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Here's a simple ebook reader with the first three chapters of A Tale of Two Cities. Press X to advance (once per paragraph / page).

Cart [#16672#] | Copy | Code | 2015-11-15 | Link

This is a tech demo of some tools I've been working on for developing text-based games. It'd be more impressive if it were an actual game, but this victory was hard won so I'm posting it. :)


  • Text is stored in cart data, not as string literals in the code.

  • The original source file does have the text as string literals in code. I use a post-processor to extract the string literals, pack them into text data stored in the cart, then replace them with string IDs. I use a custom syntax to flag which strings ought to be extracted so I can still use regular string literals elsewhere. The processing tool lets me adjust the location of the text in memory, so I can set aside space for sprites, sfx, etc. by limiting the size of the text data region.

  • Text is compressed using LZW with variable-width codes. The processor has a compressor written in Python, and it appends a decoder written in Lua to the cart code. This Tale of Two Cites excerpt is 25,613 bytes, and compresses to 12,457 bytes for storage in the cart data, 48% of its original size.

  • My LZW implementation is designed to allow random access to strings during program execution. LZW is a dictionary-based compression algorithm, and all strings share the same dictionary for efficient packing. The entire corpus is not decompressed all at once into Lua memory. Instead, the lookup dictionary is calculated from the bit stream and retained in Lua memory so that strings can be decoded on the fly as they are accessed. In the cart data, I use a simple binary layout that gives each string a header with information that helps track the code bit width during decompression, and byte-aligns each string's first and last characters.

  • This Tale of Two Cities demo gets close to the Lua memory limit with its dictionary. I maximized the size of the dictionary (7,903 entries) to minimize the size of the compressed data. In practice, I'll probably cap the dictionary size to 4,096 entries, which for this text gains a few kilobytes in cart data. But headroom in Lua RAM will be important for real games.

  • The slow scroll of the text in this reader app is artificial, originally intended for use in a text game. Decompression is quite fast after the initial dictionary is built. I have limited interest in making a usable ebook reader cart, but you're welcome to try it. This implementation uses only 292 tokens and 5017 chars, and that could probably be tightened up a bit.

  • This excerpt is 4,634 words. For comparison, Zork I is 14,214 words. Considering Zork had the luxury of paging from a 160k floppy disk and this is packed into a 16k region of cart data, that's not too shabby. :)

I don't know yet if this will actually be useful for a game project, but it was fun to make. The complete code is not ready for public consumption, but here's the Github link anyway: https://github.com/dansanderson/p8advent It's based on and requires picotool.

Happy reading!

-- Dan

P#16673 2015-11-15 05:50

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