Avoid 2 more RSS Expanded finder pattern false positive scenarios (#1692)

core/src/main/java/com/google/zxing/oned/rss/expanded/RSSExpandedReader.java

@@ -120,6 +120,10 @@ public final class RSSExpandedReader extends AbstractRSSReader {
 
   private static final int MAX_PAIRS = 11;
 
+  private static final float FINDER_PATTERN_MODULES = 15f;
+  private static final float DATA_CHARACTER_MODULES = 17f;
+  private static final float MAX_FINDER_PATTERN_DISTANCE_VARIANCE = 0.1f;
+
   private final List<ExpandedPair> pairs = new ArrayList<>(MAX_PAIRS);
   private final List<ExpandedRow> rows = new ArrayList<>();
   private final int [] startEnd = new int[2];
@@ -129,9 +133,7 @@ public final class RSSExpandedReader extends AbstractRSSReader {
   public Result decodeRow(int rowNumber,
                           BitArray row,
                           Map<DecodeHintType,?> hints) throws NotFoundException, FormatException {
-    // Rows can start with even pattern in case in prev rows there where odd number of patters.
-    // So lets try twice
-    this.pairs.clear();
+    // Rows can start with even pattern if previous rows had an odd number of patterns, so we try twice.
     this.startFromEven = false;
     try {
       return constructResult(decodeRow2pairs(rowNumber, row));
@@ -139,7 +141,6 @@ public final class RSSExpandedReader extends AbstractRSSReader {
       // OK
     }
 
-    this.pairs.clear();
     this.startFromEven = true;
     return constructResult(decodeRow2pairs(rowNumber, row));
   }
@@ -152,6 +153,7 @@ public final class RSSExpandedReader extends AbstractRSSReader {
 
   // Not private for testing
   List<ExpandedPair> decodeRow2pairs(int rowNumber, BitArray row) throws NotFoundException {
+    this.pairs.clear();
     boolean done = false;
     while (!done) {
       try {
@@ -456,6 +458,7 @@ public final class RSSExpandedReader extends AbstractRSSReader {
     }
 
     FinderPattern pattern;
+    DataCharacter leftChar = null;
 
     boolean keepFinding = true;
     int forcedOffset = -1;
@@ -463,17 +466,20 @@ public final class RSSExpandedReader extends AbstractRSSReader {
       this.findNextPair(row, previousPairs, forcedOffset);
       pattern = parseFoundFinderPattern(row, rowNumber, isOddPattern, previousPairs);
       if (pattern == null) {
-        forcedOffset = getNextSecondBar(row, this.startEnd[0]);
+        forcedOffset = getNextSecondBar(row, this.startEnd[0]); // probable false positive, keep looking
       } else {
+        try {
+          leftChar  = this.decodeDataCharacter(row, pattern, isOddPattern, true);
           keepFinding = false;
+        } catch (NotFoundException ignored) {
+          forcedOffset = getNextSecondBar(row, this.startEnd[0]); // probable false positive, keep looking
+        }
       }
     } while (keepFinding);
 
     // When stacked symbol is split over multiple rows, there's no way to guess if this pair can be last or not.
     // boolean mayBeLast = checkPairSequence(previousPairs, pattern);
 
-    DataCharacter leftChar  = this.decodeDataCharacter(row, pattern, isOddPattern, true);
-
     if (!previousPairs.isEmpty() && previousPairs.get(previousPairs.size() - 1).mustBeLast()) {
       throw NotFoundException.getNotFoundInstance();
     }
@@ -615,6 +621,21 @@ public final class RSSExpandedReader extends AbstractRSSReader {
       return null;
     }
 
+    // Check that the finder pattern that we *think* we found is not too far from where we would expect to find it,
+    // given that finder patterns are 15 modules wide and the data characters between them are 17 modules wide.
+    if (!previousPairs.isEmpty()) {
+      ExpandedPair prev = previousPairs.get(previousPairs.size() - 1);
+      int prevStart = prev.getFinderPattern().getStartEnd()[0];
+      int prevEnd = prev.getFinderPattern().getStartEnd()[1];
+      int prevWidth = prevEnd - prevStart;
+      float charWidth = (prevWidth / FINDER_PATTERN_MODULES) * DATA_CHARACTER_MODULES;
+      float minX = prevEnd + (2 * charWidth * (1 - MAX_FINDER_PATTERN_DISTANCE_VARIANCE));
+      float maxX = prevEnd + (2 * charWidth * (1 + MAX_FINDER_PATTERN_DISTANCE_VARIANCE));
+      if (start < minX || start > maxX) {
+        return null;
+      }
+    }
+
     return new FinderPattern(value, new int[] {start, end}, start, end, rowNumber);
   }
 

core/src/test/java/com/google/zxing/oned/rss/expanded/RSSExpandedBlackBox1TestCase.java

@@ -37,7 +37,7 @@ public final class RSSExpandedBlackBox1TestCase extends AbstractBlackBoxTestCase
 
   public RSSExpandedBlackBox1TestCase() {
     super("src/test/resources/blackbox/rssexpanded-1", new MultiFormatReader(), BarcodeFormat.RSS_EXPANDED);
-    addTest(33, 33, 0.0f);
-    addTest(33, 33, 180.0f);
+    addTest(35, 35, 0.0f);
+    addTest(35, 35, 180.0f);
   }
 }

core/src/test/resources/blackbox/rssexpanded-1/34.txt

@@ -0,0 +1 @@
+(420)aaeaaa"-a23455a

core/src/test/resources/blackbox/rssexpanded-1/35.txt

@@ -0,0 +1 @@
+(420)WWWWWW3/a"Wa1WW