add wip span type and new file2 interface and fstream impl.

testing was very minimal

solanaceae/util/span.hpp

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <cstdint>
+#include <stdexcept>
+#include <vector>
+
+// non owning view
+template<typename T>
+struct Span final {
+	const T* ptr {nullptr};
+	uint64_t size {0};
+
+	constexpr Span(void) {}
+	constexpr Span(const T* ptr_, uint64_t size_) : ptr(ptr_), size(size_) {}
+	constexpr explicit Span(const std::vector<T>& vec) : ptr(vec.data()), size(vec.size()) {}
+
+	explicit operator std::vector<T>() const {
+		return std::vector<T>{ptr, ptr+size};
+	}
+
+	constexpr const T& operator[](uint64_t i) const {
+		if (i > size) {
+			throw std::out_of_range("accessed span out of range");
+		}
+
+		return ptr[i];
+	}
+
+	constexpr const T* cbegin(void) const { return ptr; }
+	constexpr const T* cend(void) const { return ptr+size; }
+	constexpr const T* begin(void) const { return ptr; }
+	constexpr const T* end(void) const { return ptr+size; }
+
+	constexpr bool empty(void) const { return ptr == nullptr || size == 0; }
+};
+
+// useful alias
+using ByteSpan = Span<uint8_t>;
+