idea for a test: it's easy to spot which chunk in a city is the image, because it's the biggest. Here's a way to determine if it's looking up by IDs or offsets/indices: swap the IDs of two images

darn. turns out you can't just renumber the chunks, because they have to be in increasing order.

so maybe I just need to leave the chunk indexes as is, and instead of moving the entries around, I move where they're pointing?

Bingo! I'm in Athens, but I'm seeing the image for Baghdad, and apparently with the Baghdad palette?

So one of these other chunks must be the palette for a city. Or it selects from a selection of palettes? Maybe they've just got a couple defined.

okay I figured out the cities.dat IDs:

They're all 1XXYY (in decimal):
XX is the city number (0-29), YY is the sub-chunk-id.

So like:
YY=0: City name
YY=2: City image.

They go between 00 and 22, and not all numbers need to be present.

hmm, reading a buffer and then summing all the values of the bytes in it.

suspicious behavior.

okay I think it has a very simple 1-byte CRC check on the chunks, which are optionally not run.
I can't make the math work but I'm reasonably sure that's what it is

okay they're using a blit that's UI-aware, so it starts the coordinate system at (1,13). Fun!

looking into the blitting code I managed to steal the world map out of RAM

ugh. TODO for my eventual Good DOS Debugger:
Instant Video display.
I don't know exactly how DOSBox-X is doing it, but while single-stepping the debugger, the display never updates. I can dump the ram at A000:0000 and see what updated, but not on the screen in DOSBox

found a suspicious array, which goes:
[
(-1,0),
(-1,1),
(0,1),
(1,1),
(1,0),
(1,-1),
(0, -1),
(-1,-1),
(0,0)
]

POP QUIZ: why does the font renderer need this array? how are they being "lazy" with this array?

there's also this code in the for-loop that steps through this array:

if index==8:
color=white
else:
color=black

@dividuum got it:

they're drawing the font 9 times, offset in each of the 8 directions, and in black. then they draw it in white, with no offset.

It's a pixel-outliner! By drawing their pixel font offset in each direction, they get a black outline on their font.

The Answer to the DRM questions for Where in the world is Carmen Sandiego? Enhanced (DOS, 1990) are, in no particular order:

23
Kent
dragon
calcium
1796
Warren
revenue
1792
Willard
1937
Crater
Tanzania
Hartford
Duluth
London
Gem
Silent
squeaker

if ((0x80 >> ((byte)local_4 & 7) &
(int)(char)*(byte *)((int)((int *)param_1 + 1) + (local_4 >> 3))) != 0) {

COULD YOU USE SOME MORE CASTS MAYBE?

oh it's because ghidra's near/far pointer support is shit.

I had param2 defined as a byte*32 and it was casting it to a byte* before using it

if I define it as byte* and let the calling convention implicitly define it as 32bit, it doesn't do the cast

well I found the decompression method.

as always, I hate it. decompression routines are probably my least favorite thing to reverse engineer

I think this compression is specifically designed for ASCII text, which is annoying because they've also got compressed images... which probably use a DIFFERENT COMPRESSION!

it looks like this chunk has length 256, which means 253 usable bytes, and it expands to 374 bytes.

Not the greatest compression. a little better than just doing 6-bit ASCII.

it's some kind of shifting bit mask but it starts at encoding values in 4 bits, then it can increase (or decrease, I guess) based on the input stream.

then it has an output filter, where if the number specified wasn't 8 bits, it's actually an index into a predefined text table