#!/bin/sh -u
# interruptible parabolas, fast or slow, on any size terminal window
#
#   $0 [ slow ]
#
# This script draws several parabolas.  At any time, you can use
# SIGINT (usually CTRL-C) to interrupt a loop and provide new loop
# values for that parabola.  An argument of "slow" slows down drawing.
#
# Integer Arithmetic Only!
#  Shell scripts only deal in integers. The order in which you do
#  arithmetic matters! let "x=1/80*160" is zero, because "1/80" is zero
#  in integer artithmetic. let "x=160*1/80" is the right answer "2".
#  Always do multiplication before division in integer math.
#
# Terminal Screen Coordinates are Zero-Based
#  The "tput cup" program positions the cursor based on coordinates that
#  range from (0,0) [top left] to (xlimit,ylimit) [bottom right], where
#  xlimit=cols-1 and ylimit=lines-1. (Using "tput $lines $cols" tries to
#  position the cursor too far off the bottom right corner of the
#  screen.) Make sure your display X,Y values lie in the range 0..cols-1
#  and 0..lines-1.  Trying to draw off the edge of the terminal screen
#  will result in a character at the edge of the screen.
#
# 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

# Set speed based on optional command line argument.  Default is fast.
#
if [ "${1-fast}" = 'slow' ] ; then
    sleepstart=3
else
    sleepstart=1
fi

###########################################################################
# Function Definitions
###########################################################################

# function: NewSleep time points
# - time is the number of seconds to draw all the points
# - points is the number of points to be drawn in the time
# - calculates and echoes the floating-point value that will let all the
#   points be drawn in the given amount of time (usable by "sleep")
#
# The "dc" program is a floating-point, reverse-Polish calculator.
# "2 k" sets the precison to 2 decimal places, enough for "sleep".
# "p" prints the current calculation value ($1 divided by $2).
#
NewSleep () {
    # XXX should validate both integer arguments here
    echo 2 k "$1" "$2" / p | dc
}

# function: Prompt (no arguments - SIGINT handler)
#
# This Prompt function is used as the SIGINT trap handler.
# Modifies global variables!
#
# A SIGINT (#2) interrupt lets us over-ride the number being used.
# The number entered becomes the new parabola size and start value.
#
Prompt () {
    # Ignore SIGINT while prompting for user input.
    # Position the cursor at the top of the screen for prompting.
    # Clear the screen from cursor to the end of the line.
    # Make the cursor visible.
    # Read the user's input; quick-exit on EOF.
    # Make the cursor invisible.
    # Clear away the prompt and user's input.
    # Set the new program values.
    # XXX much necessary error checking of user input is missing here!
    # Restore the SIGINT trap handler.
    #
    trap '' 2             # ignore SIGINT
    tput cup 0 0
    tput el               # clear from cursor to end-of-line
    echo -n "Enter new num: "
    tput cnorm            # make cursor visible
    read ans || exit 0
    tput civis            # make cursor invisible
    tput cup 0 0
    tput el               # clear from cursor to end-of-line
    let num=ans           # XXX missing input validation here!
    let i=-num
    trap 'Prompt' 2       # reset SIGINT trap handler
}

# function: TerminalInit  (no arguments)
#
# Initialize the terminal.  Sets global variables!
#
TerminalInit () {
    # Fetch the current size of the terminal screen.
    # Create constants for the maximum X and Y terminal output values.
    #
    cols=$( tput cols )
    lines=$( tput lines )
    let xlimit=cols-1
    let ylimit=lines-1

    # Set the SIGINT trap (some shells allow INT instead of 2; 2 is portable)
    #
    trap 'Prompt' 2

    # Clear the terminal screen.
    # Turn off the cursor.
    #
    tput clear
    tput civis            # make cursor invisible
}

###########################################################################
# End of Function Definitions
###########################################################################

# Initialize the terminal.  Sets global variables!
#
TerminalInit

# Output some debugging info just above the last line of the screen.
# - Remember: tput is zero based and runs from 0..xlimit and 0..ylimit
#
let upone=ylimit-1
tput cup $upone 0
tput el   # clear from cursor to end-of-line
echo -n "cols $cols lines $lines, xlimit $xlimit ylimit $ylimit"

#--------------------------------------------------------------------------
# First Parabola
#--------------------------------------------------------------------------

# Run down the Y axis plotting X = Y*Y
# - make sure display X and Y are positive and lie on terminal screen
# - Remember: tput is zero based and runs from 0..xlimit and 0..ylimit
#
let num=lines/2-1
let i=-num
sleep=$( NewSleep "$sleepstart" "$num" )
while [ "$i" -lt "$num" ] ; do
    let dispy=i+num
    let dispx=i*i*xlimit/num/num
    tput cup "$dispy" "$dispx" ; echo -n '*'
    let i=i+1
    sleep "$sleep"
done

#--------------------------------------------------------------------------
# Second Parabola Pair
#--------------------------------------------------------------------------

# Run across the X axis plotting both Y = X*X and Y = LINES - X*X
# - make sure display X and Y are positive and lie on terminal screen
# - Remember: tput is zero based and runs from 0..xlimit and 0..ylimit
#
let num=cols/2-1
let i=-num
sleep=$( NewSleep "$sleepstart" "$num" )
while [ "$i" -lt "$num" ] ; do
    let dispx=i+num
    let dispy=i*i*ylimit/num/num
    # plot the main parabola (which comes out upside-down)
    tput cup "$dispy" "$dispx" ; echo -n '*'
    # plot a mirror-image parabola (right-side-up)
    let yinverse=ylimit-dispy
    tput cup "$yinverse" "$dispx" ; echo -n '+'
    let i=i+1
    sleep "$sleep"
done

# Move cursor to two lines above last line on screen.
# Switch to "bold" terminal font.
# Say goodbye.
# Switch to "normal" terminal font.
# Move cursor to start of last line.
# Make the cursor visible again.
# Exit.
#
let uptwo=ylimit-2
tput cup $uptwo 0
tput bold
echo -n "Done parabolas."
tput sgr0
tput cup $ylimit 0
tput cnorm            # make cursor visible