allow cache disabling, tweak LRU counter

pocketfft_hdronly.h

@@ -41,7 +41,10 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #error This file requires at least C++11 support.
 #endif
 
-#include <array>
+#ifndef POCKETFFT_CACHE_SIZE
+#define POCKETFFT_CACHE_SIZE 16
+#endif
+
 #include <cmath>
 #include <cstring>
 #include <cstdlib>
@@ -49,7 +52,10 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <memory>
 #include <vector>
 #include <complex>
+#if POCKETFFT_CACHE_SIZE!=0
+#include <array>
 #include <mutex>
+#endif
 #ifdef POCKETFFT_OPENMP
 #include <omp.h>
 #endif
@@ -2266,7 +2272,10 @@ template<typename T0> class pocketfft_r
 
 template<typename T> shared_ptr<T> get_plan(size_t length)
   {
-  constexpr size_t nmax=10;
+#if POCKETFFT_CACHE_SIZE==0
+  return make_shared<T>(length);
+#else
+  constexpr size_t nmax=POCKETFFT_CACHE_SIZE;
   static array<shared_ptr<T>, nmax> cache;
   static array<size_t, nmax> last_access{{0}};
   static size_t access_counter = 0;
@@ -2274,17 +2283,21 @@ template<typename T> shared_ptr<T> get_plan(size_t length)
 
   auto find_in_cache = [&]() -> shared_ptr<T>
     {
-      for (size_t i=0; i<nmax; ++i)
-        if (cache[i] && (cache[i]->length()==length))
+    for (size_t i=0; i<nmax; ++i)
+      if (cache[i] && (cache[i]->length()==length))
+        {
+        // no need to update if this is already the most recent entry
+        if (last_access[i]!=access_counter)
           {
-          last_access[i] = access_counter++;
+          last_access[i] = ++access_counter;
           // Guard against overflow
           if (access_counter == 0)
             last_access.fill(0);
-          return cache[i];
           }
+        return cache[i];
+        }
 
-      return nullptr;
+    return nullptr;
     };
 
   {
@@ -2304,9 +2317,10 @@ template<typename T> shared_ptr<T> get_plan(size_t length)
       lru = i;
 
   cache[lru] = plan;
-  last_access[lru] = access_counter++;
+  last_access[lru] = ++access_counter;
   }
   return plan;
+#endif
   }
 
 class arr_info