package prebuilds

import (
	"context"
	"fmt"
	"sync/atomic"
	"time"

	"github.com/prometheus/client_golang/prometheus"
	"golang.org/x/xerrors"

	"cdr.dev/slog"

	"github.com/coder/coder/v2/coderd/database"
	"github.com/coder/coder/v2/coderd/database/dbtime"
	"github.com/coder/coder/v2/coderd/prebuilds"
)

var (
	labels               = []string{"template_name", "preset_name", "organization_name"}
	createdPrebuildsDesc = prometheus.NewDesc(
		"coderd_prebuilt_workspaces_created_total",
		"Total number of prebuilt workspaces that have been created to meet the desired instance count of each "+
			"template preset.",
		labels,
		nil,
	)
	failedPrebuildsDesc = prometheus.NewDesc(
		"coderd_prebuilt_workspaces_failed_total",
		"Total number of prebuilt workspaces that failed to build.",
		labels,
		nil,
	)
	claimedPrebuildsDesc = prometheus.NewDesc(
		"coderd_prebuilt_workspaces_claimed_total",
		"Total number of prebuilt workspaces which were claimed by users. Claiming refers to creating a workspace "+
			"with a preset selected for which eligible prebuilt workspaces are available and one is reassigned to a user.",
		labels,
		nil,
	)
	desiredPrebuildsDesc = prometheus.NewDesc(
		"coderd_prebuilt_workspaces_desired",
		"Target number of prebuilt workspaces that should be available for each template preset.",
		labels,
		nil,
	)
	runningPrebuildsDesc = prometheus.NewDesc(
		"coderd_prebuilt_workspaces_running",
		"Current number of prebuilt workspaces that are in a running state. These workspaces have started "+
			"successfully but may not yet be claimable by users (see coderd_prebuilt_workspaces_eligible).",
		labels,
		nil,
	)
	eligiblePrebuildsDesc = prometheus.NewDesc(
		"coderd_prebuilt_workspaces_eligible",
		"Current number of prebuilt workspaces that are eligible to be claimed by users. These are workspaces that "+
			"have completed their build process with their agent reporting 'ready' status.",
		labels,
		nil,
	)
	lastUpdateDesc = prometheus.NewDesc(
		"coderd_prebuilt_workspaces_metrics_last_updated",
		"The unix timestamp when the metrics related to prebuilt workspaces were last updated; these metrics are cached.",
		[]string{},
		nil,
	)
)

const (
	metricsUpdateInterval = time.Second * 15
	metricsUpdateTimeout  = time.Second * 10
)

type MetricsCollector struct {
	database    database.Store
	logger      slog.Logger
	snapshotter prebuilds.StateSnapshotter

	latestState atomic.Pointer[metricsState]
}

var _ prometheus.Collector = new(MetricsCollector)

func NewMetricsCollector(db database.Store, logger slog.Logger, snapshotter prebuilds.StateSnapshotter) *MetricsCollector {
	log := logger.Named("prebuilds_metrics_collector")
	return &MetricsCollector{
		database:    db,
		logger:      log,
		snapshotter: snapshotter,
	}
}

func (*MetricsCollector) Describe(descCh chan<- *prometheus.Desc) {
	descCh <- createdPrebuildsDesc
	descCh <- failedPrebuildsDesc
	descCh <- claimedPrebuildsDesc
	descCh <- desiredPrebuildsDesc
	descCh <- runningPrebuildsDesc
	descCh <- eligiblePrebuildsDesc
	descCh <- lastUpdateDesc
}

// Collect uses the cached state to set configured metrics.
// The state is cached because this function can be called multiple times per second and retrieving the current state
// is an expensive operation.
func (mc *MetricsCollector) Collect(metricsCh chan<- prometheus.Metric) {
	currentState := mc.latestState.Load() // Grab a copy; it's ok if it goes stale during the course of this func.
	if currentState == nil {
		mc.logger.Warn(context.Background(), "failed to set prebuilds metrics; state not set")
		metricsCh <- prometheus.MustNewConstMetric(lastUpdateDesc, prometheus.GaugeValue, 0)
		return
	}

	for _, metric := range currentState.prebuildMetrics {
		metricsCh <- prometheus.MustNewConstMetric(createdPrebuildsDesc, prometheus.CounterValue, float64(metric.CreatedCount), metric.TemplateName, metric.PresetName, metric.OrganizationName)
		metricsCh <- prometheus.MustNewConstMetric(failedPrebuildsDesc, prometheus.CounterValue, float64(metric.FailedCount), metric.TemplateName, metric.PresetName, metric.OrganizationName)
		metricsCh <- prometheus.MustNewConstMetric(claimedPrebuildsDesc, prometheus.CounterValue, float64(metric.ClaimedCount), metric.TemplateName, metric.PresetName, metric.OrganizationName)
	}

	for _, preset := range currentState.snapshot.Presets {
		if !preset.UsingActiveVersion {
			continue
		}

		presetSnapshot, err := currentState.snapshot.FilterByPreset(preset.ID)
		if err != nil {
			mc.logger.Error(context.Background(), "failed to filter by preset", slog.Error(err))
			continue
		}
		state := presetSnapshot.CalculateState()

		metricsCh <- prometheus.MustNewConstMetric(desiredPrebuildsDesc, prometheus.GaugeValue, float64(state.Desired), preset.TemplateName, preset.Name, preset.OrganizationName)
		metricsCh <- prometheus.MustNewConstMetric(runningPrebuildsDesc, prometheus.GaugeValue, float64(state.Actual), preset.TemplateName, preset.Name, preset.OrganizationName)
		metricsCh <- prometheus.MustNewConstMetric(eligiblePrebuildsDesc, prometheus.GaugeValue, float64(state.Eligible), preset.TemplateName, preset.Name, preset.OrganizationName)
	}

	metricsCh <- prometheus.MustNewConstMetric(lastUpdateDesc, prometheus.GaugeValue, float64(currentState.createdAt.Unix()))
}

type metricsState struct {
	prebuildMetrics []database.GetPrebuildMetricsRow
	snapshot        *prebuilds.GlobalSnapshot
	createdAt       time.Time
}

// BackgroundFetch updates the metrics state every given interval.
func (mc *MetricsCollector) BackgroundFetch(ctx context.Context, updateInterval, updateTimeout time.Duration) {
	tick := time.NewTicker(time.Nanosecond)
	defer tick.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-tick.C:
			// Tick immediately, then set regular interval.
			tick.Reset(updateInterval)

			if err := mc.UpdateState(ctx, updateTimeout); err != nil {
				mc.logger.Error(ctx, "failed to update prebuilds metrics state", slog.Error(err))
			}
		}
	}
}

// UpdateState builds the current metrics state.
func (mc *MetricsCollector) UpdateState(ctx context.Context, timeout time.Duration) error {
	start := time.Now()
	fetchCtx, fetchCancel := context.WithTimeout(ctx, timeout)
	defer fetchCancel()

	prebuildMetrics, err := mc.database.GetPrebuildMetrics(fetchCtx)
	if err != nil {
		return xerrors.Errorf("fetch prebuild metrics: %w", err)
	}

	snapshot, err := mc.snapshotter.SnapshotState(fetchCtx, mc.database)
	if err != nil {
		return xerrors.Errorf("snapshot state: %w", err)
	}
	mc.logger.Debug(ctx, "fetched prebuilds metrics state", slog.F("duration_secs", fmt.Sprintf("%.2f", time.Since(start).Seconds())))

	mc.latestState.Store(&metricsState{
		prebuildMetrics: prebuildMetrics,
		snapshot:        snapshot,
		createdAt:       dbtime.Now(),
	})
	return nil
}