That said, I'm pretty sure that TeX is more than a 1000 lines of code.
This is because most modern languages allow you to write in structural style which is impossible to get wrong without noticing. If you have a loop, you know it repeats for its condition, and does not otherwise. If you have a `then` clause, you know it will only run when the `if` condition is true. If you have a function, you know it's only computed when called.
Compare it to pre-structural languages like Fortran IV or Basic, where you can freely jump into the middle of the loop, inside of what constitutes an undelimited `then` clause, or even into the middle of a function, without "calling" it properly.
This allows for potentially very clever code that saves every last byte and cycle. It also allows for totally mind-boggling, insidious bugs in code we'd now consider plain and foolproof to write.
Loops are sometimes more restricted, but even then, neither C nor Pascal prohibit it (Algol-60 did, curiously), and many languages derived from them that retained goto do the same. Off the top of my head, the only mainstream language I can think of that specifically prohibits goto into inner blocks is C#.
Can someone please tell the clang compiler folks?
“TeX was the first faily large program I had written since 1970; so it was my first nontrivial ‘structured program,’ in the sense that I wrote it while consciously applying the methodology I had learned in the early 70s from Dijkstra, Hoare, Dahl, and others. I foudn that structural programming greatly increased my confidence in the correctness of the code, while the code still existed only on paper. Therefore I could wait until the whole program was written, before trying to debug any of it. This saved lots of time, because I din't have to prepare ‘dummy’ versions of nonexistent modules while testing modules that were already written; I could test everything in its final environment.”
That said, there are still 337 instances of goto in tex.web by my quick and dirty count (grep goto tex.web | grep -v '@' | wc -l), which is perhaps an undercount since there are modules that might be included multiple times thanks to how the WEB system builds its Pascal output and I don't have a Pascal reformatter to make analyzing tex.p manageable.¹
1. The choice to "compress" the Pascal output of tangle is one of the most inexplicable choices made in the development of the TeX ecosystem.