[Add] Entity interpolation for remote players.

Added linear interpolation for remote player movement to provide a
smoother and more visually appealing experience.

Where previously remote players would snap directly to their new coords
on receiving a game state update, making it a bit jerky, now we get a
smoothe ride.

src/bettola.cpp

@@ -15,6 +15,7 @@
 
 #include "bettola.h"
 #include "bettola/network/net_common.h"
+#include "game/remote_player.h"
 #include "math/mat4.h"
 #include "network/message.h"
 #include "network/game_state_message.h"
@@ -198,6 +199,10 @@ void Bettola::update(double dt) {
 
   _player.update(dt);
 
+  for(auto& remote_player : _remote_players) {
+    remote_player.update(dt);
+  }
+
   if(_our_player_id > 0) {
     /* Send player position to server via UDP.. */
     BettolaLib::Network::MessageHeader header;
@@ -229,11 +234,6 @@ void Bettola::update(double dt) {
 }
 
 void Bettola::render(void) {
-  static int frame_count =0 ;
-  if(frame_count++ % 300 == 0) {
-    /* Print every 5 seconds or something... */
-    printf("DEBUG: Rendering %zu remote players.\n", _remote_players.size());
-  }
   glClearColor(0.1f, 0.1f, 0.3f, 1.0f);
   glClear(GL_COLOR_BUFFER_BIT);
 
@@ -278,9 +278,6 @@ void Bettola::render(void) {
 }
 
 void Bettola::process_game_state(const BettolaLib::Network::GameStateMessage& msg) {
-  printf("DEBUG: Receiving game state withj %u players.\n", msg.num_players);
-  /* First, let's handle our own player's state for reconciliation. */
-  bool found_our_player = false;
   for(unsigned int i = 0; i < msg.num_players; ++i) {
     const auto& player_state = msg.players[i];
 
@@ -291,29 +288,30 @@ void Bettola::process_game_state(const BettolaLib::Network::GameStateMessage& ms
        */
       printf("  -> Processing remote player %u at (%.2f, %.2f)\n", player_state.player_id,
              player_state.x, player_state.y);
-      found_our_player = true;
+      /* This is us! Reconcile our predicted state. */
       float dx = _player.get_x() - player_state.x;
       float dy = _player.get_y() - player_state.y;
       if((dx*dx+dy*dy) > 0.0001f) { /* If distance is not negligibale. */
         /* Simple correction, snap to server position. */
         _player.set_position(player_state.x, player_state.y);
       }
-      break;
-    }
-  }
+    } else {
+      /* Remote player. Find if we already know about them.. */
+      auto it = std::find_if(_remote_players.begin(), _remote_players.end(),
+                             [&](const RemotePlayer& p) {
+                                return p.get_id() == player_state.player_id;
+                             });
 
-  /*
-   * Update all remote players now.
-   * Might need some work later. Definately will need some work late..
-   */
-  std::vector<RemotePlayer> updated_remote_players;
-  for(unsigned int i = 0; i < msg.num_players; ++i) {
-    const auto& player_state = msg.players[i];
-    if(player_state.player_id != _our_player_id) {
-      updated_remote_players.emplace_back(player_state.player_id, player_state.x, player_state.y);
+      if(it != _remote_players.end()) {
+        /* Found 'em! update their target pos. */
+        it->set_target_position(player_state.x, player_state.y);
+      } else {
+        /* They are new, add them to our list. */
+        _remote_players.emplace_back(player_state.player_id, player_state.x, player_state.y);
       }
     }
-  _remote_players = updated_remote_players;
+  }
+  /* TODO: Remove players fomr _remote_players if they are no longer in the msg. */
 }
 
 bool Bettola::init_sdl(void) {

src/game/remote_player.cpp

@@ -1,8 +1,15 @@
 #include "remote_player.h"
 
 RemotePlayer::RemotePlayer(unsigned int id, float x, float y) :
-    _id(id), _x(x), _y(y) {}
+    _id(id), _x(x), _y(y), _target_x(x), _target_y(y) {}
 
-void RemotePlayer::set_position(float x, float y) {
-  _x = x; _y = y;
+void RemotePlayer::update(double dt) {
+  const float interp_speed = 15.0f;
+
+  _x += (_target_x - _x) * interp_speed * dt;
+  _y += (_target_y - _y) * interp_speed * dt;
+}
+
+void RemotePlayer::set_target_position(float x, float y) {
+  _target_x = x; _target_y = y;
 }

src/game/remote_player.h

@@ -4,7 +4,8 @@ class RemotePlayer {
 public:
   RemotePlayer(unsigned int id, float x, float y);
 
-  void set_position(float x, float y);
+  void update(double dt);
+  void set_target_position(float x, float y);
 
   unsigned int get_id(void) const { return _id; }
   float get_x(void) const { return _x; }
@@ -14,4 +15,6 @@ private:
   unsigned int _id;
   float _x;
   float _y;
+  float _target_x;
+  float _target_y;
 };