Bit reversal: the order an FFT reads its input in

Pick a size and an index: its bits read backwards give the place it takes at the input of a radix-2 FFT. See the whole crossing pattern at once.

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Where the numbers come from

gHashTag/t27 specs/fpga/dsp/signals.t27 (t27#8985): every value on the page is its fn bodies, compiled to wasm (logic.js)
J. W. Cooley, J. W. Tukey, An algorithm for the machine calculation of complex Fourier series, Math. Comp. 19, 1965
R. G. Lyons, Understanding Digital Signal Processing, 3rd ed., ch. 4: the radix-2 FFT and bit reversal