Do not convert existing UTF-8 sequences

Makefile

@@ -3,6 +3,7 @@ MAKE=make
 
 CFLAGS=-g
 DEFS=-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_REENTRANT -D_THREAD_SAFE -D_GNU_SOURCE
+#WARN=-Wall -Wunused
 
 all: lib test
 

README.md

@@ -1,2 +1,3 @@
 # libxconv
 Lightweight ISO-6937 to UTF-8 conversion library.
+

test.c

@@ -1,12 +1,17 @@
+
 #include <stdio.h>
 #include <string.h>
-#include "charset.h"
+#include <ctype.h>
+#include <stdint.h>
+#include "xconv.h"
 
 char *teststr[] = {
 	"The House That \243100k Built",
 	"home d\302ecor",
 	"home made p\303at\302e",
 	"scene at \251Odette\271 that",
+	"Sealgairean Sp\xc3\xacrsail/History Hunters",
+	"Ben & Hoilidh san R\xc3\xb2oghachd Bhig",
 	NULL
 };
 
@@ -54,9 +59,10 @@ main()
 	{
 		hexdump(teststr[i], 0);
 
-		xconv(teststr[i], buf, sizeof(buf));
-
+		if (xconv(teststr[i], buf, sizeof(buf)))
 			hexdump(buf, 0);
+		else
+			printf("Unchanged.\n");
 
 		printf("\n\n");
 	}

xconv.c

@@ -37,15 +37,52 @@ add_unicode(char **d, size_t *len, uint16_t u)
 	return 0;
 }
 
+static int
+is_utf_sequence(char *s)
+{
+	int utf8_len;
+	int i;
+
+	if ((*s & 0xe0) == 0xc0)      // U+0080+      110xxxxx
+		utf8_len = 1;
+	else if ((*s & 0xf0) == 0xe0) // U+0800+      1110xxxx
+		utf8_len = 2;
+	else if ((*s & 0xf8) == 0xf0) // U+10000+     11110xxx
+		utf8_len = 3;
+	else return 0;
+
+	for (i = 1; i <= utf8_len; i++)
+	{
+		// UTF-8 continuation character 10xxxxxx
+		if ((s[i] & 0xc0) != 0x80)
+			// Invalid UTF-8 sequence
+			return 0;
+	}
+	return 1;
+}
+
 int
 xconv(char *src, char *dst, size_t dstlen)
 {
-	size_t len = dstlen;
+	size_t len = dstlen - 1;
 	char *s, *d;
+	int changed = 0;
 	int i;
 
 	for (s = src, d = dst; *s && len > 0; s++)
 	{
+		if (!(*s & 0x80))
+		{
+			*d++ = *s;
+			len--;
+			continue;
+		}
+
+		if (is_utf_sequence(s))
+			return 0;
+
+		changed = 1;
+		
 		// Check for combined character.
 		if ((*s & 0xf0) == 0xc0 && s[1])
 		{
@@ -66,6 +103,9 @@ xconv(char *src, char *dst, size_t dstlen)
 
 		add_unicode(&d, &len, iso6937_map[*s & 0xff]);
 	}
+	if (!changed)
+		return 0;
+	
 	*d = '\0';
 	return dstlen - len;
 }