require 'sys/syscall.ph';  # for subsecond timing
            my $option = $ARGV[0] || ""; # simple option handling
            # filename for hdaps sensor reads
            my $hdapsFN = "/sys/devices/platform/hdaps/position";
            my $UPDATE_THRESHOLD =   4;      # threshold of force that indicates a knock
            my $INTERVAL_THRESHOLD = 100000; # microseconds of time required between knock
            # events
            my $SLEEP_INTERVAL =     0.01;   # time to pause between hdaps reads
            my $MAX_TIMEOUT_LENGTH = 4;      # maximum length in seconds of knock pattern
            # length
            my $MAX_KNOCK_DEV =      100000; # maximum aclearcase/" target="_blank" >cceptable deviation between recorded
            # pattern values and knocking values
            my $LISTEN_TIMEOUT =     2;      # timeout value in seconds between knock
            # events when in listening mode
            

my @baseKnocks = ();             # contains knock intervals currently entered
            my %knockHash = ();              # contains knock patterns, associated commands
            my $prevInterval =       0;      # previous interval of time
            my $knockCount =         0;      # current number of knocks detected
            my $restX = 0; # `resting' positiong of X axis accelerometer
            my $restY = 0; # `resting' positiong of Y axis accelerometer
            my $currX = 0; # current position of X axis accelerometer
            my $currY = 0; # current position of Y axis accelerometer
            my $lastX = 0; # most recent position of X axis accelerometer
            my $lastY = 0; # most recent position of Y axis accelerometer
            my $startTime = 0;  # to manage timeout intervals
            my $currTime  = 0;  # to manage timeout intervals
            my $timeOut   = 0;  # perpetual loop variable
            my $knockAge  = 0;  # count of knocks to cycle time interval
            

sub checkAccelerometer() {
            my $ret;
            $ret = readPosition ();
            if( $ret ){
            print "no accelerometer data available - tis bork ed\n";
            exit(1);
            }
            }#checkAccelerometer
            

## comments from Jeff Molofee in hdaps-gl.c
            #* read_position - read the (x,y) position pair from hdaps.
            #*
            #* We open and close the file on every invocation, which is lame but due to
            #* several features of sysfs files:
            #*
            #*  (a) Sysfs files are seekable.
            #*  (b) Seeking to zero and then rereading does not seem to work.
            ##
            sub readPosition() {
            my ($posX, $posY) = "";
            my $fd = open(FH," $hdapsFN");
            while( <FH> ){
            s/\(//g;
            s/\)//g;
            ($posX, $posY) = split ",";
            }# while read
            close(FH);
            return( $posX, $posY );
            }#readPosition
            

sub getEpochMicroSeconds {
            my $TIMEVAL_T = "LL";      # LL for microseconds
            my $timeVal = pack($TIMEVAL_T, ());
            syscall(&SYS_gettimeofday, $timeVal, 0) != -1 or die "micro seconds: $!";
            my @vals =  unpack( $TIMEVAL_T, $timeVal );
            $timeVal = $vals[0] . $vals[1];
            $timeVal = substr( $timeVal, 6);
            my $padLen =  10 - length($timeVal);
            $timeVal = $timeVal . "0" x $padLen;
            return($timeVal);
            }#getEpochMicroSeconds
            sub getEpochSeconds {
            my $TIMEVAL_T = "LL";      # LL for microseconds
            my $start = pack($TIMEVAL_T, ());
            syscall(&SYS_gettimeofday, $start, 0) != -1 or die "seconds: $!";
            return( (unpack($TIMEVAL_T, $start))[0] );
            }#getEpochSeconds
            

sub knockListen() {
            my $checkKnock = 0;
            ($currX, $currY) = readPosition();
            $currX -= $restX;  # adjust for rest data state
            $currY -= $restY;  # adjust for rest data state
            # require a high threshold of acceleration to ignore non-events like
            # bashing the enter key or hitting the side with the mouse
            if( abs ($currX) > $UPDATE_THRESHOLD) {
            $checkKnock = 1;
            }
            if( abs ($currY) > $UPDATE_THRESHOLD) {
            $checkKnock = 1;
            }
            if( $checkKnock == 1 ){
            my $currVal = getEpochMicroSeconds();
            my $diffInterval = abs($prevInterval - $currVal);
            # hard knock events can create continuous acceleration across a large time
            # threshold.  requiring an elapsed time between knock events effectively
            # reduces what appear as multiple events according to sleep_interval and
            # update_threshold into a singular event.
            if(  $diffInterval > $INTERVAL_THRESHOLD ){
            if( $knockCount == 0 ){ $diffInterval = 0 }
            if( $option ){
            print "Knock: $knockCount ## last: [$currVal] curr: [$prevInterval] ";
            print "difference is: $diffInterval\n";
            }
            push @baseKnocks, $diffInterval;
            $knockCount++;
            }# if the difference interval is greater than the threshold
            $prevInterval = $currVal;
            }#if checkknock passed
            }#knockListen
            

sub readKnockFile {
            open(KNCKFILE,"$ENV{HOME}/.knockFile") or die "no knock file: $!";
            while(<KNCKFILE>){
            if( !/^#/ ){
            my @arrLine = split "_#_";
            $knockHash{ $arrLine[0] }{ cmd }     = $arrLine[1];
            $knockHash{ $arrLine[0] }{ comment } = $arrLine[2];
            }#if not a comment line
            }#for each line in file
            close(KNCKFILE);
            }#readKnockFile
            

sub compareKnockSequences {
            my $countMatch = 0;  # record how many knocks matched
            # for each knock sequence in the config file
            for( keys %knockHash ){
            # get the timings between knocks
            my @confKnocks = split;
            # if the count of knocks match
            if( $knockCount eq @confKnocks ){
            my $knockDiff = 0;
            my $counter = 0;
            for( $counter=0; $counter<$knockCount; $counter++ ){
            $knockDiff = abs($confKnocks[$counter] - $baseKnocks[$counter]);
            my $knkStr = "k $counter b $baseKnocks[$counter] ".
            "c $confKnocks[$counter] d $knockDiff\n";
            # if it's an exact match, increment the matching counter
            if( $knockDiff < $MAX_KNOCK_DEV ){
            if( $option ){ print "MATCH $knkStr" }
            $countMatch++;
            # if the knocks don't match, move on to the next pattern in the list
            }else{
            if( $option ){ print "DISSONANCE $knkStr" }
            last;
            }# deviation check
            }#for each knock
            }#if number of knocks matches
            # if the count of knocks is an exact match, run the command
            if( $countMatch eq @confKnocks ){
            my $cmd = system( $knockHash{"@confKnocks "}{ cmd } );
            if( $option ){ print "$cmd\n" }
            last;
            # otherwise, make the count of matches zero, in order to not reset
            }else{
            $countMatch = 0;
            }
            }#for keys
            # if the match count is zero, exit and don't reset variables so a longer
            # knock sequence can be entered and checked
            if( $countMatch == 0 ){ return() }
            # if a match occurred, reset the variables so it won't match another pattern
            $knockCount = 0;
            @baseKnocks = ();
            }#compareKnockSequences
            

if( $option eq "-c" ){
            print "create a knock pattern:\n";
            $startTime = getEpochSeconds();  # reset time out start
            while( $timeOut == 0 ){
            $currTime = getEpochSeconds();
            # check if there has not been a knock in a while
            if( $currTime - $startTime > $MAX_TIMEOUT_LENGTH ){
            $timeOut = 1;  # exit the loop
            }else{
            # if a knock has been entered before timeout, reset timers so
            # more knocks can be entered
            if( $knockCount != $knockAge ){
            $startTime = $currTime;   # reset timer for longer delay
            $knockAge = $knockCount;  # synchronize knock counts
            }# if a new knock came in
            }# if timer not reached
            knockListen();
            select(undef, undef, undef, $SLEEP_INTERVAL);
            }#timeOut =0
            if( @baseKnocks ){
            print "place the following line in $ENV{HOME}/.knockFile\n\n";
            for( @baseKnocks ){ print "$_ " }
            print "_#_ (command here) _#_ <comments here>\n\n";
            }#if knocks entered
            

}else{
            # main code loop to listen for knocking and run commands
            readKnockFile();
            $startTime = getEpochSeconds();
            while( $timeOut == 0 ){
            $currTime = getEpochSeconds();
            if( $currTime - $startTime > $LISTEN_TIMEOUT ){
            $knockCount = 0;
            @baseKnocks = ();
            $startTime = $currTime;
            if( $option ){ print "listen timeout - resetting knocks \n" }
            }else{
            if( $knockCount != $knockAge ){
            $startTime = $currTime;   # reset timer for longer delay
            $knockAge = $knockCount;  # synchronize knock counts
            }# if a new knock came in
            compareKnockSequences();
            }#if not reset timeout
            knockListen();
            select(undef, undef, undef, $SLEEP_INTERVAL);
            }#main knock listen loop
            }# if create or listen for knocks