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The guru interface introduces one basic new data structure,
Chris@10: fftw_iodim, that is used to specify sizes and strides for
Chris@10: multi-dimensional transforms and vectors:
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typedef struct {
Chris@10: int n;
Chris@10: int is;
Chris@10: int os;
Chris@10: } fftw_iodim;
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Chris@10: Here, n is the size of the dimension, and is and os
Chris@10: are the strides of that dimension for the input and output arrays. (The
Chris@10: stride is the separation of consecutive elements along this dimension.)
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The meaning of the stride parameter depends on the type of the array
Chris@10: that the stride refers to. If the array is interleaved complex,
Chris@10: strides are expressed in units of complex numbers
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Chris@10: are expressed in units of real numbers (double). This
Chris@10: convention is consistent with the usual pointer arithmetic in the C
Chris@10: language. An interleaved array is denoted by a pointer p to
Chris@10: fftw_complex, so that p+1 points to the next complex
Chris@10: number. Split arrays are denoted by pointers to double, in
Chris@10: which case pointer arithmetic operates in units of
Chris@10: sizeof(double).
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The guru planner interfaces all take a (rank, dims[rank])
Chris@10: pair describing the transform size, and a (howmany_rank,
Chris@10: howmany_dims[howmany_rank]) pair describing the “vector” size (a
Chris@10: multi-dimensional loop of transforms to perform), where dims and
Chris@10: howmany_dims are arrays of fftw_iodim.
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For example, the howmany parameter in the advanced complex-DFT
Chris@10: interface corresponds to howmany_rank = 1,
Chris@10: howmany_dims[0].n = howmany, howmany_dims[0].is =
Chris@10: idist, and howmany_dims[0].os = odist.
Chris@10: (To compute a single transform, you can just use howmany_rank = 0.)
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A row-major multidimensional array with dimensions n[rank]
Chris@10: (see Row-major Format) corresponds to dims[i].n =
Chris@10: n[i] and the recurrence dims[i].is = n[i+1] *
Chris@10: dims[i+1].is (similarly for os). The stride of the last
Chris@10: (i=rank-1) dimension is the overall stride of the array.
Chris@10: e.g. to be equivalent to the advanced complex-DFT interface, you would
Chris@10: have dims[rank-1].is = istride and
Chris@10: dims[rank-1].os = ostride.
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In general, we only guarantee FFTW to return a non-NULL plan if
Chris@10: the vector and transform dimensions correspond to a set of distinct
Chris@10: indices, and for in-place transforms the input/output strides should
Chris@10: be the same.
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