Finish implementation of range

rt/include/python/builtins/range.hpp

@@ -7,16 +7,35 @@
 
 #include <ranges>
 
+namespace view = std::views;
+
 #include <python/basedef.hpp>
 
+auto stride = [](int n) {
+  return [s = -1, n](auto const&) mutable { s = (s + 1) % n; return !s; };
+};
+
 // todo: proper range support
 struct range_s : TyBuiltin<range_s>
 {
   template <typename T>
-  auto sync(T stop) { return std::views::iota(0, stop); }
+  auto sync(T stop) { return sync(0, stop); }
 
   template <typename T>
-  auto sync(T start, T stop) { return std::views::iota(start, stop); }
+  auto sync(T start, T stop, T step = 1) {
+    // https://www.modernescpp.com/index.php/c-20-pythons-map-function/
+    if(step == 0) {
+      throw std::invalid_argument("Step cannot be 0");
+    }
+    auto Step = start < stop ? step : -step;
+    auto Begin = std::min(start, stop);
+    auto End = Step < 0 ? Begin : std::max(start, stop);
+    return view::iota(Begin, End)
+           | view::filter(stride(std::abs(Step)))
+           | view::transform([start, stop](std::size_t i){
+               return start < stop ? i : stop - (i - start);
+             });
+  }
 } range;
 
 #endif // TYPON_RANGE_HPP