How to apply game progression changes on screen loading in a Zelda like game

Question

I'm trying to develop a 2D game like Zelda. The game is divided in screens with exits linking each other. I store each screen data (including objects and NPC positions) on a separate file, and when the main character steps on the exit, the next screen data is parsed and the screen is loaded. Everything works just fine so far.

I'm now trying to implement the game progression system, along with the save system. I've read a lot about some ideas on how to do that, finite state machine design, what to store on save files, serialization and everything, but I'm kind stuck here.

Let's say that action X triggers event Y, moving some NPC from point A to point B. The player then saves and quits the game. When the game is loaded, it must position that NPC on point B. To achieve that, should I rewrite the files that contains screen data so when the screen is parsed it draws the NPC on its new place? Or should I store its new position on a separate file, and apply the changes on the screen after its loaded? Or maybe there's a better and less ugly way.

Any suggestions are welcome.

Answer 1

A common solution for this is to have variables you save together with the players state and objects which have a conditional state depending on these variables.

Example:

1. When the player flips the switch on screen 42, you set the persistent variable "switch_42_state" to "flipped".
2. On screen 76, you have a conditional door tile. When "switch_42_state" is "flipped", it is loaded as an open door, otherwise it is loaded as a closed door.

Similarily, you can have NPCs which only spawn when certain variables are set to specific values or which have different dialog depending on the variables.

Implementation detail: It is usually advisable to not use "real" programming language variables for this. It is usually better to store the variables and their states in a key/value data structure (binary tree, hash map, dictionary, associative array or whatever is available in your programming language), because it allows you to add new variables just by mentioning them on a screen file, makes saving and loading more straight-forward and avoids name collisions with internal variables.