Increase the test coverage further and while doing so understand the …

…code better.

src/util/JoinAlgorithms/JoinAlgorithms.h

@@ -565,10 +565,6 @@ class BlockAndSubrange {
         subrange_{0, block_->size()} {}
 
   // Return a reference to the last element of the currently specified subrange.
-  reference back() {
-    AD_CORRECTNESS_CHECK(!empty() && subrange_.second <= block_->size());
-    return (*block_)[subrange_.second - 1];
-  }
   const_reference back() const {
     AD_CORRECTNESS_CHECK(!empty() && subrange_.second <= block_->size());
     return std::as_const(*block_)[subrange_.second - 1];
@@ -826,9 +822,9 @@ struct BlockZipperJoinImpl {
   // the last one, which might contain elements `> currentEl` at the end.
   auto getEqualToCurrentEl(const auto& blocks, const auto& currentEl) {
     auto result = blocks;
-    // If one of the inputs is empty, this function shouldn't have been called
-    // in the first place.
-    AD_CORRECTNESS_CHECK(!result.empty());
+    if (result.empty()) {
+      return result;
+    }
     auto& last = result.back();
     last.setSubrange(
         std::ranges::equal_range(last.subrange(), currentEl, lessThan_));
@@ -973,19 +969,16 @@ struct BlockZipperJoinImpl {
     while (!equalToCurrentElLeft.empty() && !equalToCurrentElRight.empty()) {
       addAll<DoOptionalJoin>(equalToCurrentElLeft, equalToCurrentElRight);
       switch (blockStatus.value()) {
-        case BlockStatus::allFilled: {
+        case BlockStatus::allFilled:
           removeEqualToCurrentEl(sameBlocksLeft, currentEl);
           removeEqualToCurrentEl(sameBlocksRight, currentEl);
           return;
-        }
-        case BlockStatus::rightMissing: {
+        case BlockStatus::rightMissing:
           getNextBlocks(equalToCurrentElRight, rightSide_);
           continue;
-        }
-        case BlockStatus::leftMissing: {
+        case BlockStatus::leftMissing:
           getNextBlocks(equalToCurrentElLeft, leftSide_);
           continue;
-        }
         default:
           AD_FAIL();
       }

test/JoinAlgorithmsTest.cpp

@@ -172,6 +172,16 @@ TEST(JoinAlgorithms, JoinWithBlocksMoreThanThreeBlocksPerElement) {
   testJoin(a, b, expectedResult);
 }
 
+// Test the coverage of a corner case.
+TEST(JoinAlgorithms, JoinWithBlocksExactlyFourBlocksPerElement) {
+  NestedBlock a{{{42, 0}}, {{42, 1}},          {{42, 2}},  {{42, 3}},
+                {},        {{48, 5}, {67, 0}}, {{96, 32}}, {{96, 33}}};
+  NestedBlock b{{{42, 12}, {67, 13}}};
+  JoinResult expectedResult{
+      {42, 0, 12}, {42, 1, 12}, {42, 2, 12}, {42, 3, 12}, {67, 0, 13}};
+  testJoin(a, b, expectedResult);
+}
+
 // ________________________________________________________________________________________
 TEST(JoinAlgorithms, JoinWithBlocksMultipleBlocksPerElementBothSides) {
   NestedBlock a{{{42, 0}}, {{42, 1}, {42, 2}}, {{42, 3}, {67, 0}}};