function accumulator( x ) {
		var v;
		var k;
		if ( arguments.length === 0 ) {
			if ( N === 0 ) {
				return null;
			}
			return max - min;
		}
		// Update the index for managing the circular buffer:
		i = (i+1) % W;

		if ( x === 0.0 ) {
			x = 0.0; // normalizes +-0
		}
		// Case: incoming value is NaN...
		if ( isnan( x ) ) {
			N = W; // explicitly set to avoid `N < W` branch
			min = x;
			max = x;
		}
		// Case: initial window...
		else if ( N < W ) {
			N += 1;
			if ( x < min ) {
				min = x;
			}
			if ( x > max ) {
				max = x;
			}
		}
		// Case: outgoing value is the current minimum or maximum and the new value is either greater than the minimum or less than the maximum, and, thus, we need to find a new accumulated values among the current buffer values...
		else if (
			( buf[ i ] === min && x > min ) ||
			( buf[ i ] === max && x < max ) ||
			isnan( buf[ i ] )
		) {
			min = x;
			max = x;
			for ( k = 0; k < W; k++ ) {
				if ( k !== i ) {
					v = buf[ k ];
					if ( isnan( v ) ) {
						min = v;
						max = v;
						break; // no need to continue searching
					}
					if ( v < min ) {
						min = v;
					}
					if ( v > max ) {
						max = v;
					}
				}
			}
		}
		// Case: incoming value is less than current minimum value...
		else if ( x < min ) {
			min = x;
		}
		// Case: incoming value is greater than current maximum value...
		else if ( x > max ) {
			max = x;
		}
		// Case: updating existing window; however, the minimum and maximum values do not change so nothing to do but update our buffer...
		buf[ i ] = x;

		return max - min;
	}
If provided a value, the accumulator function returns an updated range. If not provided a value, the accumulator function returns the current range.

@private
@param {number} [x] - input value
@returns {(number|null)} range or null