Computes the CRC-32 checksum of character or raw input: the CRC of ITU
V.42, zip and PNG (reflected polynomial 0xEDB88320, initial value
and final XOR 0xFFFFFFFF). Use it as a fast first check that a
large file or capsule member arrived intact, or to compare with the CRC
a zip archive records for its members.
Value
A numeric vector of unsigned 32-bit checksums (one per element
for character input, NA for NA; length 1 for raw input).
Returned as double rather than integer because values
above 2^31 - 1 are not representable as an R integer.
Details
A CRC is NOT a cryptographic digest: it detects accidental corruption
– a truncated download, a flipped bit on disk – but anyone can
construct a different input with the same value, so it must never be
used where csha256() is meant. It is
here because it is far cheaper than SHA-256 over gigabyte-scale capsule
members, which makes it the right first pass when the question is only
“did this file arrive intact”.
Character input is checked element by element, as its bytes, with
NA giving NA; other atomic input is converted with
as.character(). A raw vector gives one checksum. For files use
crc32file().
See also
csha256() for integrity against
tampering rather than accident.
Examples
# The standard check value: CRC-32("123456789") is 0xCBF43926.
ccrc32("123456789")
#> [1] 3421780262
ccrc32("123456789") == 0xCBF43926
#> [1] TRUE
# The empty input has checksum zero.
ccrc32("")
#> [1] 0
# Vectorised, and sensitive to a single changed byte.
ccrc32(c("brick", "brack"))
#> [1] 928041788 960731780
# Raw bytes work the same way.
ccrc32(charToRaw("123456789"))
#> [1] 3421780262