Yes, understood. But if you need a check-sum, it does not have to be a check-digit. ISBN uses a check-digit, and to achieve its base-11 (first prime after 10), they need only add the symbol X to their alphabet, and it will only appear in the last place.
Crockford does a similar thing, but because 37 is five more than his 32 “digits” (0 - 9, A - Z, less I, L, O, and U), he adds five new symbols to his alphabet, all five of which can only appear in the last position: *, ~, $, =, and U/u. That seems overkill. There are lots of alternatives if you don’t insist on a single digit. One possibility, use the U, which was excluded not because it was easily confused with a digit, as were I, L, and O, but mostly because he needed to remove one more, and I guess he was worried about people saying FU too often, which, because he adds it back, can still happen at the end of the word. Here’s one possibility, which uses a U in one of the last two places when you need a check-sum.
| u-first | u-second |
|---|---|
| UA - 0 | AU - 19 |
| UB - 1 | BU - 20 |
| UC - 2 | CU - 21 |
| UD - 3 | DU - 22 |
| UE - 4 | EU - 23 |
| UF - 5 | FU - 24 |
| UG - 6 | GU - 25 |
| UH - 7 | HU - 26 |
| UJ - 8 | JU - 27 |
| UK - 9 | KU - 28 |
| UM - 10 | MU - 29 |
| UN - 11 | NU - 30 |
| UP - 12 | PU - 31 |
| UQ - 13 | QU - 32 |
| UQ - 14 | RU - 33 |
| US - 15 | SU - 34 |
| UT - 16 | TU - 35 |
| UV - 17 | VU - 36 |
| UW - 18 |
I don’t know why he insisted on a check-digit.
I’m sure we could get them together. The last state of play was that Jeremy wanted to consider more general-purpose tools to carry information in the URL fragment. Mine would fit in fine, but wouldn’t be the whole story. This POC would certainly fit in as well. I didn’t have the skills at the time to make the necessary changes in the TW codebase, and it got dropped for 5.4. I think my skills have grown enough that I could reasonably consider this now. I’ll try to squeeze out some time.