer returns a measurer with the default grouping window.
func NewMeasurer() *Measurer { return &Measurer{ChainGap: 30 * time.Second} }

// Measure computes the standard metrics per revision over a window.
//
// Only events inside the window count. Blueprint section 18 requires the
// measurement window to be retained, and quietly including events outside it
// would make a comparison irreproducible.
func (m *Measurer) Measure(events []contract.FluidTelemetry, window Window) []Observation {
	type acc struct {
		requests int
		errors   int
		tasks    map[string]int
		latency  []float64
	}
	byRevision := map[contract.RevisionID]*acc{}

	for _, ev := range events {
		if !inWindow(ev.OccurredAt, window) {
			continue
		}
		if ev.Revision == nil {
			continue
		}
		rev := *ev.Revision

		a, ok := byRevision[rev]
		if !ok {
			a = &acc{tasks: map[string]int{}}
			byRevision[rev] = a
		}

		a.requests++
		if ev.Error != nil {
			a.errors++
		}
		if ev.Request != nil && ev.Request.LatencyMS != nil {
			a.latency = append(a.latency, *ev.Request.LatencyMS)
		}
		a.tasks[taskKey(ev, m.ChainGap)]++
	}

	revisions := make([]contract.RevisionID, 0, len(byRevision))
	for r := range byRevision {
		revisions = append(revisions, r)
	}
	sort.Slice(revisions, func(i, j int) bool { return revisions[i] < revisions[j] })

	var out []Observation
	for _, rev := range revisions {
		a := byRevision[rev]
		if a.requests == 0 {
			continue
		}

		// Requests per completed task is the metric that catches an interface
		// making consumers assemble what it could have handed them.
		tasks := len(a.tasks)
		if tasks > 0 {
			out = append(out, Observation{
				Metric:   MetricRequestsPerTask,
				Revision: rev,
				Value:    round4(float64(a.requests) / float64(tasks)),
				Samples:  tasks,
			})
		}

		out = append(out, Observation{
			Metric:   MetricErrorRate,
			Revision: rev,
			Value:    round4(float64(a.errors) / float64(a.requests)),
			Samples:  a.requests,
		})
		out = append(out, Observation{
			Metric:   MetricSuccessRate,
			Revision: rev,
			Value:    round4(float64(a.requests-a.errors) / float64(a.requests)),
			Samples:  a.requests,
		})

		if len(a.latency) > 0 {
			out = append(out, Observation{
				Metric:   MetricP95LatencyMS,
				Revision: rev,
				Value:    round4(percentile(a.latency, 0.95)),
				Samples:  len(a.latency),
			})
		}
	}
	return out
}

// taskKey groups events into completed tasks.
func taskKey(ev contract.FluidTelemetry, gap time.Duration) string {
	consumer := ev.ConsumerRef
	if consumer == "" {
		consumer = ev.CorrelationID
	}
	if ev.Sequence != nil && ev.Sequence.ChainID != "" {
		return consumer + "/" + ev.Sequence.ChainID
	}
	// Without a chain id, bucket by consumer and elapsed gap. This is a
	// heuristic, which is the reason to prefer chain ids from agentic
	// consumers where they can supply them.
	bucket := ev.OccurredAt.Truncate(gap).UTC().Format(time.RFC3339)
	return consumer + "/" + bucket
}

func inWindow(t time.Time, w Window) bool {
	if !w.Start.IsZero() && t.Before(w.Start) {
		return false
	}
	if w.End != nil && t.After(*w.End) {
		return false
	}
	return true
}

// percentile returns the nearest-rank percentile of values.
func percentile(values []float64, p float64) float64 {
	sorted := make([]float64, len(values))
	copy(sorted, values)
	sort.Float64s(sorted)

	if len(sorted) == 1 {
		return sorted[0]
	}
	rank := int(p*float64(len(sorted)-1) + 0.5)
	if rank < 0 {
		rank = 0
	}
	if rank >= len(sorted) {
		rank = len(sorted) - 1
	}
	return sorted[rank]
}