package protocol

import (
	"bytes"
	"encoding/json"
	"io"
	"testing"
	"time"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
)

func TestCodec_Encode_Decode(t *testing.T) {
	codec := NewCodec()

	tests := []struct {
		name    string
		message *Message
	}{
		{
			name: "challenge request (empty payload)",
			message: &Message{
				Type:    ChallengeRequest,
				Payload: []byte{},
			},
		},
		{
			name: "challenge response with payload",
			message: &Message{
				Type:    ChallengeResponse,
				Payload: []byte(`{"challenge":{"timestamp":1640995200,"difficulty":4}}`),
			},
		},
		{
			name: "error response",
			message: &Message{
				Type:    ErrorResponse,
				Payload: []byte(`{"code":"INVALID_SOLUTION","message":"Invalid nonce"}`),
			},
		},
	}

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

			// Encode message
			err := codec.Encode(&buf, tt.message)
			require.NoError(t, err)

			// Decode message
			decoded, err := codec.Decode(&buf)
			require.NoError(t, err)

			assert.Equal(t, tt.message.Type, decoded.Type)
			if len(tt.message.Payload) == 0 && len(decoded.Payload) == 0 {
				// Both are empty (nil or empty slice)
				assert.True(t, true)
			} else {
				assert.Equal(t, tt.message.Payload, decoded.Payload)
			}
		})
	}
}

func TestCodec_Encode_Errors(t *testing.T) {
	codec := NewCodec()

	tests := []struct {
		name    string
		message *Message
		wantErr string
	}{
		{
			name:    "nil message",
			message: nil,
			wantErr: "message cannot be nil",
		},
		{
			name: "payload too large",
			message: &Message{
				Type:    ChallengeRequest,
				Payload: make([]byte, MaxPayloadSize+1),
			},
			wantErr: "payload size",
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			var buf bytes.Buffer
			err := codec.Encode(&buf, tt.message)
			assert.Error(t, err)
			assert.Contains(t, err.Error(), tt.wantErr)
		})
	}
}

func TestCodec_Decode_Errors(t *testing.T) {
	codec := NewCodec()

	tests := []struct {
		name    string
		data    []byte
		wantErr string
	}{
		{
			name:    "empty data",
			data:    []byte{},
			wantErr: "EOF",
		},
		{
			name:    "invalid message type",
			data:    []byte{0xFF, 0x00, 0x00, 0x00, 0x00},
			wantErr: "invalid message type",
		},
		{
			name:    "incomplete header",
			data:    []byte{0x01, 0x00, 0x00},
			wantErr: "failed to read payload length",
		},
		{
			name:    "payload too large",
			data:    append([]byte{0x01}, encodeBigEndianUint32(MaxPayloadSize+1)...),
			wantErr: "payload length",
		},
		{
			name:    "incomplete payload",
			data:    []byte{0x01, 0x00, 0x00, 0x00, 0x05, 0x01, 0x02},
			wantErr: "failed to read payload",
		},
		{
			name:    "invalid UTF-8 in payload",
			data:    []byte{0x01, 0x00, 0x00, 0x00, 0x03, 0xFF, 0xFE, 0xFD},
			wantErr: "invalid UTF-8",
		},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			buf := bytes.NewBuffer(tt.data)
			_, err := codec.Decode(buf)
			assert.Error(t, err)
			assert.Contains(t, err.Error(), tt.wantErr)
		})
	}
}


func TestCodec_RoundTrip_RealPayloads(t *testing.T) {
	codec := NewCodec()

	t.Run("challenge response round trip", func(t *testing.T) {
		original := &ChallengeResponsePayload{
			Timestamp:  time.Now().Unix(),
			Difficulty: 4,
			Resource:   "quotes",
			Random:     []byte("random123"),
			HMAC:       []byte("hmac_signature"),
		}

		// Marshal payload
		jsonData, err := json.Marshal(original)
		require.NoError(t, err)

		msg := &Message{
			Type:    ChallengeResponse,
			Payload: jsonData,
		}

		// Simulate network transmission
		var buf bytes.Buffer
		err = codec.Encode(&buf, msg)
		require.NoError(t, err)

		// Decode message
		decoded, err := codec.Decode(&buf)
		require.NoError(t, err)

		// Unmarshal payload
		var result ChallengeResponsePayload
		err = json.Unmarshal(decoded.Payload, &result)
		require.NoError(t, err)

		assert.Equal(t, original.Timestamp, result.Timestamp)
		assert.Equal(t, original.Difficulty, result.Difficulty)
		assert.Equal(t, original.Resource, result.Resource)
		assert.Equal(t, original.Random, result.Random)
		assert.Equal(t, original.HMAC, result.HMAC)
	})

	t.Run("quote response round trip", func(t *testing.T) {
		original := &QuoteResponsePayload{
			Text:   "Test quote",
			Author: "Test author",
		}

		// Marshal payload
		jsonData, err := json.Marshal(original)
		require.NoError(t, err)

		msg := &Message{
			Type:    QuoteResponse,
			Payload: jsonData,
		}

		var buf bytes.Buffer
		err = codec.Encode(&buf, msg)
		require.NoError(t, err)

		decoded, err := codec.Decode(&buf)
		require.NoError(t, err)

		var result QuoteResponsePayload
		err = json.Unmarshal(decoded.Payload, &result)
		require.NoError(t, err)

		assert.Equal(t, original.Text, result.Text)
		assert.Equal(t, original.Author, result.Author)
	})
}

func TestCodec_WriteError_Handling(t *testing.T) {
	codec := NewCodec()

	// Create a writer that fails after a certain number of bytes
	failAfter := 3
	writer := &failingWriter{failAfter: failAfter}

	msg := &Message{
		Type:    ChallengeResponse,
		Payload: []byte("test payload"),
	}

	err := codec.Encode(writer, msg)
	assert.Error(t, err)
}

func TestCodec_ReadError_Handling(t *testing.T) {
	codec := NewCodec()

	// Create a reader that fails after reading header
	reader := &failingReader{
		data:      []byte{0x01, 0x00, 0x00, 0x00, 0x05},
		failAfter: 5,
	}

	_, err := codec.Decode(reader)
	assert.Error(t, err)
	assert.Contains(t, err.Error(), "failed to read payload")
}

// Helper functions and types for testing

func encodeBigEndianUint32(val uint32) []byte {
	return []byte{
		byte(val >> 24),
		byte(val >> 16),
		byte(val >> 8),
		byte(val),
	}
}

type failingWriter struct {
	written   int
	failAfter int
}

func (w *failingWriter) Write(data []byte) (int, error) {
	if w.written >= w.failAfter {
		return 0, io.ErrShortWrite
	}

	remaining := w.failAfter - w.written
	if len(data) <= remaining {
		w.written += len(data)
		return len(data), nil
	}

	w.written = w.failAfter
	return remaining, io.ErrShortWrite
}

type failingReader struct {
	data      []byte
	pos       int
	failAfter int
}

func (r *failingReader) Read(buf []byte) (int, error) {
	if r.pos >= r.failAfter {
		return 0, io.ErrUnexpectedEOF
	}

	if r.pos >= len(r.data) {
		return 0, io.EOF
	}

	n := copy(buf, r.data[r.pos:])
	r.pos += n

	if r.pos >= r.failAfter {
		return n, io.ErrUnexpectedEOF
	}

	return n, nil
}