package protocol

import (
	"bytes"
	"encoding/binary"
	"encoding/json"
	"errors"
	"reflect"
	"strings"
	"testing"
)

// TestEncodeDecodeMessageTextASCII 验证 ASCII 文本可以按 text 消息往返编解码。
func TestEncodeDecodeMessageTextASCII(t *testing.T) {
	original := Message{
		Type: MessageTypeText,
		ID:   42,
		From: "peer-a",
		To:   "peer-b",
		Body: []byte("hello"),
	}

	data, err := EncodeMessage(original)
	if err != nil {
		t.Fatalf("EncodeMessage() error = %v", err)
	}

	decoded, err := DecodeMessage(data)
	if err != nil {
		t.Fatalf("DecodeMessage() error = %v", err)
	}

	if !reflect.DeepEqual(decoded, original) {
		t.Fatalf("round trip mismatch: got %+v want %+v", decoded, original)
	}
}

// TestEncodeDecodeMessageTextUTF8 验证 text 消息允许合法 UTF-8，
// 从而天然兼容 ASCII 之外的普通文本。
func TestEncodeDecodeMessageTextUTF8(t *testing.T) {
	original := Message{
		Type: MessageTypeText,
		ID:   43,
		From: "peer-a",
		To:   "peer-b",
		Body: []byte("你好, world"),
	}

	data, err := EncodeMessage(original)
	if err != nil {
		t.Fatalf("EncodeMessage() error = %v", err)
	}

	decoded, err := DecodeMessage(data)
	if err != nil {
		t.Fatalf("DecodeMessage() error = %v", err)
	}

	if !reflect.DeepEqual(decoded, original) {
		t.Fatalf("round trip mismatch: got %+v want %+v", decoded, original)
	}
}

// TestEncodeDecodeMessageFile 验证 file 消息会保留文件名和原始二进制正文。
func TestEncodeDecodeMessageFile(t *testing.T) {
	original := Message{
		Type:     MessageTypeFile,
		ID:       44,
		From:     "peer-a",
		To:       "peer-b",
		FileName: "data.bin",
		Body:     []byte{0x00, 0xff, 0x10, 0x7f},
	}

	data, err := EncodeMessage(original)
	if err != nil {
		t.Fatalf("EncodeMessage() error = %v", err)
	}

	decoded, err := DecodeMessage(data)
	if err != nil {
		t.Fatalf("DecodeMessage() error = %v", err)
	}

	if !reflect.DeepEqual(decoded, original) {
		t.Fatalf("round trip mismatch: got %+v want %+v", decoded, original)
	}
}

// TestEncodeDecodeMessageRegister 验证 register 控制消息也能正常编解码。
func TestEncodeDecodeMessageRegister(t *testing.T) {
	original := Message{
		Type: MessageTypeRegister,
		ID:   45,
		From: "peer-a",
		To:   ServerPeerID,
		Body: []byte{},
	}

	data, err := EncodeMessage(original)
	if err != nil {
		t.Fatalf("EncodeMessage() error = %v", err)
	}

	decoded, err := DecodeMessage(data)
	if err != nil {
		t.Fatalf("DecodeMessage() error = %v", err)
	}

	if !reflect.DeepEqual(decoded, original) {
		t.Fatalf("round trip mismatch: got %+v want %+v", decoded, original)
	}
}

// TestEncodeDecodeMessageError 验证 error 控制消息会保留 UTF-8 错误文本。
func TestEncodeDecodeMessageError(t *testing.T) {
	original := Message{
		Type: MessageTypeError,
		ID:   46,
		From: ServerPeerID,
		To:   "peer-a",
		Body: []byte("unknown target"),
	}

	data, err := EncodeMessage(original)
	if err != nil {
		t.Fatalf("EncodeMessage() error = %v", err)
	}

	decoded, err := DecodeMessage(data)
	if err != nil {
		t.Fatalf("DecodeMessage() error = %v", err)
	}

	if !reflect.DeepEqual(decoded, original) {
		t.Fatalf("round trip mismatch: got %+v want %+v", decoded, original)
	}
}

// TestWriteReadFrame 单独验证最底层的长度前缀帧逻辑，
// 不依赖 Message 结构，方便确认 TCP 粘包拆包问题是否被正确处理。
func TestWriteReadFrame(t *testing.T) {
	var buf bytes.Buffer
	payload := []byte("header+body")

	if err := WriteFrame(&buf, payload); err != nil {
		t.Fatalf("WriteFrame() error = %v", err)
	}

	got, err := ReadFrame(&buf)
	if err != nil {
		t.Fatalf("ReadFrame() error = %v", err)
	}

	if !bytes.Equal(got, payload) {
		t.Fatalf("payload mismatch: got %q want %q", got, payload)
	}
}

// TestWriteReadMessageAllowsEmptyBody 验证空文本和空文件都可以正常通过协议层，
// 因为外层帧非空的前提下，空正文是合法业务内容。
func TestWriteReadMessageAllowsEmptyBody(t *testing.T) {
	tests := []struct {
		name    string
		message Message
	}{
		{
			name: "empty text",
			message: Message{
				Type: MessageTypeText,
				ID:   1,
				From: "peer-a",
				To:   "peer-b",
				Body: []byte(""),
			},
		},
		{
			name: "empty file",
			message: Message{
				Type:     MessageTypeFile,
				ID:       2,
				From:     "peer-a",
				To:       "peer-b",
				FileName: "empty.txt",
				Body:     []byte{},
			},
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			var buf bytes.Buffer

			if err := WriteMessage(&buf, tt.message); err != nil {
				t.Fatalf("WriteMessage() error = %v", err)
			}

			got, err := ReadMessage(&buf)
			if err != nil {
				t.Fatalf("ReadMessage() error = %v", err)
			}

			if !reflect.DeepEqual(got, tt.message) {
				t.Fatalf("round trip mismatch: got %+v want %+v", got, tt.message)
			}
		})
	}
}

// TestWriteReadMessageRejectsInvalidMessages 验证协议层会在编码前拦住明显非法的消息。
func TestWriteReadMessageRejectsInvalidMessages(t *testing.T) {
	tests := []struct {
		name    string
		message Message
		wantErr error
	}{
		{
			name: "invalid type",
			message: Message{
				Type: MessageType("unknown"),
				ID:   1,
				From: "peer-a",
				To:   "peer-b",
			},
			wantErr: ErrInvalidMessageType,
		},
		{
			name: "missing from",
			message: Message{
				Type: MessageTypeText,
				ID:   2,
				To:   "peer-b",
			},
			wantErr: ErrMissingFrom,
		},
		{
			name: "missing to",
			message: Message{
				Type: MessageTypeText,
				ID:   3,
				From: "peer-a",
			},
			wantErr: ErrMissingTo,
		},
		{
			name: "text with file name",
			message: Message{
				Type:     MessageTypeText,
				ID:       4,
				From:     "peer-a",
				To:       "peer-b",
				FileName: "bad.txt",
				Body:     []byte("hello"),
			},
			wantErr: ErrUnexpectedFileName,
		},
		{
			name: "text with invalid utf8",
			message: Message{
				Type: MessageTypeText,
				ID:   5,
				From: "peer-a",
				To:   "peer-b",
				Body: []byte{0xff, 0xfe},
			},
			wantErr: ErrInvalidTextBody,
		},
		{
			name: "file without file name",
			message: Message{
				Type: MessageTypeFile,
				ID:   6,
				From: "peer-a",
				To:   "peer-b",
				Body: []byte{0x01},
			},
			wantErr: ErrMissingFileName,
		},
		{
			name: "register with wrong target",
			message: Message{
				Type: MessageTypeRegister,
				ID:   7,
				From: "peer-a",
				To:   "peer-b",
			},
			wantErr: ErrInvalidRegisterTarget,
		},
		{
			name: "register with body",
			message: Message{
				Type: MessageTypeRegister,
				ID:   8,
				From: "peer-a",
				To:   ServerPeerID,
				Body: []byte("unexpected"),
			},
			wantErr: ErrUnexpectedBody,
		},
		{
			name: "error with wrong source",
			message: Message{
				Type: MessageTypeError,
				ID:   9,
				From: "peer-a",
				To:   "peer-b",
				Body: []byte("bad"),
			},
			wantErr: ErrInvalidErrorSource,
		},
		{
			name: "error with file name",
			message: Message{
				Type:     MessageTypeError,
				ID:       10,
				From:     ServerPeerID,
				To:       "peer-a",
				FileName: "bad.txt",
				Body:     []byte("bad"),
			},
			wantErr: ErrUnexpectedFileName,
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			_, err := EncodeMessage(tt.message)
			if !errors.Is(err, tt.wantErr) {
				t.Fatalf("EncodeMessage() error = %v, want %v", err, tt.wantErr)
			}
		})
	}
}

// TestReadFrameRejectsInvalidLength 验证长度为 0 的帧会被当成非法输入，
// 而不是被当成一条合法的空消息。
func TestReadFrameRejectsInvalidLength(t *testing.T) {
	var buf bytes.Buffer

	if err := binary.Write(&buf, binary.BigEndian, uint32(0)); err != nil {
		t.Fatalf("binary.Write() error = %v", err)
	}

	_, err := ReadFrame(&buf)
	if !errors.Is(err, ErrInvalidFrameLength) {
		t.Fatalf("ReadFrame() error = %v, want %v", err, ErrInvalidFrameLength)
	}
}

// TestReadFrameRejectsTooLargeFrame 验证超大帧会在分配消息体前被拒绝，
// 从而保证最大长度限制真正生效。
func TestReadFrameRejectsTooLargeFrame(t *testing.T) {
	var buf bytes.Buffer

	if err := binary.Write(&buf, binary.BigEndian, uint32(MaxFrameSize+1)); err != nil {
		t.Fatalf("binary.Write() error = %v", err)
	}

	_, err := ReadFrame(&buf)
	if !errors.Is(err, ErrFrameTooLarge) {
		t.Fatalf("ReadFrame() error = %v, want %v", err, ErrFrameTooLarge)
	}
}

// TestWriteFrameRejectsEmptyPayload 验证写入端和读取端的约束保持一致：
// 既然读取端不接受 0 长度帧，写入端也不应该产生这种帧。
func TestWriteFrameRejectsEmptyPayload(t *testing.T) {
	var buf bytes.Buffer

	err := WriteFrame(&buf, nil)
	if !errors.Is(err, ErrInvalidFrameLength) {
		t.Fatalf("WriteFrame() error = %v, want %v", err, ErrInvalidFrameLength)
	}
}

// TestDecodeMessageRejectsInvalidHeaderLength 验证无法切出完整头部时会被立即拒绝。
func TestDecodeMessageRejectsInvalidHeaderLength(t *testing.T) {
	tests := []struct {
		name string
		data []byte
	}{
		{
			name: "too short for header len",
			data: []byte{0x00, 0x00, 0x00},
		},
		{
			name: "zero header len",
			data: []byte{0x00, 0x00, 0x00, 0x00},
		},
		{
			name: "header len exceeds payload",
			data: []byte{0x00, 0x00, 0x00, 0x10, '{', '}'},
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			_, err := DecodeMessage(tt.data)
			if !errors.Is(err, ErrInvalidHeaderLength) {
				t.Fatalf("DecodeMessage() error = %v, want %v", err, ErrInvalidHeaderLength)
			}
		})
	}
}

// TestDecodeMessageRejectsInvalidHeaderJSON 验证头部 JSON 非法时能返回明确错误。
func TestDecodeMessageRejectsInvalidHeaderJSON(t *testing.T) {
	data := append([]byte{0x00, 0x00, 0x00, 0x09}, []byte("{invalid}")...)

	_, err := DecodeMessage(data)
	if !errors.Is(err, ErrInvalidHeaderJSON) {
		t.Fatalf("DecodeMessage() error = %v, want %v", err, ErrInvalidHeaderJSON)
	}
}

// TestDecodeMessageRejectsContentLengthMismatch 验证头部声明长度和实际正文不一致时会失败。
func TestDecodeMessageRejectsContentLengthMismatch(t *testing.T) {
	headerPayload, err := json.Marshal(messageHeader{
		Type:          MessageTypeText,
		ID:            7,
		From:          "peer-a",
		To:            "peer-b",
		ContentLength: 10,
	})
	if err != nil {
		t.Fatalf("json.Marshal() error = %v", err)
	}

	var data bytes.Buffer
	if err := binary.Write(&data, binary.BigEndian, uint32(len(headerPayload))); err != nil {
		t.Fatalf("binary.Write() error = %v", err)
	}
	if _, err := data.Write(headerPayload); err != nil {
		t.Fatalf("data.Write(headerPayload) error = %v", err)
	}
	if _, err := data.Write([]byte("hello")); err != nil {
		t.Fatalf("data.Write(body) error = %v", err)
	}

	_, err = DecodeMessage(data.Bytes())
	if !errors.Is(err, ErrInvalidContentLength) {
		t.Fatalf("DecodeMessage() error = %v, want %v", err, ErrInvalidContentLength)
	}
}

// TestReadMultipleMessages 模拟同一条流中连续写入 text 和 file，
// 验证读取端每次都能严格停在当前帧边界，不会串包。
func TestReadMultipleMessages(t *testing.T) {
	var buf bytes.Buffer

	first := Message{
		Type: MessageTypeText,
		ID:   1,
		From: "peer-a",
		To:   "peer-b",
		Body: []byte("hello"),
	}

	second := Message{
		Type:     MessageTypeFile,
		ID:       2,
		From:     "peer-b",
		To:       "peer-a",
		FileName: "payload.bin",
		Body:     []byte{0x01, 0x02, 0x03},
	}

	if err := WriteMessage(&buf, first); err != nil {
		t.Fatalf("WriteMessage(first) error = %v", err)
	}
	if err := WriteMessage(&buf, second); err != nil {
		t.Fatalf("WriteMessage(second) error = %v", err)
	}

	gotFirst, err := ReadMessage(&buf)
	if err != nil {
		t.Fatalf("ReadMessage(first) error = %v", err)
	}
	gotSecond, err := ReadMessage(&buf)
	if err != nil {
		t.Fatalf("ReadMessage(second) error = %v", err)
	}

	if !reflect.DeepEqual(gotFirst, first) {
		t.Fatalf("first message mismatch: got %+v want %+v", gotFirst, first)
	}
	if !reflect.DeepEqual(gotSecond, second) {
		t.Fatalf("second message mismatch: got %+v want %+v", gotSecond, second)
	}
}

// TestReadMessageWrapsDecodeError 验证 ReadMessage 在返回错误时会保留解码阶段上下文。
func TestReadMessageWrapsDecodeError(t *testing.T) {
	var buf bytes.Buffer

	if err := WriteFrame(&buf, append([]byte{0x00, 0x00, 0x00, 0x09}, []byte("{invalid}")...)); err != nil {
		t.Fatalf("WriteFrame() error = %v", err)
	}

	_, err := ReadMessage(&buf)
	if err == nil {
		t.Fatal("ReadMessage() error = nil, want non-nil")
	}
	if !strings.Contains(err.Error(), "decode message") {
		t.Fatalf("ReadMessage() error = %v, want wrapped decode error", err)
	}
}