在 javascript 中对大数使用按位运算符

Question

我正在编写此 Microsoft string decoding algorithm 的 Javascript 版本，但它在大量失败。这似乎是因为大小（整数/长）问题。如果我单步执行 C# 中的代码，我会看到 JS 实现在此行失败

n |= (b & 31) << k;

当值为（并且 C# 结果为 240518168576）时会发生这种情况

(39 & 31) << 35

如果我在 C# 中使用这些值，如果 b 是 int，我可以复制 JS 问题。如果我将 b 设置为 long 它可以正常工作。

然后我检查了一个JS数字的最大大小，并将其与C# long result

进行了比较

240518168576 < Number.MAX_SAFE_INTEGER = true

所以..我可以看到发生了某种数字大小问题，但不知道如何强制 JS 将此数字视为长整数。

完整的 JS 代码：

private getPointsFromEncodedString(encodedLine: string): number[][] {

    const EncodingString = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-";

    var points: number[][] = [];

    if (!encodedLine) {
        return points;
    }

    var index = 0;
    var xsum = 0;
    var ysum = 0;

    while (index < encodedLine.length) {
        var n = 0;
        var k = 0;

        debugger;

        while (true) {
            if (index >= encodedLine.length) {
                return points;
            }

            var b = EncodingString.indexOf(encodedLine[index++]);

            if (b == -1) {
                return points;
            }

            n |= (b & 31) << k;
            k += 5;
            if (b < 32) {
                break;
            }
        }

        var diagonal = ((Math.sqrt(8 * n + 5) - 1) / 2);

        n -= diagonal * (diagonal + 1) / 2;

        var ny = n;
        var nx = diagonal - ny;

        nx = (nx >> 1) ^ -(nx & 1);
        ny = (ny >> 1) ^ -(ny & 1);

        xsum += nx;
        ysum += ny;

        points.push([ysum * 0.000001, xsum * 0.000001]);
    }

    console.log(points);

    return points;
}

预期输入输出：

编码字符串

qkoo7v4q-lmB0471BiuuNmo30B

解码点数：

35.89431, -110.72522
35.89393, -110.72578
35.89374, -110.72606
35.89337, -110.72662

Answer 1

Bitwise operators treat their operands as a sequence of 32 bits (zeroes and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.

(39 & 31) << 35 试图在只有 32

时移动 35 位

Bitwise Operators

要解决此问题，您可以使用 BigInt 执行这些操作，然后将其向下转换回 Number

Number((39n & 31n) << 35n)

你可以试试这个：

function getPointsFromEncodedString(encodedLine) {

    const EncodingString = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-";

    var points = [];

    if (!encodedLine) {
        return points;
    }

    var index = 0;
    var xsum = 0;
    var ysum = 0;

    while (index < encodedLine.length) {
        var n = 0n;
        var k = 0n;


        while (true) {
            if (index >= encodedLine.length) {
                return points;
            }

            var b = EncodingString.indexOf(encodedLine[index++]);

            if (b === -1) {
                return points;
            }

            n |= (b & 31n) << k;
            k += 5n;
            if (b < 32n) {
                break;
            }
        }

        var diagonal = ((Math.sqrt(8 * Number(n) + 5) - 1) / 2);

        n -= diagonal * (diagonal + 1) / 2;

        var ny = n;
        var nx = diagonal - ny;

        nx = (nx >> 1) ^ -(nx & 1);
        ny = (ny >> 1) ^ -(ny & 1);

        xsum += Number(nx);
        ysum += Number(ny);

        points.push([ysum * 0.000001, xsum * 0.000001]);
    }

    console.log(points);

    return points;
}

在 javascript 中对大数使用按位运算符

Using bitwise operators with large numbers in javascript

javascript

compression

long-integer