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bash/atbash-cipher/README.md

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+# Atbash Cipher
+
+Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
+
+The Atbash cipher is a simple substitution cipher that relies on
+transposing all the letters in the alphabet such that the resulting
+alphabet is backwards. The first letter is replaced with the last
+letter, the second with the second-last, and so on.
+
+An Atbash cipher for the Latin alphabet would be as follows:
+
+```text
+Plain:  abcdefghijklmnopqrstuvwxyz
+Cipher: zyxwvutsrqponmlkjihgfedcba
+```
+
+It is a very weak cipher because it only has one possible key, and it is
+a simple monoalphabetic substitution cipher. However, this may not have
+been an issue in the cipher's time.
+
+Ciphertext is written out in groups of fixed length, the traditional group size
+being 5 letters, and punctuation is excluded. This is to make it harder to guess
+things based on word boundaries.
+
+## Examples
+
+- Encoding `test` gives `gvhg`
+- Decoding `gvhg` gives `test`
+- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`
+
+
+Run the tests with:
+
+```bash
+bats atbash_cipher_test.sh
+```
+
+After the first test(s) pass, continue by commenting out or removing the
+`[[ $BATS_RUN_SKIPPED == true ]] || skip` 
+annotations prepending other tests.
+
+To run all tests, including the ones with `skip` annotations, run:
+
+```bash
+BATS_RUN_SKIPPED=true bats atbash_cipher_test.sh
+```
+
+## Source
+
+Wikipedia [http://en.wikipedia.org/wiki/Atbash](http://en.wikipedia.org/wiki/Atbash)
+
+
+## External utilities
+`Bash` is a language to write "scripts" -- programs that can call
+external tools, such as
+[`sed`](https://www.gnu.org/software/sed/),
+[`awk`](https://www.gnu.org/software/gawk/),
+[`date`](https://www.gnu.org/software/coreutils/manual/html_node/date-invocation.html)
+and even programs written in other programming languages, 
+like [`Python`](https://www.python.org/).
+This track does not restrict the usage of these utilities, and as long
+as your solution is portable between systems and does not require
+installation of third party applications, feel free to use them to solve
+the exercise.
+
+For an extra challenge, if you would like to have a better understanding
+of the language, try to re-implement the solution in pure `Bash`,
+without using any external tools. Note that there are some types of
+problems that bash cannot solve, such as performing floating point
+arithmetic and manipulating dates: for those, you must call out to an
+external tool.
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others
+have completed the exercise.

bash/atbash-cipher/atbash_cipher.sh

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+#!/usr/bin/env bash
+#
+# External tools: none.
+# Subshell: no.
+#
+# Tested on: bash 5.0, 3.2
+#
+# V1: initial version
+# V2: changed +() to @() in args check. Fixed typo on lines 48-52 formulas.
+
+shopt -s extglob
+
+# set to mask to enable logs. 0: none, 255: all
+(( debug=2#00000000 ))
+#(( debug=2#00001111 ))
+# $1: log level (mask), then strings to display.
+debug () {
+    (( debug & $1 )) && shift && echo Line ${BASH_LINENO[0]}: "${@}" >&2
+}
+
+die () {
+    echo "${@}" >&2
+    exit 1
+}
+usage() {
+    die "usage: atbash_cypher.sh encode|decode text [...]"
+}
+
+# we will accept the syntax:
+# atbash_cypher.sh encode|decode text [...]
+# in case text is separated as multiple arguments, we will consider a space is
+# in between.
+# example:
+#   atbash_cypher.sh encode "I am happy"
+# will also be accepted as:
+#   atbash_cypher.sh encode I am happy
+
+# basic args check & set all chars to lowercase
+(($# < 2)) || [[ "$1" != @(encode|decode) ]] && usage
+action="$1"
+shift
+string="${*,,}"
+len="${#string}"
+debug 1 "string=$string" "len=$len"
+
+# algorithm we will use:
+# to get rev element c' of c in a consecutive integer n-m list :
+#     c    = n+i                        i is distance from n to c
+#     c'   = m-i                        and also reverse distance from m to c'
+#     ------------
+#     c+c' = n+m                        sum the 2
+#     c'   = n+m-c                      after simplification
+
+# for printf, a '<char> arg is the ascii value of <char>
+# and yes, we should use a constant, any programmer knows ascii value of 'a'
+# and # of letters in alphabet, we could simply set tval to 219
+printf -v aval "%d" "'a"                # 'a' ascii value
+printf -v zval "%d" "'z"                # 'z' ascii value
+printf -v tval "%d" $((aval+zval))      # their sum
+
+# tval=219                              # correct code for this exercise, for me
+
+debug 1 "aval:$aval zval:$zval tval:$tval"
+
+# many options here:
+#   - for encode, add <spaces> in loop, or at the end
+#   - use a table with all alphabet ({a..z}), to avoid multiple printf
+#   - fill acceptable chars only (my choice), insert spaces at end.
+#     I preferred that one to avoid multiple encode/decode tests in loop.
+#   - likely many others
+result=""                               # resulting string, with no spaces
+for ((i=0; i<len; ++i)); do
+    c=${string:$i:1}                    # current char
+    printf -v cval "%d" "'$c"           # $c's ascii value
+    debug 2 "c='$c' cval=$cval"
+
+    case "$c" in
+        [[:alpha:]])                    # rev $c's in alphabet (with hex value)
+            printf -v rval "%x" $((tval - cval))
+            printf -v c "\\x$rval"      # shellcheck unhappy here
+            debug 4 "rev c=$c"
+            ;;
+        [![:digit:]])                   # we avoid 1 case (digits)
+            continue
+            ;;
+    esac
+    result+="$c"
+done
+
+# for encoding, we split in 5 chars groups.
+if [[ $action == encode ]]; then
+    [[ $result =~ ${result//?/(.)} ]]
+    # we use REMATCH here, to avoid loop, but we could end with a space.
+    printf -v result "%c%c%c%c%c " "${BASH_REMATCH[@]:1}"
+fi
+
+debug 1 "result: =${result%% }="
+echo "${result%% }"                     # fix possible REMATCH extra final space
+exit 0
+
+# emacs/vim settings.
+# Local Variables:
+# sh-basic-offset: 4
+# indent-tabs-mode: nil
+# comment-column: 40
+# fill-column: 80
+# End:
+# vim: set tabstop=4 expandtab: