Module fft 

Radix-2 1D FFT GPU compute kernel for OxiGDAL.

This module implements a monomorphic Cooley-Tukey Decimation-In-Time (DIT) radix-2 FFT using WGSL compute shaders dispatched via wgpu. Each workgroup processes exactly one transform; the batch variant dispatches multiple workgroups in parallel.

Constraints

• Size must be a power of two in the range [4, 2048].
• All butterfly stages and the bit-reversal function are unrolled in the emitted WGSL because WGSL workgroup-shared memory paths do not allow fully dynamic loop bounds at all WGSL validation levels.

Sign convention

Mode	twiddle angle	normalisation
Forward	-2π · k / N	none
Inverse	+2π · k / N	divide by N

Structs

Fft1d

GPU-accelerated radix-2 1D FFT (or IFFT) kernel.

Functions

bit_reverse

Reverse the bottom log2_n bits of x.

make_fft_shader_source

Emit a monomorphic WGSL compute shader for a radix-2 Cooley-Tukey DIT FFT of exactly size points.

twiddle_factor

Compute the twiddle factor for position k of an n-point transform.

validate_size

Validate that size satisfies the FFT kernel constraints (power of two, in the range [4, 2048]).