#!/bin/bash
# Base64 decoder - pure bash function
# <C> 2024 F-Hauri.ch - felix@f-hauri.ch
# Licensed under terms of GPL v3. www.gnu.org
# shellcheck disable=SC2154  # not assigned 'myVar' ??
shopt -s extglob

declare -a B64=( {A..Z} {a..z} {0..9} + / '=' )
declare -Ai 'B64R=()'
for i in "${!B64[@]}"; do B64R["${B64[i]}"]=i%64; done
# shellcheck disable=SC2034 # Unused 'B64R' ??
declare -r B64R
unset B64

b64dec() {
    local _4B _Tail _hVal _v _opt OPTIND
    local -i iFd _24b _ar
    while getopts "av:" _opt; do case $_opt in
	a) _ar=1;; v) _v=${OPTARG};; *) return 1;; esac; done
    shift $((OPTIND-1))
    if [[ $_v ]];then local -n _res=${_v}; else local _res; fi
    if [[ $1 ]]; then	exec {iFd}<<<"$1"	# Open Input FD from string
    else		exec {iFd}<&0	; fi	# Open Input FD from STDIN
    _res=()
    while read -rn4 -u $iFd _4B; do
	if [[ "$_4B" ]]; then
	    _Tail=$_4B
	    _24b=" B64R['${_4B::1}'] << 18 | B64R['${_4B:1:1}'] << 12 |
                   B64R['${_4B:2:1}'] << 6 | B64R['${_4B:3:1}'] "
	    printf -v _hval %02x\  $((_24b>>16)) $((_24b>>8&255)) $((_24b&255))
	    read -ra _hval <<<"$_hval"
	    _res+=("${_hval[@]}")
	fi
    done
    exec {iFd}<&-
    _Tail=${_Tail##*([^=])}
    while [[ $_Tail ]]; do
	unset "_res[-1]"
	_Tail=${_Tail:1}
    done
    ((_ar==0)) && printf -v _res %b "${_res[@]/#/\\x}" && _res=("${_res[0]}")
    [[ -z $_v ]] && echo "${_res[@]}"
}

## Exit here if script is sourced
[[ $0 = "${BASH_SOURCE[0]}" ]] || { true;return 0;}

cat <<"eoUsage"
Usage: b64dec [-v varname] [-a] [string]
    -v VARNAME  Assign result to $VARNAME instead of print them on STDOUT
    -a          Return an array of hexadecimal values for hangling binary
If no "string" are submited as argument, b64dec will wait for input on STDIN.
eoUsage

set -v
echo -n '1 '
b64dec 'dGVzdA=='

echo -n '2 '
b64dec <<<'dGVzdA=='

b64dec -v myVar 'dGVzdA=='
echo "3 $myVar"

b64dec -av myVar 'dGVzdA=='
printf -v str %b "${myVar[@]/#/\\x}"
printf '4 %s <= [%s].\n' "$str" "${myVar[*]}"

lstr='H4sIAAAAAAACA/NIzcnJVyjPL8pJU'
for str in ${lstr}{eQCAEHkqbINAAAA,VTkAgAYI85FDgAAAA==,VRU5AIAj62OVw8AAAA=}; do
    b64dec -v test -a <<<"$str"
    zcat < <(printf %b "${test[@]/#/\\x}")
done

b64dec -av myVar < <( seq 1 100 | column | gzip | base64 )
gzip -d < <(printf %b "${myVar[@]/#/\\x}")