#!/bin/bash
#
# A interactive "bc" calculator with 8 decimal places and error handling.
#
# It is an example of setting up a co-process with completely separate handling
# of stdin, stdout, and stderr channels.  Basically this example completely
# wrappers the "bc" program allowing for full program control without lockups.
#
# This requires an small perl program "shell_select" to allow the shell script
# to access the select() library function. You can download it from...
#    https://antofthy.gitlab.io/software/#shell_select
#
# Try it with this example input.
#   a = 3
#   10 / a
#   some error
#   print "some output\n"
#
####
#
# NOTE: Without the '-t 0.0001' in the shell "read" builtins below,
# a print without a newline will lockup the program!  In that event
# you will not get a complete line from the read!
#
# Anthony Thyssen <Anthony.Thyssen@gmail.com>
#
mkfifo /tmp/bc_in.$$ /tmp/bc_out.$$ /tmp/bc_err.$$

bc </tmp/bc_in.$$ >/tmp/bc_out.$$ 2>/tmp/bc_err.$$ &

bc_pid=$!  bc_in=10  bc_out=11  bc_err=12
eval "exec  $bc_in>/tmp/bc_in.$$"
eval "exec  $bc_out</tmp/bc_out.$$"
eval "exec  $bc_err</tmp/bc_err.$$"
rm -f /tmp/bc_in.$$ /tmp/bc_out.$$ /tmp/bc_err.$$    # named pipes done

#trap 'echo SIGCHLD; exit 10'  SIGCHLD
trap 'echo SIGPIPE; exit 10'  SIGPIPE

while read -r -p "bc expr> " user_input; do    # read request from user

  # send user input to "bc" with -- assuming it is always okay to send
  echo >&$bc_in "scale=8; $user_input"

  # wait for any output, or timeout,  -- sets $rd_ready
  eval `shell_select -t 1 -r $bc_out,$bc_err`  # wait/timeout
  #echo "rd_ready = $rd_ready"

  if kill -0 $bc_pid 2>/dev/null; then         # has co-processor exited?
    : all is fine
  else
    wait $bc_pid
    status=$?
    echo "BC EXIT"
    exit $status
  fi

  if [ -z "$rd_ready" ]; then                  # any output ready to read?
    echo >&2 "TIMEOUT: no results for expression"
    continue
  fi

  # handle results (stdout) or errors (stderr) from appropriate source
  while true; do
    case ",$rd_ready," in
      *,$bc_out,*) read -r -t 0.0001 -u $bc_out result
                   echo "Result => $result"
                   ;;
      *,$bc_err,*) read -r -t 0.0001 -u $bc_err error
                   echo "ERROR => $error"
                   ;;
      *) break ;;  # no valid file descriptor - end of output
    esac
    eval `shell_select -t 0 -r $bc_out,$bc_err`   # poll for more output
    #echo "rd_ready = $rd_ready"
  done

done
echo "NO MORE USER INPUT"
echo >&$bc_in "quit"
wait $bc_pid
exit "$?"