// Package stats implements a background goroutine that periodically samples
// Docker container and host-level resource usage and persists the samples
// into SQLite. It reads its interval and retention from settings on every
// tick so configuration changes take effect without a restart.
package stats

import (
	"context"
	"log/slog"
	"sync"
	"time"

	"github.com/alexei/tinyforge/internal/docker"
	"github.com/alexei/tinyforge/internal/store"
)

// Defaults applied when settings values are outside their valid range.
const (
	DefaultIntervalSeconds = 15
	DefaultRetentionHours  = 2
	MinIntervalSeconds     = 5
	MaxIntervalSeconds     = 300
	// Hard cap on parallel container stat requests to avoid overwhelming
	// the Docker daemon when the user has many containers.
	maxParallelSamples = 8
)

// OwnerType values for ContainerStatsSample.OwnerType.
const (
	OwnerTypeInstance = "instance"
	OwnerTypeSite     = "site"
)

// Collector runs the background sampling loop.
type Collector struct {
	store  *store.Store
	docker *docker.Client

	startOnce sync.Once
	stopOnce  sync.Once
	started   bool
	stop      chan struct{}
	done      chan struct{}
}

// New creates a new stats collector. Call Start to begin sampling.
func New(s *store.Store, d *docker.Client) *Collector {
	return &Collector{
		store:  s,
		docker: d,
		stop:   make(chan struct{}),
		done:   make(chan struct{}),
	}
}

// Start launches the background loop. Returns immediately. The loop exits
// when Stop is called. Safe to call multiple times — only the first call has
// an effect.
func (c *Collector) Start() {
	c.startOnce.Do(func() {
		c.started = true
		go c.run()
	})
}

// Stop signals the collector to exit and blocks until it has finished the
// in-flight tick. If Start was never called, Stop returns immediately.
func (c *Collector) Stop() {
	c.stopOnce.Do(func() {
		close(c.stop)
		if !c.started {
			close(c.done)
		}
	})
	<-c.done
}

// run is the main loop. It reads the interval from settings on every tick,
// which lets configuration changes propagate within one tick without a
// dedicated reload mechanism.
func (c *Collector) run() {
	defer close(c.done)

	// Derive a base context that's cancelled when Stop is called so in-flight
	// Docker requests abort instead of waiting out their timeout.
	baseCtx, cancel := context.WithCancel(context.Background())
	defer cancel()
	go func() {
		<-c.stop
		cancel()
	}()

	// Wait a few seconds before the first sample so the app has settled.
	select {
	case <-time.After(3 * time.Second):
	case <-c.stop:
		return
	}

	for {
		interval, retention := c.readConfig()
		if interval == 0 || retention == 0 {
			// Collection disabled. Poll settings every minute in case the
			// user re-enables it.
			select {
			case <-time.After(time.Minute):
				continue
			case <-c.stop:
				return
			}
		}

		c.tick(baseCtx, retention)

		select {
		case <-time.After(time.Duration(interval) * time.Second):
		case <-c.stop:
			return
		}
	}
}

// readConfig reads the current interval + retention from settings, applying
// defaults and clamping to the valid range.
func (c *Collector) readConfig() (intervalSeconds, retentionHours int) {
	settings, err := c.store.GetSettings()
	if err != nil {
		slog.Warn("stats collector: failed to read settings — using defaults", "error", err)
		return DefaultIntervalSeconds, DefaultRetentionHours
	}
	intervalSeconds = settings.StatsIntervalSeconds
	retentionHours = settings.StatsRetentionHours
	if intervalSeconds < 0 || retentionHours < 0 {
		return 0, 0
	}
	if intervalSeconds > 0 && intervalSeconds < MinIntervalSeconds {
		intervalSeconds = MinIntervalSeconds
	}
	if intervalSeconds > MaxIntervalSeconds {
		intervalSeconds = MaxIntervalSeconds
	}
	return intervalSeconds, retentionHours
}

// tick samples all known containers, aggregates workload-level totals,
// persists samples, and prunes rows beyond the retention window. When
// the Docker daemon is unreachable the whole tick is skipped with a
// single debug log instead of one warning per container.
func (c *Collector) tick(parent context.Context, retentionHours int) {
	ctx, cancel := context.WithTimeout(parent, 30*time.Second)
	defer cancel()

	pingCtx, pingCancel := context.WithTimeout(ctx, 2*time.Second)
	defer pingCancel()
	if err := c.docker.Ping(pingCtx); err != nil {
		slog.Debug("stats collector: docker unreachable, skipping tick", "error", err)
		return
	}

	targets := c.buildTargets()
	if len(targets) == 0 {
		// No containers to sample, but still record a system sample so the
		// host history isn't empty.
		c.recordSystemSample(ctx, 0, 0, 0)
		c.prune(retentionHours)
		return
	}

	samples := c.sampleAll(ctx, targets)

	var (
		totalCPU float64
		totalMem int64
		running  int
	)
	for _, s := range samples {
		if err := c.store.InsertContainerStatsSample(s); err != nil {
			slog.Warn("stats collector: insert container sample",
				"container", s.ContainerID, "error", err)
			continue
		}
		totalCPU += s.CPUPercent
		totalMem += s.MemoryUsage
		running++
	}

	c.recordSystemSample(ctx, totalCPU, totalMem, running)
	c.prune(retentionHours)
}

// target describes a single container to sample.
type target struct {
	ContainerID string
	OwnerType   string
	OwnerID     string
}

// buildTargets fetches container rows that have a docker container_id bound.
// Project containers and stack containers are surfaced as OwnerTypeInstance
// (the stats sample owner_type is kept for back-compat with the persisted
// schema and the dashboard's group-by semantics).
func (c *Collector) buildTargets() []target {
	var out []target

	containers, err := c.store.ListContainers(store.ContainerFilter{})
	if err != nil {
		slog.Warn("stats collector: list containers", "error", err)
	} else {
		for _, row := range containers {
			if row.ContainerID == "" {
				continue
			}
			ownerType := OwnerTypeInstance
			if row.WorkloadKind == string(store.WorkloadKindSite) {
				ownerType = OwnerTypeSite
			}
			out = append(out, target{
				ContainerID: row.ContainerID,
				OwnerType:   ownerType,
				OwnerID:     row.ID,
			})
		}
	}

	return out
}

// sampleAll fetches Docker stats for every target in bounded parallelism.
// Failed samples are logged and skipped — a missing container must not kill
// the whole tick.
func (c *Collector) sampleAll(ctx context.Context, targets []target) []store.ContainerStatsSample {
	sem := make(chan struct{}, maxParallelSamples)
	results := make([]store.ContainerStatsSample, len(targets))
	found := make([]bool, len(targets))

	var wg sync.WaitGroup
loop:
	for i, t := range targets {
		// Acquire the semaphore in the parent loop so ctx cancellation
		// short-circuits the queue rather than spawning goroutines that
		// block on an unreachable slot. The labelled break exits the for
		// loop directly; a bare `break` inside `select` would only break
		// the select and let the loop continue.
		select {
		case sem <- struct{}{}:
		case <-ctx.Done():
			break loop
		}
		wg.Add(1)
		go func(i int, t target) {
			defer wg.Done()
			defer func() { <-sem }()

			sampleCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
			defer cancel()

			stats, err := c.docker.GetContainerStats(sampleCtx, t.ContainerID)
			if err != nil {
				slog.Debug("stats collector: get container stats",
					"container", t.ContainerID, "owner_type", t.OwnerType, "error", err)
				return
			}
			ts := stats.Timestamp.Unix()
			if ts <= 0 {
				ts = time.Now().UTC().Unix()
			}
			results[i] = store.ContainerStatsSample{
				ContainerID:     t.ContainerID,
				OwnerType:       t.OwnerType,
				OwnerID:         t.OwnerID,
				TS:              ts,
				CPUPercent:      stats.CPUPercent,
				MemoryUsage:     stats.MemoryUsage,
				MemoryLimit:     stats.MemoryLimit,
				NetworkRxBytes:  stats.NetworkRxBytes,
				NetworkTxBytes:  stats.NetworkTxBytes,
				BlockReadBytes:  stats.BlockReadBytes,
				BlockWriteBytes: stats.BlockWriteBytes,
			}
			found[i] = true
		}(i, t)
	}
	wg.Wait()

	out := results[:0]
	for i := range results {
		if found[i] {
			out = append(out, results[i])
		}
	}
	return out
}

// recordSystemSample fetches host info + disk usage and persists a combined
// system-level sample. Failures are warned but do not propagate.
func (c *Collector) recordSystemSample(ctx context.Context, workloadCPU float64, workloadMem int64, running int) {
	sysStats, err := c.docker.GetSystemStats(ctx)
	if err != nil {
		slog.Warn("stats collector: get system stats", "error", err)
		return
	}
	ts := sysStats.Timestamp.Unix()
	if ts <= 0 {
		ts = time.Now().UTC().Unix()
	}
	sample := store.SystemStatsSample{
		TS:                 ts,
		NCPU:               sysStats.NCPU,
		MemoryTotal:        sysStats.MemoryTotal,
		WorkloadCPUPercent: workloadCPU,
		WorkloadMemUsage:   workloadMem,
		ContainersRunning:  running,
		DiskTotalBytes:     sysStats.DiskTotalBytes,
	}
	// Prefer the Docker-reported running count when we have no running samples
	// (e.g., very first tick may race with container readiness).
	if running == 0 && sysStats.Running > 0 {
		sample.ContainersRunning = sysStats.Running
	}
	if err := c.store.InsertSystemStatsSample(sample); err != nil {
		slog.Warn("stats collector: insert system sample", "error", err)
	}
}

// prune drops rows older than the retention window.
func (c *Collector) prune(retentionHours int) {
	if retentionHours <= 0 {
		return
	}
	cutoff := time.Now().UTC().Add(-time.Duration(retentionHours) * time.Hour).Unix()
	if _, err := c.store.PruneStatsSamplesBefore(cutoff); err != nil {
		slog.Warn("stats collector: prune", "error", err)
	}
}