Fix crash in #103

[sgn]

See #103 and void-linux/void-packages#39624

[tsjensen]

Thank you for providing us with a fix! 👍

Before I merge, I'd like to understand the change a little better, if that's OK with you. In the call to u32_move(), you changed the number of characters moved. It is calculated as input.lines[j].num_chars - input.lines[j].posmap[c] + 1, where num_chars is in characters, but posmap[c] is in bytes. How does that result in a valid number? Or is my assumption about num_chars incorrect?

[sgn]

Your assumption about num_chars is correct. It's counted by characters. However, posmap[c] is in characters, too.
Quoting your code:

boxes/src/boxes.in.h

Line 146 in 7de6854

size_t *posmap; /* for each character in `text`, position of corresponding char in `mbtext`. Needed for box removal. */

[tsjensen]

Ah, right, I get it now. I literally wrote this in the previous millennium, so bear with me. 😊

posmap gives the position in mbtext for each visible character. All the invisible characters, like ANSI color codes, are ignored. The full posmap looks like this:

In this test case, we want to remove 3 spaces from the beginning of the string, which means that we must remove 55 characters, including lots of invisible ones. So, as you said from the start, we were moving too many characters.