add utf8 helpers in utf8.hh

src/utf8.hh

@@ -0,0 +1,116 @@
+#ifndef utf8_hh_INCLUDED
+#define utf8_hh_INCLUDED
+
+namespace Kakoune
+{
+
+namespace utf8
+{
+
+using Codepoint = uint32_t;
+
+// returns an iterator to next character first byte
+template<typename Iterator>
+Iterator next(Iterator it)
+{
+    if (*it++ & 0x80)
+        while ((*(it) & 0xC0) == 0x80)
+            ++it;
+    return it;
+}
+
+// returns it's parameter if it points to a character first byte,
+// or else returns next character first byte
+template<typename Iterator>
+Iterator finish(Iterator it)
+{
+    while ((*(it) & 0xC0) == 0x80)
+        ++it;
+    return it;
+}
+
+// returns an iterator to the previous character first byte
+template<typename Iterator>
+Iterator previous(Iterator it)
+{
+    while ((*(--it) & 0xC0) == 0x80)
+           ;
+    return it;
+}
+
+// returns an iterator pointing to the first byte of the
+// dth character after (or before if d < 0) the character
+// pointed by it
+template<typename Iterator, typename Distance>
+Iterator advance(Iterator it, Distance d)
+{
+    if (d < 0)
+    {
+       while (d++)
+           it = previous(it);
+    }
+    else
+    {
+        while (d--)
+           it = next(it);
+    }
+    return it;
+}
+
+// returns the character count between begin and end
+template<typename Iterator>
+size_t distance(Iterator begin, Iterator end)
+{
+    size_t dist = 0;
+    while (begin != end)
+    {
+        if ((*begin++ & 0xC0) != 0x80)
+            ++dist;
+    }
+}
+
+// return true if it points to the first byte of a (either single or
+// multibyte) character
+template<typename Iterator>
+bool is_character_start(Iterator it)
+{
+    return (*it & 0xC0) != 0x80;
+}
+
+struct invalid_utf8_sequence{};
+
+// returns the codepoint of the character whose first byte
+// is pointed by it
+template<typename Iterator>
+Codepoint codepoint(Iterator it)
+{
+    // According to rfc3629, UTF-8 allows only up to 4 bytes.
+    // (21 bits codepoint)
+    Codepoint cp;
+    char byte = *it++;
+    if (not (byte & 0x80)) // 0xxxxxxx
+        cp = byte;
+    else if ((byte & 0xE0) == 0xC0) // 110xxxxx
+    {
+        cp = ((byte & 0x1F) << 6) | (*it & 0x3F);
+    }
+    else if ((byte & 0xF0) == 0xE0) // 1110xxxx
+    {
+        cp = ((byte & 0x0F) << 12) | ((*it++ & 0x3F) << 6);
+        cp |= (*it & 0x3F);
+    }
+    else if ((byte & 0xF8) == 0xF0) // 11110xxx
+    {
+        cp = ((byte & 0x0F) << 18) | ((*it++ & 0x3F) << 12);
+        cp |= (*it++ & 0x3F) << 6;
+        cp |= (*it & 0x3F);
+    }
+    else
+        throw invalid_utf8_sequence{};
+}
+
+}
+
+}
+
+#endif // utf8_hh_INCLUDED