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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