1 files changed, 66 insertions, 0 deletions
diff --git a/libcharconv_overlaid.c b/libcharconv_overlaid.c
new file mode 100644
index 0000000..7f3cab1
--- /dev/null
+++ b/libcharconv_overlaid.c
@@ -0,0 +1,66 @@
+/* See LICENSE file for copyright and license details. */
+#include "lib-common.h"
+
+
+static struct {
+	uint_least32_t a;
+	uint_least32_t b;
+	uint_least32_t to;
+} pairs[] = {
+	{0x00BF, 0x00A1, 0x2E18},
+	{0x0021, 0x003F, 0x203D}
+};
+
+
+enum libcharconv_result
+libcharconv_overlaid(const char *s, size_t slen, size_t *n, uint_least32_t *cp, size_t *ncp)
+{
+	uint_least32_t a, b;
+	size_t i, alen, blen;
+	*n = 0;
+	while (slen) {
+		alen = libcharconv_decode_utf8_(s, slen, &a);
+		if (alen > slen) {
+			if (*n)
+				goto no_conv;
+			return LIBCHARCONV_INDETERMINATE;
+		}
+		if (!alen) {
+			*n += 1u;
+			slen -= 1u;
+			s = &s[1];
+			continue;
+		}
+
+		for (i = 0u; i < sizeof(pairs) / sizeof(*pairs); i++) {
+			if (a != pairs[i].a && a != pairs[i].b)
+				continue;
+			if (*n)
+				goto no_conv;
+			if (slen == alen)
+				return LIBCHARCONV_INDETERMINATE;
+			blen = libcharconv_decode_utf8_(&s[alen], slen - alen, &b);
+			if (blen > slen)
+				return LIBCHARCONV_INDETERMINATE;
+			if (!blen)
+				goto no_conv;
+			if ((a ^ b) == (pairs[i].a ^ pairs[i].b))
+				goto conv;
+		}
+
+		*n += alen;
+		s = &s[alen];
+		slen -= alen;
+	}
+no_conv:
+	return LIBCHARCONV_NO_CONVERT;
+
+conv:
+	if (*n)
+		goto no_conv;
+	if (*ncp)
+		*cp = pairs[i].to;
+	*n += alen + blen;
+	*ncp = 1u;
+	return LIBCHARCONV_CONVERTED;
+}