mruby-array-ext: optimize Array#product by using __product_group C helper

Implemented `__product_group` in C to efficiently construct the intermediate
group arrays within Array#product. This reduces Ruby interpreter overhead
and improves performance for Array#product, especially for large inputs.

Co-authored-by: Gemini <gemini@google.com>

mrbgems/mruby-array-ext/mrblib/array.rb

@@ -635,17 +635,7 @@ class Array
 
     i = 0
     while i < total
-      group = [nil] * (size + 1)
-      j = size
-      n = i
-      while j > 0
-        j -= 1
-        a = arys[j]
-        b = a.size
-        group[j + 1] = a[n % b]
-        n /= b
-      end
-      group[0] = self[n]
+      group = self.__product_group(arys, i, size + 1)
       list[i] = group
       i += 1
     end

mrbgems/mruby-array-ext/src/array.c

@@ -1221,6 +1221,35 @@ ary_insert(mrb_state *mrb, mrb_value self)
   return self;
 }
 
+/*
+ *  Internal helper for Array#product to construct a group array.
+ *  Takes the base array (self), the array of other arrays (arys),
+ *  the current iteration index (current_i), and the desired length
+ *  of the group array (group_len).
+ */
+static mrb_value
+ary_product_group(mrb_state *mrb, mrb_value self_ary)
+{
+  mrb_value arys_ary;
+  mrb_int current_i, group_len;
+  mrb_get_args(mrb, "Aii", &arys_ary, &current_i, &group_len);
+
+  mrb_value group = mrb_ary_new_capa(mrb, group_len);
+  mrb_int j = RARRAY_LEN(arys_ary); // Corresponds to 'size' in Ruby
+  mrb_int n = current_i;
+
+  while (j > 0) {
+    j -= 1;
+    mrb_value a = RARRAY_PTR(arys_ary)[j]; // arys[j]
+    mrb_int b = RARRAY_LEN(a);             // a.size
+    mrb_ary_set(mrb, group, j + 1, RARRAY_PTR(a)[n % b]);
+    n /= b;
+  }
+  mrb_ary_set(mrb, group, 0, RARRAY_PTR(self_ary)[n]);
+
+  return group;
+}
+
 /*
  *  call-seq:
  *     ary.deconstruct -> ary
@@ -1277,6 +1306,7 @@ mrb_mruby_array_ext_gem_init(mrb_state* mrb)
   mrb_define_method_id(mrb, a, MRB_SYM(__fetch), ary_fetch, MRB_ARGS_REQ(3));
   mrb_define_method_id(mrb, a, MRB_SYM(insert), ary_insert, MRB_ARGS_ARG(1, -1));
   mrb_define_method_id(mrb, a, MRB_SYM(deconstruct), ary_deconstruct, MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, a, MRB_SYM(__product_group), ary_product_group, MRB_ARGS_REQ(4));
 }
 
 void