--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Modules/_decimal/libmpdec/literature/six-step.txt Sat Mar 10 18:12:20 2012 +0100 @@ -0,0 +1,63 @@ + + +(* Copyright (c) 2011 Stefan Krah. All rights reserved. *) + + +The Six Step Transform: +======================= + +In libmpdec, the six-step transform is the Matrix Fourier Transform (See +matrix-transform.txt) in disguise. It is called six-step transform after +a variant that appears in [1]. The algorithm requires that the input +array can be viewed as an R*C matrix. + + +Algorithm six-step (forward transform): +--------------------------------------- + + 1a) Transpose the matrix. + + 1b) Apply a length R FNT to each row. + + 1c) Transpose the matrix. + + 2) Multiply each matrix element (addressed by j*C+m) by r**(j*m). + + 3) Apply a length C FNT to each row. + + 4) Transpose the matrix. + +Note that steps 1a) - 1c) are exactly equivalent to step 1) of the Matrix +Fourier Transform. For large R, it is faster to transpose twice and do +a transform on the rows than to perform a column transpose directly. + + + +Algorithm six-step (inverse transform): +--------------------------------------- + + 0) View the matrix as a C*R matrix. + + 1) Transpose the matrix, producing an R*C matrix. + + 2) Apply a length C FNT to each row. + + 3) Multiply each matrix element (addressed by i*C+n) by r**(i*n). + + 4a) Transpose the matrix. + + 4b) Apply a length R FNT to each row. + + 4c) Transpose the matrix. + +Again, steps 4a) - 4c) are equivalent to step 4) of the Matrix Fourier +Transform. + + + +-- + + [1] David H. Bailey: FFTs in External or Hierarchical Memory + http://crd.lbl.gov/~dhbailey/dhbpapers/ + +