Merge branch 'tweak_dcst4' into 'master'

Misc updates and simplifications

See merge request !26

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
+
 }
 
 //
@@ -2752,8 +2774,6 @@ template<typename T0> class T_dcst23
 
 template<typename T0> class T_dcst4
   {
-  // even length algorithm from
-  // https://www.appletonaudio.com/blog/2013/derivation-of-fast-dct-4-algorithm-based-on-dft/
   private:
     size_t N;
     unique_ptr<pocketfft_c<T0>> fft;
@@ -2789,9 +2809,8 @@ template<typename T0> class T_dcst4
         {
         // The following code is derived from the FFTW3 function apply_re11()
         // and is released under the 3-clause BSD license with friendly
-        // permission of Matteo Frigo.
+        // permission of Matteo Frigo and Steven G. Johnson.
 
-        auto SGN_SET = [](T x, size_t i) {return (i%2) ? -x : x;};
         arr<T> y(N);
         {
         size_t i=0, m=n2;
@@ -2803,30 +2822,27 @@ template<typename T0> class T_dcst4
           y[i] = -c[m-2*N];
         for (; m<4*N; ++i, m+=4)
           y[i] = c[4*N-m-1];
-        m -= 4*N;
         for (; i<N; ++i, m+=4)
-          y[i] = c[m];
+          y[i] = c[m-4*N];
         }
         rfft->exec(y.data(), fct, true);
         {
-        size_t i=0;
-        for (; i+i+1<n2; ++i)
+        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)
           {
-          size_t k = i+i+1;
-          T c1=y[2*k-1], s1=y[2*k], c2=y[2*k+1], s2=y[2*k+2];
-          c[i] = sqrt2 * (SGN_SET(c1, (i+1)/2) + SGN_SET(s1, i/2));
-          c[N-(i+1)] = sqrt2 * (SGN_SET(c1, (N-i)/2) - SGN_SET(s1, (N-(i+1))/2));
-          c[n2-(i+1)] = sqrt2 * (SGN_SET(c2, (n2-i)/2) - SGN_SET(s2, (n2-(i+1))/2));
-          c[n2+(i+1)] = sqrt2 * (SGN_SET(c2, (n2+i+2)/2) + SGN_SET(s2, (n2+(i+1))/2));
+          c[i    ] = y[2*k-1]*SGN(i1)     + y[2*k  ]*SGN(i);
+          c[N -i1] = y[2*k-1]*SGN(N -i)   - y[2*k  ]*SGN(N -i1);
+          c[n2-i1] = y[2*k+1]*SGN(n2-i)   - y[2*k+2]*SGN(n2-i1);
+          c[n2+i1] = y[2*k+1]*SGN(n2+i+2) + y[2*k+2]*SGN(n2+i1);
           }
-        if (i+i+1 == n2)
+        if (k == n2)
           {
-          T cx=y[2*n2-1], sx=y[2*n2];
-          c[i] = sqrt2 * (SGN_SET(cx, (i+1)/2) + SGN_SET(sx, i/2));
-          c[N-(i+1)] = sqrt2 * (SGN_SET(cx, (i+2)/2) + SGN_SET(sx, (i+1)/2));
+          c[i   ] = y[2*k-1]*SGN(i+1) + y[2*k]*SGN(i);
+          c[N-i1] = y[2*k-1]*SGN(i+2) + y[2*k]*SGN(i1);
           }
         }
-        c[n2] = sqrt2 * SGN_SET(y[0], (n2+1)/2);
 
         // FFTW-derived code ends here
         }

setup.py

@@ -37,7 +37,7 @@ def get_extension_modules():
     return [Extension('pypocketfft',
                       language='c++',
                       sources=['pypocketfft.cc'],
-                      depends=['pocketfft_hdronly.h'],
+                      depends=['pocketfft_hdronly.h', 'setup.py'],
                       include_dirs=include_dirs,
                       define_macros=define_macros,
                       extra_compile_args=extra_compile_args,