initial commit

bash/atbash-cipher/atbash_cipher.sh

@@ -0,0 +1,108 @@
+#!/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: