Janet Exercism: Reverse a string

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Blog 2020/8/4

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Janet Exercism: Reverse a string

Here are Janet solutions (recursive and iterative) to the exercism "reverse a string" problem.

The rules are that you are not allowed to use any built-in reverse functions.

Makefile

default: test-all bin-all

test-all: test-recur test-iter

test-recur:
        ./rev-str-recur.janet

test-iter:
        ./rev-str-iter.janet

bin-all: rev-str-recur rev-str-iter

rev-str-recur: rev-str-recur.janet
        jpm quickbin rev-str-recur.janet rev-str-recur

rev-str-iter: rev-str-iter.janet
        jpm quickbin rev-str-iter.janet rev-str-iter

clean:
        rm -f rev-str-recur.c rev-str-recur rev-str-iter rev-str-iter.c

.PHONY: default test-all test-recur test-iter bin-all clean

rev-str-iter.janet

#!/usr/bin/env janet

# reverse a string, without using the built-in reverse functions.

(defn rev-str-iter [s]
  (var b @"")
  (var i (length s))
  (while (> i 0)
    (buffer/push-byte b (get s (- i 1)))
    (-- i))
  b)

(defn main [& args]
  (print (rev-str-iter ""))
  (print (rev-str-iter "a"))
  (print (rev-str-iter "ab"))
  (print (rev-str-iter "abc")))

rev-str-recur.janet

#!/usr/bin/env janet

# reverse a string, without using the built-in reverse functions.

# conceptually, what we want is this:
# (defn reverse [s]
#  (join (last s) (reverse (all-but-last s))))

(defn reverse-str [s]
  (defn join [& ss]
    (string (splice ss)))
  (defn last [s]
    (string/slice s -2 -1))
  (defn all-but-last [s]
    (string/slice s 0 -2))

  (if (= 0 (length s))
    ""
    (join (last s) (reverse-str (all-but-last s)))))


(defn main [& args]
  (print (reverse-str ""))
  (print (reverse-str "a"))
  (print (reverse-str "ab"))
  (print (reverse-str "abc")))