#!/bin/bash

# implementing a stack or a queue in bash.

# push    = pushes an element onto the stack
# show    = shows all elements in the stack
# peek    = shows the last element in the stack
# peekf   = shows the first element in the stack
# pop     = pops the last element off the stack
            
# popf    = pops the first element off the queue
            
# isempty = returns 0 (false) if stack or queue has 
#           elements in it; 1 (true) otherwise
#           example call: 
#           if [[ $(outputstack_isempty) == 1 ]]; 
#           then echo "is empty"; 
#           else echo "is not empty"; 
#           fi; 
          
# size    = size of the stack

outputstack=();

outputstack_push()    { outputstack+=("$@"); }
outputstack_show()    { echo ${outputstack[@]}; }
outputstack_peek()    { echo "${outputstack[-1]}"; }
outputstack_peekf()   { echo $outputstack; }
outputstack_pop()     { unset outputstack[${#outputstack[@]}-1]; }
# the following method is currently buggy in a for loop as of 02-25-2022
outputstack_popf()    { unset outputstack[0]; }
outputstack_isempty() { if (( ${#outputstack[@]} )); then echo 0; else echo 1; fi; }
outputstack_size()    { echo ${#outputstack[@]}; }

# demo usage

outputstack_push mercury;
outputstack_push venus;
outputstack_push earth;

outputstack_show;
if [[ $(outputstack_isempty) == 1 ]]; then echo "is empty"; else echo "is not empty"; fi;
echo its size is $(outputstack_size);
echo .;

echo "------------";
popstring+=$(outputstack_peek)" ";
outputstack_pop;
outputstack_show;
popstring+=$(outputstack_peek)" ";
outputstack_pop;
outputstack_show;
popstring+=$(outputstack_peek)" ";
outputstack_pop;
outputstack_show;
if [[ $(outputstack_isempty) == 1 ]]; then echo "is empty"; else echo "is not empty"; fi;
echo $popstring;