ts-ds-tool/src/algorithms/max-sub-array/maxSubArray.ts

Data structure and algorithm of TypeScript

export function maxSubArray<T>(arr: Array<T>, key?: keyof T){
    if (!arr || !arr.length){
        return {
            low: -1,
            high: -1,
            sum: -Infinity,
        };
    }
    return divideConquerMaxSubArray(arr, 0, arr.length - 1, key);
}

function divideConquerMaxSubArray<T>(
    arr: Array<T>,
    low: number,
    high: number,
    key?: keyof T): {low: number, high: number, sum: number}{
    const getValTemp = getVal(key);
    if (low === high){
        return {
            low,
            high,
            sum: getValTemp(arr[low]),
        };
    }

    const mid = Math.floor((low + high) / 2);
    const left = divideConquerMaxSubArray(arr, low, mid, key);
    const right = divideConquerMaxSubArray(arr, mid + 1, high, key);
    const cross = crossMaxSubArray(arr, low, mid, high, key);

    if (left.sum >= right.sum && left.sum >= cross.sum){
        return left;
    }
    if (right.sum >= left.sum &&  right.sum >= cross.sum){
        return right;
    }
    return cross;
}

function getVal<T>(key?: keyof T): (val: T) => any{
    if (key){
        return (val: T) => val[key];
    }
    return (val: T) => val;
}

function crossMaxSubArray<T>(arr: Array<T>, low: number, mid: number, high: number, key?: keyof T){
    const getValTemp = getVal(key);
    let leftMax = -Infinity;
    let sumLeft = 0;
    let maxLeftIndex: number;
    for (let i = mid; i >= low; i--){
        sumLeft += getValTemp(arr[i]);
        if (sumLeft > leftMax ){
            leftMax = sumLeft;
            maxLeftIndex = i;
        }
    }

    let rightMax = -Infinity;
    let sumRight = 0;
    let maxRightIndex: number;
    for (let i = mid + 1; i <= high; i++){
        sumRight += getValTemp(arr[i]);
        if (sumRight > rightMax ){
            rightMax = sumRight;
            maxRightIndex = i;
        }
    }
    return {
        low: maxLeftIndex,
        high: maxRightIndex,
        sum: leftMax + rightMax,
    };
}