feat(furlang): add view

Add a basic view (span from C++20).

furlang/include/furlang/view.hpp

@@ -0,0 +1,38 @@
+#ifndef FURLANG_VIEW_HPP
+#define FURLANG_VIEW_HPP
+
+#include <algorithm>
+#include <cstddef>
+#include <limits>
+#include <stdexcept>
+
+namespace furlang {
+
+template <typename T>
+class view {
+public:
+    constexpr view() noexcept = default;
+
+    constexpr view(const T* data, std::size_t size) noexcept
+      : m_data(data), m_size(size) {}
+public:
+    constexpr view subview(std::size_t offset, std::size_t count = std::numeric_limits<std::size_t>::max()) {
+        if (count > 0 && offset >= m_size) throw std::runtime_error("offset too large");
+        return { m_data + offset, std::min(m_size - offset, count) };
+    }
+
+    const T& operator[](std::size_t offset) const {
+        if (offset >= m_size) throw std::runtime_error("out of bounds");
+        return m_data[offset];
+    }
+
+    constexpr const T*    data() const { return m_data; }
+    constexpr std::size_t size() const { return m_size; }
+private:
+    const T*    m_data = nullptr;
+    std::size_t m_size = 0;
+};
+
+} // namespace furlang
+
+#endif // FURLANG_VIEW_HPP