如何将数组中的最后4个字节转换为整数？ [英] How to convert last 4 bytes in an array to an integer?

问题描述

如果我在JavaScript中有一个 Uint8Array 数组，我如何获得最后四个字节然后将其转换为int？使用C＃我会这样做：

If I have an Uint8Array array in JavaScript, how would I get the last four bytes and then convert that to an int? Using C# I would do something like this:

int count = BitConverter.ToInt32(array, array.Length - 4);

是否有使用JavaScript的不等效方法？

Is there an inequivalent way to do this using JavaScript?

推荐答案

访问底层的 ArrayBuffer 并创建一个新的 TypedArray 用一小块字节：

Access the underlying ArrayBuffer and create a new TypedArray with a slice of its bytes:

var u8 = new Uint8Array([1,2,3,4,5,6]); // original array
var u32bytes = u8.buffer.slice(-4); // last four bytes as a new `ArrayBuffer`
var uint = new Uint32Array(u32bytes)[0];

如果 TypedArray 未覆盖整个缓冲区，你需要有点棘手但不多：

If the TypedArray does not cover the entire buffer, you need to be a little trickier, but not much:

var startbyte = u8.byteOffset + u8.byteLength - Uint32Array.BYTES_PER_ELEMENT;
var u32bytes = u8.buffer.slice(startbyte, startbyte + Uint32Array.BYTES_PER_ELEMENT);

这适用于两种情况。

如果您想要的字节适合数据类型的底层缓冲区的对齐边界（例如，您需要底层缓冲区的字节4-8的32位值），则可以避免使用 slice（）并且只为视图构造函数提供一个byteoffset，就像@ Bergi的回答一样。

If the bytes you want fit in the alignment boundary of your underlying buffer for the datatype (e.g., you want the 32-bit value of bytes 4-8 of the underlying buffer), you can avoid copying the bytes with slice() and just supply a byteoffset to the view constructor, as in @Bergi's answer.

下面是一个非常轻微的测试应该获得所需偏移量的标量值的函数。如果可能，它将避免复制。

Below is a very-lightly-tested function that should get the scalar value of any offset you want. It will avoid copying if possible.

function InvalidArgument(msg) {
    this.message = msg | null;
}

function scalarValue(buf_or_view, byteOffset, type) {
    var buffer, bufslice, view, sliceLength = type.BYTES_PER_ELEMENT;
    if (buf_or_view instanceof ArrayBuffer) {
        buffer = buf_or_view;
        if (byteOffset < 0) {
            byteOffset = buffer.byteLength - byteOffset;
        }
    } else if (buf_or_view.buffer instanceof ArrayBuffer) {
        view = buf_or_view;
        buffer = view.buffer;
        if (byteOffset < 0) {
            byteOffset = view.byteOffset + view.byteLength + byteOffset;
        } else {
            byteOffset = view.byteOffset + byteOffset;
        }
        return scalarValue(buffer, view.byteOffset + byteOffset, type);
    } else {
        throw new InvalidArgument('buf_or_view must be ArrayBuffer or have a .buffer property');
    }
    // assert buffer instanceof ArrayBuffer
    // assert byteOffset > 0
    // assert byteOffset relative to entire buffer
    try {
        // try in-place first
        // only works if byteOffset % slicelength === 0
        return (new type(buffer, byteOffset, 1))[0]
    } catch (e) {
        // if this doesn't work, we need to copy the bytes (slice them out)
        bufslice = buffer.slice(byteOffset, byteOffset + sliceLength);
        return (new type(bufslice, 0, 1))[0]
    }
}

您可以这样使用它：

// positive or negative byte offset
// relative to beginning or end *of a view*
100992003 === scalarValueAs(u8, -4, Uint32Array)
// positive or negative byte offset
// relative to the beginning or end *of a buffer*
100992003 === scalarValue(u8.buffer, -4, Uint32Array)

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