src/vs/editor/common/model/prefixSumComputer.ts/ConstantTimePrefixSumComputer/getPrefixSum
class ConstantTimePrefixSumComputer:	public getPrefixSum(count: number): number {
		this._ensureValid();
		if (count === 0) {
			return 0;
		}
		return this._prefixSum[count - 1];
	}