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Another approach for this is to explore the format through Apple's tools for building dictionaries – as they provide a "Dictionary Development Kit" in Xcode's downloadable "Additional Tools" package (which has documentation for the XML format and a bunch of scripts/binaries for building the bundle).

I wound up doing this a while ago for a similar toy project. After some poking around, it turned out that dictionary bundles are entirely supported by system APIs in CoreServices! The APIs are private, but Apple accidentally shipped a header file with documentation for them in the 10.7 SDK [1]. You can load a dictionary with `IDXCreateIndexObject()`, read through its indices with the search methods (and the convenient `kIDXSearchAllMatch`), and get pointers to its entry data with `IDXGetFieldDataPtrs()`.

It takes a bit of fiddling to figure out the structure (there are multiple indices for headwords, search keywords, cross-references, etc., and the API is a general-purpose trie library) and request the right fields, but those property lists in the bundle are there to help! (As the author of this article discovered, the entries are compressed and are proceeded with a 4-byte length marker.)

[1] https://github.com/phracker/MacOSX-SDKs/blob/master/MacOSX10...

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I have memories of using the Dictionary Development Kit to create custom dictionaries (I remember creating one for medical jargon) about ten years ago. (At that time custom dictionaries are placed in ~/Library/Dictionaries, and system dictionaries in /System/Library/Dictionaries, not some obfuscated path like now.)

To find the Kit in question, simply Google "com.apple.TrieAccessMethod" and you should find it online.