function fmt = snc_format(theFile)
%SNC_FORMAT  Determine the file format if possible.
%
% which file format do we employ?  Many possibilities exist.
%
%   [fmt] = snc_format(ncfile)
%
% returns format for specified file, http or url.
%
%   [fmt] = snc_format()
%
% returns all available fmt options
%
%See also: snctools, snc_read_backend

fmts.URL                   = 'URL';
fmts.GRIB                  = 'GRIB';
fmts.GRIB2                 = 'GRIB2';
fmts.netcdf_java           = 'oct_netcdf-java'; % not used here, but in snc_read_backend
fmts.HDF4                  = 'HDF4';
fmts.NetCDF                = 'NetCDF';
fmts.NetCDF4               = 'NetCDF-4';
fmts.unknown               = 'unknown';

if nargin==0
   fmt              = fmts;
   return
else   
   fmt              = '';
end

% Must check for a URL first, because it doesn't exist as a local file.

if ~isempty(regexp(theFile,'https*://', 'once'))
	fmt = fmts.URL;
	return;
end


% For the GRIBs and oct_netcdf formats, we need to read a certain number
% of bytes.
afid = fopen(theFile,'r');
if afid == -1
    error('snctools:format:cannotOpenFile', ...
        'Cannot open %s.  Make sure it is on your path.', ...
        theFile);
end
[signature,count] = fread(afid,8,'uint8=>uint8');
fclose(afid);

if count < 4
    error('snctools:snc_format:truncatedFile', ...
        '''%s'' is less than four bytes long, so it must have been truncated.', ...
        theFile)
elseif (strcmp(char(signature(1:4))', 'GRIB')) && (signature(8) == 1)
    % GRIB : bytes 1-4 are 'GRIB', and byte 8 is 1.
	fmt = fmts.GRIB;
	return
elseif (strcmp(char(signature(1:4))', 'GRIB')) && (signature(8) == 2)
    % GRIB 2 : bytes 1-4 are 'GRIB', and byte 8 is 2.
	fmt = fmts.GRIB2;
	return
elseif (strcmp(char(signature(1:3))', 'CDF')) && (signature(4) == 1)
    % oct_netcdf classic : bytes 1-3 are 'CDF', and byte 4 is 1.
	fmt = fmts.NetCDF;
	return
elseif (strcmp(char(signature(1:3))', 'CDF')) && (signature(4) == 2)
    % oct_netcdf 64bit offset : bytes 1-3 are 'CDF', and byte 4 is 2.
	fmt = fmts.NetCDF;
	return
elseif (strcmp(char(signature(2:4))', 'HDF'))
    % oct_netcdf-4 : bytes 2-4 are 'HDF'.  
	fmt = fmts.NetCDF4;
	return
end

% Could it be HDF4?  Do this check last, because non-windows versions
% of matlab would identify a oct_netcdf file as HDF thru 2009b.

fid = fopen(theFile,'r');
filename = fopen(fid);
fclose(fid);

fid = hdfh('open',filename,'read',0);
if fid >= 0
	fmt = 'HDF4';
	hdfh('close',fid);
	return
end

% We couldn't figure out what it was.
fmt = fmts.unknown;

return