Fix inserting 3 byte wchar into String

* Fix inserting 3 byte wchar into String
* Improve documentation

source/tanya/container/string.d

@@ -3,7 +3,19 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 /**
- * UTF-8 string.
+ * UTF-8 encoded string.
+ *
+ * You can create a $(D_PSYMBOL String) from a literal string, single character
+ * or character range. Characters can be of the type $(D_KEYWORD char),
+ * $(D_KEYWORD wchar) or $(D_KEYWORD dchar). Literal strings, characters and
+ * character ranges can be also inserted into an existing string.
+ *
+ * $(D_PSYMBOL String) is always valid UTF-8. Inserting an invalid sequence
+ * or working on a corrupted $(D_PSYMBOL String) causes
+ * $(D_PSYMBOL UTFException) to be thrown.
+ *
+ * Internally $(D_PSYMBOL String) is represented by a sequence of
+ * $(D_KEYWORD char)s.
  *
  * Copyright: Eugene Wissner 2017.
  * License: $(LINK2 https://www.mozilla.org/en-US/MPL/2.0/,
@@ -537,7 +549,7 @@ struct String
     if (is(C == wchar) || is(C == dchar))
     in
     {
-        assert(capacity - length >= C.sizeof);
+        assert(capacity - length >= 3);
     }
     body
     {
@@ -555,7 +567,7 @@ struct String
             this.length_ += 2;
             return 2;
         }
-        else if (chr < 0xd800 || chr - 0xe000 < 0x2000)
+        else if (chr < 0xd800 || (chr >= 0xe000 && chr <= 0xffff))
         {
             *dst++ = 0xe0 | (chr >> 12) & 0xff;
             *dst++ = 0x80 | ((chr >> 6) & 0x3f);
@@ -593,9 +605,9 @@ struct String
     /// Ditto.
     size_t insertBack(const wchar chr) @trusted @nogc
     {
-        reserve(length + wchar.sizeof);
+        reserve(length + 3);
 
-        auto ret = insertWideChar(chr);
+        const ret = insertWideChar(chr);
         if (ret == 0)
         {
             throw defaultAllocator.make!UTFException("Invalid UTF-16 sequeunce");
@@ -603,12 +615,34 @@ struct String
         return ret;
     }
 
+    // Allocates enough space for 3-byte character.
+    private @safe @nogc unittest
+    {
+        String s;
+        s.insertBack('\u8100');
+    }
+
+    private @safe @nogc unittest
+    {
+        UTFException exception;
+        try
+        {
+            auto s = String(1, cast(wchar) 0xd900);
+        }
+        catch (UTFException e)
+        {
+            exception = e;
+        }
+        assert(exception !is null);
+        defaultAllocator.dispose(exception);
+    }
+
     /// Ditto.
     size_t insertBack(const dchar chr) @trusted @nogc
     {
         reserve(length + dchar.sizeof);
 
-        auto ret = insertWideChar(chr);
+        const ret = insertWideChar(chr);
         if (ret > 0)
         {
             return ret;
@@ -624,6 +658,21 @@ struct String
         }
     }
 
+    private @safe @nogc unittest
+    {
+        UTFException exception;
+        try
+        {
+            auto s = String(1, cast(dchar) 0xd900);
+        }
+        catch (UTFException e)
+        {
+            exception = e;
+        }
+        assert(exception !is null);
+        defaultAllocator.dispose(exception);
+    }
+
     /**
      * Inserts a stringish range at the end of the string.
      *