This game would be the concept demo to end all concept demos. Blue Sphere (the game didn't even have an official name until it was released as part of Sonic Mega Collection) is what you get from locking on Sonic 1 to Sonic & Knuckles, and it consists of an endless loop of special stages using S3&K's engine.
The trick is...there are 134,217,728 levels before it goes back to level 1. I believe that puts it in the running for longest TAS, ever.
The idea I had here wasn't to manually play through all the levels one at a time; that would take far too long. There would need to be some kind of program that looks to see what level it is, and generates a valid input stream for that level, continuing until all levels are complete.
For maps of each level, you can look up
http://drspud.no-ip.com/bluesphere/ . Basically, each map is a 32x32 wrapping square, divided into four 16x16 tiles, and there are 128 possible tile designs numbered from 0 to 127. For level N, the top-right tile is (N-1) % 128, the bottom-right is (3N-2) % 127, the top-left in (5N-3) % 126, and the bottom-left is (7N-4) % 125. 128 designs per tile and 4 tiles would dictate 268435456 possible levels. But tile 127 can only ever appear in the top right slot, as none of the other mod numbers can produce 127. Likewise, tile 126 can only appear in the two right-side slots, and tile 125 can't appear in the bottom left. Furthermore, half of the remaining layouts are disqualified, as the mod-128 and mod-126 tracks can never have differing parities, so the actual number of unique stages is 128*127*126*125/2 = 128016000. Level 128016001 is the same as level 1, but with a different number and code. Basically, you play the 128016000 unique levels, then you play clones of the first 6201728 levels, then you start again from level 1.
The easiest way to go about constructing a Blue Sphere run would be to take the same amount of time for all possible tiles, found by calculating the most time-consuming tile and if faced with anything else, killing time until you've taken up exactly as much as the longest tile takes, then moving on to the next. This is because the player is forced to speed up at regular intervals, and the amount of time spent on the first tile can cause a variance in how far you are into the second tile when the speedup hits, and therefore a desync. The next step would be to go through each tile "as fast as possible", with the program figuring out how much time has passed when it's done with each tile and planning for speedups accordingly. A third step would be to see if time could be saved with unusual cross-tile cuts, but that's significantly more advanced than the other two.
One goal I have in mind for this run is "uses no warps". If you get all the rings in a level, you're awarded a "Perfect," and you skip ahead 10 levels instead of 1. The run is intended to show a game with 134217728 levels in its full glory, not just 13,421,780 of them. Care must be taken
not to obtain all rings. In particular, tile 49's natural path goes right through the only ring on that tile. For level 25403954, which consists of four copies of tile 49, one of the tile paths (preferably the last one) will have to be rewritten, probably as a hard-coded exception, to take a detour through the bumpers rather than collecting the last ring.
The biggest problem with such a run, besides being boring when watched for any purpose besides as a concept demo for large-scale computational construction of a TAS, is that TASvideos likely doesn't even have enough space for it, and neither does the typical viewer. Even if each level and its completion screen can be done in an average of 1 minute each, the complete GMV file is going to be somewhere in the range of 1.5 terabytes. ZIP compression could knock a good deal off the size--the bytes representing the path through the first tile will in all cases be one of 128 chunks, and the second and subsequent tiles will be similar except that the button presses involved will become closer together after some point--but at some point the file will have to be decompressed in some form. TASvideos currently limits movie file uploads to 100KB, and there's no way in hell the file would be compressed down to less than 1 bit per level.
Oh well, at least it's nice to think such a thing can be done in theory.
