Fix QR decode failure for small/native-size images (#2061)

* Fix QR decode failure for small/native-size images (#1976)

QRCodeWriter.encode(..., 0, 0) produces a 33x33 native-size QR that
ZXing cannot decode. The CENTER_QUORUM filter in selectBestPatterns()
(added in 110ef9e to fix #1567) removes finder patterns with count < 2.
For small images, the row-skip interval (iSkip=3, MIN_SKIP) means the
scanner only crosses the top two finder pattern centers on a single row
each, giving them count=1. The bottom-left pattern gets count=2 because
it's scanned after iSkip drops to 2. The filter then removes the two
count=1 patterns, leaving 1 of the 3 needed, and throws
NotFoundException.

The fix: only apply the CENTER_QUORUM filter when either (a) at least 3
confirmed patterns would survive, or (b) the total candidate list
exceeds a size threshold. This preserves #1567 protection while
allowing small images through.

Why a size threshold: the filter was added to prevent O(n^3) blowup
in the triangle-selection loop when noisy images produce thousands of
spurious candidates. When the candidate list is small (<= 25 entries,
yielding <= ~2300 loop iterations), the cubic cost is negligible
regardless of whether the patterns are real or spurious. A clean
single-QR image produces ~3 candidates; even a noisy image below the
threshold is computationally trivial. Above the threshold, the filter
applies unconditionally, matching today's behavior.

Alternatives considered:
- Removing the filter entirely: all 561 existing tests pass, but
  re-opens #1567 for pathological images with thousands of spurious
  count=1 candidates.
- Reducing MIN_SKIP from 3 to 1 for small images: addresses the
  symptom (more scan rows = higher counts) but doesn't fix the
  fundamental interaction between iSkip and the quorum filter,
  and changes scan behavior for all small images.
- Fallback (try with filter, retry without on failure): re-opens
  #1567 whenever fewer than 3 confirmed patterns exist, which is
  exactly the scenario that produces thousands of unconfirmed
  candidates in noisy images.

* Increase noisy-image test timeout from 5s to 30s

GitHub shared runners timed out at 5s. Local measurement shows ~1.2s,
implying at least 4x slowdown on CI. 30s gives a comfortable margin
while still catching the O(n^3) regression from #1567.

1 files changed, 12 insertions, 3 deletions

diff --git a/core/src/main/java/com/google/zxing/qrcode/detector/FinderPatternFinder.java b/core/src/main/java/com/google/zxing/qrcode/detector/FinderPatternFinder.java
index 951bd197f..2eeef5c8d 100755
--- a/core/src/main/java/com/google/zxing/qrcode/detector/FinderPatternFinder.java
+++ b/core/src/main/java/com/google/zxing/qrcode/detector/FinderPatternFinder.java
@@ -42,6 +42,7 @@ import java.util.Map;
 public class FinderPatternFinder {
 
   private static final int CENTER_QUORUM = 2;
+  private static final int MAX_FINDER_PATTERN_CANDIDATES = 25;
   private static final EstimatedModuleComparator moduleComparator = new EstimatedModuleComparator();
   protected static final int MIN_SKIP = 3; // 1 pixel/module times 3 modules/center
   protected static final int MAX_MODULES = 97; // support up to version 20 for mobile clients
@@ -623,9 +624,17 @@ public class FinderPatternFinder {
       throw NotFoundException.getNotFoundInstance();
     }
 
-    for (Iterator<FinderPattern> it = possibleCenters.iterator(); it.hasNext();) {
-      if (it.next().getCount() < CENTER_QUORUM) {
-        it.remove();
+    int confirmedCount = 0;
+    for (FinderPattern pattern : possibleCenters) {
+      if (pattern.getCount() >= CENTER_QUORUM) {
+        confirmedCount++;
+      }
+    }
+    if (confirmedCount >= 3 || startSize > MAX_FINDER_PATTERN_CANDIDATES) {
+      for (Iterator<FinderPattern> it = possibleCenters.iterator(); it.hasNext();) {
+        if (it.next().getCount() < CENTER_QUORUM) {
+          it.remove();
+        }
       }
     }