absFile2relFile(abs_file, rel_to_abs_file)

function [rel_ref_file_path level] = absFile2relFile(abs_file, rel_to_abs_file)
% absFile2relFile  Returns a relative path to given absolute file.
% Input:
% * abs_file ... absolute file for which to find the realtive path
% * rel_to_abs_file ... this is the basis for the relative path
%
% Output:
% * rel_ref_file_path ... relative path
% * level ... if in the same directory level equals zero, one directory
% below returns level -1, above returns +1
% 
% Note: In case any of the files is not found or they are on different
% drives empty parameters are returned.
%
% Example
% first create diretories 
%+ mkdir('c:\test\a\b\'); 
% and test files
%+ cd('c:\test\a\b\'); fid=fopen('c.txt','w'); fclose(fid);
%+ cd('c:\test\'); fid=fopen('d.txt','w'); fclose(fid);
% now execute the command
%+ [rel_ref_file_path level] = absFile2relFile('c:\test\a\b\c.txt', 'c:\test\d.txt')
% The file c.txt is to levels (directories) below d.txt, hence the result:
% a/b and level -2.
%
% Continue the example with ...
%+ mkdir('c:\test\x\y\'); 
%+ cd('c:\test\x\y\'); fid=fopen('z.txt','w'); fclose(fid);
% now execute the command
%+ [rel_ref_file_path level] = absFile2relFile('c:\test\a\b\c.txt', 'c:\test\x\y\z.txt')
%The absolute file is "abc", and "xyz" is relative to "abc", hence
% go twice up to 'c:\test' (../../) and then go to "z" (x/y)
% together we got ../../x/y
%
% See also: uigetfile, uiputfile, dir
%
%% Signature
% Author: W.Garn
% E-Mail: [e-mail]
% Date: 2005/12/05 20:00:00 
% 
% Copyright 2005 W.Garn
%  
if nargin<1
    help absFile2relFile;
end
rel_ref_file_path=[]; level=0;

if exist(abs_file,'file') && exist(rel_to_abs_file,'file')
    abs_dirs = getDirs(abs_file);
    rel_dirs = getDirs(rel_to_abs_file);
    
    n = length(abs_dirs);
    m = length(rel_dirs);
    % common basis but distinct branches
    basis = min( find ( (abs_dirs(1:min(n,m)) == rel_dirs(1:min(n,m))) == 0 ) )-1;
    % common basis but what goes deeper
    if isempty(basis), 
        basis = max( find ( (abs_dirs(1:min(n,m)) == rel_dirs(1:min(n,m))) == 1 ) );        
    end
    %if basis== min(n,m) % then one contains the other
        if n<=m %above or same level
            rel_ref_file_path = rel_dirs(basis+1:m);
            level = length(allSlashes(rel_ref_file_path));
            pp = '';
            for k=1:level
                pp = [pp '../'];
            end
            rel_ref_file_path = [pp rel_ref_file_path];
        else %within
            rel_ref_file_path = abs_dirs(basis+1:n);
            level = -length(allSlashes(rel_ref_file_path));
        end
    %end
end

function dirs = getDirs(abs_file)
% Returns the directories including the slash or backslash.
    Ib = strfind(abs_file,'\');
    Is = strfind(abs_file,'/');
    I = union(Ib,Is);
    if ~isempty(I)
        dirs = abs_file(1:max(I));
        dirs(I) = '/'; % replace backslahs by slash
    else
        dirs=[];
    end

function pos = allSlashes(str)
Ib = strfind(str,'\');
Is = strfind(str,'/');
pos = union(Ib,Is);