diff --git a/src/multibyte/mbrtowc.c b/src/multibyte/mbrtowc.c
index cc49781..d552652 100644
--- a/src/multibyte/mbrtowc.c
+++ b/src/multibyte/mbrtowc.c
@@ -18,6 +18,7 @@ size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate
 	const unsigned char *s = (const void *)src;
 	const unsigned N = n;
 
+	if (!n) return -2;
 	if (!st) st = (void *)&internal_state;
 	c = *(unsigned *)st;
 	
@@ -27,9 +28,9 @@ size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate
 		n = 1;
 	} else if (!wc) wc = (void *)&wc;
 
-	if (!n) return -2;
+	/* This condition can only be true if *s<0x80 and c==0 */
+	if (*s + c < 0x80) return !!(*wc = *s);
 	if (!c) {
-		if (*s < 0x80) return !!(*wc = *s);
 		if (*s-SA > SB-SA) goto ilseq;
 		c = bittab[*s++-SA]; n--;
 	}
diff --git a/src/multibyte/mbtowc.c b/src/multibyte/mbtowc.c
index b5dd7e3..5ce9281 100644
--- a/src/multibyte/mbtowc.c
+++ b/src/multibyte/mbtowc.c
@@ -11,9 +11,43 @@
 
 #include "internal.h"
 
-int mbtowc(wchar_t *restrict wc, const char *restrict s, size_t n)
+int mbtowc(wchar_t *restrict wc, const char *restrict src, size_t n)
 {
-	mbstate_t st = { 0 };
-	n = mbrtowc(wc, s, n, &st);
-	return n+2 ? n : -1;
+	unsigned c;
+	const unsigned char *s = (const void *)src;
+
+	if (!s) return 0;
+	if (!n) goto ilseq;
+	if (!wc) wc = (void *)&wc;
+
+	if (*s < 0x80) return !!(*wc = *s);
+	if (*s-SA > SB-SA) goto ilseq;
+	c = bittab[*s++-SA];
+
+	/* Avoid excessive checks against n: If shifting the state n-1
+	 * times does not clear the high bit, then the value of n is
+	 * insufficient to read a character */
+	if (n<4 && ((c<<(6*n-6)) & (1U<<31))) goto ilseq;
+
+	if (OOB(c,*s)) goto ilseq;
+	c = c<<6 | *s++-0x80;
+	if (!(c&(1U<<31))) {
+		*wc = c;
+		return 2;
+	}
+
+	if (*s-0x80u >= 0x40) goto ilseq;
+	c = c<<6 | *s++-0x80;
+	if (!(c&(1U<<31))) {
+		*wc = c;
+		return 3;
+	}
+
+	if (*s-0x80u >= 0x40) goto ilseq;
+	*wc = c<<6 | *s++-0x80;
+	return 4;
+
+ilseq:
+	errno = EILSEQ;
+	return -1;
 }