Add segy endian check#21
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Fix undef access
SEGY may have BIG or LITTLE endian byte order. Machine also may have BIG or LITTLE endian order. If machine and SEGY have different byte order then we have to swap bytes while reading data. To check whether machine and SEGY byte order are same we can use 3225-3226 bytes (Data Sample Format). According to SEGY revision document this value may have only value in range 1-16. Than concerns any type greater > 1 byte (even INT16, FLOAT32 or IBMFLOAT32 or any other). For `bswap_needed` correct work it is important that stream (`s::IO)` is opened from the begining of the file. Thus we can't use the stream when reading traces or trace header (there we pass stream that is opened from some offset). That is why in some functions we don't have default value for `swap_bytes::Bool` argument.
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mloubout
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Jun 7, 2023
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As a reference the following is the breaking tests (lots of errors) in JUDI GROUP=test_judiVector.jl julia --color=yes --check-bounds=no --project -e 'using Pkg;Pkg.test("JUDI", coverage=false)' |
If we swap bytes of traces and headers then SEGY will always be of another endianness than machine. No need to swap bytes when writing SEGY.
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Seems to me that tests passed but could please test it on your own as I'm not 100% sure that I didn't messed up with branches. |
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Reopen PR.
The poing is now SegyIO assumes that all SEGY are big endian.
It doesn't work with little endian.
Moreover it doesn't check the endianness of the machine.
This PR aims to check whether endianness of the machine and SEGY file are different or not.
Based on this we should make a decision whether to swap bytes or not.
Swap bytes or not depends on the value in file binary header 3224-3225 bytes (Data Sample Format).
This header is mandatory for all SEGY.
It must be in range [1,16].
If not -> then swap bytes and check again.
Also it is important to be careful with IBM32 format.
Does the function ntoh in IBM32->FLOAT32 converter works as expected on both BIG and LITTLE endian machines?
Finally we have to check carefully the following cases:
My machines are LE only :)