package tunnel

import (
	"testing"
	"time"
)

// TestBackoffValues 驗證退避函式輸出。
//
// 這組測試的核心動機是 POC 的 backoff bug：math.Min 單位 mix 造成 attempt>=1
// 永遠回 30s。修復後應該是純指數退避，clamp 在 [base, cap]。
func TestBackoffValues(t *testing.T) {
	cases := []struct {
		attempt int
		want    time.Duration
	}{
		{attempt: 0, want: backoffBase},     // 不應發生，但定義行為
		{attempt: 1, want: 1 * time.Second}, // base × 2^0 = 1s
		{attempt: 2, want: 2 * time.Second}, // base × 2^1 = 2s
		{attempt: 3, want: 4 * time.Second},
		{attempt: 4, want: 8 * time.Second},
		{attempt: 5, want: 16 * time.Second},
		{attempt: 6, want: backoffCap}, // 32s > 30s → cap
		{attempt: 7, want: backoffCap},
		{attempt: 10, want: backoffCap},
		{attempt: 30, want: backoffCap},
		{attempt: 100, want: backoffCap}, // 極大 attempt 應不 overflow
	}
	for _, tc := range cases {
		got := backoff(tc.attempt)
		if got != tc.want {
			t.Errorf("backoff(%d) = %v, want %v", tc.attempt, got, tc.want)
		}
	}
}

// TestBackoffMonotonicUntilCap 驗證退避在 cap 之前是遞增的。
// 這是 POC bug 會失敗的測試（POC 從 attempt=1 起就固定 30s，根本不遞增）。
func TestBackoffMonotonicUntilCap(t *testing.T) {
	var prev time.Duration
	for attempt := 1; attempt <= 5; attempt++ {
		got := backoff(attempt)
		if got <= prev {
			t.Errorf("backoff not monotonically increasing: backoff(%d)=%v, backoff(%d)=%v", attempt-1, prev, attempt, got)
		}
		if got > backoffCap {
			t.Errorf("backoff(%d)=%v exceeds cap %v", attempt, got, backoffCap)
		}
		prev = got
	}
}

// TestKeepAliveIntervalDefault 固定住 10s 預設值，避免未來不小心改回 POC 的 30s。
// 10s 是 tunnel.md §4.2 M-5 統一的心跳週期。
func TestKeepAliveIntervalDefault(t *testing.T) {
	if KeepAliveInterval != 10*time.Second {
		t.Errorf("KeepAliveInterval = %v, want 10s (tunnel.md §4.2 M-5)", KeepAliveInterval)
	}
}