Some logic and planning help requested

[gin]

I'm building a scheduler, something along the lines of Google Calendar, where a user can input events and generally plan their day. One of the features I wish to add is a check for time clashing, where if two events overlap a small image alert will display.

What I currently have is like so:

1) query db and loop thru results
2) for each and every event, query db again and look for clashes
3) if there are clashes, add image to $output
4) echo $output

The actual problem is the query within a loop, which is an awful example of coding, I know. Is there some kind of... uh, funky recursive loop or something... that I can use? Any advise appreciated.

This is the actual code, if anyone's interested:
[code]
<?php
// sort of pseudo code
// Get all events happening in a day.
// For each event check if another event clashes with it, then output.

$query = "SELECT * FROM events WHERE date='$today'";
$result = mysql_query($query) or die ("Query failed: " . mysql_error());
if ($myrow = mysql_fetch_array($result)) {
do {
$this_id = $myrow['id'];
$ts = $myrow['timestart'];
$te = $myrow['timeend'];

$query2 = "SELECT id,timestart,timeend FROM events WHERE date='$today' AND id<>$this_id";
$result2 = mysql_query($query2) or die ("Query failed: " . mysql_error());
while ($myrow2 = mysql_fetch_array($result2)) {
$ts2 = $myrow['timestart'];
$te2 = $myrow['timeend'];

// if event2 starts and ends totally before or after event1, then no clash
if ( ($te2<=$ts) || ($ts2>=$te) ) {
// no clash
} else { // clash!
$clash[$j] = $myrow['id'];
$j++;
}
}
if ($j>0)
$clash = '<img src="clash.gif">';

echo $clash.' event details here'; // output event details

} while ($myrow = mysql_fetch_array($result));
}
?>
[/code]

Edit: Edited for clarity.

[gin]

bump

[btherl]

Depending on the granularity of your times, you can use an associative array to record which events are at which times, like

[code=php:0]$arr['20:05'] = 'event1';
$arr['20:10'] = 'event1';[/code]


And so on.. then, if any event needs to go in an array slot that's used, you know there's a clash.  This approach requires only one pass through the events.  But if you're allowing fine-grained times, it may be cumbersome to make all the array entries.

[gin]

I'm not entirely sure what you mean by granularity but I think I get it. Users can only select times at half hour intervals, incidentally.
So one pass to create an $output array and a $clash array, then loop thru the $output array while checking with the $clash array whether to output the additional image, right?

Wow, still something of a headache, but doable now. Thank, I'll try this!

[btherl]

Yep, exactly :)  I think you get what I mean by granularity.  You're using half-hour granularity, and I used 5 minutes in my example.

I use this kind of technique a lot.. instead of looping over an array multiple times, I make an associative array where the array index is exactly what I'm looking for.

If I want to check if a string is in an array (and will be doing it several times), then I do this:

[code=php:0]$arr = array(
  'string1' => true,
  'string2' => true,
....
  'string100' => true,
);

if ($arr[$test_string] === true) {
  print "Matched $test_string\n";
}[/code]

That's much faster than checking against all of 100 array entries for every string you want to check.  It only works for exact matches though.