core: irrt add unchecked ndarray broadcasting

nac3core/irrt/irrt/numpy/ndarray_broadcast.hpp

@@ -54,5 +54,63 @@ namespace ndarray {
             return true;
         }
     }
+
+    // Similar to `np.broadcast_to(<ndarray>, <target_shape>)`
+    // Assumptions:
+    //   - `src_ndarray` has to be fully initialized.
+    //   - `dst_ndarray->ndims` has to be set.
+    //   - `dst_ndarray->shape` has to be set, this determines the shape `this` broadcasts to.
+    //
+    // Other notes:
+    //   - `dst_ndarray->data` does not have to be set, it will be set to `src_ndarray->data`.
+    //   - `dst_ndarray->itemsize` does not have to be set, it will be set to `src_ndarray->data`.
+    //   - `dst_ndarray->strides` does not have to be set, it will be overwritten.
+    //
+    // Cautions:
+    //   ```
+    //   xs = np.zeros((4,))
+    //   ys = np.zero((4, 1))
+    //   ys[:] = xs # ok
+    // 
+    //   xs = np.zeros((1, 4))
+    //   ys = np.zero((4,))
+    //   ys[:] = xs # allowed
+    //   # However `np.broadcast_to(xs, (4,))` would fails, as per numpy's broadcasting rule.
+    //   # and apparently numpy will "deprecate" this? SEE https://github.com/numpy/numpy/issues/21744
+    //   # This implementation will NOT support this assignment.
+    //   ```
+    template <typename SizeT>
+    void broadcast_to(NDArray<SizeT>* src_ndarray, NDArray<SizeT>* dst_ndarray) {
+        dst_ndarray->data = src_ndarray->data;
+        dst_ndarray->itemsize = src_ndarray->itemsize;
+
+        // irrt_assert(
+        //     ndarray_util::can_broadcast_shape_to(
+        //         dst_ndarray->ndims,
+        //         dst_ndarray->shape,
+        //         src_ndarray->ndims,
+        //         src_ndarray->shape
+        //     )
+        // );
+
+        SizeT stride_product = 1;
+        for (SizeT i = 0; i < max(src_ndarray->ndims, dst_ndarray->ndims); i++) {
+            SizeT this_dim_i = src_ndarray->ndims - i - 1;
+            SizeT dst_dim_i = dst_ndarray->ndims - i - 1;
+
+            bool this_dim_exists = this_dim_i >= 0;
+            bool dst_dim_exists = dst_dim_i >= 0;
+
+            // TODO: Explain how this works
+            bool c1 = this_dim_exists && src_ndarray->shape[this_dim_i] == 1;
+            bool c2 = dst_dim_exists && dst_ndarray->shape[dst_dim_i] != 1;
+            if (!this_dim_exists || (c1 && c2)) {
+                dst_ndarray->strides[dst_dim_i] = 0; // Freeze it in-place
+            } else {
+                dst_ndarray->strides[dst_dim_i] = stride_product * src_ndarray->itemsize;
+                stride_product *= src_ndarray->shape[this_dim_i]; // NOTE: this_dim_exist must be true here.
+            }
+        }
+    }
 }
 }