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