62 lines
1.9 KiB
Scheme
62 lines
1.9 KiB
Scheme
(import (srfi :43)) ; vector extensions
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(load "../common.scm")
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;; naive version, as per the example
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; simulate lanterfish growth and returns the number of fishes after given days
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;(define (simulate-growth fishes days)
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; (let days-loop [(day 0) (fishes fishes)]
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; (if (= day days)
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; (length fishes)
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; (days-loop
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; (+ day 1)
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; (let grow [(new '()) (old fishes)]
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; (if (null? old)
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; new
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; (if (= (car old) 0)
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; (grow (cons 6 (cons 8 new)) (cdr old))
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; (grow (cons (- (car old) 1) new) (cdr old)))))))))
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;; optimized version
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;; instead of keeping track of fishes
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;; we just keep track of how many are living at each age.
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;; so, the living vector is actually indexed by age
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(define (sum living)
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(vector-fold (lambda (i sum value) (+ sum value)) 0 living))
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; returns a new vector with the new number of fishes at each age
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(define (grow living)
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(let [(new (make-vector 9 0))]
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(vector-for-each
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(lambda (age count)
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(if (= age 0)
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(begin
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(vector-set! new 8 count)
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(vector-set! new 6 count))
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(vector-set! new (- age 1)
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(+ count (vector-ref new (- age 1))))))
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living)
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new))
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; simulate lanterfish growth and returns the number of fishes after given days
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(define (simulate-growth fishes days)
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(let [(living-per-day (make-vector 9 0))]
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(let init-loop [(fishes fishes)]
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(when (not (null? fishes))
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(vector-set! living-per-day (car fishes)
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(+ 1 (vector-ref living-per-day (car fishes))))
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(init-loop (cdr fishes))))
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(let loop [(day 0)
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(living-per-day living-per-day)]
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(if (= day days)
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(sum living-per-day)
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(loop (+ day 1) (grow living-per-day))))))
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(call-with-input-file
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"input"
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(lambda (file)
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(let [(input (read-comma-separated-numbers file))]
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(printf "part 1:~% count after 80 days: ~a~%"
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(simulate-growth input 80))
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(printf "part 2:~% count after 256 days: ~a~%"
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(simulate-growth input 256)))))
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