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The guru interface introduces one basic new data structure,
cannam@95: fftw_iodim, that is used to specify sizes and strides for
cannam@95: multi-dimensional transforms and vectors:
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typedef struct {
cannam@95: int n;
cannam@95: int is;
cannam@95: int os;
cannam@95: } fftw_iodim;
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cannam@95: Here, n is the size of the dimension, and is and os
cannam@95: are the strides of that dimension for the input and output arrays. (The
cannam@95: 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
cannam@95: that the stride refers to. If the array is interleaved complex,
cannam@95: strides are expressed in units of complex numbers
cannam@95: (fftw_complex). If the array is split complex or real, strides
cannam@95: are expressed in units of real numbers (double). This
cannam@95: convention is consistent with the usual pointer arithmetic in the C
cannam@95: language. An interleaved array is denoted by a pointer p to
cannam@95: fftw_complex, so that p+1 points to the next complex
cannam@95: number. Split arrays are denoted by pointers to double, in
cannam@95: which case pointer arithmetic operates in units of
cannam@95: sizeof(double).
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The guru planner interfaces all take a (rank, dims[rank])
cannam@95: pair describing the transform size, and a (howmany_rank,
cannam@95: howmany_dims[howmany_rank]) pair describing the “vector” size (a
cannam@95: multi-dimensional loop of transforms to perform), where dims and
cannam@95: howmany_dims are arrays of fftw_iodim.
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For example, the howmany parameter in the advanced complex-DFT
cannam@95: interface corresponds to howmany_rank = 1,
cannam@95: howmany_dims[0].n = howmany, howmany_dims[0].is =
cannam@95: idist, and howmany_dims[0].os = odist.
cannam@95: (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]
cannam@95: (see Row-major Format) corresponds to dims[i].n =
cannam@95: n[i] and the recurrence dims[i].is = n[i+1] *
cannam@95: dims[i+1].is (similarly for os). The stride of the last
cannam@95: (i=rank-1) dimension is the overall stride of the array.
cannam@95: e.g. to be equivalent to the advanced complex-DFT interface, you would
cannam@95: have dims[rank-1].is = istride and
cannam@95: dims[rank-1].os = ostride.
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In general, we only guarantee FFTW to return a non-NULL plan if
cannam@95: the vector and transform dimensions correspond to a set of distinct
cannam@95: indices, and for in-place transforms the input/output strides should
cannam@95: be the same.
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