如何正确解析 R 中的字节流？

Question

我正在访问 API，其中 returns 一长串原始字节。

我的 Q 不适合 API 本身的简单代表，但这是我最好的镜头：

raw_bytes <-
 as.raw(c("0x43","0xb7","0x01","0x48","0x43","0xb7","0x01","0x48","0x43","0xb7","0x01","0x48","0x43","0xb7","0x01","0x48","0x3f","0x80","0x00","0x00","0x00","0x00","0x01","0x5e","0xa9","0x3e","0x83","0x80"))

   > str(raw_bytes)
     raw [1:28] 43 b7 01 48 ...

现在，从 API 文档中，我知道这个 28 字节的块将按如下方式解析，"big" endian-ness：

字节类型

4 浮动

8 长整数（这是一个日期对象，定义为从 1970 年 1 月 1 日开始的毫秒数）

writeBin(raw_bytes, "myfile.txt")

con <- file("myfile.txt", "rb") # create connection object; specify raw binary

> readBin(con, "double", size = 4, n = 5, endian = "big") # get those first 5 objects from the chunk
[1] 366.00 366.00 365.75 366.00  10.70

到目前为止一切顺利；这些与我的预期一致。

> readBin(con, "integer", size = 8, n = 1, endian = "big") # get the last 8 byte chunk
[1] -1453180896

嗯，看起来不对。在线 8 字节十六进制转换器建议正确的十进制值为 1506080340000，这与我期望的日期相匹配（2017 年 9 月 22 日）

仔细看看最后 8 个字节：

> (con2 <- tail(raw_bytes, 8))

[1] 00 00 01 5e a9 62 38 20

并尝试对 readBin() 进行一些不同的尝试：

> readBin(con2, "double", size = 8, n = 1, endian = "big")
[1] 7.441026e-312

> readBin(con2, "numeric", size = 8, n = 1, endian = "little")
[1] 1.818746e-153

> readBin(con2, "integer", size = 8, n = 1, endian = "little")
[1] 1577123840

没有。

我可以使用外部库从这些字节中生成预期的十进制数：

str <- paste(con2, collapse = "")

> bit64::as.integer64(as.numeric(paste0("0x",str)))
integer64
[1] 1506080340000

无论如何，这是我的问题：有没有办法使用 base R 正确解析我的比特流，尤其是 readBin()？

而且，更一般地说，是否有关于如何在 R 会话中解析字节流的自以为是的方式？

Answer 1

您可以使用类似问题的答案：。它实际上还尝试读取日期。

一个更 hacky 的答案是这样的：

library( bit64 )
con <- file("myfile.txt", "rb")
readBin(con, "double", size = 4, n = 5, endian = "big")
a = readBin(con, "double", size = 8, n = 1, endian = "big")
class(a) = "integer64"
a
# 1506078000000

呸！或者：

library( bit64 )
con <- file("myfile.txt", "rb")
readBin(con, "double", size = 4, n = 5, endian = "big")
sum( as.integer64( readBin(con,"integer",size=2,n=4,endian="big",signed=F) ) * 
     as.integer64(65536)^(3:0) )

如何正确解析 R 中的字节流？

How can correctly parse a byte stream in R?

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