// SnapshotHolder provides atomic hot-reload for SnapshotReader with drain
// grace — solves the classic "Get concurrent with Close/munmap" race that
// would otherwise produce uncatchable SIGBUS / SIGSEGV under reload.
//
// Pattern source: slask-finder's `RawItemsStore` and similar mmap+reload
// stores, which delay unmap behind a configurable drain-grace timer so any
// in-flight Get on the previously-current reader completes before the
// underlying pages become unmapped. See docs/inventory-shape.md & the
// finder WARP doc on mmap reload for the prior art.
//
// Use:
//
//	holder := catalog.NewSnapshotHolder(30 * time.Second) // drain grace
//	if err := holder.Open("/var/lib/catalog/bygglagret.bin"); err != nil { ... }
//	// later, when a fresh snapshot lands:
//	if err := holder.Reload("/var/lib/catalog/bygglagret.bin"); err != nil { ... }
//	// pulls the projection; always hits the current reader:
//	p, ok := holder.Get(sku)
//
// holder.Get / Len are safe for parallel use. Open and Reload are also
// safe for parallel use. Close stops the reaper goroutine and shuts the
// current reader + drains down.
package catalog

import (
	"sync"
	"sync/atomic"
	"time"
)

// DefaultDrainGrace is the default drain period for SnapshotHolder. Finder's
// analogous reload path uses ~30s; we mirror that value.
const DefaultDrainGrace = 30 * time.Second

// SnapshotHolder swaps a SnapshotReader under load without unmapping pages
// out from under in-flight Get()s. Set retains the previous reader behind a
// drain timer; after the timer elapses the reaper goroutine closes the old
// reader (and runs its munmap).
type SnapshotHolder struct {
	current atomic.Pointer[SnapshotReader]

	drainsMu  sync.Mutex
	drains    []*pendingClose
	drainGrace time.Duration

	stopCh chan struct{}
	once   sync.Once
}

type pendingClose struct {
	r        *SnapshotReader
	expireAt time.Time
}

// NewSnapshotHolder returns a holder with the given drain grace (the time
// the previous reader lives behind Set before being closed). Zero or
// negative grace falls back to DefaultDrainGrace.
func NewSnapshotHolder(grace time.Duration) *SnapshotHolder {
	if grace <= 0 {
		grace = DefaultDrainGrace
	}
	h := &SnapshotHolder{
		drainGrace: grace,
		stopCh:     make(chan struct{}),
	}
	go h.sweepLoop()
	return h
}

// Open is shorthand for Set(ReadSnapshotFile(path)). Errors leave the
// holder's current reader unchanged.
func (h *SnapshotHolder) Open(path string) error {
	r, err := ReadSnapshotFile(path)
	if err != nil {
		return err
	}
	h.Set(r)
	return nil
}

// Reload atomically swaps the current reader for one freshly opened from
// path. The old reader (if any) is queued for close after the drain grace
// elapses; in-flight Get(s) on the old reader continue to work.
func (h *SnapshotHolder) Reload(path string) error {
	r, err := ReadSnapshotFile(path)
	if err != nil {
		return err
	}
	h.Set(r)
	return nil
}

// Set replaces the current reader with r. The previous reader is queued
// for close after the drain grace elapses. r may be nil to clear.
func (h *SnapshotHolder) Set(r *SnapshotReader) {
	if r == nil {
		h.current.Store(nil)
		return
	}
	old := h.current.Swap(r)
	if old == nil {
		return
	}
	h.drainsMu.Lock()
	h.drains = append(h.drains, &pendingClose{
		r:        old,
		expireAt: time.Now().Add(h.drainGrace),
	})
	h.drainsMu.Unlock()
}

// Get is the concurrent-safe shim over the current reader's Get. Returns
// (zero, false) when the holder is empty.
func (h *SnapshotHolder) Get(sku string) (Projection, bool) {
	r := h.current.Load()
	if r == nil {
		return Projection{}, false
	}
	return r.Get(sku)
}

// Len returns the count of the current reader (0 if empty).
func (h *SnapshotHolder) Len() int {
	r := h.current.Load()
	if r == nil {
		return 0
	}
	return r.Len()
}

// Close stops the reaper goroutine and closes the current reader + any
// pending drains without waiting for the grace period. Safe to call
// multiple times.
func (h *SnapshotHolder) Close() error {
	var firstErr error
	h.once.Do(func() {
		close(h.stopCh)
		r := h.current.Swap(nil)
		if r != nil {
			if err := r.Close(); err != nil {
				firstErr = err
			}
		}
		h.drainsMu.Lock()
		drained := h.drains
		h.drains = nil
		h.drainsMu.Unlock()
		for _, d := range drained {
			if err := d.r.Close(); err != nil && firstErr == nil {
				firstErr = err
			}
		}
	})
	return firstErr
}

// DrainCount returns the number of readers currently in the drain queue.
// Useful for tests and operational metrics.
func (h *SnapshotHolder) DrainCount() int {
	h.drainsMu.Lock()
	defer h.drainsMu.Unlock()
	return len(h.drains)
}

// reapNow scans the drain queue and Closes any readers whose grace has
// elapsed. Called by the sweeper goroutine on a tick.
func (h *SnapshotHolder) reapNow() {
	now := time.Now()
	var alive []*pendingClose
	var reaped []*pendingClose
	h.drainsMu.Lock()
	for _, d := range h.drains {
		if !d.expireAt.After(now) {
			reaped = append(reaped, d)
		} else {
			alive = append(alive, d)
		}
	}
	h.drains = alive
	h.drainsMu.Unlock()
	for _, d := range reaped {
		_ = d.r.Close()
	}
}

func (h *SnapshotHolder) sweepLoop() {
	interval := h.drainGrace / 4
	if interval < 10*time.Millisecond {
		interval = 10 * time.Millisecond
	}
	t := time.NewTicker(interval)
	defer t.Stop()
	for {
		select {
		case <-h.stopCh:
			return
		case <-t.C:
			h.reapNow()
		}
	}
}