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js实现packed

2026-01-31 18:22:17JavaScript

实现 Packed 编码的 JavaScript 方法

Packed 编码通常用于将多个较小的数值高效地存储在一个较大的数值中（如 32 位整数）。以下是几种常见的实现方式：

位操作实现

使用位操作将多个数值打包到一个整数中：

function packNumbers(a, b, c, d) {
    return (a << 24) | (b << 16) | (c << 8) | d;
}

function unpackNumbers(packed) {
    return [
        (packed >> 24) & 0xff,
        (packed >> 16) & 0xff,
        (packed >> 8) & 0xff,
        packed & 0xff
    ];
}

TypedArray 实现

使用 ArrayBuffer 和 TypedArray 进行更灵活的打包：

function packToBuffer(values, bitsPerValue) {
    const buffer = new ArrayBuffer(Math.ceil(values.length * bitsPerValue / 8));
    const view = new DataView(buffer);

    let bitOffset = 0;
    for (const value of values) {
        const byteIndex = Math.floor(bitOffset / 8);
        const bitIndex = bitOffset % 8;

        const remainingBits = 8 - bitIndex;
        if (bitsPerValue <= remainingBits) {
            view.setUint8(byteIndex, view.getUint8(byteIndex) | (value << bitIndex));
        } else {
            const lowPart = value & ((1 << remainingBits) - 1);
            const highPart = value >> remainingBits;

            view.setUint8(byteIndex, view.getUint8(byteIndex) | (lowPart << bitIndex));
            view.setUint8(byteIndex + 1, highPart);
        }

        bitOffset += bitsPerValue;
    }

    return buffer;
}

字符串打包

将数值打包为紧凑字符串表示：

function packToBase64(numbers) {
    const buffer = new Uint8Array(numbers.length * 4);
    const view = new DataView(buffer.buffer);

    for (let i = 0; i < numbers.length; i++) {
        view.setUint32(i * 4, numbers[i]);
    }

    return btoa(String.fromCharCode(...buffer));
}

function unpackFromBase64(str) {
    const binary = atob(str);
    const buffer = new Uint8Array(binary.length);

    for (let i = 0; i < binary.length; i++) {
        buffer[i] = binary.charCodeAt(i);
    }

    const view = new DataView(buffer.buffer);
    const result = [];

    for (let i = 0; i < buffer.length; i += 4) {
        result.push(view.getUint32(i));
    }

    return result;
}