interpolating_map.h

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// Copyright (c) FIRST and other WPILib contributors.
// Open Source Software; you can modify and/or share it under the terms of
// the WPILib BSD license file in the root directory of this project.
 
#pragma once
 
#include <map>
#include <utility>
 
namespace wpi {
 
/**
 * Implements a table of key-value pairs with linear interpolation between
 * values.
 *
 * If there's no matching key, the value returned will be a linear interpolation
 * between the keys before and after the provided one.
 *
 * @tparam Key   The key type.
 * @tparam Value The value type.
 */
template <typename Key, typename Value>
class interpolating_map {
 public:
  /**
   * Inserts a key-value pair.
   *
   * @param key   The key.
   * @param value The value.
   */
  void insert(const Key& key, const Value& value) {
    m_container.insert(std::make_pair(key, value));
  }
 
  /**
   * Inserts a key-value pair.
   *
   * @param key   The key.
   * @param value The value.
   */
  void insert(Key&& key, Value&& value) {
    m_container.insert(std::make_pair(key, value));
  }
 
  /**
   * Returns the value associated with a given key.
   *
   * If there's no matching key, the value returned will be a linear
   * interpolation between the keys before and after the provided one.
   *
   * @param key The key.
   */
  Value operator[](const Key& key) const {
    using const_iterator = typename std::map<Key, Value>::const_iterator;
 
    // Get iterator to upper bound key-value pair for the given key
    const_iterator upper = m_container.upper_bound(key);
 
    // If key > largest key in table, return value for largest table key
    if (upper == m_container.end()) {
      return (--upper)->second;
    }
 
    // If key <= smallest key in table, return value for smallest table key
    if (upper == m_container.begin()) {
      return upper->second;
    }
 
    // Get iterator to lower bound key-value pair
    const_iterator lower = upper;
    --lower;
 
    // Perform linear interpolation between lower and upper bound
    const double delta = (key - lower->first) / (upper->first - lower->first);
    return delta * upper->second + (1.0 - delta) * lower->second;
  }
 
  /**
   * Clears the contents.
   */
  void clear() { m_container.clear(); }
 
 private:
  std::map<Key, Value> m_container;
};
 
}  // namespace wpi