add switch to disable multithreading; fix lambda capture for MSVC

README.md

@@ -17,4 +17,4 @@ Features
 - makes use of CPU vector instructions when performing 2D and higher-dimensional
   transforms
 - supports prime-length transforms without degrading to O(N**2) performance
-- has optional OpenMP support for multidimensional transforms
+- has optional multi-threading support for multidimensional transforms

pocketfft_hdronly.h

@@ -63,6 +63,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <mutex>
 #endif
 
+#ifndef POCKETFFT_NO_MULTITHREADING
 #include <mutex>
 #include <condition_variable>
 #include <thread>
@@ -73,7 +74,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #ifdef POCKETFFT_PTHREADS
 #  include <pthread.h>
 #endif
-
+#endif
 
 #if defined(__GNUC__)
 #define POCKETFFT_NOINLINE __attribute__((noinline))
@@ -694,21 +695,39 @@ struct util // hack to avoid duplicate symbols
     if (axis>=shape.size()) throw invalid_argument("bad axis number");
     }
 
-    static size_t thread_count (size_t nthreads, const shape_t &shape,
-      size_t axis, size_t vlen)
-      {
-      if (nthreads==1) return 1;
-      size_t size = prod(shape);
-      size_t parallel = size / (shape[axis] * vlen);
-      if (shape[axis] < 1000)
-        parallel /= 4;
-      size_t max_threads = nthreads == 0 ?
-        thread::hardware_concurrency() : nthreads;
-      return max(size_t(1), min(parallel, max_threads));
-      }
+#ifdef POCKETFFT_NO_MULTITHREADING
+  static size_t thread_count (size_t /*nthreads*/, const shape_t &/*shape*/,
+    size_t /*axis*/, size_t /*vlen*/)
+    { return 1; }
+#else
+  static size_t thread_count (size_t nthreads, const shape_t &shape,
+    size_t axis, size_t vlen)
+    {
+    if (nthreads==1) return 1;
+    size_t size = prod(shape);
+    size_t parallel = size / (shape[axis] * vlen);
+    if (shape[axis] < 1000)
+      parallel /= 4;
+    size_t max_threads = nthreads == 0 ?
+      thread::hardware_concurrency() : nthreads;
+    return max(size_t(1), min(parallel, max_threads));
+    }
+#endif
   };
 
 namespace threading {
+
+#ifdef POCKETFFT_NO_MULTITHREADING
+
+constexpr size_t thread_id = 0;
+constexpr size_t num_threads = 1;
+
+template <typename Func>
+void thread_map(size_t /* nthreads */, Func f)
+  { f(); }
+
+#else
+
 thread_local size_t thread_id = 0;
 thread_local size_t num_threads = 1;
 
@@ -892,6 +911,9 @@ void thread_map(size_t nthreads, Func f)
   if (ex)
     rethrow_exception(ex);
   }
+
+#endif
+
 }
 
 //
@@ -2789,7 +2811,6 @@ template<typename T0> class T_dcst4
         // and is released under the 3-clause BSD license with friendly
         // permission of Matteo Frigo and Steven G. Johnson.
 
-        auto SGN = [](size_t i) {return (i&2) ? -sqrt2 : sqrt2;};
         arr<T> y(N);
         {
         size_t i=0, m=n2;
@@ -2806,6 +2827,7 @@ template<typename T0> class T_dcst4
         }
         rfft->exec(y.data(), fct, true);
         {
+        auto SGN = [sqrt2](size_t i) {return (i&2) ? -sqrt2 : sqrt2;};
         c[n2] = y[0]*SGN(n2+1);
         size_t i=0, i1=1, k=1;
         for (; k<n2; ++i, ++i1, k+=2)