/**
 * Fourier analysis.
 *
 * An iterative radix-2 Cooley-Tukey FFT. Recursive implementations read more
 * clearly but allocate two arrays per level and blow the call stack on large
 * inputs; this one works in place on preallocated buffers, which is what makes
 * a spectrogram over a whole track practical.
 *
 * Twiddle factors are cached per size, since a spectrogram calls the same
 * transform size thousands of times in a row.
 */
/**
 * In-place complex FFT. `real` and `imag` must both have a power-of-two length.
 *
 * For real input, leave `imag` zeroed; the output's second half mirrors the
 * first, so only bins `0 .. size/2` carry independent information.
 */
export declare function fft(real: Float64Array, imag: Float64Array): void;
export interface SpectrumOptions {
    /** Transform size; must be a power of two. Defaults to 2048. */
    fftSize?: number;
    /**
     * Analysis window. Defaults to `'hann'`.
     *
     * Windowing is not optional in practice: a rectangular window on a signal
     * whose period does not divide the frame length smears energy across every
     * bin ("spectral leakage"), which buries real detail under an artefact.
     */
    window?: 'hann' | 'hamming' | 'blackman' | 'none';
}
/**
 * Magnitude spectrum of one frame, in linear units.
 *
 * Returns `fftSize / 2 + 1` bins. Bin `k` is centred on
 * `k * sampleRate / fftSize` Hz.
 */
export declare function spectrum(samples: Float32Array, options?: SpectrumOptions): Float64Array;
/**
 * Magnitude spectra over successive overlapping frames.
 *
 * @param hopSize Frames advanced between windows. Defaults to `fftSize / 4`.
 */
export declare function spectrogram(samples: Float32Array, options?: SpectrumOptions & {
    hopSize?: number;
}): Float64Array[];
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