Bagaimana cara memecahkan kode hex kolom byte PostgreSQL ke int16/uint16 di r?

Anda dapat mulai dengan fungsi konversi ini, ganti dengan strsplit yang lebih cepat dan gunakan readBin pada hasil:

byteArray <- "\\xffff00000100020003000400050006000700080009000a00"

## Split a long string into a a vector of character pairs
Rcpp::cppFunction( code = '
CharacterVector strsplit2(const std::string& hex) {
  unsigned int length = hex.length()/2;
  CharacterVector res(length);
  for (unsigned int i = 0; i < length; ++i) {
    res(i) = hex.substr(2*i, 2);
  }
  return res;
}')

## A function to convert one string to an array of raw
f <- function(x)  {
  ## Split a long string into a a vector of character pairs
  x <- strsplit2(x)
  ## Remove the first element, "\\x"
  x <- x[-1]
  ## Complete the conversion
  as.raw(as.hexmode(x))
}

raw <- f(byteArray)
# int16
readBin(con = raw,
        what = "integer",
        n = length(raw) / 2,
        size = 2,
        signed = TRUE,
        endian = "little")
# -1  0  1  2  3  4  5  6  7  8  9 10

# uint16
readBin(con = raw,
        what = "integer",
        n = length(raw) / 2,
        size = 2,
        signed = FALSE,
        endian = "little")
# 65535     0     1     2     3     4     5     6     7     8     9    10

# int32
readBin(con = raw,
        what = "integer",
        n = length(raw) / 4,
        size = 4,
        signed = TRUE,
        endian = "little")
# 65535 131073 262147 393221 524295 655369

Ini tidak akan berfungsi untuk uint32 dan (u)int64 , meskipun, karena R menggunakan int32 secara internal. Namun, R juga dapat menggunakan numerics untuk menyimpan bilangan bulat di bawah 2^52. Jadi kita bisa menggunakan ini:

# uint32
byteArray <- "\\xffffffff0100020003000400050006000700080009000a00"
int32 <- readBin(con = f(byteArray),
                 what = "integer",
                 n = length(raw) / 4,
                 size = 4,
                 signed = TRUE,
                 endian = "little")

ifelse(int32 < 0, int32 + 2^32, int32)
# 4294967295     131073     262147     393221     524295     655369

Dan untuk gzip data terkompresi:

# gzip
byteArray <- "\\x1f8b080000000000000005c1870100200800209a56faffbd41d30dd3b285e37a52f9d033018818000000"
con <- gzcon(rawConnection(f(byteArray)))
readBin(con = con,
        what = "integer",
        n = length(raw) / 2,
        size = 2,
        signed = TRUE,
        endian = "little")
close(con = con)

Karena ini adalah koneksi nyata, kita harus memastikan untuk menutupnya.