/**
 * Simple Moving Average (SMA).
 * Computes the arithmetic mean over a sliding window.
 * When `skipna` is true, NaNs inside the window are ignored; when false
 * a dense fast-path is used (assumes no NaNs).
 * @param source Input series
 * @param period Window length (must be > 0)
 * @param skipna Whether to ignore NaNs inside windows (default: true)
 * @returns Float64Array of SMA values (NaN before window fills)
 */
export declare function sma(source: ArrayLike<number>, period: number, skipna?: boolean): Float64Array;