[ruby/prism] Make pm_integer -> Integer faster

ruby/prism@47601e7928

prism/templates/ext/prism/api_node.c.erb

@@ -39,12 +39,21 @@ pm_string_new(const pm_string_t *string, rb_encoding *encoding) {
 
 static VALUE
 pm_integer_new(const pm_integer_t *integer) {
-    VALUE result = UINT2NUM(integer->head.value);
-    size_t shift = 0;
-
-    for (const pm_integer_word_t *node = integer->head.next; node != NULL; node = node->next) {
-        VALUE receiver = rb_funcall(UINT2NUM(node->value), rb_intern("<<"), 1, ULONG2NUM(++shift * 32));
-        result = rb_funcall(receiver, rb_intern("|"), 1, result);
+    VALUE result;
+    if (integer->head.next) {
+        size_t length = integer->length + 1;
+        VALUE str = rb_str_new(NULL, length * 8);
+        unsigned char *buf = (unsigned char *)RSTRING_PTR(str);
+        size_t offset = length * 8;
+        for (const pm_integer_word_t *node = &integer->head; node != NULL; node = node->next) {
+            for (int i = 0; i < 8; i++) {
+                int n = (node->value >> (4 * i)) & 0xf;
+                buf[--offset] = n < 10 ? n + '0' : n - 10 + 'a';
+            }
+        }
+        result = rb_funcall(str, rb_intern("to_i"), 1, UINT2NUM(16));
+    } else {
+        result = UINT2NUM(integer->head.value);
     }
 
     if (integer->negative) {