The best way to do this kind of thing involves having the file format itself tell you (or imply) where the record you want to change is found. In all examples, I will assume that your file is binary, but this is also technically possible with a text (ASCII) file. Typically, there are a few solutions:
Solution A:
This solution is relatively easy to visualize and quick to program. It has some inflexibility and limitations, but you can work around those relatively easily.
1. Separate your files into sections. Each section holds exactly one type of data (doors, or chests, but never "doors and chests"; have separate sections for each).
2. Make it so that each record always consumes the exact same number of bytes. For example, a "chest" record takes up 10 bytes -- 4 are "CHST", 2 are X-coord, 2 are Y-coord, 2 are flags (open, destroyed, key, etc).
3. The "header" of your file -- the first bit you read -- is a constant length. It describes the initial location of each section and the number of elements in that section.
4. Now, to change chest 423, you just need to edit starting at (start of chests section) + 423 * (size of a chest record), and you need to write exactly (size of a chest record) bytes.
Solution B:
This kind of thing is more useful when your objects are logically grouped, but contain varying amounts of information -- for example, items include swords and rings and potions, but each stores a different amount of information and is therefore a different length. However, it is considerably more complicated.
1. The header of your file stores the number of items and the offset to an "index table".
2. The "index table" is just a bunch of offsets into the file, describing the start of each item. So the first 4 bytes of the index table describe the offset of the first item.
3. When you go to the offset described in the table, you read a "record header" that tells you the type of the record and how long it is. This gives you enough information to figure out how much to read or write.
4. Now, to change chest (record number 423, chest number 1), you would go to the offset table at index 423 and read 4 bytes of offset. Then, at that offset, you would read "CHST" as the type and 6 as the length. Now you know you can write 6 bytes (X coord, Y coord, flags).
If you absolutely must pursue this, I recommend you start with solution A. However, I recommend that you consider just rewriting the file every time you save -- this is how professionally-developed games do it, and for good reason: small random writes to files are disproportionately expensive compared to large, consecutive writes. Another good reason is that doing it this way is considerably harder than just rewriting the entire file -- and therefore, more prone to bugs that damage saved games so they can't be loaded again. |
), and one of my problems is that I'm using a text file for the map, but I want to be able to change the states of a certain place on the map (for instance you would use the code chst+ for a chest that's closed and chst- for a chest that's opened). However, I'm not quite sure how to find a specific spot and then go about changing it.
