/**
 * The ONE radix-2 Cooley-Tukey core for this package, on flat Float64Arrays.
 *
 * There used to be two. This flat one (used by `parallelFFT`), and a second inside
 * `signal/fft.ts` that did the butterfly arithmetic in Complex OBJECTS — a `{ re, im }`
 * allocation per twiddle step and per butterfly. Same transform, same machine:
 *
 * | n         | flat Float64Array core | Complex-object core |
 * | --------- | ---------------------- | ------------------- |
 * | 262,144   |                  80 ms |              607 ms |
 * | 1,048,576 |                 358 ms |             2987 ms |
 *
 * The public `fft()` used the SLOW one, so the FFT every consumer reaches by default was
 * ~8x slower than the library's own fast path. Its `ComplexNumber[]` return type was not
 * the cause: materialising the objects once at the boundary is cheap — doing the
 * ARITHMETIC in them is what cost 8x. Both surfaces now share this core and box at the
 * edge.
 *
 * Internal. Not part of the public API.
 *
 * @packageDocumentation
 */
/** 32-bit int / 64-bit float aliases, matching the annotations used across `signal/`. */
type i32 = number;
/**
 * Bit reverse for FFT
 */
export declare function bitReverse(x: i32, bits: i32): i32;
/**
 * Radix-2 FFT core using Float64Array for WASM compatibility
 */
export declare function fftCoreFloat64(realIn: Float64Array, imagIn: Float64Array, inverse?: boolean): {
    real: Float64Array;
    imag: Float64Array;
};
export {};
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