This module exports two functions, adjust_crc and adjust_crc!, which allow you to write 4 bytes to a file or array in order to adjust the 32-bit CRC checksum (either CRC32 or CRC32c) to equal any desired value.

This is useful, for example, to store the checksum of data within the data itself, or simply to set the checksum to be a hard-coded value like 0x01020304 for ease of later checks.

adjust_crc!(crc, a::AbstractVector{UInt8}, wantcrc::UInt32, fixpos::Integer)

Write 4 bytes to a[fixpos:fixpos+3] so that crc(a) becomes equal to wantcrc. This is especially useful if you want to store the checksum of some data within the data itself, or simply to set the crc to an arbitrary predetermined value. Here, crc is a function that computes a 32-bit CRC checksum: either crc32c from the CRC32c standard library or crc32 from the CRC32.jl package.

Note that the adjust_crc! function is most efficient when fixpos is close to the end of the array a, and is slowest for fixpos near the beginning. (Though in all cases the cost should scale linearly with length(a).) See also adjust_crc, below, to append similar padding bytes to the end of a file or I/O stream (which has the advantage of not requiring you to read the entire file into memory at once).

adjust_crc(crc, filename::AbstractString, wantcrc::UInt32)
adjust_crc(crc, io::IO, wantcrc::UInt32)

Write 4 bytes of "padding" to the end of the the I/O stream io (which must be seekable and read/write) or the file filename, in order to cause the crc checksum of the whole stream/file to equal wantcrc. Here, crc is a function that computes a 32-bit CRC checksum: either crc32c from the CRC32c standard library or crc32 from the CRC32.jl package.

(This is mainly useful if you want to store the checksum of the file within the file: simply set wantcrc to be an arbitrary number, such as rand(UInt32), store it within the file as desired, and then call adjust_crc to write padding bytes that force the checksum to match wantcrc. Even more simply, you could force all of your files to have a checksum that matches a hard-coded value like 0x01020304, in which case you don't need to store the checksum in the file itself.) See also adjust_crc!, above, to write similar padding bytes to an arbitrary position within an array.

For example, suppose that we are writing some data to a file, and want to force the file's CRC-32c checksum to be 0x01020304, using the CRC32c standard library (which should usually be the fastest CRC available in Julia). Then we can use adjust_crc to append 4 bytes which force this checksum, either while we have the file still open or afterwards given the filename:

julia> using AdjustCRC, CRC32c

julia> open("foo.dat", "w+") do io   # create/open foo.dat for reading and writing
           println(io, "Hello, world!\n")
           adjust_crc(crc32c, io, 0x01020304)
       end
IOStream(<file foo.dat>)

julia> open(crc32c, "foo.dat") # check the CRC-32c checksum of the file "foo.dat"
0x01020304

julia> read("foo.dat", String) # note that it has 4 bytes of "garbage" at the end
"Hello, world!\n\n\xf3B_\xe4"

You can also pass a filename to adjust_crc to modify a file after the fact:

julia> adjust_crc(crc32c, "foo.dat", 0x05060708)
IOStream(<file foo.dat>)

julia> open(crc32c, "foo.dat") # check that it has the new checksum
0x05060708

julia> read("foo.dat", String) # another 4 bytes of "garbage" have been appended
"Hello, world!\n\n\xf3B_\xe4ޕ\x15\e"

The adjust_crc function always appends 4 bytes to the end of a file or stream in order to change the CRC to the desired value. If you have an array of bytes, you can also write these 4 "garbage" bytes anywhere in the array to adjust its CRC using the adjust_crc! function. For example, the following overwrites 4 bytes of the string "Hello, world!" starting at the 8th byte ('w') to make the checksum equal 0x01020304:

julia> bytes = Vector{UInt8}("Hello, world!") # a string converted to mutable bytes
13-element Vector{UInt8}:
 0x48
 0x65
 0x6c
 0x6c
 0x6f
 0x2c
 0x20
 0x77
 0x6f
 0x72
 0x6c
 0x64
 0x21

julia> adjust_crc!(crc32c, bytes, 0x01020304, 8) # adjust CRC to 0x01020304 via bytes[8:11]
13-element Vector{UInt8}:
 0x48
 0x65
 0x6c
 0x6c
 0x6f
 0x2c
 0x20
 0xf3
 0xe7
 0xf1
 0xe6
 0x64
 0x21

julia> s = String(bytes) # let's view the new bytes as a string; note the 4 "garbage" bytes
"Hello, \xf3\xe7\xf1\xe6d!"

julia> crc32c(s) # check that it has the desired CRC
0x01020304

All of the above examples used the CRC-32c checksum by passing the crc32c argument. You can instead do the same thing with the CRC-32 (ISO 3309 / ITU-T V.42 / CRC-32-IEEE) checksum by doing using CRC32 (from CRC32.jl) and passing crc32 instead.

This package was written by Steven G. Johnson based on the algorithm described in:

• Martin Stigge, Henryk Plötz, Wolf Müller, and Jens-Peter Redlich, Reversing CRC – Theory and Practice, HU Berlin Public Report SAR-PR-2006-05 (May 2006).

as suggested on StackOverflow.