Add solution for "Day 19: Linen Layout", part 2

src/LinenLayout.cpp

@@ -31,10 +31,12 @@ void LinenLayout::processDataLine(const std::string& line)
 {
     if (isReadingDesigns_)
     {
-        if (patterns_.isPossible(line))
+        auto nArrangements = patterns_.countArrangements(line);
+        if (nArrangements > 0)
         {
             part1++;
         }
+        part2 += nArrangements;
     }
     else if (line.empty())
     {

src/extra/LinenTowelPatterns.cpp

@@ -46,50 +46,48 @@ void LinenTowelPatterns::add(const std::string& towelPattern)
     patterns_[current].second = true;
 }
 
-bool LinenTowelPatterns::isPossible(const std::string& design)
+int64_t LinenTowelPatterns::countArrangements(const std::string& design)
 {
-    std::vector<bool> canStartFrom(design.size(), false);
+    std::vector<bool> canStartFrom(design.size() + 1, false);
     canStartFrom[0] = true;
-    std::set<size_t> unexploredStart;
-    unexploredStart.insert(0);
+    std::vector<int64_t> nArrangements(design.size() + 1, 0);
+    nArrangements[0] = 1;
+    size_t startPosition{ 0 };
 
-    while (!unexploredStart.empty())
+    while (startPosition < design.size())
     {
-        size_t designPosition = *unexploredStart.rbegin();
-        unexploredStart.erase(designPosition);
+        size_t designPosition{ startPosition };
 
         size_t i{ stripes_[design[designPosition]] };
         while (++designPosition < design.size() && patterns_[i].first > 0)
         {
-            if (patterns_[i].second && !canStartFrom[designPosition])
+            // Checks if there is a towel with the current accumulated stripe pattern.
+            if (patterns_[i].second)
             {
-                // The current accumulated stripe pattern exists and it ends at a new position within the design.
                 canStartFrom[designPosition] = true;
-                unexploredStart.insert(designPosition);
+                nArrangements[designPosition] += nArrangements[startPosition];
             }
+            // Checks if the current accumulated stripe pattern can be extended further.
             if (patterns_[i].first > 0)
             {
-                // The current accumulated stripe pattern can be extended further.
                 i = patterns_[i].first + stripes_[design[designPosition]];
             }
         }
+        // Checks if there is a towel with the current accumulated stripe pattern, after its final extension.
         if (patterns_[i].second)
         {
-            if (designPosition == design.size())
-            {
-                // The current accumulated stripe pattern exists and finishes the design.
-                return true;
-            }
-            else if (!canStartFrom[designPosition])
-            {
-                // The current accumulated stripe pattern exists and it ends at a new position within the design.
-                canStartFrom[designPosition] = true;
-                unexploredStart.insert(designPosition);
-            }
+            canStartFrom[designPosition] = true;
+            nArrangements[designPosition] += nArrangements[startPosition];
         }
+
+        // Seeks the next start position.
+        do
+        {
+            startPosition++;
+        } while (startPosition < canStartFrom.size() && !canStartFrom[startPosition]);
     }
 
-    return false;
+    return *nArrangements.rbegin();
 }
 
 void LinenTowelPatterns::addBranch()