1 files changed, 44 insertions, 0 deletions
diff --git a/lib/core/c++/flatten.hpp b/lib/core/c++/flatten.hpp
new file mode 100644
index 0000000..164bb77
--- /dev/null
+++ b/lib/core/c++/flatten.hpp
@@ -0,0 +1,44 @@
+#pragma once
+
+#include <forstio/error.hpp>
+#include <forstio/codec/data.hpp>
+
+namespace kel {
+namespace lbm {
+namespace sch {
+using namespace saw::schema;
+}
+
+template<typename T, uint64_t D>
+struct flatten_index {
+public:
+	template<uint64_t i>
+	static constexpr saw::data<sch::UInt64> stride(const saw::data<sch::FixedArray<sch::UInt64,D>>& meta) {
+		if constexpr (i > 0u){
+			return stride<i-1u>(meta) * meta.at({i-1u});
+		}
+
+		return 1u;
+	}
+
+private:
+	/// 2,3,4 => 2,6,24
+	/// i + j * 2 + k * 3*2
+	/// 1 + 2 * 2 + 3 * 3*2 = 1+4+18 = 23
+	template<uint64_t i>
+	static void apply_i(saw::data<sch::UInt64>& flat_ind, const saw::data<sch::FixedArray<T,D>>& index, const saw::data<sch::FixedArray<T,D>>& meta){
+		if constexpr ( D > i ) {
+			flat_ind = flat_ind + index.at({i}) * stride<i>(meta);
+			apply_i<i+1u>(flat_ind,index,meta);
+		}
+	}
+public:
+	static saw::data<T> apply(const saw::data<sch::FixedArray<T,D>>& index, const saw::data<sch::FixedArray<T,D>>& meta){
+		saw::data<T> flat_ind{0u};
+
+		apply_i<0u>(flat_ind, index, meta);
+		return flat_ind;
+	}
+};
+}
+}