add UTF<char16_t>::utf_length(). Be more pedantic about surrogates where they're not allowed.

src/nfc.cc

@@ -378,6 +378,10 @@ void UTF<char>::generate(const char32_t c, OutIter& out)
 			*out++ = char( 0x80 + (c & 63));
 		}else if(c<=0xFFFF)
 		{
+			if(c>=0xD800 && c<=0xDFFF)
+			{
+				throw unexpected_surrogate(c);
+			}
 			*out++ = char( 0xE0 + (c>>12) );
 			*out++ = char( 0x80 + ((c>>6) & 63));
 			*out++ = char( 0x80 + (c & 63));
@@ -637,6 +641,10 @@ size_t UTF<char>::utf_length(u32string_view s)
 			len += 2;
 		}else if(c<=0xffff)
 		{
+			if(c>=0xD800 && c<=0xDFFF)
+			{
+				throw unexpected_surrogate(c);
+			}
 			len += 3;
 		}else if(c<=0x10ffff)
 		{
@@ -650,6 +658,31 @@ size_t UTF<char>::utf_length(u32string_view s)
 }
 
 
+template<>
+size_t UTF<char16_t>::utf_length(u32string_view s)
+{
+	size_t len = 0;
+	for(const char32_t c : s)
+	{
+		if(c <= 0xffff)
+		{
+			if(c>=0xD800 && c<=0xDFFF)
+			{
+				throw unexpected_surrogate(c);
+			}
+			len += 1;
+		}else if(c<=0x10ffff)
+		{
+			len += 2;
+		}else{
+			throw too_big(0, c);
+		}
+	}
+	
+	return len;
+}
+
+
 // convenience function to avoid ::strdup(pEp::toNFC<char>(text).c_str());
 // and unecessary temporary std::string etc.
 char* strdup_NFC(string_view s)