Escape all special characters when emitting GTKWave strings.

Fixes #17.

tests/test_writer.py

@@ -923,11 +923,12 @@ def test_vcd_string_var(capsys):
         vcd.change(v0, 1, 'hello')
         vcd.change(v0, 2, '')
         vcd.change(v0, 3, 'world')
-        with pytest.raises(ValueError):
-            vcd.change(v0, 4, 'no string allowed')
-        vcd.change(v0, 4, None)
-        vcd.change(v0, 5, '!')
-        vcd.dump_off(6)
+        vcd.change(v0, 4, 'spaces are\tok')
+        vcd.change(v0, 5, 'newlines\r\ntoo')
+        vcd.change(v0, 6, 'slash\\slash')
+        vcd.change(v0, 7, None)
+        vcd.change(v0, 8, '!')
+        vcd.dump_off(8)
     expected = [
         '#0',
         '$dumpvars',
@@ -941,10 +942,15 @@ def test_vcd_string_var(capsys):
         '#3',
         'sworld !',
         '#4',
-        's !',
+        'sspaces\\x20are\\tok !',
         '#5',
-        's! !',
+        'snewlines\\r\\ntoo !',
         '#6',
+        'sslash\\\\slash !',
+        '#7',
+        's !',
+        '#8',
+        's! !',
         '$dumpoff',
         '$end',
     ]

vcd/writer.py

@@ -665,10 +665,11 @@ def format_value(self, value: StringValue, check: bool = True) -> str:
         """
         if value is None:
             value = ''
-        if check and (not isinstance(value, str) or ' ' in value):
-            raise ValueError(f'Invalid string value ({value})')
+        if check and not isinstance(value, str):
+            raise ValueError(f'Invalid string value ({value!r})')
 
-        return f's{value} {self.ident}'
+        value_str = _escape_string(value)
+        return f's{value_str} {self.ident}'
 
 
 class RealVariable(Variable[RealValue]):
@@ -814,3 +815,16 @@ def _encode_identifier(v: int) -> str:
         encoded += chr((v % 94) + 33)
         v //= 94
     return encoded
+
+
+def _escape_string(v: str) -> str:
+    """Escape special characters for GTKWave."""
+    return v.translate(
+        {
+            9: "\\t",
+            10: "\\n",
+            13: "\\r",
+            32: "\\x20",
+            92: "\\\\",
+        }
+    )