if I define it as byte* and let the calling convention implicitly define it as 32bit, it doesn't do the cast
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well I found the decompression method.
as always, I hate it. decompression routines are probably my least favorite thing to reverse engineer
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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!
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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.
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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
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the predefined table starts with NUL, space, then:
aetonisrdlhugfcwypbmk,vSA.T'PMxBCIRGDWHqE-zNFKL0j:51YJ8\U?73Q;2!469
\r\nOVXZ()*+"#$%&<=>/@[]^_`
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given that the most comment symbols are near the beginning, this is presumably a sort of lazy huffman coding
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but I've got the predefined table, an input file, an output file, and now I need to write some python code to replicate this, hopefully without crying
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"vs ses oa is isgit's tc eital and largest t u anhtA ttggh os nnotosnhrdsmarosogdn ss drte tishoth's isdhsceohtsnthminder of isgit's t nuorhdhtpast\x00 geru is slightltsn oaller than ndhd na and is o nnsgtgstbtst oa dotlalssaaolootbiaoht Sal gh, sonuhvia and sl ghh\x00isgit, ontvdn ss nhsiaalgarsnadlfnaatawlarst oadrlhrs i is a rugged land dooousr'casrbhe nrdsgs fountainsnht iah"
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I mean, it's not 100% wrong, but it's not right either
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that's supposed to read:
"\x03Lima is Peru's capital and largest city. A well-known landmark is the Archbishop's Palace, a reminder of Peru's colonial past\x00Peru is slightly smaller than Alaska and is bordered by Ecuador, Colombia, Brazil, Bolivia and Chile\x00Peru, once the center of the mighty Incan Empire, is a rugged land dominated by the Andes Mountains. Forests and jungles cover half its land area\x00"
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I somehow confused the dosbox-x debugger into not accepting letters anymore
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it was a trivial off-by-one error.
I was doing saved_byte=input[3]
but while I needed the 3rd byte, that's at input[2]
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yess!
C:\DOSBox-X\drive_c\carmen\py>python datfile.py cities.dat --dump=12803 --decompress
"\x03Sydney, with a population of more than 3.3 million people, is Australia's largest city. A well-known sight is Sydney's distinctively designed Opera House\x00An island continent, Australia is nearly as large as the United States but has only one-fifteenth the population\x00The capital of Australia is Canberra, located in the southeast corner of the country between Sydney and Melbourne\x00"
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It starts with \x03 to indicate there's three strings: then it describes the city three times. at runtime it uses select_string function with a random input to select one of the three strings
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okay now that I can decode the chunks (well, most of them) I can identify a lot more of them:
00 Name and (some other info)
01 ???
02 Image
03 City descriptions
04 Items to steal
10 ???
11&up: Hints leading here
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So like, the 12 chunk for Tokyo says:
b'\x05asked about the exchange rate for yen\x00was practicing Japanese characters\x00said\x81planned to take photographs of Mount Fuji\x00asked about tours of the Imperial Palace\x00was interested in visiting Shinto shrines\x00'
So it picks from one of those 5 options
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and then 13 will be:
b'\x02asked questions about Shinto rituals\x00said\x81was researching an archipelago\x00'
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so when it sets up a city that has hints to lead to Tokyo, it picks 3 of these sets of questions, then picks a question in each set.
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tool that'd really be handy right now:
a "live" version of binxelview, so I can step through the DOSBox-x debugger and see how memory is changing in real time, as an image.
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that might not be TOO hard to hack in, hmm.
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I'm stepping through a high-level loading routine I don't understand yet, trying to figure out when it decompresses an image by watching the RAM it uses for file loading and decompression and spotting when the image appears
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sadly DOSBox-X's memory breakpoints don't let you set up a breakpoint that covers a whole 64k. you only get one byte. A shame.
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ooh, I'd also need to be able to watch multiple address ranges at once. that'd be sweet, multiple windows of visibility into RAM
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I'm in Paris, I look at work ram, I see the image of the Eiffel. I head to Rome, and before I load the next image, I can see that the Eiffle tower in workram now has the wrong stride.
That's odd, because it means it had to rewrite the image in memory, the image it's about to unload.
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I think this might be the GUI system doing a screenshot of the image under a window, so it can restore it at the end. And it still does that here, even though we'll never need to restore that image: we're about to overwrite it
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Here's what I want a tool to do:
I hit a breakpoint in the debugger, I turn it on, set another breakpoint, and hit go.
between those two breakpoints, every time a CALL instruction is hit, it dumps my selected memory region. If it's identical to the last dump, it's ignored.
At the end, each dump is rendered as an image, and the combined set are an animation I can scroll through.
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I need a higher order debugger. I'm doing too much shit manually
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GOT YOU, YOU SON OF A BITCH! I FOUND YOU.
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it's in a function I already found, temporarily named "blit_related".
I guess they don't decode the image until RIGHT before it needs to go up on the screen!
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if definitely decompresses and then blits the image as two parts, which aren't evenly sized, and it starts from the bottom
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I think they're just trying to keep their RAM usage down by not having both halves in memory at once
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wait is this image format vertically interlaced!?
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It loads the half-width version, then a few functions later, it's been replaced with a full-width version.
Strange!
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wait no, the colors are wrong... I bet I'm seeing it decompress the binary, but that's using the full width of the bytes. it then gets expanded out to a 16-color image.
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well the good news is that I think I've found the decompress_image function. the bad news is that now I have to reverse engineer it :(
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it's currently doing the obvious thing for a decompressor to do:
write the byte 04 every 69 bytes
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oh sweet jesus, that's the left two pixels of the image.
it's loading the image vertically!
at least it's top to bottom.
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yeah, doom did that too, but Doom was a 2.5D image that had to do pseudo-raycasting.
THIS GAME DOES NOT
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it allocates a 1024 byte buffer, then makes a pointer to the end of it, minus -0x42?
why would you need a link to the end of a new, freshly cleared buffer, minus 62?
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@foone I have seen that behavior in OS development, when you need to reserve space for a stack and the first pointer needs to match a specific global alignment.
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