1 files changed, 45 insertions, 0 deletions
diff --git a/lib/mbstring/u8prev.c b/lib/mbstring/u8prev.c
new file mode 100644
index 0000000..507f7e1
--- /dev/null
+++ b/lib/mbstring/u8prev.c
@@ -0,0 +1,45 @@
+#include <stddef.h>
+
+#include "mbstring.h"
+#include "rune.h"
+
+int
+u8prev(rune *ch, const char8_t **p, const char8_t *start)
+{
+	int off;
+	const char8_t *s = *p;
+	ptrdiff_t d = s - start;
+
+	if (d <= 0) {
+		return 0;
+	} else if (U8_BYTE_1(s[-1])) {
+		if (ch)
+			*ch = s[-1];
+		off = 1;
+	} else if (d > 1 && U8_BYTE_C(s[-1]) && U8_BYTE_2(s[-2])) {
+		if (ch)
+			*ch = ((s[-2] & 0x1F) << 6) | (s[-1] & 0x3F);
+		off = 2;
+	} else if (d > 2 && U8_BYTE_C(s[-1]) && U8_BYTE_C(s[-2])
+			&& U8_BYTE_3(s[-3])) {
+		if (ch) {
+			*ch =
+				((s[-3] & 0x0F) << 12) | ((s[-2] & 0x3F) << 6) | (s[-1] & 0x3F);
+		}
+		off = 3;
+	} else if (d > 3 && U8_BYTE_C(s[-1]) && U8_BYTE_C(s[-2]) && U8_BYTE_C(s[-3])
+			&& U8_BYTE_4(s[-4])) {
+		if (ch) {
+			*ch = ((s[-4] & 0x07) << 18) | ((s[-3] & 0x3F) << 12)
+			    | ((s[-2] & 0x3F) << 6) | (s[-1] & 0x3F);
+		}
+		off = 4;
+	} else {
+		if (ch)
+			*ch = RUNE_ERROR;
+		off = 1;
+	}
+
+	*p -= off;
+	return off;
+}