Merkle tree implementation in Racket

merkle-tree.rkt

#lang racket
(require openssl/sha1)

(struct node (value left right) #:transparent)

(define (hash s)
  (sha1 (open-input-string s)))

(define (build-foundation l [hash hash])
  (let loop ([l l])
    (if (null? l)
        null
        (if (= 1 (length l))
            (list (node (hash (first l)) null null))
            (let ([vals (take l 2)])
              (cons (node (hash (string-append (first vals) (second vals)))
                          (node (hash (first vals)) null null)
                          (node (hash (second vals)) null null))
                    (loop (drop l 2))))))))

(define (build-next-node a b)
  (node (hash (string-append (node-value a) (node-value b)))
        a
        b))

(define (merkle-tree l [hash hash])
  (define (aux l)
    (if (= 1 (length l))
        l
        (aux (let loop ([l l])
               (if (null? l)
                   null
                   (if (= 1 (length l))
                       (list (node (hash (node-value (first l))) null null))
                       (let ([vals (take l 2)])
                         (cons (build-next-node (first vals) (second vals)) (loop (drop l 2))))))))))
  (first (aux (build-foundation l hash))))

(module+ test
  (define test-1 (build-list 10000 (lambda (i) (number->string i))))
  (merkle-tree test-1))