给定一个64位(8字节)的little-endian ArrayBuffer字节,我们如何在JavaScript中读取64位整数值?
我进行了实验并想出了这个,但是有一个更优雅的解决方案,因为DataView还没有提供getUint64()吗?
const bytes = new Uint8Array([ 0xff,0xff,0xff,0xff, 0xff,0xff,0xff,0xff ]);
// [----- left -----] [---- right ----]
const view = new DataView(bytes.buffer);
// split 64-bit number into two 32-bit numbers
const left = view.getUint32(0, true); // 4294967295
const right = view.getUint32(4, true); // 4294967295
// combine the 2 32-bit numbers using max 32-bit val 0xffffffff
const combined = left + 2**32*right;
console.log('combined', combined);
// 18,446,744,073,709,552,000 is returned Javascript for "combined"
// 18,446,744,073,709,551,615 is max uint64 value
// some precision is lost since JS doesn't support 64-bit ints, but it's close enough
解决方法:
基于原始实验和Sebastian Speitel
的建议/修复,此函数返回64位值,直到Number.MAX_SAFE_INTEGER
之后精度丢失
DataView.prototype.getUint64 = function(byteOffset, littleEndian) {
// split 64-bit number into two 32-bit parts
const left = this.getUint32(byteOffset, littleEndian);
const right = this.getUint32(byteOffset+4, littleEndian);
// combine the two 32-bit values
const combined = littleEndian? left + 2**32*right : 2**32*left + right;
if (!Number.isSafeInteger(combined))
console.warn(combined, 'exceeds MAX_SAFE_INTEGER. Precision may be lost');
return combined;
}
测试如下:
// [byteArray, littleEndian, expectedValue]
const testValues = [
// big-endian
[new Uint8Array([0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff]), false, 255],
[new Uint8Array([0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff]), false, 65535],
[new Uint8Array([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]), false, 4294967295],
[new Uint8Array([0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00]), false, 4294967296],
[new Uint8Array([0x00, 0x1f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff]), false, 9007199254740991], // maximum precision
[new Uint8Array([0x00, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]), false, 9007199254740992], // precision lost
[new Uint8Array([0x00, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01]), false, 9007199254740992], // precision lost
// little-endian
[new Uint8Array([0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]), true, 255],
[new Uint8Array([0xff, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]), true, 65535],
[new Uint8Array([0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0x00]), true, 4294967295],
[new Uint8Array([0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00]), true, 4294967296],
[new Uint8Array([0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00]), true, 1099511627776],
[new Uint8Array([0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00]), true, 281474976710656],
[new Uint8Array([0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x1f, 0x00]), true, 9007199254740991], // maximum precision
];
testValues.forEach(testGetUint64);
function testGetUint64([bytes, littleEndian, expectedValue]) {
const val = new DataView(bytes.buffer).getUint64(0, littleEndian);
console.log(val === expectedValue? 'pass' : 'FAIL. expected '+expectedValue+', received '+val);
}
标签:arraybuffer,javascript 来源: https://codeday.me/bug/20191008/1871184.html
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