fix: isbitset reading bytes in wrong order

packet/readcoilsresponse_test.go

@@ -275,7 +275,8 @@ func TestReadCoilsResponse_IsCoilSet(t *testing.T) {
 		},
 		{
 			name:             "ok, first byte, second bit, start 1",
-			whenStartAddress: 1, whenCoilAddress: 2, expect: true},
+			whenStartAddress: 1, whenCoilAddress: 2, expect: true,
+		},
 		{
 			name:             "ok, first byte, second bit, start 100",
 			whenStartAddress: 100, whenCoilAddress: 101, expect: true,
@@ -308,7 +309,7 @@ func TestReadCoilsResponse_IsCoilSet(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			given := ReadCoilsResponse{
 				CoilsByteLength: 2,
-				Data:            []byte{0b10000001, 0b00010010},
+				Data:            []byte{0b00010010, 0b10000001}, // byte0, byte1 - bytes are read left to right
 			}
 			result, err := given.IsCoilSet(tc.whenStartAddress, tc.whenCoilAddress)
 

packet/readdiscreteinputsresponse_test.go

@@ -308,7 +308,7 @@ func TestReadDiscreteInputsResponse_IsInputSet(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			given := ReadDiscreteInputsResponse{
 				InputsByteLength: 2,
-				Data:             []byte{0b10000001, 0b00010010},
+				Data:             []byte{0b00010010, 0b10000001},
 			}
 			result, err := given.IsInputSet(tc.whenStartAddress, tc.whenCoilAddress)
 
@@ -369,7 +369,7 @@ func TestReadDiscreteInputsResponse_IsCoilSet(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			given := ReadDiscreteInputsResponse{
 				InputsByteLength: 2,
-				Data:             []byte{0b10000001, 0b00010010},
+				Data:             []byte{0b00010010, 0b10000001},
 			}
 			result, err := given.IsCoilSet(tc.whenStartAddress, tc.whenCoilAddress)
 

packet/response.go

@@ -100,16 +100,16 @@ func ParseRTUResponse(data []byte) (Response, error) {
 	}
 }
 
-// isBitSet checks if N-th bit is set in data. NB: Bits are counted from `startBit` and right to left.
+// isBitSet checks if N-th bit is set in data. NB: Bits are counted from `startBit` and left to right (bytes).
 func isBitSet(data []byte, startBit uint16, bit uint16) (bool, error) {
-	targetBit := bit - startBit
+	targetBit := int(bit) - int(startBit)
 	if bit < startBit {
 		return false, errors.New("bit can not be before startBit")
 	}
-	if len(data)*8 <= int(targetBit) {
+	if len(data)*8 <= targetBit {
 		return false, errors.New("bit value more than data contains bits")
 	}
-	nThByte := len(data) - 1 - int(targetBit/8)
+	nThByte := targetBit / 8
 	nThBit := targetBit % 8
 	b := data[nThByte]
 	return b&(1<<nThBit) != 0, nil

packet/response_test.go

@@ -402,3 +402,35 @@ func TestParseRTUResponseWithCRC(t *testing.T) {
 		})
 	}
 }
+
+func TestIsBitSet(t *testing.T) {
+	var testCases = []struct {
+		name         string
+		givenData    []byte
+		whenStartBit uint16
+		whenBit      uint16
+		expect       bool
+		expectError  string
+	}{
+		{
+			name:         "ok", // 16 = 0000000000010000
+			givenData:    []byte{16, 0, 0, 128, 0, 33, 192, 128, 29, 0, 0, 32, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 40, 0, 0, 128, 144, 0},
+			whenStartBit: 6161,
+			whenBit:      6165, // idx=4 bit, in first byte
+			expect:       true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			result, err := isBitSet(tc.givenData, tc.whenStartBit, tc.whenBit)
+
+			assert.Equal(t, tc.expect, result)
+			if tc.expectError != "" {
+				assert.EqualError(t, err, tc.expectError)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}