#!/bin/bash
# stack.sh: push-down stack simulation

# Similar to the CPU stack, a push-down stack stores data items
#+ sequentially, but releases them in reverse order, last-in first-out.

BP=100 # Base Pointer of stack array.
# Begin at element 100.

SP=$BP # Stack Pointer.
# Initialize it to "base" (bottom) of stack.

Data= # Contents of stack location.
# Must use local variable,
#+ because of limitation on function return range.

declare -a stack


push() # Push item on stack.
{
if [ -z "$1" ] # Nothing to push?
then
return
fi

let "SP -= 1" # Bump stack pointer.
stack[$SP]=$1

return
}

pop() # Pop item off stack.
{
Data= # Empty out data item.

if [ "$SP" -eq "$BP" ] # Stack empty?
then
return
fi # This also keeps SP from getting past 100,
#+ i.e., prevents a runaway stack.

Data=${stack[$SP]}
let "SP += 1" # Bump stack pointer.
return
}

status_report() # Find out what's happening.
{
echo "-------------------------------------"
echo "REPORT"
echo "Stack Pointer = $SP"
echo "Just popped \""$Data"\" off the stack."
echo "-------------------------------------"
echo
}


# =======================================================
# Now, for some fun.

echo

# See if you can pop anything off empty stack.
pop
status_report

echo

push garbage
pop
status_report # Garbage in, garbage out.

value1=23; push $value1
value2=skidoo; push $value2
value3=FINAL; push $value3

pop # FINAL
status_report
pop # skidoo
status_report
pop # 23
status_report # Last-in, first-out!

# Notice how the stack pointer decrements with each push,
#+ and increments with each pop.

echo
# =======================================================


# Exercises:
# ---------

# 1) Modify the "push()" function to permit pushing
# + multiple element on the stack with a single function call.

# 2) Modify the "pop()" function to permit popping
# + multiple element from the stack with a single function call.

# 3) Using this script as a jumping-off point,
# + write a stack-based 4-function calculator.

exit 0