package server

import (
	"context"
	"sync"
	"time"

	"git.eeqj.de/sneak/routewatch/internal/database"
)

const (
	// statsRefreshInterval is how often the cached database statistics are
	// recomputed. The scans behind GetStatsContext grow with the tables, so a
	// request serves the cached copy instead of running them.
	statsRefreshInterval = 30 * time.Second

	// statsComputeTimeout bounds a single statistics computation so a stuck scan
	// cannot block the refresh forever.
	statsComputeTimeout = 20 * time.Second
)

// statsFetch computes fresh statistics. It is the expensive database scan that
// the cache runs at most once per interval.
type statsFetch func(ctx context.Context) (database.Stats, error)

// statsCache serves the most recent database statistics and recomputes them at
// most once per interval. The first request computes synchronously so it has
// real data to return; afterwards requests serve the cached copy immediately
// and a stale copy triggers a single background refresh, so no request waits on
// the scans.
type statsCache struct {
	fetch    statsFetch
	interval time.Duration
	now      func() time.Time

	mu         sync.Mutex
	stats      database.Stats
	haveStats  bool
	fetchedAt  time.Time
	refreshing bool
}

// newStatsCache returns a cache that recomputes statistics with fetch no more
// than once per statsRefreshInterval.
func newStatsCache(fetch statsFetch) *statsCache {
	return &statsCache{
		fetch:    fetch,
		interval: statsRefreshInterval,
		now:      time.Now,
	}
}

// get returns the cached statistics. On the first call it computes them
// synchronously and returns any error. Later calls return the cached copy, and
// when that copy is older than the interval they start one background refresh.
func (c *statsCache) get() (database.Stats, error) {
	c.mu.Lock()

	if !c.haveStats {
		// Cold start: compute once under the lock so concurrent first callers
		// wait for this single computation rather than each starting their own.
		stats, err := c.compute()
		if err != nil {
			c.mu.Unlock()

			return database.Stats{}, err
		}
		c.store(stats)
		c.mu.Unlock()

		return stats, nil
	}

	if c.now().Sub(c.fetchedAt) >= c.interval && !c.refreshing {
		c.refreshing = true
		go c.refresh()
	}

	stats := c.stats
	c.mu.Unlock()

	return stats, nil
}

// refresh recomputes the statistics in the background and replaces the cached
// copy. A failed computation leaves the previous copy in place.
func (c *statsCache) refresh() {
	stats, err := c.compute()

	c.mu.Lock()
	defer c.mu.Unlock()
	c.refreshing = false
	if err == nil {
		c.store(stats)
	}
}

// compute runs the fetch with its own bounded context, independent of any
// request, so one request's cancellation cannot abort a shared refresh.
func (c *statsCache) compute() (database.Stats, error) {
	ctx, cancel := context.WithTimeout(context.Background(), statsComputeTimeout)
	defer cancel()

	return c.fetch(ctx)
}

// store records a fresh result. The caller must hold the mutex.
func (c *statsCache) store(stats database.Stats) {
	c.stats = stats
	c.haveStats = true
	c.fetchedAt = c.now()
}