package backend

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

// health_test.go exercises health.go's degraded/healthy state machine
// (component-failover-adoption task 4.5, boundary: ApiserverBackendHealth,
// Req 4.1-4.2). Deliberately no build tag, matching health.go itself: pure
// Go, no CGO -- runs under both CGO_ENABLED=0 and =1, so this file is the
// one this package's tests can run without any backend/nix CGO setup at
// all (see the task's own validation command: `CGO_ENABLED=0 go test
// ./backend/...`).

// resetHealthForTest restores degradedUntilNanos to its zero value (never
// degraded) between tests, mirroring reconnect_test.go's
// resetNeedsRejoinForTest for the sibling needsRejoin state.
func resetHealthForTest() {
	atomic.StoreInt64(&degradedUntilNanos, 0)
}

// ════════════════════════════════════════════════════════════════════════
// Req 4.1 -- basic degraded/healthy transitions
// ════════════════════════════════════════════════════════════════════════

func TestIsDegraded_FreshState_ReportsHealthy(t *testing.T) {
	resetHealthForTest()

	if IsDegraded() {
		t.Error("expected IsDegraded() to report false before MarkDegraded is ever called")
	}
}

func TestMarkDegraded_ThenIsDegraded_ReportsTrue(t *testing.T) {
	resetHealthForTest()

	MarkDegraded(1 * time.Minute)

	if !IsDegraded() {
		t.Error("expected IsDegraded() to report true immediately after MarkDegraded")
	}
}

// ════════════════════════════════════════════════════════════════════════
// design.md's own stated validation requirement: "unit tests cover
// degraded->healthy transitions via both the TTL expiry and explicit
// MarkHealthy()"
// ════════════════════════════════════════════════════════════════════════

func TestMarkHealthy_ClearsDegradedStateExplicitly(t *testing.T) {
	resetHealthForTest()
	MarkDegraded(1 * time.Minute)
	if !IsDegraded() {
		t.Fatal("setup: expected degraded immediately after MarkDegraded")
	}

	MarkHealthy()

	if IsDegraded() {
		t.Error("expected IsDegraded() to report false immediately after MarkHealthy()")
	}
}

func TestMarkDegraded_TTLExpiry_ClearsAutomaticallyWithoutMarkHealthy(t *testing.T) {
	resetHealthForTest()

	MarkDegraded(20 * time.Millisecond)
	if !IsDegraded() {
		t.Fatal("setup: expected degraded immediately after MarkDegraded")
	}

	time.Sleep(60 * time.Millisecond)

	if IsDegraded() {
		t.Error("expected IsDegraded() to report false once the TTL has elapsed, " +
			"without any explicit MarkHealthy() call -- the half-open circuit " +
			"breaker's own timeout must clear it")
	}
}

func TestMarkDegraded_ExtendsAnExistingDegradedWindow(t *testing.T) {
	resetHealthForTest()

	MarkDegraded(20 * time.Millisecond)
	time.Sleep(10 * time.Millisecond)
	// A second call (e.g. a second call's own retry-budget exhaustion)
	// resets the window from now, not additively -- confirms MarkDegraded
	// always sets an absolute deadline, not a relative extension bug.
	MarkDegraded(20 * time.Millisecond)
	time.Sleep(10 * time.Millisecond)

	if !IsDegraded() {
		t.Error("expected still degraded 10ms into a freshly-reset 20ms window")
	}
}

// ════════════════════════════════════════════════════════════════════════
// Concurrent-request safety (a go test -race concern, though -race itself
// is known not to work in this ASAN-linked CGO environment -- see
// tasks.md's task 4.2 Implementation Note. This proves race-freedom by
// construction: every access goes through sync/atomic, so this test's
// value is in exercising heavy concurrent read/write without a panic or
// data corruption, not in relying on -race actually running clean here).
// ════════════════════════════════════════════════════════════════════════

func TestHealthState_ConcurrentReadersAndWriters_NoDataCorruption(t *testing.T) {
	resetHealthForTest()

	const goroutines = 50
	const iterations = 200

	var wg sync.WaitGroup
	wg.Add(goroutines * 3)

	for i := 0; i < goroutines; i++ {
		go func() {
			defer wg.Done()
			for j := 0; j < iterations; j++ {
				_ = IsDegraded()
			}
		}()
		go func() {
			defer wg.Done()
			for j := 0; j < iterations; j++ {
				MarkDegraded(time.Millisecond)
			}
		}()
		go func() {
			defer wg.Done()
			for j := 0; j < iterations; j++ {
				MarkHealthy()
			}
		}()
	}

	wg.Wait()
	// No assertion on the final state (inherently racy which goroutine's
	// write lands last) -- reaching here without a panic/deadlock, under
	// concurrent atomic reads and writes from 150 goroutines, is the proof
	// this test exists for.
}