Only reserve output size in path::walk

include/tc/core.hpp

@@ -29,6 +29,25 @@ namespace tc {
         [[nodiscard]] Action get(int to_idx) const;
 
         void put(int from_idx, int gen, int to_idx);
+      
+        template<class C, class T, class E>
+        void walk(
+            C& res,
+            T start,
+            std::vector<E> gens,
+            std::function<T(const T &, const E &)> op
+        ) const {
+            size_t s = size();
+            res.reserve(s);
+            res.push_back(start);
+
+            for (int i = 1; i < s; ++i) {
+                auto &action = path[i];
+                auto &from = res.get(action.from_idx);
+                auto &val = gens[action.gen];
+                res.push_back(op(from,val));
+            }
+        }
 
         template<class T, class E>
         [[nodiscard]] std::vector<T> walk(
@@ -36,14 +55,15 @@ namespace tc {
             std::vector<E> gens,
             std::function<T(const T &, const E &)> op
         ) const {
-            std::vector<T> res(size());
-            res[0] = start;
+            std::vector<T> res;
+            res.reserve(size());
+            res.push_back(start);
 
-            for (int i = 1; i < res.size(); ++i) {
+            for (int i = 1; i < size(); ++i) {
                 auto &action = path[i];
                 auto &from = res[action.from_idx];
                 auto &val = gens[action.gen];
-                res[i] = op(from, val);
+                res.push_back(op(from, val));
             }
 
             return res;
@@ -54,10 +74,11 @@ namespace tc {
             T start,
             std::function<T(const T &, const int &)> op
         ) const {
-            std::vector<T> res(size());
-            res[0] = start;
+            std::vector<T> res;
+            res.reserve(size());
+            res.push_back(start);
 
-            for (int i = 1; i < res.size(); ++i) {
+            for (int i = 1; i < size(); ++i) {
                 auto &action = path[i];
                 auto &from = res[action.from_idx];
                 auto &val = action.gen;
@@ -108,7 +129,7 @@ namespace tc {
     struct SubGroup;
 
     struct Group {
-        const int ngens;
+        int ngens;
         std::vector<std::vector<int>> _mults;
         std::string name;