The XMPP site has a document describing a process for deterministically mapping strings to colors with [XEP-0392](https://xmpp.org/extensions/xep-0392.html#algorithm-genpalette). I've had this document saved in my notes for two years now and I haven't really had a good excuse to apply it until now.
I won't get too detailed here but the short version is
- I've got a spreadsheet of addresses.
- Each entry has a category and I'm turning that spreadsheet into a KML document.
- I want each KML placemark to be colored according to its category but I don't want to manually assign colors to categories. I don't have much of an eye for design and categories might change over time.
All of this adds up to finding a way to programmatically turn a string into a color.
I made an attempt at implementing XEP-0392 verbatim but I got walled by two details:
1. Go doesn't have any HSL handling in the standard library.
2. I don't understand HSLuv well enough to parse the instructions for converting the SHA1 hash to a color. HSL is comprised of 3 components ( hue, saturation, and lightness ) but the XEP-0392 document only talks about generating an angle.
I do understand RGB and how hashes work so I was able to synthesize that into a close-enough solution:
`ToRGB` is pretty straightforward, implementing the algorithm described in the previous section. We take the SHA1 of the string, break it into chunks, convert those into `uint64`s to account for the 7 bytes ( 56 bits total ) we're turning into an integer. The blue value overlaps the green by a few bits but that this isn't a security system so that doesn't matter much.
`ToRGBA` functions almost identically, with a few small changes. Instead of 7 byte chunks, we use 5 byte chunks and adjust the `Interpolate` call accordingly for the 40 bit chunks.
