function [overlap_cat, data_cat, meandata_cat] = ...
    colloc_concatenate_colloc_data_mean(...
    overlap_old, overlap_new, ...
    data_old, data_new, ...
    meandata_old, meandata_new)

% colloc_concatenate_colloc_data_mean Concatenate collocation structures
%
% Concatenate collocation structures: a structure where each field is a
% matrix, and those are concatenated vertically by field. 
% Meandata is a special case.
% Meandata, as processed by process_meandata_*, contains columns (the first
% two) referring to the first and the last line in the collocation matrix
% that is being averaged over. For example, each row in the
% meandata_cpr_mhs describes between what two rows all the CloudSat pixels
% in the corresponding collocation matrix are described. Since this works
% by row-numbers, when the collocation tables and the corresponding
% meandata tables are concatenated, those row numbers need to be corrected.
%
% FORMAT
%
%    [overlap_cat, data_cat, meandata_cat] = ...
%       colloc_concatenate_colloc_data_mean(...
%           overlap_old, overlap_new, ...
%           data_old, data_new, ...
%           meandata_old, meandata_new)
%
% IN
%
%   overlap_old     structure
%   overlap_new     structure
%   data_old        structure
%   data_new        structure
%   meandata_old    structure
%   meandata_new    structure
%
% OUT
%
%   overlap_cat     structure
%   data_cat        structure
%   meandata_cat    structure
%
% $Id$

% in case nothing to be done
overlap_cat = overlap_old;
data_cat = data_old;
meandata_cat = meandata_old;

fields = fieldnames(overlap_new); % fields that exist in NEW one will be used
for n = 1:length(fields) % loop through all fields
    sat = fields{n};
    if strcmp(sat, 'version') % FIXME/UGLY: special case
%         if isfield(overlap_old, 'version')
%             assert(isequal(overlap_old.version, overlap_new.version), ...
%                 'atmlab:colloc_concatenate_colloc_data_mean', ...
%                 'Concatenating different versions, no good');
%         end
        overlap_cat.version = overlap_new.version;
        continue
    end
    
    if ~isfield(overlap_new, sat) || isempty(overlap_new.(sat)) % ignore
        continue
    end
    
    overlap_cat.(sat) = [overlap_old.(sat); overlap_new.(sat)];
    
    if ~isempty(data_new.(sat)) % that have anything this iteration
        data_cat.(sat) = [data_old.(sat); data_new.(sat)];
    end
    
    %% correct meandata FIRST/LAST columns
    % the first two columns of meandata should always be FIRST and LAST;
    % those refer to line numbers in the overlap matrices. From
    % collocate_granule, this is per granule. When concatenating
    % granules, those numbers need to be corrected.
    if any(meandata_new.(sat)(:)) % that have anything this iteration
        n_before = size(overlap_old.(sat), 1);

        meandata_new.(sat)(:, 1:2) = meandata_new.(sat)(:, 1:2) + n_before;
        % and proceed with cat'ing
        meandata_cat.(sat) = [meandata_cat.(sat); meandata_new.(sat)];
    end
end