add Encoding::decode()

src/core/Encoding.cc

@@ -96,3 +96,63 @@ const uint8_t * Encoding::beginning_of_code_point(Type type, const uint8_t * enc
 
     return nullptr;
 }
+
+uint32_t Encoding::decode(Type type, const uint8_t * encoded)
+{
+    switch (type)
+    {
+    case UTF_8:
+        {
+            const uint8_t c = *encoded;
+            uint8_t following_bytes = 0u;
+            uint32_t v;
+            if ((c & 0x80u) == 0u)
+            {
+                return c;
+            }
+            else if ((c & 0xE0u) == 0xC0u)
+            {
+                v = c & 0x1Fu;
+                following_bytes = 1u;
+            }
+            else if ((c & 0xF0u) == 0xE0u)
+            {
+                v = c & 0x0Fu;
+                following_bytes = 2u;
+            }
+            else if ((c & 0xF8u) == 0xF0u)
+            {
+                v = c & 0x07u;
+                following_bytes = 3u;
+            }
+            else if ((c & 0xFCu) == 0xF8u)
+            {
+                v = c & 0x03u;
+                following_bytes = 4u;
+            }
+            else if ((c & 0xFEu) == 0xFCu)
+            {
+                v = c & 0x01u;
+                following_bytes = 5u;
+            }
+            else
+            {
+                return 0u;
+            }
+            while (following_bytes-- > 0u)
+            {
+                encoded++;
+                v <<= 6u;
+                v |= *encoded & 0x3Fu;
+            }
+            return v;
+        }
+        break;
+    case CP_1252:
+        /* TODO: map byte to code point */
+        return *encoded;
+        break;
+    }
+
+    return 0u;
+}