package icssub

import (
	"net/http"
	"net/http/httptest"
	"strings"
	"sync"
	"sync/atomic"
	"testing"
	"time"

	ical "github.com/emersion/go-ical"
)

const sampleICS = "BEGIN:VCALENDAR\r\n" +
	"VERSION:2.0\r\n" +
	"PRODID:-//nidus//test//EN\r\n" +
	"BEGIN:VEVENT\r\n" +
	"UID:ev1@nidus.test\r\n" +
	"DTSTAMP:20260101T000000Z\r\n" +
	"DTSTART:20260805T090000Z\r\n" +
	"DTEND:20260805T100000Z\r\n" +
	"SUMMARY:Original title\r\n" +
	"END:VEVENT\r\n" +
	"END:VCALENDAR\r\n"

func mustParseEvent(raw string) ical.Event {
	cal, err := ical.NewDecoder(strings.NewReader(raw)).Decode()
	if err != nil {
		panic("mustParseEvent: " + err.Error())
	}
	evs := cal.Events()
	if len(evs) == 0 {
		panic("mustParseEvent: no events")
	}
	return evs[0]
}

// TestCacheFirstFetchPopulatesEntry verifies that the very first Get for a
// URL does a foreground fetch and returns the parsed calendar.
func TestCacheFirstFetchPopulatesEntry(t *testing.T) {
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusOK)
		_, _ = w.Write([]byte(sampleICS))
	}))
	defer srv.Close()

	c := NewCache(time.Hour)
	cal, err := c.Get(srv.URL)
	if err != nil {
		t.Fatalf("first Get: %v", err)
	}
	if len(cal.Events()) == 0 {
		t.Fatalf("expected >=1 event, got 0")
	}
}

// TestCacheStaleReturnsWithBackgroundRefresh verifies that once a cached
// entry is stale, Get keeps returning the *stale* copy immediately while a
// background refresh is spawned in a separate goroutine (rather than
// blocking the caller on the network).
func TestCacheStaleReturnsWithBackgroundRefresh(t *testing.T) {
	var hits atomic.Int32
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		hits.Add(1)
		time.Sleep(50 * time.Millisecond) // make the fetch slow enough to
		w.WriteHeader(http.StatusOK)      // observe as background work
		_, _ = w.Write([]byte(sampleICS))
	}))
	defer srv.Close()

	c := NewCache(1 * time.Millisecond) // very short TTL
	if _, err := c.Get(srv.URL); err != nil {
		t.Fatalf("first Get: %v", err)
	}
	firstHits := hits.Load()

	// Force staleness.
	time.Sleep(3 * time.Millisecond)

	// Subsequent Get must return the stale copy immediately without waiting
	// for the slow server to respond — if it blocked, this call would take
	// >= 50ms, which we bound with a deadline below via the hit counter.
	st := time.Now()
	cal, err := c.Get(srv.URL)
	if err != nil {
		t.Fatalf("second Get: %v", err)
	}
	if len(cal.Events()) == 0 {
		t.Fatalf("expected event in stale response")
	}
	if time.Since(st) > 40*time.Millisecond {
		t.Fatalf("second Get appears to have blocked on the network for %v", time.Since(st))
	}

	// Wait for the background refresh to complete.
	deadline := time.Now().Add(2 * time.Second)
	for hits.Load() == firstHits && time.Now().Before(deadline) {
		time.Sleep(5 * time.Millisecond)
	}
	if hits.Load() == firstHits {
		t.Fatalf("background refresh did not fire (hits stayed at %d)", firstHits)
	}
}

// TestCacheConcurrentFirstFetchCoalesce ensures N concurrent first calls
// for the same URL coalesce to a small number of actual origin fetches.
func TestCacheConcurrentFirstFetchCoalesce(t *testing.T) {
	var hits atomic.Int32
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		hits.Add(1)
		time.Sleep(40 * time.Millisecond) // widen the coalescing window
		w.WriteHeader(http.StatusOK)
		_, _ = w.Write([]byte(sampleICS))
	}))
	defer srv.Close()

	c := NewCache(time.Hour)

	const n = 8
	errs := make([]error, n)
	var wg sync.WaitGroup
	start := make(chan struct{})
	for i := 0; i < n; i++ {
		wg.Add(1)
		go func(i int) {
			defer wg.Done()
			<-start
			_, err := c.Get(srv.URL)
			errs[i] = err
		}(i)
	}
	close(start)
	wg.Wait()
	for _, err := range errs {
		if err != nil {
			t.Fatalf("goroutine error: %v", err)
		}
	}
	// Without coalescing we'd see n=8 hits; with it we expect a small
	// constant (the first caller fetches; the rest either return the
	// in-progress result or kick off a coalesced background refresh).
	if hits.Load() > 3 {
		t.Fatalf("expected coalescing to reduce fetches (got %d)", hits.Load())
	}
}

// TestEventIDStable verifies that the same event hashes to the same ID
// across calls, and is the right shape (32 hex chars + ".ics").
func TestEventIDStable(t *testing.T) {
	ev := mustParseEvent(sampleICS)
	id1 := EventID(ev)
	id2 := EventID(ev)
	if id1 != id2 {
		t.Fatalf("EventID not stable: %q vs %q", id1, id2)
	}
	if !strings.HasSuffix(id1, ".ics") {
		t.Fatalf("EventID should end in .ics, got %q", id1)
	}
	if len(id1) != 32+4 {
		t.Fatalf("expected 32 hex chars + .ics, got %q (len %d)", id1, len(id1))
	}
}

// TestEventIDDiffersForDifferentEvents verifies that two events with
// different titles or different start times hash to different IDs, while
// two events with only a different UID hash to the same ID.
func TestEventIDDiffersForDifferentEvents(t *testing.T) {
	base := EventID(mustParseEvent(sampleICS))

	// Different title.
	rawA := strings.Replace(sampleICS, "SUMMARY:Original title", "SUMMARY:Other title", 1)
	if EventID(mustParseEvent(rawA)) == base {
		t.Fatal("same start, different title must hash differently")
	}

	// Different start time.
	rawB := strings.Replace(sampleICS, "DTSTART:20260805T090000Z", "DTSTART:20260806T090000Z", 1)
	if EventID(mustParseEvent(rawB)) == base {
		t.Fatal("different start times must hash differently")
	}

	// Same start, same title, different UID → same hash.
	rawC := strings.Replace(sampleICS, "UID:ev1@nidus.test", "UID:ev2@nidus.test", 1)
	if EventID(mustParseEvent(rawC)) != base {
		t.Fatal("UID should not affect the hash — same start/dur/title must be equal")
	}
}

// TestEventIDWorksWithoutUID verifies EventID still produces a value (from
// DTSTART+DTEND+SUMMARY) when the event has no UID property.
func TestEventIDWorksWithoutUID(t *testing.T) {
	raw := strings.Replace(sampleICS, "UID:ev1@nidus.test\r\n", "", 1)
	ev := mustParseEvent(raw)
	id := EventID(ev)
	if id == "" {
		t.Fatal("EventID should be non-empty even without UID")
	}
}