//go:build darwin || linux

package daemon

import (
	"bytes"
	"net"
	"sync"
	"testing"
	"time"
)

// startMockDNSServer starts a UDP server that echoes back whatever it receives,
// prefixed with "RESP:" to distinguish responses from queries.
// Returns the server's address and a cleanup function.
func startMockDNSServer(t *testing.T) (string, func()) {
	t.Helper()

	addr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
	if err != nil {
		t.Fatalf("Failed to resolve address: %v", err)
	}

	conn, err := net.ListenUDP("udp", addr)
	if err != nil {
		t.Fatalf("Failed to start mock DNS server: %v", err)
	}

	done := make(chan struct{})
	go func() {
		buf := make([]byte, maxDNSPacketSize)
		for {
			n, remoteAddr, err := conn.ReadFromUDP(buf)
			if err != nil {
				select {
				case <-done:
					return
				default:
					continue
				}
			}
			// Echo back with "RESP:" prefix.
			resp := append([]byte("RESP:"), buf[:n]...)
			_, _ = conn.WriteToUDP(resp, remoteAddr) // best-effort in test
		}
	}()

	cleanup := func() {
		close(done)
		_ = conn.Close()
	}

	return conn.LocalAddr().String(), cleanup
}

// startSilentDNSServer starts a UDP server that accepts connections but never
// responds, simulating an upstream timeout.
func startSilentDNSServer(t *testing.T) (string, func()) {
	t.Helper()

	addr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
	if err != nil {
		t.Fatalf("Failed to resolve address: %v", err)
	}

	conn, err := net.ListenUDP("udp", addr)
	if err != nil {
		t.Fatalf("Failed to start silent DNS server: %v", err)
	}

	cleanup := func() {
		_ = conn.Close()
	}

	return conn.LocalAddr().String(), cleanup
}

func TestDNSRelay_ForwardPacket(t *testing.T) {
	// Start a mock upstream DNS server.
	upstreamAddr, cleanupUpstream := startMockDNSServer(t)
	defer cleanupUpstream()

	// Create and start the relay.
	relay, err := NewDNSRelay("127.0.0.1:0", upstreamAddr, true)
	if err != nil {
		t.Fatalf("Failed to create DNS relay: %v", err)
	}

	if err := relay.Start(); err != nil {
		t.Fatalf("Failed to start DNS relay: %v", err)
	}
	defer relay.Stop()

	// Send a query through the relay.
	clientConn, err := net.Dial("udp", relay.ListenAddr())
	if err != nil {
		t.Fatalf("Failed to connect to relay: %v", err)
	}
	defer clientConn.Close() //nolint:errcheck // test cleanup

	query := []byte("test-dns-query")
	if _, err := clientConn.Write(query); err != nil {
		t.Fatalf("Failed to send query: %v", err)
	}

	// Read the response.
	if err := clientConn.SetReadDeadline(time.Now().Add(5 * time.Second)); err != nil {
		t.Fatalf("Failed to set read deadline: %v", err)
	}

	buf := make([]byte, maxDNSPacketSize)
	n, err := clientConn.Read(buf)
	if err != nil {
		t.Fatalf("Failed to read response: %v", err)
	}

	expected := append([]byte("RESP:"), query...)
	if !bytes.Equal(buf[:n], expected) {
		t.Errorf("Unexpected response: got %q, want %q", buf[:n], expected)
	}
}

func TestDNSRelay_UpstreamTimeout(t *testing.T) {
	// Start a silent upstream server that never responds.
	upstreamAddr, cleanupUpstream := startSilentDNSServer(t)
	defer cleanupUpstream()

	// Create and start the relay.
	relay, err := NewDNSRelay("127.0.0.1:0", upstreamAddr, false)
	if err != nil {
		t.Fatalf("Failed to create DNS relay: %v", err)
	}

	if err := relay.Start(); err != nil {
		t.Fatalf("Failed to start DNS relay: %v", err)
	}
	defer relay.Stop()

	// Send a query through the relay.
	clientConn, err := net.Dial("udp", relay.ListenAddr())
	if err != nil {
		t.Fatalf("Failed to connect to relay: %v", err)
	}
	defer clientConn.Close() //nolint:errcheck // test cleanup

	query := []byte("test-dns-timeout")
	if _, err := clientConn.Write(query); err != nil {
		t.Fatalf("Failed to send query: %v", err)
	}

	// The relay should not send back a response because upstream timed out.
	// Set a short deadline on the client side; we expect no data.
	if err := clientConn.SetReadDeadline(time.Now().Add(6 * time.Second)); err != nil {
		t.Fatalf("Failed to set read deadline: %v", err)
	}

	buf := make([]byte, maxDNSPacketSize)
	_, err = clientConn.Read(buf)
	if err == nil {
		t.Fatal("Expected timeout error reading from relay, but got a response")
	}

	// Verify it was a timeout error.
	netErr, ok := err.(net.Error)
	if !ok || !netErr.Timeout() {
		t.Fatalf("Expected timeout error, got: %v", err)
	}
}

func TestDNSRelay_ConcurrentQueries(t *testing.T) {
	// Start a mock upstream DNS server.
	upstreamAddr, cleanupUpstream := startMockDNSServer(t)
	defer cleanupUpstream()

	// Create and start the relay.
	relay, err := NewDNSRelay("127.0.0.1:0", upstreamAddr, false)
	if err != nil {
		t.Fatalf("Failed to create DNS relay: %v", err)
	}

	if err := relay.Start(); err != nil {
		t.Fatalf("Failed to start DNS relay: %v", err)
	}
	defer relay.Stop()

	const numQueries = 20
	var wg sync.WaitGroup
	errors := make(chan error, numQueries)

	for i := range numQueries {
		wg.Add(1)
		go func(id int) {
			defer wg.Done()

			clientConn, err := net.Dial("udp", relay.ListenAddr())
			if err != nil {
				errors <- err
				return
			}
			defer clientConn.Close() //nolint:errcheck // test cleanup

			query := []byte("concurrent-query-" + string(rune('A'+id))) //nolint:gosec // test uses small range 0-19, no overflow
			if _, err := clientConn.Write(query); err != nil {
				errors <- err
				return
			}

			if err := clientConn.SetReadDeadline(time.Now().Add(5 * time.Second)); err != nil {
				errors <- err
				return
			}

			buf := make([]byte, maxDNSPacketSize)
			n, err := clientConn.Read(buf)
			if err != nil {
				errors <- err
				return
			}

			expected := append([]byte("RESP:"), query...)
			if !bytes.Equal(buf[:n], expected) {
				errors <- &unexpectedResponseError{got: buf[:n], want: expected}
			}
		}(i)
	}

	wg.Wait()
	close(errors)

	for err := range errors {
		t.Errorf("Concurrent query error: %v", err)
	}
}

func TestDNSRelay_ListenAddr(t *testing.T) {
	// Use port 0 to get an ephemeral port.
	relay, err := NewDNSRelay("127.0.0.1:0", "1.1.1.1:53", false)
	if err != nil {
		t.Fatalf("Failed to create DNS relay: %v", err)
	}
	defer relay.Stop()

	addr := relay.ListenAddr()
	if addr == "" {
		t.Fatal("ListenAddr returned empty string")
	}

	host, port, err := net.SplitHostPort(addr)
	if err != nil {
		t.Fatalf("ListenAddr returned invalid address %q: %v", addr, err)
	}
	if host != "127.0.0.1" {
		t.Errorf("Expected host 127.0.0.1, got %q", host)
	}
	if port == "0" {
		t.Error("Expected assigned port, got 0")
	}
}

func TestNewDNSRelay_InvalidDNSAddr(t *testing.T) {
	tests := []struct {
		name    string
		dnsAddr string
	}{
		{"missing port", "1.1.1.1"},
		{"empty string", ""},
		{"empty host", ":53"},
		{"empty port", "1.1.1.1:"},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			_, err := NewDNSRelay("127.0.0.1:0", tt.dnsAddr, false)
			if err == nil {
				t.Errorf("Expected error for DNS address %q, got nil", tt.dnsAddr)
			}
		})
	}
}

func TestNewDNSRelay_InvalidListenAddr(t *testing.T) {
	_, err := NewDNSRelay("invalid-addr", "1.1.1.1:53", false)
	if err == nil {
		t.Error("Expected error for invalid listen address, got nil")
	}
}

// unexpectedResponseError is used to report mismatched responses in concurrent tests.
type unexpectedResponseError struct {
	got  []byte
	want []byte
}

func (e *unexpectedResponseError) Error() string {
	return "unexpected response: got " + string(e.got) + ", want " + string(e.want)
}