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+PocketFFT
+---------
+
+This is a heavily modified implementation of FFTPack [1,2], with the following
+advantages:
+
+- strictly C99 compliant
+- more accurate twiddle factor computation
+- very fast plan generation
+- worst case complexity for transform sizes with large prime factors is
+  `N*log(N)`, because Bluestein's algorithm [3] is used for these cases.
+
+License
+-------
+
+3-clause BSD (see LICENSE.md)
+
+
+Some code details
+-----------------
+
+Twiddle factor computation:
+
+- making use of symmetries to reduce number of sin/cos evaluations
+- all angles are reduced to the range `[0; pi/4]` for higher accuracy
+- an adapted implementation of `sincospi()` is used, which actually computes
+  `sin(x)` and `(cos(x)-1)`.
+- if `n` sin/cos pairs are required, the adjusted `sincospi()` is only called
+  `2*sqrt(n)` times; the remaining values are obtained by evaluating the
+  angle addition theorems in a numerically accurate way.
+
+Parallel invocation:
+
+- Plans only contain read-only data; all temporary arrays are allocated and
+  deallocated during an individual FFT execution. This means that a single plan
+  can be used in several threads at the same time.
+
+Efficient codelets are available for the factors:
+
+- 2, 3, 4, 5, 7, 11 for complex-valued FFTs
+- 2, 3, 4, 5 for real-valued FFTs
+
+Larger prime factors are handled by somewhat less efficient, generic routines.
+
+For lengths with very large prime factors, Bluestein's algorithm is used, and
+instead of an FFT of length `n`, a convolution of length `n2 >= 2*n-1`
+is performed, where `n2` is chosen to be highly composite.
+
+
+[1] Swarztrauber, P. 1982, Vectorizing the Fast Fourier Transforms
+    (New York: Academic Press), 51
+[2] https://www.netlib.org/fftpack/
+[3] https://en.wikipedia.org/wiki/Chirp_Z-transform