Day 20: Weekly challenge. Persistent player data. Java date woes.
Weekly challenge will be to collect some amount of stars during a week and get some nice reward, like 8 atomic bombs, 5 shields or similar. I made a nice gold star in Gimp. I tried different particle effects on it and also some diffused star-light, but it did not look really good. So I went back to the particle effect used for powerups and tweaked that until I got something distinct for the star. Stars show up on their own pacing, so you can have both a star and a powerup on the screen at the same time.
I also worked on loading and saving player data. It was much easier than I expected. I expected to have to learn some Android data storage API, but for simple key-value storage, libGDX provides the Preferences class. Just init with:
Preferences prefs = Gdx.app.getPreferences("DroneInvaders");
and then use get("key", defaultValute) and set(key, value) to read and write the values.
The only thing I had problems with are the dates. To keep track of daily and weekly challenges, the game stores the date of last play. When player launches the game, it compares that and resets some counters. Theoretically, I could prevent players from changing the system's calendar to past date, but I don't want to. What I am doing, is setting new set of daily and weekly challenges when day rolls over and resetting the counters for number of stars and letters picked up.
To make that work, I had to get the day difference between the previous play and current date. It matters whether it's the same day, exactly one day apart or more than one day. Googling got me to various websites and StackOverflow questions. Answers are funny. Many programmers simply calculate difference in seconds and then divide by 60 * 60 * 24 to get the days, completely ignorant about issues with daylight savings and leap seconds. One could argue that it does not matter that much for a game, but still getting many bug reports twice a year doesn't sound like fun to me. Some other guys simply count the days by adding one-by-one from start until they reach the end. While those loops might look correct, they still miss sometimes as they do not take the time out of the calculation. If one object stored 01.Jan at 5am, and you calculate it vs 02.Jan at 23pm, adding one day to first object is still less than the second, so they add two and get 2 day difference.
One of the tricks I use in this situation is to always set the time of the earlier Date to be something like 10am, and set the time for the later Date object to 5pm. Since daylight changes always happen at night, this is safe. And we also have 7 hours in between, just in case someone in future decides to do daylight saving changes in the middle of the day.
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