src/vs/editor/common/model/prefixSumComputer.ts/PrefixSumComputer/_getPrefixSum
class PrefixSumComputer:	private _getPrefixSum(index: number): number {
		if (index <= this.prefixSumValidIndex[0]) {
			return this.prefixSum[index];
		}

		let startIndex = this.prefixSumValidIndex[0] + 1;
		if (startIndex === 0) {
			this.prefixSum[0] = this.values[0];
			startIndex++;
		}

		if (index >= this.values.length) {
			index = this.values.length - 1;
		}

		for (let i = startIndex; i <= index; i++) {
			this.prefixSum[i] = this.prefixSum[i - 1] + this.values[i];
		}
		this.prefixSumValidIndex[0] = Math.max(this.prefixSumValidIndex[0], index);
		return this.prefixSum[index];
	}