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Country-coding from Lat/Long, using GeoPoly and/or R-Tree
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I got the data from GitHub.  There is a lot of polygon files wafting around the Internet.  Many of them claim varying levels of GeoJSON compliance.

https://github.com/evansiroky/timezone-boundary-builder/releases

is where I found the geojson files I used.  The geojson files contained multiple polygons and sometimes multiple holes so they required a bit of pre-processing.