[encoding/gsm7] Fix the bug of confusion with '@' when the first 7 bi…

go.sum

@@ -3,6 +3,7 @@ github.com/cpuguy83/go-md2man/v2 v2.0.0-20190314233015-f79a8a8ca69d h1:U+s90UTSY
 github.com/cpuguy83/go-md2man/v2 v2.0.0-20190314233015-f79a8a8ca69d/go.mod h1:maD7wRr/U5Z6m/iR4s+kqSMx2CaBsrgA7czyZG/E6dU=
 github.com/fiorix/go-smpp v0.0.0-20210403173735-2894b96e70ba h1:vBqABUa2HUSc6tj22Tw+ZMVGHuBzKtljM38kbRanmrM=
 github.com/fiorix/go-smpp v0.0.0-20210403173735-2894b96e70ba/go.mod h1:VfKFK7fGeCP81xEhbrOqUEh45n73Yy6jaPWwTVbxprI=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/russross/blackfriday/v2 v2.0.1 h1:lPqVAte+HuHNfhJ/0LC98ESWRz8afy9tM/0RK8m9o+Q=
 github.com/russross/blackfriday/v2 v2.0.1/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=

smpp/encoding/gsm7.go

@@ -209,6 +209,10 @@ func (g *gsm7Decoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, er
 		}
 	}
 
+	if g.packed && len(septets)%8 == 0 && septets[len(septets)-1] == 0x0d {
+		septets = septets[:len(septets)-1]
+	}
+
 	nSeptet := 0
 	builder := bytes.NewBufferString("")
 	for nSeptet < len(septets) {
@@ -271,6 +275,7 @@ func (g *gsm7Encoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, er
 	}
 
 	nDst = len(septets)
+	unpackedLen := nDst
 	if g.packed {
 		nDst = int(math.Ceil(float64(len(septets)) * 7 / 8))
 	}
@@ -354,5 +359,14 @@ func (g *gsm7Encoder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, er
 		}
 		remain = len(septets) - nSeptet
 	}
+
+	// When there are 7 spare bits in the last octet of a message, these bits are set to the 7-bit code of the CR control
+	// (also used as a padding filler) instead of being set to zero (where they would be confused with the 7-bit code of an '@' character).
+	if g.packed && (unpackedLen*7)%8 == 1 {
+		if v := dst[nDst-1]; v == 0x01 || v == 0x00 {
+			dst[nDst-1] = v | 0x0d<<1
+		}
+	}
+
 	return nDst, nSrc, err
 }

smpp/encoding/gsm7_test.go

@@ -41,7 +41,7 @@ var packedTests = []struct {
 	{Text: "1234", Buff: []byte{0x31, 0xD9, 0x8C, 0x06}},
 	{Text: "12345", Buff: []byte{0x31, 0xD9, 0x8C, 0x56, 0x03}},
 	{Text: "123456", Buff: []byte{0x31, 0xD9, 0x8C, 0x56, 0xB3, 0x01}},
-	{Text: "1234567", Buff: []byte{0x31, 0xD9, 0x8C, 0x56, 0xB3, 0xDD, 0x00}},
+	{Text: "1234567", Buff: []byte{0x31, 0xD9, 0x8C, 0x56, 0xB3, 0xDD, 0x1A}},
 	{Text: "12345678", Buff: []byte{0x31, 0xD9, 0x8C, 0x56, 0xB3, 0xDD, 0x70}},
 	{Text: "123456789", Buff: []byte{0x31, 0xD9, 0x8C, 0x56, 0xB3, 0xDD, 0x70, 0x39}},
 	{Text: "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Curabitur nec nunc venenatis, ultricies ipsum id, volutpat ante. Sed pretium ac metus a interdum metus.", Buff: []byte("\xCC\xB7\xBC\xDC\x06\xA5\xE1\xF3\x7A\x1B\x44\x7E\xB3\xDF\x72\xD0\x3C\x4D\x07\x85\xDB\x65\x3A\x0B\x34\x7E\xBB\xE7\xE5\x31\xBD\x4C\xAF\xCB\x41\x61\x72\x1A\x9E\x9E\x8F\xD3\xEE\x33\xA8\xCC\x4E\xD3\x5D\xA0\x61\x5D\x1E\x16\xA7\xE9\x75\x39\xC8\x5D\x1E\x83\xDC\x75\xF7\x18\x64\x2F\xBB\xCB\xEE\x30\x3D\x3D\x67\x81\xEA\x6C\xBA\x3C\x3D\x4E\x97\xE7\xA0\x34\x7C\x5E\x6F\x83\xD2\x64\x16\xC8\xFE\x66\xD7\xE9\xF0\x30\x1D\x14\x76\xD3\xCB\x2E\xD0\xB4\x4C\x06\xC1\xE5\x65\x7A\xBA\xDE\x06\x85\xC7\xA0\x76\x99\x5E\x9F\x83\xC2\xA0\xB4\x9B\x5E\x96\x93\xEB\x6D\x50\xBB\x4C\xAF\xCF\x5D")},
@@ -204,3 +204,34 @@ func TestInvalidByte(t *testing.T) {
 		}
 	}
 }
+
+func TestExtraCR(t *testing.T) {
+	for _, item := range []struct {
+		name    string
+		content string
+	}{
+		{name: "would have CR", content: "1234567890abcdefghijklm"},
+		{name: "would have CR, last byte is \\r", content: "1234567890abcdefghijkl\r"},
+		{name: "would not have CR, but last byte is 1", content: "123456"},
+		{name: "woud have CR, also has escape", content: "12345678[abcdefghijklmn"},
+		{name: "would not have CR, not have escape", content: "012345678[abcdefghijklmno"},
+	} {
+		encoder := GSM7(true).NewEncoder()
+		es, _, err := transform.Bytes(encoder, []byte(item.content))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// t.Log(es, len(es))
+
+		decoder := GSM7(true).NewDecoder()
+		res, _, err := transform.Bytes(decoder, es)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if string(res) != item.content {
+			t.Fatalf("== %s not equal. %s vs %s", item.name, item.content, string(res))
+		}
+	}
+}