I know you're probably thinking 2D array or 2D vector, but hear me out.
I'm actually already using a 2D array for tilemaps, which works great. However, I'm trying to develop a data structure that can store game objects positioned on top of such a tilemap.
My requirements are as follows:
The object map must allow many objects to be positioned at the same cell
The object map must be iterable by rows and columns, so that I can cull the iteration space to certain coordinates (this is so I can render only the objects that are actually on-screen, among other things)
Preferably, the object map should also offer fast lookup to determine 1: What objects are at a given cell, 2: Whether a given object is currently on the object map somewhere and 3: The position of a given object on the object map
I have drafted up a basic data structure that uses two STL containers:
A 3D
std::map<int, std::map<int, std::vector<Object*>>>is used to provide an iterable, easily culled container. The std::vector is used so that many objects can be contained at the same cell. A cell can also be accessed via _map[x][y].Additionally, I am using a 1D
std::map<Object*, Vec2<int>*>, which contains the very same objects as the 3D std::map, except I think it will allow faster searching since it is 1D. The reasonVec2<int>*is a pointer is so that a GameObject may ask the ObjectMap for its position on the map, possibly save it, and then in future immediately access it without the need for a search.
Given my requirements, are there more suitable containers available than the ones I've used?
If it helps, I've pasted my code for the ObjectMap below:
#pragma once
#include <vector>
#include <map>
template<typename T>
struct Vec2 {
Vec2() { x = 0; y = 0; }
Vec2(T xVal, T yVal) : x(xVal), y(yVal) {}
void Set(T xVal, T yVal) { x = xVal; y = yVal; }
T x, y;
Vec2& operator+=(const Vec2& rhs) { x += rhs.x; y += rhs.y; return *this; }
Vec2 operator+(const Vec2& rhs) { return Vec2<T>(x + rhs.x, y + rhs.y); }
};
/// <summary>
/// Represents a map of objects that can be layered on top of a cell-based map
/// Allows for multiple objects per map cell
/// </summary>
template <typename Object>
class ObjectMap {
public:
/// <summary>
/// Gets the objects located at the given map cell
/// </summary>
/// <param name="row">The row of the cell to inspect</param>
/// <param name="column">The column of the cell to inspect</param>
/// <returns>
/// A pointer to a vector of objects residing at the given cell.
/// Returns a nullptr if there are no objects at the cell.
/// </returns>
std::vector<Object*>* At(int row, int column);
/// <summary>
/// Checks whether the ObjectMap contains the given object
/// </summary>
/// <param name="object">A pointer to the object to check for</param>
/// <returns>True if the ObjectMap contains the object</returns>
bool Contains(Object* object);
/// <summary>
/// Adds the given object to the ObjectMap at the given cell
/// </summary>
/// <param name="object">The object to add to the map</param>
/// <param name="row">The row of the cell to add the object to</param>
/// <param name="column">The column of the cell to add the object to</param>
/// <returns>True if successful, false if the object is already in the ObjectMap</returns>
bool Add(Object* object, int row, int column);
/// <summary>
/// Moves the given object by some number of rows and columns
/// </summary>
/// <param name="object">The object to move</param>
/// <param name="rows">The number of rows to move the object by</param>
/// <param name="columns">The number of columns to move the object by</param>
/// <returns>True if successful, false if the object does not exist in the ObjectMap</returns>
bool MoveBy(Object* object, int rows, int columns);
/// <summary>
/// Moves the given object to the given cell
/// </summary>
/// <param name="object">The object to move</param>
/// <param name="row">The row of the cell to move the object to</param>
/// <param name="column">The column of the cell to move the object to</param>
/// <returns>True if successful, false if the object does not exist in the ObjectMap</returns>
bool MoveTo(Object* object, int row, int column);
/// <summary>
/// Gets the position of the given object
/// </summary>
/// <param name="object">A pointer to the object to check the position of</param>
/// <returns>
/// A pointer to the position of the object.
/// Returns a nullptr if the object does not exist in the ObjectMap.
/// </returns>
Vec2<int>* GetPosition(Object* object);
private:
/// <summary>
/// A 3D container allowing object access via cell positions
/// Provides the ability to iterate across sections of the map
/// Useful for object culling and rendering
/// Useful for object lookup when the position is known
/// Example: _map[a][b] is a vector objects positioned at the map cell (x=a,y=b)
/// </summary>
std::map<int, std::map<int, std::vector<Object*>>> _map;
/// <summary>
/// A 1D container of all objects and pointers to their positions
/// Useful for quickly checking whether an object exists
/// Useful for quickly getting the location of an object
/// </summary>
std::map<Object*, Vec2<int>*> _objects;
};
///
/// ObjectMap.tpp
/// The implementation has not been separated into a .cpp file because templated
/// functions must be implemented in header files.
///
/// See http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file
///
#include <algorithm>
template <typename Object>
std::vector<Object*>* ObjectMap<Object>::At(int column, int row) {
// Checks whether a key exists for the given column
if (_map.find(column) != _map.end()) {
// Checks whether a key exists for the given row
if (_map.at(column).find(row) != _map.at(column).end()) {
// Return the objects residing in the cell
return &_map.at(column).at(row);
}
}
return nullptr;
}
template <typename Object>
bool ObjectMap<Object>::Contains(Object* object) {
return _objects.find(object) != _objects.end();
}
template <typename Object>
bool ObjectMap<Object>::Add(Object* object, int column, int row) {
if (!Contains(object)) {
_objects[object] = new Vec2<int>(column, row);
_map[column][row].push_back(object);
return true;
}
return false;
}
template <typename Object>
bool ObjectMap<Object>::MoveBy(Object* object, int columns, int rows) {
Vec2<int> newPosition = *_objects[object] + Vec2<int>(columns, rows);
return MoveTo(object, newPosition.x, newPosition.y);
}
template <typename Object>
bool ObjectMap<Object>::MoveTo(Object* object, int column, int row) {
if (Contains(object)) {
// Get the position reference of the object
Vec2<int>* position = _objects[object];
// Erase the object from its current position in the map
auto *oldTile = &_map[position->x][position->y];
oldTile->erase(std::remove(oldTile->begin(), oldTile->end(), object), oldTile->end());
// Erase any newly-empty keys from the map
if (oldTile->size() == 0) {
_map[position->x].erase(_map[position->x].find(position->y));
if (_map[position->x].size() == 0) {
_map.erase(_map.find(position->x));
}
}
// Add the object to its new position on the map
_map[column][row].push_back(object);
// Set the position of the object
position->Set(column, row);
return true;
}
return false;
}
template <typename Object>
Vec2<int>* ObjectMap<Object>::GetPosition(Object * object) {
if (Contains(object)) {
return _objects[object];
}
return nullptr;
}
