[wombat/utils] Created a Cached<T> API for lazily loading values

This allows us to create optimisations for values that do not need to be
called regularly.

wombat/src/main/cpp/utils/Cache.cpp

@@ -0,0 +1,31 @@
+#include "utils/Cache.h"
+
+#include "utils/Util.h"
+
+template <typename T>
+wom::utils::Cached<T>::Cached(std::function<T()> supplier)
+    : m_supplier{supplier}, k_timestamp{wom::utils::now()}, k_dt{20_ms} {};
+
+template <typename T>
+wom::utils::Cached<T>::Cached(std::function<T()> supplier, units::second_t dt)
+    : m_supplier{supplier}, k_timestamp{wom::utils::now()}, k_dt{dt} {};
+
+template <typename T>
+wom::utils::Cached<T>& wom::utils::Cached<T>::Update() {
+  this->k_timestamp = wom::utils::now();
+  this->m_value = m_supplier();
+  return this;
+}
+
+template <typename T>
+T& wom::utils::Cached<T>::GetValue() {
+  if (IsStale()) {
+    Update();
+  }
+  return m_value;
+}
+
+template <typename T>
+bool wom::utils::Cached<T>::IsStale() {
+  return wom::utils::now() - this->k_timestamp > k_dt;
+}

wombat/src/main/include/utils/Cache.h

@@ -0,0 +1,57 @@
+#pragma once
+
+#include <functional>
+
+#include <units/time.h>
+
+namespace wom {
+namespace utils {
+
+/* A class for a cached value that can be updated lazily */
+template <typename T>
+class Cached {
+ public:
+  /**
+   * Creates a new Cached<T> with the given supplier. Sets the validity period to 20 milliseconds by default.
+   *
+   * @param supplier A function that sources the value.
+   */
+  Cached(std::function<T()> supplier);
+
+  /**
+   * Creates a new Cached<T> with the given supplier.
+   *
+   * @param supplier A function that sources the value.
+   * @param dt The validity length of the stored value.
+   */
+  Cached(std::function<T()> supplier, units::second_t dt);
+
+  Cached(Cached&&) = default;
+  Cached& operator=(Cached&&) = default;
+
+  /**
+   * Updates the Cached class with the current timestamp and value.
+   *
+   * @return The updated class.
+   */
+  Cached<T>& Update();
+
+  /**
+   * Gets the cached value.
+   */
+  T& GetValue();
+
+  /**
+   * Checks whether or not the stored value is stale.
+   */
+  inline bool IsStale();
+
+ private:
+  T& m_value;
+  std::function<T()> m_supplier;
+  const units::second_t k_timestamp;
+  const units::second_t k_dt;
+};
+
+}  // namespace utils
+}  // namespace wom