2 files changed, 12 insertions, 6 deletions
diff --git a/src/numerics.cpp b/src/numerics.cpp
index f992873..608a724 100644
--- a/src/numerics.cpp
+++ b/src/numerics.cpp
@@ -43,7 +43,7 @@ bool Fraction::operator==(const Fraction &other)
   return numerator == other.numerator && denominator == other.denominator;
 }
 
-Node::Node(Fraction val, int64_t left, int64_t right)
+Node::Node(Fraction val, index_t left, index_t right)
     : value{val}, left{left}, right{right}
 {
 }
@@ -107,10 +107,13 @@ void indent_depth(int depth, std::stringstream &ss)
     ss << "  ";
 }
 
-std::string to_string(const NodeAllocator &allocator, const word_t n, int depth)
+std::string to_string(const NodeAllocator &allocator, const index_t n,
+                      int depth)
 {
+  if (!n.has_value())
+    return "NIL";
   std::stringstream ss;
-  Node x = allocator.vec[n];
+  Node x = allocator.getVal(n.value());
   ss << "(" << to_string(x.value) << "\n";
   indent_depth(depth, ss);
   if (x.left == -1)
diff --git a/src/numerics.hpp b/src/numerics.hpp
index 9fdaf56..3d74202 100644
--- a/src/numerics.hpp
+++ b/src/numerics.hpp
@@ -19,6 +19,7 @@
 
 #include <cstdint>
 
+#include <optional>
 #include <queue>
 #include <string>
 #include <vector>
@@ -26,6 +27,7 @@
 #define MIN(A, B) ((A) < (B) ? (A) : (B))
 #define MAX(A, B) ((B) < (A) ? (A) : (B))
 typedef uint64_t word_t;
+typedef std::optional<word_t> index_t;
 
 struct Fraction
 {
@@ -40,9 +42,10 @@ struct Fraction
 struct Node
 {
   Fraction value;
-  int64_t left, right;
+  index_t left, right;
 
-  Node(Fraction val = {}, int64_t left = -1, int64_t right = -1);
+  Node(Fraction val = {}, index_t left = std::nullopt,
+       index_t right = std::nullopt);
 };
 
 struct NodeAllocator
@@ -57,6 +60,6 @@ word_t gcd(word_t a, word_t b);
 Fraction iterate(std::queue<word_t> &queue, NodeAllocator &allocator);
 
 std::string to_string(const Fraction &);
-std::string to_string(const NodeAllocator &, const word_t, int depth = 1);
+std::string to_string(const NodeAllocator &, const index_t, int depth = 1);
 
 #endif