Scheme/Guile
Takes about 5 seconds.
(import (rnrs io ports (6))
(srfi srfi-1))
#!curly-infix
(define (parse-file file-name)
(let ((sequence (map string->number (string-split (string-trim-both (call-with-input-file file-name get-string-all)) #\,))))
(zip sequence (cdr sequence))))
(let loop ((sequence (parse-file "notes/everybody_codes_e2025_q08_p1.txt")) (count 0))
(if (null? sequence)
(format #t "P1 Answer: ~a\n\n" count)
(loop (cdr sequence) (+ count (if (and last (eq? (modulo (- (cadar sequence) (caar sequence)) 32) 16)) 1 0)))))
(define (crosses-over? a b)
(let ((a1 (car a))
(a2 (cadr a))
(b1 (car b))
(b2 (cadr b)))
(let ((a2 (modulo {a2 - a1} 256))
(b1 (modulo {b1 - a1} 256))
(b2 (modulo {b2 - a1} 256)))
(and (not (eq? b1 0)) (not (eq? b2 0))
(or
(and {b1 < a2} {b2 > a2})
(and {b1 > a2} {b2 < a2}))))))
(define (count-cross-overs sequence a)
(let loop ((sequence sequence) (count 0))
(if (null? sequence)
count
(loop (cdr sequence) (+ count (if (crosses-over? (car sequence) a) 1 0))))))
(let loop ((sequence (parse-file "notes/everybody_codes_e2025_q08_p2.txt")) (passed '()) (count 0))
(if (null? sequence)
(format #t "P2 Answer: ~a\n\n" count)
(loop (cdr sequence) (cons (car sequence) passed) (+ count (count-cross-overs passed (car sequence))))))
(let ((sequence (parse-file "notes/everybody_codes_e2025_q08_p3.txt")))
(let loop ((i 1) (greatest 0))
(if {i > 256}
(format #t "P3 Answer: ~a\n\n" greatest)
(loop (1+ i) (max greatest (let loop ((j i) (greatest 0))
(if {j > 256}
greatest
(loop (1+ j) (max greatest (count-cross-overs sequence (list i j)))))))))))