+void gdcmHeader::CheckSwap()
+{
+ // Fourth semantics:
+ //
+ // ---> Warning : This fourth field is NOT part
+ // of the 'official' Dicom Dictionnary
+ // and should NOT be used.
+ // (Not defined for all the groups
+ // may be removed in a future release)
+ //
+ // CMD Command
+ // META Meta Information
+ // DIR Directory
+ // ID
+ // PAT Patient
+ // ACQ Acquisition
+ // REL Related
+ // IMG Image
+ // SDY Study
+ // VIS Visit
+ // WAV Waveform
+ // PRC
+ // DEV Device
+ // NMI Nuclear Medicine
+ // MED
+ // BFS Basic Film Session
+ // BFB Basic Film Box
+ // BIB Basic Image Box
+ // BAB
+ // IOB
+ // PJ
+ // PRINTER
+ // RT Radio Therapy
+ // DVH
+ // SSET
+ // RES Results
+ // CRV Curve
+ // OLY Overlays
+ // PXL Pixels
+ // DL Delimiters
+ //
+
+ // The only guaranted way of finding the swap code is to find a
+ // group tag since we know it's length has to be of four bytes i.e.
+ // 0x00000004. Finding the swap code in then straigthforward. Trouble
+ // occurs when we can't find such group...
+ guint32 s;
+ guint32 x=4; // x : for ntohs
+ bool net2host; // true when HostByteOrder is the same as NetworkByteOrder
+
+ int lgrLue;
+ char * entCur;
+ char deb[HEADER_LENGTH_TO_READ];
+
+ // First, compare HostByteOrder and NetworkByteOrder in order to
+ // determine if we shall need to swap bytes (i.e. the Endian type).
+ if (x==ntohs(x))
+ net2host = true;
+ else
+ net2host = false;
+ //cout << net2host << endl;
+
+ // The easiest case is the one of a DICOM header, since it possesses a
+ // file preamble where it suffice to look for the string "DICM".
+ lgrLue = fread(deb, 1, HEADER_LENGTH_TO_READ, fp);
+
+ entCur = deb + 128;
+ if(memcmp(entCur, "DICM", (size_t)4) == 0) {
+ dbg.Verbose(1, "gdcmHeader::CheckSwap:", "looks like DICOM Version3");
+ // Next, determine the value representation (VR). Let's skip to the
+ // first element (0002, 0000) and check there if we find "UL"
+ // - or "OB" if the 1st one is (0002,0001) -,
+ // in which case we (almost) know it is explicit VR.
+ // WARNING: if it happens to be implicit VR then what we will read
+ // is the length of the group. If this ascii representation of this
+ // length happens to be "UL" then we shall believe it is explicit VR.
+ // FIXME: in order to fix the above warning, we could read the next
+ // element value (or a couple of elements values) in order to make
+ // sure we are not commiting a big mistake.
+ // We need to skip :
+ // * the 128 bytes of File Preamble (often padded with zeroes),
+ // * the 4 bytes of "DICM" string,
+ // * the 4 bytes of the first tag (0002, 0000),or (0002, 0001)
+ // i.e. a total of 136 bytes.
+ entCur = deb + 136;
+ // FIXME
+ // Use gdcmHeader::dicom_vr to test all the possibilities
+ // instead of just checking for UL, OB and UI !?
+ if( (memcmp(entCur, "UL", (size_t)2) == 0) ||
+ (memcmp(entCur, "OB", (size_t)2) == 0) ||
+ (memcmp(entCur, "UI", (size_t)2) == 0) )
+ {
+ filetype = ExplicitVR;
+ dbg.Verbose(1, "gdcmHeader::CheckSwap:",
+ "explicit Value Representation");
+ } else {
+ filetype = ImplicitVR;
+ dbg.Verbose(1, "gdcmHeader::CheckSwap:",
+ "not an explicit Value Representation");
+ }
+ if (net2host) {
+ sw = 4321;
+ dbg.Verbose(1, "gdcmHeader::CheckSwap:",
+ "HostByteOrder != NetworkByteOrder");
+ } else {
+ sw = 0;
+ dbg.Verbose(1, "gdcmHeader::CheckSwap:",
+ "HostByteOrder = NetworkByteOrder");
+ }
+
+ // Position the file position indicator at first tag (i.e.
+ // after the file preamble and the "DICM" string).
+ rewind(fp);
+ fseek (fp, 132L, SEEK_SET);
+ return;
+ } // End of DicomV3
+
+ // Alas, this is not a DicomV3 file and whatever happens there is no file
+ // preamble. We can reset the file position indicator to where the data
+ // is (i.e. the beginning of the file).
+ dbg.Verbose(1, "gdcmHeader::CheckSwap:", "not a DICOM Version3 file");
+ rewind(fp);
+
+ // Our next best chance would be to be considering a 'clean' ACR/NEMA file.
+ // By clean we mean that the length of the first tag is written down.
+ // If this is the case and since the length of the first group HAS to be
+ // four (bytes), then determining the proper swap code is straightforward.
+
+ entCur = deb + 4;
+ // We assume the array of char we are considering contains the binary
+ // representation of a 32 bits integer. Hence the following dirty
+ // trick :
+ s = *((guint32 *)(entCur));
+
+ switch (s) {
+ case 0x00040000 :
+ sw = 3412;
+ filetype = ACR;
+ return;
+ case 0x04000000 :
+ sw = 4321;
+ filetype = ACR;
+ return;
+ case 0x00000400 :
+ sw = 2143;
+ filetype = ACR;
+ return;
+ case 0x00000004 :
+ sw = 0;
+ filetype = ACR;
+ return;
+ default :
+ dbg.Verbose(0, "gdcmHeader::CheckSwap:",
+ "ACR/NEMA unfound swap info (time to raise bets)");
+ }
+
+ // We are out of luck. It is not a DicomV3 nor a 'clean' ACR/NEMA file.
+ // It is time for despaired wild guesses. So, let's assume this file
+ // happens to be 'dirty' ACR/NEMA, i.e. the length of the group is
+ // not present. Then the only info we have is the net2host one.
+ filetype = Unknown;
+ if (! net2host )
+ sw = 0;
+ else
+ sw = 4321;
+ return;