将字节数组转换为整数而不循环的方法? [英] Method to convert byte array to integer without looping?
问题描述
(我已经看过这些问题(将字节转换为整数/C中的uint 和将字节数组转换为Int24 ),但它们似乎比我想做的要复杂得多,所以我想问一下.)
(I've seen these questions (Convert Bytes to Int / uint in C and Converting Byte Array To Int24) but they seem more complicated than what I am trying to do, so I thought I would ask.)
我正在Arduino/Wiring中进行一些LED矩阵编程.由于这里可能无关紧要的原因,我有一个字节数组,表示一个LED行的位状态",我用它来缓冲其他操作的结果.要实际设置LED(我使用的是Maxim 7219芯片),我需要从字节数组中得出一个整数.
I'm doing some LED matrix programming in Arduino/Wiring. For reasons probably not relevant here I have a byte array representing a "bit state" of a LED row, which I use to buffer the result of other operations. To actually set the LEDs (I'm using the Maxim 7219 chip), I need to derive an integer from the byte array.
使用arduino/wiring bitWrite 方法,下面的示例程序很简单,但是我想知道是否有比循环更快的C方法.
Using the Arduino/Wiring bitWrite method, my little stripped down example routine below works but I am wondering if there is a C method which would be faster than looping.
byte dog[8] = {0,0,1,1,1,1,1,0};
byte cat;
for (int i = 0; i < 8; i++){
bitWrite(cat, i, dog[7-i]);
}
推荐答案
您可以展开循环:
bitWrite(cat, 0, dog[7]);
bitWrite(cat, 1, dog[6]);
bitWrite(cat, 2, dog[5]);
bitWrite(cat, 3, dog[4]);
bitWrite(cat, 4, dog[3]);
bitWrite(cat, 5, dog[2]);
bitWrite(cat, 6, dog[1]);
bitWrite(cat, 7, dog[0]);
或设置不带库功能的位(仅在保证字节为0或1的情况下有效):
Or set the bits without a library function (only works if the bytes are guaranteed to be either 0 or 1):
cat = (dog[7] << 0) |
(dog[6] << 1) |
(dog[5] << 2) |
(dog[4] << 3) |
(dog[3] << 4) |
(dog[2] << 5) |
(dog[1] << 6) |
(dog[0] << 7);
但是C中没有内置任何命令可以执行此操作,因此它可能不会比这快得多.
But there's nothing built into C to do this with a single command so it probably won't get much faster than that.
编辑:通过一些琐碎的技巧,可以(可能)加快速度.如下所示的内容应在低端32位处理器上工作:
With some bit-twiddling tricks this can be (probably) sped up a little. Something like the following should work on a little-endian 32-bit processor:
uint32_t int_dog = (uint32_t*)dog;
uint32_t t0, t1;
t0 = int_dog[0]; // .......3.......2.......1.......0
t0 |= t0 << 9; // ......23......12......01.......0
t0 |= t0 << 18; // ....0123.....012......01.......0
t1 = int_dog[1]; // .......7.......6.......5.......4
t1 |= t1 << 9; // ......67......56......45.......4
t1 |= t1 << 18; // ....4567.....456......45.......4
cat = (t0 >> 20) | (t1 >> 24);
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