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function varargout = polylineSelfIntersections(poly, varargin)
%POLYLINESELFINTERSECTIONS Find self-intersection points of a polyline.
%
%   Computes self-intersections of a polyline, eventually specifying if
%   polyline is closed or open, and eventually returning position of
%   intersection points on polyline.
%   For common use cases, the intersectPolylines function may return the
%   desired result in a faster way.
%
%
%   PTS = polylineSelfIntersections(POLY);
%   Returns the position of self intersections of the given polyline.
%
%   PTS = polylineSelfIntersections(POLY, CLOSED);
%   Adds an options to specify if the polyline is closed (i.e., is a
%   polygon), or open (the default). CLOSED can be a boolean, or one of
%   'closed' or 'open'.
%
%   [PTS, POS1, POS2] = polylineSelfIntersections(POLY);
%   Also return the 2 positions of each intersection point (the position
%   when meeting point for first time, then position when meeting point
%   for the second time).
%
%   [...] = polylineSelfIntersections(POLY, 'tolerance', TOL)
%   Specifies an additional parameter to decide whether two intersection
%   points should be considered the same, based on their Euclidean
%   distance.  
%
%
%   Example
%       % use a gamma-shaped polyline
%       poly = [0 0;0 10;20 10;20 20;10 20;10 0];
%       polylineSelfIntersections(poly)
%       ans = 
%           10 10
%
%       % use a 'S'-shaped polyline
%       poly = [10 0;0 0;0 10;20 10;20 20;10 20];
%       polylineSelfIntersections(poly)
%       ans =
%           Empty matrix: 0-by-2
%       polylineSelfIntersections(poly, 'closed')
%       ans = 
%           10 10
%
%   See also
%   polygons2d, intersectPolylines, polygonSelfIntersections
%

% ------
% Author: David Legland
% e-mail: david.legland@inra.fr
% Created: 2009-06-15,    using Matlab 7.7.0.471 (R2008b)
% Copyright 2009 INRA - Cepia Software Platform.

%   HISTORY
%   2009/06/18 check bounding boxes before computing intersections


%% Initialisations

% flag indicating whether the polyline is closed (polygon) or not
closed = false;

% the tolerance for comparing positions based on distances
tol = 1e-14;

% determine whether the polyline is open or closed
if ~isempty(varargin)
    closed = varargin{1};
    if ischar(closed)
        if strcmp(closed, 'closed')
            closed = true;
            varargin(1) = [];
        elseif strcmp(closed, 'open')
            closed = false;
            varargin(1) = [];
        end
    end
end

% parse optional arguments
while length(varargin) > 1
    pname = varargin{1};
    if ~ischar(pname)
        error('Expect optional arguments as name-value pairs');
    end
    
    if strcmpi(pname, 'tolerance')
        tol = varargin{2};
    else
        error(['Unknown parameter name: ' pname]);
    end
    varargin(1:2) = [];
end

% if polyline is closed, ensure the last point equals the first one
if closed
    if sum(abs(poly(end, :) - poly(1,:)) < tol) ~= 2
        poly = [poly; poly(1,:)];
    end
end

% arrays for storing results
points  = zeros(0, 2);
pos1    = zeros(0, 1);
pos2    = zeros(0, 1);

% number of edges
nEdges = size(poly, 1) - 1;


%% Main processing

% index of current intersection
ip = 0;

% iterate over each couple of edge ( (N-1)*(N-2)/2 iterations)
for iEdge1 = 1:nEdges-1
    % create first edge
    edge1 = [poly(iEdge1, :) poly(iEdge1+1, :)];
    for iEdge2 = iEdge1+2:nEdges
        % create second edge
        edge2 = [poly(iEdge2, :) poly(iEdge2+1, :)];

        % check conditions on bounding boxes, to avoid computing the
        % intersections
        if min(edge1([1 3])) > max(edge2([1 3]))
            continue;
        end
        if max(edge1([1 3])) < min(edge2([1 3]))
            continue;
        end
        if min(edge1([2 4])) > max(edge2([2 4]))
            continue;
        end
        if max(edge1([2 4])) < min(edge2([2 4]))
            continue;
        end
        
        % compute intersection point
        inter = intersectEdges(edge1, edge2, tol);
        
        if sum(isfinite(inter)) == 2
            % add point to the list
            ip = ip + 1;
            points(ip, :) = inter;
            
            % also compute positions on the polyline
            pos1(ip, 1) = iEdge1 - 1 + edgePosition(inter, edge1);
            pos2(ip, 1) = iEdge2 - 1 + edgePosition(inter, edge2);
        end
    end
end


%% Post-processing

% if polyline is closed, the first vertex was found as an intersection, so
% we need to remove it
if closed
    % identify the intersection between first and last edges using position
    % indices (pos1 < pos2 by construction)
    ind = pos1 == 0 & pos2 == size(poly,1)-1;
    points(ind,:) = [];
    pos1(ind)   = [];
    pos2(ind)   = [];
end

% remove multiple intersections
[points, I, J] = unique(points, 'rows', 'first'); %#ok<ASGLU>
pos1 = pos1(I);
pos2 = pos2(I);

% remove multiple intersections, using tolerance on distance
iInter = 0;
while iInter < size(points, 1) - 1
    iInter = iInter + 1;
% for iInter = 1:size(points, 1)-1
    % determine distance between current point and remaining points
    inds = iInter+1:size(points, 1);
    dists = distancePoints(points(iInter,:), points(inds, :));
    
    % identify index of other points located in a close neighborhood
    inds = inds(dists < tol);
    
    % remove redundant intersection points
    if ~isempty(inds)
        points(inds, :) = [];
        pos1(inds) = [];
        pos2(inds) = [];
    end
end


%% process output arguments

if nargout <= 1
    varargout{1} = points;
    
elseif nargout == 3
    varargout{1} = points;
    varargout{2} = pos1;
    varargout{3} = pos2;
end
