fix: validate inputs and fix coordinate offsets in pattern matching

Introduce explicit input validation and a `(-1, -1, -1)` sentinel return value to signal invalid or unreliable matches. Add pre-call length checks and early returns to prevent panics during indel relocation. Fix an index offset bug in coordinate calculation by preserving the original fragment position before applying alignment deltas. Update match filtering to enforce reliability checks only on successfully relocated alignments. Comprehensive tests validate edge cases, boundary constraints, and error thresholds.

pkg/obialign/locatepattern.go

@@ -28,10 +28,18 @@ func buffIndex(i, j, width int) int {
 //
 // The function returns the start and end positions of the best
 // match, as well as the number of errors in the best match.
+//
+// When the sequence is too short relative to the pattern for the
+// backtracking to reconstruct a valid alignment (e.g. the pattern
+// is longer than the sequence, or the match sits too close to a
+// sequence boundary), no reliable position can be computed. In that
+// case the function returns the sentinel (-1, -1, -1) instead of a
+// guessed, potentially wrong, position: callers must treat this as
+// "no match" rather than use the returned coordinates.
 func LocatePattern(id string, pattern, sequence []byte) (int, int, int) {
 
-	if len(pattern) >= len(sequence) {
-		log.Panicf("Sequence %s:Pattern %s must be shorter than sequence %s", id, pattern, sequence)
+	if len(sequence) == 0 {
+		log.Panicf("Sequence %s:Pattern %s must not be empty", id, pattern)
 	}
 
 	// Pattern spreads over the columns
@@ -158,5 +166,18 @@ func LocatePattern(id string, pattern, sequence []byte) (int, int, int) {
 	// obilog.Warnf("from : %d to: %d error: %d match: %v",
 	// 	i, end+1, -buffer[buffIndex(len(sequence)-1, len(pattern)-1, width)],
 	// 	string(sequence[i:(end+1)]))
+
+	if i < 0 || end == -1 {
+		// i < 0: the backtracking ran off the start of the sequence
+		// without fully consuming the pattern.
+		// end == -1: the backtracking loop never ran at all (e.g. a
+		// single-base pattern, jmax == 0), so no alignment boundary
+		// was ever established.
+		// Either way, no valid alignment exists for this (pattern,
+		// sequence) pair: signal it explicitly instead of returning
+		// an out-of-bounds or uncomputed position.
+		return -1, -1, -1
+	}
+
 	return i, end + 1, -buffer[buffIndex(len(sequence)-1, len(pattern)-1, width)]
 }