Gas Station Problem in Racket

Racket solution to the gas station problem:

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1. #lang racket
2.  
3. ;; http://programmingpraxis.com/2015/07/17/the-gas-station-problem/
4.  
5. (define (rotate li)
6.   (append (cdr li) (list (car li))))
7.  
8.  
9. (define gallons #(15 8 2 6 18 9 21 30))
10.  
11. (define miles #(8 6 30 9 15 21 2 18))
12.  
13. (define deltas (vector-map - gallons miles))
14.  
15. ;;trivial case with no solution
16. (define deltas-error #(-1 -1 -1 -1 -1 -1 -1))
17.  
18. (define (find-start deltas)
19.   (define len (vector-length deltas))
20.   (define (loop start i sum)
21.     (cond
22.       ;;our return condition
23.       ;;if we've looped around to start,
24.       ;;return start
25.       [(= start (modulo i len))
26.        start]
27.       ;;error condition, there is no solution.
28.       ;;return #f
29.       [(> i (* 2 len))
30.        #f]
31.       ;;reset condition
32.       ;;if sum < 0
33.       ;;start = i. i will loop around to 0.
34.       [(< sum 0)
35.        (loop (modulo i len) (+ 1 i) (vector-ref deltas (modulo i len)))]
36.       [else
37.        (loop start (+ 1 i) (+ sum (vector-ref deltas (modulo i len))))]))
38.   ;;start our loop with some
39.   ;;data to avoid early
40.   ;;termination.
41.   (loop 0 1 (vector-ref deltas 0)))
42.  
43. (find-start deltas)