Add hb_utf_prev()

src/hb-utf-private.hh

@@ -72,6 +72,39 @@ hb_utf_next (const uint8_t *text,
   }
 }
 
+static inline const uint8_t *
+hb_utf_prev (const uint8_t *text,
+	     const uint8_t *start,
+	     hb_codepoint_t *unicode)
+{
+  const uint8_t *end = text;
+  while (start < text && (*--text & 0xc0) == 0x80 && end - text < 4)
+    text--;
+
+  hb_codepoint_t c = *text, mask;
+  unsigned int len;
+
+  /* TODO check for overlong sequences? */
+
+  HB_UTF8_COMPUTE (c, mask, len);
+  if (unlikely (!len || (unsigned int) (end - text) != len)) {
+    *unicode = -1;
+    return end - 1;
+  } else {
+    hb_codepoint_t result;
+    unsigned int i;
+    result = c & mask;
+    for (i = 1; i < len; i++)
+      {
+	result <<= 6;
+	result |= (text[i] & 0x3f);
+      }
+    *unicode = result;
+    return text;
+  }
+}
+
+
 static inline unsigned int
 hb_utf_strlen (const uint8_t *text)
 {
@@ -105,6 +138,31 @@ hb_utf_next (const uint16_t *text,
   return text;
 }
 
+static inline const uint16_t *
+hb_utf_prev (const uint16_t *text,
+	     const uint16_t *start,
+	     hb_codepoint_t *unicode)
+{
+  hb_codepoint_t c = *--text;
+
+  if (unlikely (hb_in_range<hb_codepoint_t> (c, 0xdc00, 0xdfff)))
+  {
+    /* low surrogate */
+    hb_codepoint_t h;
+    if (start < text && ((h = *(text - 1)), likely (hb_in_range<hb_codepoint_t> (h, 0xd800, 0xdbff))))
+    {
+      /* high surrogate */
+      *unicode = (h << 10) + c - ((0xd800 << 10) - 0x10000 + 0xdc00);
+       text--;
+    } else
+      *unicode = -1;
+  } else
+    *unicode = c;
+
+  return text;
+}
+
+
 static inline unsigned int
 hb_utf_strlen (const uint16_t *text)
 {
@@ -121,8 +179,17 @@ hb_utf_next (const uint32_t *text,
 	     const uint32_t *end,
 	     hb_codepoint_t *unicode)
 {
-  *unicode = *text;
+  *unicode = *text++;
-  return text + 1;
+  return text;
+}
+
+static inline const uint32_t *
+hb_utf_prev (const uint32_t *text,
+	     const uint32_t *start,
+	     hb_codepoint_t *unicode)
+{
+  *unicode = *--text;
+  return text;
 }
 
 static inline unsigned int