#!/bin/sh -u
# falling object demo showing use of acceleration and deltay variables
#
#    $0 [ deltay [ accel ] ]
#
# This loop demonstrates an object "falling" from the top of the
# screen to the bottom.  Each iteration of the loop causes the
# acceleration of the object to be added to the change in Y ($deltay).
#
# You can start the deltay at the value given on the command line. A
# value of 0 lets the object fall from a standing start. A negative
# value (try -6) causes the object start with upward motion, as if it
# had been thrown up in the air. (Of course you won't see the object
# until it falls back into the range of the terminal screen.)
#
# The accel argument determines how fast the object falls (how strong
# gravity is pulling it down each iteration).  Default is "1".
# Negative acceleration causes the object to be "pushed" up instead of
# falling (and the loop will never finish).  Try this command line:  $0 5 -1
# and watch the object fall down briefly, slow, stop, then go up forever.
#
# NOTE: This script fragment is incomplete - it does not follow all
# the script guidelines set out in "script_style.txt".  It also
# contains many "Instructor-Style" comments that you must not use in
# your own scripts.  See script_style.txt for details.
#
# -IAN! idallen@idallen.ca

PATH=/bin:/usr/bin ; export PATH
LC_COLLATE=C ; export LC_COLLATE
LANG=C ; export LANG
umask 022

cols=$( tput cols )
lines=$( tput lines )
let xlimit=cols-1
let ylimit=lines-1

tput clear

# Set X to display the output values half-way across the terminal
# screen. Object's Y position starts at top of screen with y=0. Current
# change in Y (Delta-Y) starts at value given on command line (default
# is 0) and increases by the value of $accel on each iteration.
# Acceleration can be second argument on command line (default is 1).
# Acceleration is added to $deltay each loop iteration.  Negative
# acceleration will "push" the object up, and the loop will never end.
#
let x=xlimit/2
let y=0
let deltay=${1-"0"}     # default deltay value is 0 if argument $1 is missing
let accel=${2-"1"}      # default accel  value is 1 if argument $2 is missing
while : ; do
    let y=y+deltay
    let deltay=deltay+accel

    # In this script, the display values of X,Y are the same as the
    # program values, except when the program values don't fit on the
    # terminal screen. The display values must be non-negative and fit
    # on terminal screen, so we have code that limits the display values
    # (not the program values!) if the display values go out-of-bounds.
    # The "tput el" clears from cursor to end-of-line.
    #
    let dispx=x
    [ $dispx -lt 0 ] && let dispx=0
    [ $dispx -gt $xlimit ] && let dispx=xlimit
    let dispy=y
    [ $dispy -lt 0 ] && let dispy=0
    [ $dispy -gt $ylimit ] && let dispy=ylimit
    tput cup $dispy $dispx 
    tput el                 # clear from cursor position to end-of-line
    echo -n "*  y=$y dispy=$dispy deltay=$deltay"

    # Break out of the loop when the object hits the bottom line.
    # If $accel is negative, this may never happen!
    #
    [ $dispy -ge $ylimit ] && break

    sleep 0.5
done

# Put the cursor at the start of the last line on the screen.
#
tput cup $ylimit 0