Add an Exercism exercise from Bash track

structured/challenges.exercism.bash.org

@@ -3,11 +3,11 @@
 :END:
 #+title: Exercism track: Bash
 #+date: "2021-05-11 15:06:43 +08:00"
-#+date_modified: "2021-06-05 11:13:09 +08:00"
+#+date_modified: "2021-06-22 17:50:43 +08:00"
 #+language: en
 #+source: https://exercism.io/my/tracks/bash
 #+property: header-args  :cache yes
-#+property: header-args:bash  :shebang "#/usr/bin/env bash"
+#+property: header-args:bash  :shebang "#!/usr/bin/env bash"
 
 
 These are my submitted solutions for the Bash track in Exercism.
@@ -343,3 +343,114 @@ else
     echo "$resistance" ohms
 fi
 #+end_src
+
+
+
+
+* Raindrops
+
+Your task is to convert a number into a string that contains raindrop sounds corresponding to certain potential factors. A factor is a number that evenly divides into another number, leaving no remainder. The simplest way to test if a one number is a factor of another is to use the modulo operation.
+
+The rules of raindrops are that if a given number:
+
+- has 3 as a factor, add 'Pling' to the result.
+- has 5 as a factor, add 'Plang' to the result.
+- has 7 as a factor, add 'Plong' to the result.
+- does not have any of 3, 5, or 7 as a factor, the result should be the digits of the number.
+
+Examples:
+- 28 has 7 as a factor, but not 3 or 5, so the result would be "Plong".
+- 30 has both 3 and 5 as factors, but not 7, so the result would be "PlingPlang".
+- 34 is not factored by 3, 5, or 7, so the result would be "34".
+
+
+** Initial working solution
+
+After finding out that Bash arithmetic expression exists, we'll have to use the modulo operator.
+
+#+begin_src bash  :tangle (my/concat-assets-folder "raindrops.sh")
+n=$1
+valid=0
+function is_factor {
+    local factor=$1
+    shift
+
+    local msg=$@
+
+    [[ $(( $n % $factor )) -eq 0 ]] && printf $msg && valid=1
+}
+
+is_factor 3 "Pling"
+is_factor 5 "Plang"
+is_factor 7 "Plong"
+
+[[ $valid -eq 0 ]] && echo $n || printf '\n'
+#+end_src
+
+
+** Findings after solution
+
+The [[https://exercism.io/my/solutions/04bcb4f6de304cd891afd87f1fcf9830][solution]] was approved by the mentor.
+They did point out a few details such as ~[\[...]]~ being string-oriented.
+You can also use
+
+#+begin_src bash
+# Instead of this which compares numbers...
+n=10
+factor=10
+
+[[ $(( $n % $factor )) -eq 0 ]] && echo "Hello"
+
+# Do it like this...
+(( $n % $factor == 0 )) && echo "Hello"
+#+end_src
+
+#+results[2f32aabc4f71df02cff19faef77296081516089b]:
+: Hello
+: Hello
+
+Secondly, assigning a vector variable (i.e., ~$@~) to a scalar variable can be problematic.
+It is advisable to put the vector variable to another vector variable instead.
+
+#+begin_src bash
+msg=$@ # Wrong
+msg=("$@") # Right
+#+end_src
+
+Apparently, Bash really is a picky interpreter.
+I really like [[https://exercism.io/tracks/bash/exercises/raindrops/solutions/b882430d0b4841a4acaf7c8d5ba24a24][this simple solution]].
+If improved with the tips given by the mentor, this is my improved solution.
+
+#+begin_src bash
+if (( $1 % 3 == 0 )); then
+    result+="Pling"
+fi
+if (( $1 % 5 == 0 )); then
+    result+="Plang"
+fi
+if (( $1 % 7 == 0 )); then
+    result+="Plong"
+fi
+
+echo ${result:-$1}
+#+end_src
+
+But without entirely rewriting my solution, here's the improved solution of it.
+
+#+begin_src bash  :tangle (my/concat-assets-folder "raindrops-improved.sh")
+n=$1
+valid=0
+function is_factor {
+    local factor=$1 msg=$2
+
+    (( n % factor == 0 )) && printf $msg && valid=1
+}
+
+is_factor 3 "Pling"
+is_factor 5 "Plang"
+is_factor 7 "Plong"
+
+(( valid == 0 )) && echo $n || printf '\n'
+#+end_src
+
+Also, the [[https://github.com/koalaman/shellcheck/wiki/SC2124][ShellCheck wiki]] is a pretty handy resource.