[exams] Add check and warning for colliding exams

If two (or more) exams share at least one location and their start/end
times overlap, display a warning to the user.

1 files changed, 50 insertions, 2 deletions

diff --git a/modules-available/exams/page.inc.php b/modules-available/exams/page.inc.php
index f42711a2..2bd7a215 100644
--- a/modules-available/exams/page.inc.php
+++ b/modules-available/exams/page.inc.php
@@ -224,8 +224,51 @@ class Page_Exams extends Page
 	{
 		$out = [];
 		$now = time();
+
+		// Pre-compute collisions: exams overlap in time and share at least one location
+		$collisionFlags = [];
+		$parsedExams = [];
 		if (is_array($this->exams)) {
-			foreach ($this->exams as $exam) {
+			foreach ($this->exams as $idx => $exam) {
+				IF ($exam['endtime'] < $now)
+					continue; // Don't care about past exams
+				// Parse location IDs: treat NULL as global (0) to be consistent with permission checks
+				if (empty($exam['locationids'])) {
+					$lids = Location::getAllLocationIds(0);
+				} else {
+					$lids = array_map(function($val) {
+						return Location::getAllLocationIds($val, true);
+					}, explode(',', (string)$exam['locationids']));
+					$lids = array_unique(array_merge(...$lids));
+				}
+				$parsedExams[$idx] = [
+					'start' => (int)$exam['starttime'],
+					'end'   => (int)$exam['endtime'],
+					'lids'  => $lids,
+				];
+				$collisionFlags[$idx] = false;
+			}
+
+			$indices = array_keys($parsedExams);
+			$cnt = count($indices);
+			for ($i = 0; $i < $cnt; $i++) {
+				for ($j = $i + 1; $j < $cnt; $j++) {
+					$a = $parsedExams[$indices[$i]];
+					$b = $parsedExams[$indices[$j]];
+					// Time overlap if startA < endB and startB < endA
+					if ($a['start'] < $b['end'] && $b['start'] < $a['end']) {
+						// Location overlap
+						if (!empty(array_intersect($a['lids'], $b['lids']))) {
+							$collisionFlags[$indices[$i]] = true;
+							$collisionFlags[$indices[$j]] = true;
+						}
+					}
+				}
+			}
+
+			// Build output, attaching visual classes and collision flag
+			$numCollisions = 0;
+			foreach ($this->exams as $idx => $exam) {
 				if ($exam['endtime'] < $now) {
 					$exam['rowClass'] = 'text-muted';
 					$exam['btnClass'] = 'btn-default';
@@ -238,10 +281,15 @@ class Page_Exams extends Page
 				}
 				$exam['starttime_s'] = date('Y-m-d H:i', $exam['starttime']);
 				$exam['endtime_s'] = date('Y-m-d H:i', $exam['endtime']);
+				if ($collisionFlags[$idx] ?? false) {
+					$exam['rowClass'] = 'danger slx-bold';
+					$exam['collision'] = true;
+					$numCollisions++;
+				}
 				$out[] = $exam;
 			}
 		}
-		return ['exams' => $out];
+		return ['exams' => $out, 'collisions' => $numCollisions];
 	}
 
 	protected function makeLectureExamList(): array