regions.scad

//////////////////////////////////////////////////////////////////////
// LibFile: regions.scad
//   This file provides 2D boolean set operations on polygons, where you can
//   compute, for example, the intersection or union of the shape defined by point lists, producing
//   a new point list.  Of course, such operations may produce shapes with multiple
//   components.  To handle that, we use "regions" which are lists of paths representing the polygons.
//   In addition to set operations, you can calculate offsets, determine whether a point is in a
//   region and you can decompose a region into parts.  
// Includes:
//   include <BOSL2/std.scad>
// FileGroup: Advanced Modeling
// FileSummary: Offsets and boolean geometry of 2D paths and regions.
// FileFootnotes: STD=Included in std.scad
//////////////////////////////////////////////////////////////////////


// CommonCode:
//   include <BOSL2/rounding.scad>


// Section: Regions
//   A region is a list of polygons meeting these conditions:
//   .
//   - Every polygon on the list is simple, meaning it does not intersect itself
//   - Two polygons on the list do not cross each other
//   - A vertex of one polygon never meets the edge of another one except at a vertex
//   .
//   Note that this means vertex-vertex touching between two polygons is acceptable
//   to define a region.  Note, however, that regions with vertex-vertex contact usually
//   cannot be rendered with CGAL.  See {{is_valid_region()}} for examples of valid regions and
//   lists of polygons that are not regions.  Note that {{is_region_simple()}} will identify
//   regions with no polygon intersections at all, which should render successfully witih CGAL.  
//   .
//   The actual geometry of the region is defined by XORing together
//   all of the polygons in the list.  This may sound obscure, but it simply means that nested
//   boundaries make rings in the obvious fashion, and non-nested shapes simply union together.
//   Checking that a list of polygons is a valid region, meaning that it satisfies all of the conditions
//   above, can be a time consuming test, so it is not done automatically.  It is your responsibility to ensure that your regions are
//   compliant.  You can construct regions by making a suitable list of polygons, or by using
//   set operation function such as union() or difference(), which all acccept polygons, as
//   well as regions, as their inputs.  And if you must you can clean up an ill-formed region using make_region(),
//   which will break up self-intersecting polygons and polygons that cross each other.  


// Function: is_region()
// Usage:
//   bool = is_region(x);
// Description:
//   Returns true if the given item looks like a region.  A region is a list of non-crossing simple polygons.  This test just checks
//   that the argument is a list whose first entry is a path.  
function is_region(x) = is_list(x) && is_path(x.x);


// Function: is_valid_region()
// Usage:
//   bool = is_valid_region(region, [eps]);
// Description:
//   Returns true if the input is a valid region, meaning that it is a list of simple polygons whose segments do not cross each other.
//   This test can be time consuming with regions that contain many points.
//   It differs from `is_region()` which simply checks that the object is a list whose first entry is a path
//   because it searches all the list polygons for any self-intersections or intersections with each other.  
//   Will also return true if given a single simple polygon.  Use {{make_region()}} to convert sets of self-intersecting polygons into
//   a region.  
// Arguments:
//   region = region to check
//   eps = tolerance for geometric comparisons.  Default: `EPSILON` = 1e-9
// Example(2D,NoAxes):  In all of the examples each polygon in the region appears in a different color.  Two non-intersecting squares make a valid region.
//   region = [square(10), right(11,square(8))];
//   rainbow(region)stroke($item, width=.2,closed=true);
//   back(11)text(is_valid_region(region) ? "region" : "non-region", size=2);
// Example(2D,NoAxes):  Nested squares form a region
//   region = [for(i=[3:2:10]) square(i,center=true)];
//   rainbow(region)stroke($item, width=.2,closed=true);
//   back(6)text(is_valid_region(region) ? "region" : "non-region", size=2,halign="center");
// Example(2D,NoAxes):  Also a region:
//   region= [square(10,center=true), square(5,center=true), right(10,square(7))];
//   rainbow(region)stroke($item, width=.2,closed=true);
//   back(8)text(is_valid_region(region) ? "region" : "non-region", size=2);
// Example(2D,NoAxes):  The squares cross each other, so not a region
//   object = [square(10), move([8,8], square(8))];
//   rainbow(object)stroke($item, width=.2,closed=true);
//   back(17)text(is_valid_region(object) ? "region" : "non-region", size=2);
// Example(2D,NoAxes): A union is one way to fix the above example and get a region.  (Note that union is run here on two simple polygons, which are valid regions themselves and hence acceptable inputs to union.
//   region = union([square(10), move([8,8], square(8))]);
//   rainbow(region)stroke($item, width=.25,closed=true);
//   back(12)text(is_valid_region(region) ? "region" : "non-region", size=2);
// Example(2D,NoAxes):  Not a region due to a self-intersecting (non-simple) hourglass polygon
//   object = [move([-2,-2],square(14)), [[0,0],[10,0],[0,10],[10,10]]];
//   rainbow(object)stroke($item, width=.2,closed=true);
//   move([-1.5,13])text(is_valid_region(object) ? "region" : "non-region", size=2);
// Example(2D,NoAxes):  Breaking hourglass in half fixes it.  Now it's a region:
//   region = [move([-2,-2],square(14)), [[0,0],[10,0],[5,5]], [[5,5],[0,10],[10,10]]];
//   rainbow(region)stroke($item, width=.2,closed=true);
// Example(2D,NoAxes):  A single polygon corner touches an edge, so not a region:
//   object = [[[-10,0], [-10,10], [20,10], [20,-20], [-10,-20],
//              [-10,-10], [0,0], [10,-10], [10,0]]];
//   rainbow(object)stroke($item, width=.3,closed=true);
//   move([-4,12])text(is_valid_region(object) ? "region" : "non-region", size=3);
// Example(2D,NoAxes):  Corners touch in the same polygon, so the polygon is not simple and the object is not a region.
//   object = [[[0,0],[10,0],[10,10],[-10,10],[-10,0],[0,0],[-5,5],[5,5]]];
//   rainbow(object)stroke($item, width=.3,closed=true);
//   move([-10,12])text(is_valid_region(object) ? "region" : "non-region", size=3);
// Example(2D,NoAxes):  The shape above as a valid region with two polygons:
//   region = [  [[0,0],[10,0],[10,10],[-10,10],[-10,0]],
//               [[0,0],[5,5],[-5,5]]  ];
//   rainbow(region)stroke($item, width=.3,closed=true);
//   move([-5.5,12])text(is_valid_region(region) ? "region" : "non-region", size=3);
// Example(2D,NoAxes):  As with the "broken" hourglass, Touching at corners is OK.  This is a region.
//   region = [square(10), move([10,10], square(8))];
//   rainbow(region)stroke($item, width=.25,closed=true);
//   back(12)text(is_valid_region(region) ? "region" : "non-region", size=2);
// Example(2D,NoAxes): These two squares share part of an edge, hence not a region
//   object = [square(10), move([10,2], square(7))];
//   stroke(object[0], width=0.2,closed=true);
//   color("red")dashed_stroke(object[1], width=0.25,closed=true);
//   back(12)text(is_valid_region(object) ? "region" : "non-region", size=2);
// Example(2D,NoAxes): These two squares share a full edge, hence not a region
//   object = [square(10), right(10, square(10))];
//   stroke(object[0], width=0.2,closed=true);
//   color("red")dashed_stroke(object[1], width=0.25,closed=true);
//   back(12)text(is_valid_region(object) ? "region" : "non-region", size=2);
// Example(2D,NoAxes): Sharing on edge on the inside, also not a regionn
//   object = [square(10), [[0,0], [2,2],[2,8],[0,10]]];
//   stroke(object[0], width=0.2,closed=true);
//   color("red")dashed_stroke(object[1], width=0.25,closed=true);
//   back(12)text(is_valid_region(object) ? "region" : "non-region", size=2);
// Example(2D,NoAxes): Crossing at vertices is also bad
//   object = [square(10), [[10,0],[0,10],[8,13],[13,8]]];
//   rainbow(object)stroke($item, width=.2,closed=true);
//   back(14)text(is_valid_region(object) ? "region" : "non-region", size=2);
// Example(2D,NoAxes): One polygon touches another in the middle of an edge
//   object = [square(10), [[10,5],[15,0],[15,10]]];
//   rainbow(object)stroke($item, width=.2,closed=true);
//   back(11)text(is_valid_region(object) ? "region" : "non-region", size=2);
// Example(2D,NoAxes): The polygon touches the side, but the side has a vertex at the contact point so this is a region
//   poly1 = [ each square(30,center=true), [15,0]];
//   poly2 = right(10,circle(5,$fn=4));
//   poly3 = left(0,circle(5,$fn=4));
//   poly4 = move([0,-8],square([10,3]));
//   region = [poly1,poly2,poly3,poly4];
//   rainbow(region)stroke($item, width=.25,closed=true);
//   move([-5,16.5])text(is_valid_region(region) ? "region" : "non-region", size=3);
//   color("black")move_copies(region[0]) circle(r=.4);
// Example(2D,NoAxes): The polygon touches the side, but not at a vertex so this is not a region
//   poly1 = fwd(4,[ each square(30,center=true), [15,0]]);
//   poly2 = right(10,circle(5,$fn=4));
//   poly3 = left(0,circle(5,$fn=4));
//   poly4 = move([0,-8],square([10,3]));
//   object = [poly1,poly2,poly3,poly4];
//   rainbow(object)stroke($item, width=.25,closed=true);
//   move([-9,12.5])text(is_valid_region(object) ? "region" : "non-region", size=3);
//   color("black")move_copies(object[0]) circle(r=.4);
// Example(2D,NoAxes): The inner polygon touches the middle of the edges, so not a region
//   poly1 = square(20,center=true);
//   poly2 = circle(10,$fn=8);
//   object=[poly1,poly2];
//   rainbow(object)stroke($item, width=.25,closed=true);
//   move([-10,11.4])text(is_valid_region(object) ? "region" : "non-region", size=3);
// Example(2D,NoAxes): The above shape made into a region using {{difference()}} now has four components that touch at corners
//   poly1 = square(20,center=true);
//   poly2 = circle(10,$fn=8);
//   region = difference(poly1,poly2);
//   rainbow(region)stroke($item, width=.25,closed=true);
//   move([-5,11.4])text(is_valid_region(region) ? "region" : "non-region", size=3);
function is_valid_region(region, eps=EPSILON) =
   let(region=force_region(region))
   assert(is_region(region), "Input is not a region")
   // no short paths
   [for(p=region) if (len(p)<3) 1] == []
   &&
   // all paths are simple
   [for(p=region) if (!is_path_simple(p,closed=true,eps=eps)) 1] == []
   &&
   // paths do not cross each other
   [for(i=[0:1:len(region)-2])
            if (_polygon_crosses_region(list_tail(region,i+1),region[i], eps=eps)) 1] == []
   &&
   // one path doesn't touch another in the middle of an edge
   [for(i=idx(region), j=idx(region))
       if (i!=j) for(v=region[i], edge=pair(region[j],wrap=true))
           let(
               v1 = edge[1]-edge[0],
               v0 = v - edge[0],
               t = v0*v1/(v1*v1)
           )
           if (abs(cross(v0,v1))<eps*norm(v1) && t>eps && t<1-eps) 1
   ]==[];



// internal function:
// returns true if the polygon crosses the region so that part of the 
// polygon is inside the region and part is outside.  
function _polygon_crosses_region(region, poly, eps=EPSILON) =
    let(  
        subpaths = flatten(split_region_at_region_crossings(region,[poly],eps=eps)[1])
    )
    [for(path=subpaths)
      let(isect=
         [for (subpath = subpaths)
          let(
                midpt = mean([subpath[0], subpath[1]]),
                rel = point_in_region(midpt,region,eps=eps)
          )
          rel
         ])
       if (!all_equal(isect) || isect[0]==0) 1 ] != [];


// Function: is_region_simple()
// Usage:
//   bool = is_region_simple(region, [eps]);
// Description:
//   We extend the notion of the simple path to regions: a simple region is entirely
//   non-self-intersecting, meaning that it is formed from a list of simple polygons that
//   don't intersect each other at all&mdash;not even with corner contact points.
//   Regions with corner contact are valid but may fail CGAL.  Simple regions
//   should not create problems with CGAL.  
// Arguments:
//   region = region to check
//   eps = tolerance for geometric comparisons.  Default: `EPSILON` = 1e-9
// Example(2D,NoAxes):  Corner contact means it's not simple
//   region = [move([-2,-2],square(14)), [[0,0],[10,0],[5,5]], [[5,5],[0,10],[10,10]]];
//   rainbow(region)stroke($item, width=.2,closed=true);
//   move([-1,13])text(is_region_simple(region) ? "simple" : "not-simple", size=2);
// Example(2D,NoAxes):  Moving apart the triangles makes it simple:
//   region = [move([-2,-2],square(14)), [[0,0],[10,0],[5,4.5]], [[5,5.5],[0,10],[10,10]]];
//   rainbow(region)stroke($item, width=.2,closed=true);
//   move([1,13])text(is_region_simple(region) ? "simple" : "not-simple", size=2);
function is_region_simple(region, eps=EPSILON) =
   let(region=force_region(region))
   assert(is_region(region), "Input is not a region")
   [for(p=region) if (!is_path_simple(p,closed=true,eps=eps)) 1] == []
   &&
   [for(i=[0:1:len(region)-2])
       if (_region_region_intersections([region[i]], list_tail(region,i+1), eps=eps)[0][0] != []) 1
   ] ==[];
  
  
// Function: make_region()
// Usage:
//   region = make_region(polys, [nonzero], [eps]);
// Description:
//   Takes a list of polygons that may intersect themselves or cross each other 
//   and converts it into a properly defined region without
//   these defects.
// Arguments:
//   polys = list of polygons to use
//   nonzero = set to true to use nonzero rule for polygon membership.  Default: false
//   eps = Epsilon for geometric comparisons.  Default: `EPSILON` (1e-9)
// Example(2D,NoAxes):  The pentagram is self-intersecting, so it is not a region.  Here it becomes five triangles:
//   pentagram = turtle(["move",100,"left",144], repeat=4);
//   region = make_region(pentagram);
//   rainbow(region)stroke($item, width=1,closed=true);
// Example(2D,NoAxes):  Alternatively with the nonzero option you can get the perimeter:
//   pentagram = turtle(["move",100,"left",144], repeat=4);
//   region = make_region(pentagram,nonzero=true);
//   rainbow(region)stroke($item, width=1,closed=true);
// Example(2D,NoAxes):  Two crossing squares become two L-shaped components
//   region = make_region([square(10), move([5,5],square(8))]);
//   rainbow(region)stroke($item, width=.3,closed=true);

function make_region(polys,nonzero=false,eps=EPSILON) =
     let(polys=force_region(polys))
     assert(is_region(polys), "Input is not a region")
     exclusive_or(
                  [for(poly=polys) each polygon_parts(poly,nonzero,eps)],
                  eps=eps);

// Function: force_region()
// Usage:
//   region = force_region(poly)
// Description:
//   If the input is a polygon then return it as a region.  Otherwise return it unaltered.
// Arguments:
//   poly = polygon to turn into a region
function force_region(poly) = is_path(poly) ? [poly] : poly;


// Section: Turning a region into geometry

// Module: region()
// Usage:
//   region(r, [anchor], [spin=], [cp=], [atype=]) [ATTACHMENTS];
// Description:
//   Creates the 2D polygons described by the given region or list of polygons.  This module works on
//   arbitrary lists of polygons that cross each other and hence do not define a valid region.  The
//   displayed result is the exclusive-or of the polygons listed in the input. 
// Arguments:
//   r = region to create as geometry
//   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `"origin"`
//   ---
//   spin = Rotate this many degrees after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
//   cp = Centerpoint for determining intersection anchors or centering the shape.  Determintes the base of the anchor vector.  Can be "centroid", "mean", "box" or a 2D point.  Default: "centroid"
//   atype = Set to "hull" or "intersect" to select anchor type.  Default: "hull"
// Example(2D): Displaying a region
//   region([circle(d=50), square(25,center=true)]);
// Example(2D): Displaying a list of polygons that intersect each other, which is not a region
//   rgn = concat(
//       [for (d=[50:-10:10]) circle(d=d-5)],
//       [square([60,10], center=true)]
//   );
//   region(rgn);
module region(r, anchor="origin", spin=0, cp="centroid", atype="hull")
{
    assert(in_list(atype, _ANCHOR_TYPES), "Anchor type must be \"hull\" or \"intersect\"");
    r = force_region(r);
    dummy=assert(is_region(r), "Input is not a region");
    points = flatten(r);
    lengths = [for(path=r) len(path)];
    starts = [0,each cumsum(lengths)];
    paths = [for(i=idx(r)) count(s=starts[i], n=lengths[i])];
    attachable(anchor, spin, two_d=true, region=r, extent=atype=="hull", cp=cp){
      polygon(points=points, paths=paths);
      children();
    }
}



// Section: Gometrical calculations with regions

// Function: point_in_region()
// Usage:
//   check = point_in_region(point, region, [eps]);
// Description:
//   Tests if a point is inside, outside, or on the border of a region.  
//   Returns -1 if the point is outside the region.
//   Returns 0 if the point is on the boundary.
//   Returns 1 if the point lies inside the region.
// Arguments:
//   point = The point to test.
//   region = The region to test against, as a list of polygon paths.
//   eps = Acceptable variance.  Default: `EPSILON` (1e-9)
// Example(2D,Med):  Red points are in the region.
//   region = [for(i=[2:4:10]) hexagon(r=i)];
//   color("#ff7") region(region);
//   for(x=[-10:10], y=[-10:10])
//     if (point_in_region([x,y], region)>=0)
//       move([x,y]) color("red") circle(0.15, $fn=12);
//     else
//       move([x,y]) color("#ddf") circle(0.1, $fn=12);
function point_in_region(point, region, eps=EPSILON) =
    let(region=force_region(region))
    assert(is_region(region), "Region given to point_in_region is not a region")
    assert(is_vector(point,2), "Point must be a 2D point in point_in_region")
    _point_in_region(point, region, eps);

function _point_in_region(point, region, eps=EPSILON, i=0, cnt=0) =
      i >= len(region) ? ((cnt%2==1)? 1 : -1)
    : let(
          pip = point_in_polygon(point, region[i], eps=eps)
      )
      pip == 0 ? 0
   : _point_in_region(point, region, eps=eps, i=i+1, cnt = cnt + (pip>0? 1 : 0));


// Function: region_area()
// Usage:
//   area = region_area(region);
// Description:
//   Computes the area of the specified valid region. (If the region is invalid and has self intersections
//   the result is meaningless.)
// Arguments:
//   region = region whose area to compute
// Examples:
//   area = region_area([square(10), right(20,square(8))]);  // Returns 164
function region_area(region) =
  assert(is_region(region), "Input must be a region")
  let(
      parts = region_parts(region)
  )
  -sum([for(R=parts, poly=R) polygon_area(poly,signed=true)]);



function _clockwise_region(r) = [for(p=r) clockwise_polygon(p)];

// Function: are_regions_equal()
// Usage:
//    b = are_regions_equal(region1, region2, [either_winding])
// Description:
//    Returns true if the components of region1 and region2 are the same polygons (in any order). 
// Arguments:
//    region1 = first region
//    region2 = second region
//    either_winding = if true then two shapes test equal if they wind in opposite directions.  Default: false
function are_regions_equal(region1, region2, either_winding=false) =
    let(
        region1=force_region(region1),
        region2=force_region(region2)
    )
    assert(is_region(region1) && is_region(region2), "One of the inputs is not a region")
    len(region1) != len(region2)? false :
    __are_regions_equal(either_winding?_clockwise_region(region1):region1,
                        either_winding?_clockwise_region(region2):region2,
                        0);

function __are_regions_equal(region1, region2, i) =
    i >= len(region1)? true :
    !_is_polygon_in_list(region1[i], region2)? false :
    __are_regions_equal(region1, region2, i+1);


/// Internal Function: _region_region_intersections()
/// Usage:
///    risect = _region_region_intersections(region1, region2, [closed1], [closed2], [eps]
/// Description:
///    Returns a pair of sorted lists such that risect[0] is a list of intersection
///    points for every path in region1, and similarly risect[1] is a list of intersection
///    points for the paths in region2.  For each path the intersection list is
///    a sorted list of the form [PATHIND, SEGMENT, U].  You can specify that the paths in either
///    region be regarded as open paths if desired.  Default is to treat them as
///    regions and hence the paths as closed polygons.
///    .
///    Included as intersection points are points where region1 touches itself at a vertex or
///    region2 touches itself at a vertex.  (The paths are assumed to have no self crossings.
///    Self crossings of the paths in the regions are not returned.)
function _region_region_intersections(region1, region2, closed1=true,closed2=true, eps=EPSILON) =
   let(
       intersections =   [
           for(p1=idx(region1))
              let(
                  path = closed1?close_path(region1[p1]):region1[p1]
              )
              for(i = [0:1:len(path)-2])
                  let(
                      a1 = path[i],
                      a2 = path[i+1],
                      nrm = norm(a1-a2)
                  )
                  if( nrm>eps )  // ignore zero-length path edges
                       let( 
                           seg_normal = [-(a2-a1).y, (a2-a1).x]/nrm,
                           ref = a1*seg_normal
                       )
                           // `signs[j]` is the sign of the signed distance from
                           // poly vertex j to the line [a1,a2] where near zero
                           // distances are snapped to zero;  poly edges 
                           //  with equal signs at its vertices cannot intersect
                           // the path edge [a1,a2] or they are collinear and 
                           // further tests can be discarded.
                       for(p2=idx(region2))
                           let(
                               poly  = closed2?close_path(region2[p2]):region2[p2],
                               signs = [for(v=poly*seg_normal) abs(v-ref) < eps ? 0 : sign(v-ref) ]
                           ) 
                           if(max(signs)>=0 && min(signs)<=0) // some edge intersects line [a1,a2]
                               for(j=[0:1:len(poly)-2]) 
                                   if(signs[j]!=signs[j+1])
                                        let( // exclude non-crossing and collinear segments
                                            b1 = poly[j],
                                            b2 = poly[j+1],
                                            isect = _general_line_intersection([a1,a2],[b1,b2],eps=eps) 
                                        )
                                        if (isect 
                                            && isect[1]>= -eps 
                                            && isect[1]<= 1+eps 
                                            && isect[2]>= -eps
                                            && isect[2]<= 1+eps)       
                                         [[p1,i,isect[1]], [p2,j,isect[2]]]
         ],
         regions=[region1,region2],
         // Create a flattened index list corresponding to the points in region1 and region2
         // that gives each point as an intersection point
         ptind = [for(i=[0:1])   
                    [for(p=idx(regions[i]))
                       for(j=idx(regions[i][p])) [p,j,0]]],
         points = [for(i=[0:1]) flatten(regions[i])],
         // Corner points are those points where the region touches itself, hence duplicate
         // points in the region's point set
         cornerpts = [for(i=[0:1])
                         [for(k=vector_search(points[i],eps,points[i]))
                             each if (len(k)>1) select(ptind[i],k)]],
         risect = [for(i=[0:1]) concat(column(intersections,i), cornerpts[i])],
         counts = [count(len(region1)), count(len(region2))],
         pathind = [for(i=[0:1]) search(counts[i], risect[i], 0)]
       )
       [for(i=[0:1]) [for(j=counts[i]) _sort_vectors(select(risect[i],pathind[i][j]))]];
         

// Section: Breaking up regions into subregions


// Function: split_region_at_region_crossings()
// Usage:
//   split_region = split_region_at_region_crossings(region1, region2, [closed1], [closed2], [eps])
// Description:
//   Splits region1 at the places where polygons in region1 touches each other at corners and at locations
//   where region1 intersections region2.  Split region2 similarly with respect to region1.
//   The return is a pair of results of the form [split1, split2] where split1=[frags1,frags2,...]
//   and frags1 is a list of paths that when placed end to end (in the given order), give the first polygon of region1.
//   Each path in the list is either entirely inside or entirely outside region2.  
//   Then frags2 is the decomposition of the second polygon into path pieces, and so on.  Finally split2 is
//   the same list, but for the polygons in region2.  
//   You can pass a single polygon in for either region, but the output will be a singleton list, as if
//   you passed in a singleton region.  If you set the closed parameters to false then the region components
//   will be treated as open paths instead of polygons.  
// Arguments:
//   region1 = first region
//   region2 = second region
//   closed1 = if false then treat region1 as list of open paths.  Default: true
//   closed2 = if false then treat region2 as list of open paths.  Default: true
//   eps = Acceptable variance.  Default: `EPSILON` (1e-9)
// Example(2D): 
//   path = square(50,center=false);
//   region = [circle(d=80), circle(d=40)];
//   paths = split_region_at_region_crossings(path, region);
//   color("#aaa") region(region);
//   rainbow(paths[0][0]) stroke($item, width=2);
//   right(110){
//     color("#aaa") region([path]);
//     rainbow(flatten(paths[1])) stroke($item, width=2);
//   }
function split_region_at_region_crossings(region1, region2, closed1=true, closed2=true, eps=EPSILON) = 
    let(
        region1=force_region(region1),
        region2=force_region(region2)
    )
    assert(is_region(region1) && is_region(region2),"One of the inputs is not a region")
    let(
        xings = _region_region_intersections(region1, region2, closed1, closed2, eps),
        regions = [region1,region2],
        closed = [closed1,closed2]
    )
    [for(i=[0:1])
      [for(p=idx(xings[i]))
        let(
            crossings = deduplicate([
                                     [p,0,0],
                                     each xings[i][p],
                                     [p,len(regions[i][p])-(closed[i]?1:2), 1],
                                    ],eps=eps),
            subpaths = [
                for (frag = pair(crossings)) 
                    deduplicate(
                        _path_select(regions[i][p], frag[0][1], frag[0][2], frag[1][1], frag[1][2], closed=closed[i]),
                        eps=eps
                    )
            ]
        )
        [for(s=subpaths) if (len(s)>1) s]
       ]
    ];
                
                

// Function: region_parts()
// Usage:
//   rgns = region_parts(region);
// Description:
//   Divides a region into a list of connected regions.  Each connected region has exactly one clockwise outside boundary
//   and zero or more counter-clockwise outlines defining internal holes.  Note that behavior is undefined on invalid regions whose
//   components cross each other.
// Example(2D,NoAxes):
//   R = [for(i=[1:7]) square(i,center=true)];
//   region_list = region_parts(R);
//   rainbow(region_list) region($item);
// Example(2D,NoAxes):
//   R = [back(7,square(3,center=true)),
//        square([20,10],center=true),
//        left(5,square(8,center=true)),
//        for(i=[4:2:8])
//          right(5,square(i,center=true))];
//   region_list = region_parts(R);
//   rainbow(region_list) region($item);
function region_parts(region) =
   let(
       region = force_region(region)
   )
   assert(is_region(region), "Input is not a region")
   let(
       inside = [for(i=idx(region))
                    let(pt = mean([region[i][0], region[i][1]]))
                    [for(j=idx(region))  i==j ? 0
                                       : point_in_polygon(pt,region[j]) >=0 ? 1 : 0]
                ],
       level = inside*repeat(1,len(region))
   )
   [ for(i=idx(region))
      if(level[i]%2==0)
         let(
             possible_children = search([level[i]+1],level,0)[0],
             keep=search([1], select(inside,possible_children), 0, i)[0]
         )
         [
           clockwise_polygon(region[i]),
           for(good=keep)
              ccw_polygon(region[possible_children[good]])
         ]
    ];




// Section: Offset and 2D Boolean Set Operations


function _offset_chamfer(center, points, delta) =
    let(
        dist = sign(delta)*norm(center-line_intersection(select(points,[0,2]), [center, points[1]])),
        endline = _shift_segment(select(points,[0,2]), delta-dist)
    ) [
        line_intersection(endline, select(points,[0,1])),
        line_intersection(endline, select(points,[1,2]))
    ];


function _shift_segment(segment, d) =
    assert(!approx(segment[0],segment[1]),"Path has repeated points")
    move(d*line_normal(segment),segment);


// Extend to segments to their intersection point.  First check if the segments already have a point in common,
// which can happen if two colinear segments are input to the path variant of `offset()`
function _segment_extension(s1,s2) =
    norm(s1[1]-s2[0])<1e-6 ? s1[1] : line_intersection(s1,s2,LINE,LINE);


function _makefaces(direction, startind, good, pointcount, closed) =
    let(
        lenlist = list_bset(good, pointcount),
        numfirst = len(lenlist),
        numsecond = sum(lenlist),
        prelim_faces = _makefaces_recurse(startind, startind+len(lenlist), numfirst, numsecond, lenlist, closed)
    )
    direction? [for(entry=prelim_faces) reverse(entry)] : prelim_faces;


function _makefaces_recurse(startind1, startind2, numfirst, numsecond, lenlist, closed, firstind=0, secondind=0, faces=[]) =
    // We are done if *both* firstind and secondind reach their max value, which is the last point if !closed or one past
    // the last point if closed (wrapping around).  If you don't check both you can leave a triangular gap in the output.
    ((firstind == numfirst - (closed?0:1)) && (secondind == numsecond - (closed?0:1)))? faces :
    _makefaces_recurse(
        startind1, startind2, numfirst, numsecond, lenlist, closed, firstind+1, secondind+lenlist[firstind],
        lenlist[firstind]==0? (
            // point in original path has been deleted in offset path, so it has no match.  We therefore
            // make a triangular face using the current point from the offset (second) path
            // (The current point in the second path can be equal to numsecond if firstind is the last point)
            concat(faces,[[secondind%numsecond+startind2, firstind+startind1, (firstind+1)%numfirst+startind1]])
            // in this case a point or points exist in the offset path corresponding to the original path
        ) : (
            concat(faces,
                // First generate triangular faces for all of the extra points (if there are any---loop may be empty)
                [for(i=[0:1:lenlist[firstind]-2]) [firstind+startind1, secondind+i+1+startind2, secondind+i+startind2]],
                // Finish (unconditionally) with a quadrilateral face
                [
                    [
                        firstind+startind1,
                        (firstind+1)%numfirst+startind1,
                        (secondind+lenlist[firstind])%numsecond+startind2,
                        (secondind+lenlist[firstind]-1)%numsecond+startind2
                    ]
                ]
            )
        )
    );


// Determine which of the shifted segments are good
function _good_segments(path, d, shiftsegs, closed, quality) =
    let(
        maxind = len(path)-(closed ? 1 : 2),
        pathseg = [for(i=[0:maxind]) select(path,i+1)-path[i]],
        pathseg_len =  [for(seg=pathseg) norm(seg)],
        pathseg_unit = [for(i=[0:maxind]) pathseg[i]/pathseg_len[i]],
        // Order matters because as soon as a valid point is found, the test stops
        // This order works better for circular paths because they succeed in the center
        alpha = concat([for(i=[1:1:quality]) i/(quality+1)],[0,1])
    ) [
        for (i=[0:len(shiftsegs)-1])
            (i>maxind)? true :
            _segment_good(path,pathseg_unit,pathseg_len, d - 1e-7, shiftsegs[i], alpha)
    ];


// Determine if a segment is good (approximately)
// Input is the path, the path segments normalized to unit length, the length of each path segment
// the distance threshold, the segment to test, and the locations on the segment to test (normalized to [0,1])
// The last parameter, index, gives the current alpha index.
//
// A segment is good if any part of it is farther than distance d from the path.  The test is expensive, so
// we want to quit as soon as we find a point with distance > d, hence the recursive code structure.
//
// This test is approximate because it only samples the points listed in alpha.  Listing more points
// will make the test more accurate, but slower.
function _segment_good(path,pathseg_unit,pathseg_len, d, seg,alpha ,index=0) =
    index == len(alpha) ? false :
    _point_dist(path,pathseg_unit,pathseg_len, alpha[index]*seg[0]+(1-alpha[index])*seg[1]) > d ? true :
    _segment_good(path,pathseg_unit,pathseg_len,d,seg,alpha,index+1);


// Input is the path, the path segments normalized to unit length, the length of each path segment
// and a test point.  Computes the (minimum) distance from the path to the point, taking into
// account that the minimal distance may be anywhere along a path segment, not just at the ends.
function _point_dist(path,pathseg_unit,pathseg_len,pt) =
    min([
        for(i=[0:len(pathseg_unit)-1]) let(
            v = pt-path[i],
            projection = v*pathseg_unit[i],
            segdist = projection < 0? norm(pt-path[i]) :
                projection > pathseg_len[i]? norm(pt-select(path,i+1)) :
                norm(v-projection*pathseg_unit[i])
        ) segdist
    ]);


// Function: offset()
// Usage:
//   offsetpath = offset(path, [r=|delta=], [chamfer=], [closed=], [check_valid=], [quality=], [same_length=])
//   path_faces = offset(path, return_faces=true, [r=|delta=], [chamfer=], [closed=], [check_valid=], [quality=], [firstface_index=], [flip_faces=])
// Description:
//   Takes a 2D input path, polygon or region and returns a path offset by the specified amount.  As with the built-in
//   offset() module, you can use `r` to specify rounded offset and `delta` to specify offset with
//   corners.  If you used `delta` you can set `chamfer` to true to get chamfers.
//   For paths and polygons positive offsets make the polygons larger.  For paths, 
//   positive offsets shift the path to the left, relative to the direction of the path.  Note
//   that the path must not include any 180 degree turns, where the path reverses direction.  
//   .
//   When offsets shrink the path, segments cross and become invalid.  By default `offset()` checks
//   for this situation.  To test validity the code checks that segments have distance larger than (r
//   or delta) from the input path.  This check takes O(N^2) time and may mistakenly eliminate
//   segments you wanted included in various situations, so you can disable it if you wish by setting
//   check_valid=false.  Another situation is that the test is not sufficiently thorough and some
//   segments persist that should be eliminated.  In this case, increase `quality` to 2 or 3.  (This
//   increases the number of samples on the segment that are checked.)  Run time will increase.  In
//   some situations you may be able to decrease run time by setting quality to 0, which causes only
//   segment ends to be checked.
//   .
//   When invalid segments are eliminated, the path length decreases.  If you use chamfering or rounding, then
//   the chamfers and roundings can increase the length of the output path.  Hence points in the output may be 
//   difficult to associate with the input.  If you want to maintain alignment between the points you
//   can use the `same_length` option.  This option requires that you use `delta=` with `chamfer=false` to ensure
//   that no points are added.  When points collapse to a single point in the offset, the output includes
//   that point repeated to preserve the correct length.  
//   .
//   Another way to obtain alignment information is to use the return_faces option, which can
//   provide alignment information for all offset parameters: it returns a face list which lists faces between
//   the original path and the offset path where the vertices are ordered with the original path
//   first, starting at `firstface_index` and the offset path vertices appearing afterwords.  The
//   direction of the faces can be flipped using `flip_faces`.  When you request faces the return
//   value is a list: [offset_path, face_list].
// Arguments:
//   path = the path to process.  A list of 2d points.
//   ---
//   r = offset radius.  Distance to offset.  Will round over corners.
//   delta = offset distance.  Distance to offset with pointed corners.
//   chamfer = chamfer corners when you specify `delta`.  Default: false
//   closed = if true path is treate as a polygon. Default: False.
//   check_valid = perform segment validity check.  Default: True.
//   quality = validity check quality parameter, a small integer.  Default: 1.
//   same_length = return a path with the same length as the input.  Only compatible with `delta=`.  Default: false
//   return_faces = return face list.  Default: False.
//   firstface_index = starting index for face list.  Default: 0.
//   flip_faces = flip face direction.  Default: false
// Example(2D,NoAxes):
//   star = star(5, r=100, ir=30);
//   #stroke(closed=true, star, width=3);
//   stroke(closed=true, width=3, offset(star, delta=10, closed=true));
// Example(2D,NoAxes):
//   star = star(5, r=100, ir=30);
//   #stroke(closed=true, star, width=3);
//   stroke(closed=true, width=3,
//          offset(star, delta=10, chamfer=true, closed=true));
// Example(2D,NoAxes):
//   star = star(5, r=100, ir=30);
//   #stroke(closed=true, star, width=3);
//   stroke(closed=true, width=3,
//          offset(star, r=10, closed=true));
// Example(2D,NoAxes):
//   star = star(7, r=120, ir=50);
//   #stroke(closed=true, width=3, star);
//   stroke(closed=true, width=3,
//          offset(star, delta=-15, closed=true));
// Example(2D,NoAxes):
//   star = star(7, r=120, ir=50);
//   #stroke(closed=true, width=3, star);
//   stroke(closed=true, width=3,
//          offset(star, delta=-15, chamfer=true, closed=true));
// Example(2D,NoAxes):
//   star = star(7, r=120, ir=50);
//   #stroke(closed=true, width=3, star);
//   stroke(closed=true, width=3,
//          offset(star, r=-15, closed=true, $fn=20));
// Example(2D,NoAxes):  This case needs `quality=2` for success
//   test = [[0,0],[10,0],[10,7],[0,7], [-1,-3]];
//   polygon(offset(test,r=-1.9, closed=true, quality=2));
//   //polygon(offset(test,r=-1.9, closed=true, quality=1));  // Fails with erroneous 180 deg path error
//   %down(.1)polygon(test);
// Example(2D,NoAxes): This case fails if `check_valid=true` when delta is large enough because segments are too close to the opposite side of the curve.  
//   star = star(5, r=22, ir=13);
//   stroke(star,width=.3,closed=true);                                                           
//   color("green")
//     stroke(offset(star, delta=-9, closed=true),width=.3,closed=true); // Works with check_valid=true (the default)
//   color("red")
//     stroke(offset(star, delta=-10, closed=true, check_valid=false),   // Fails if check_valid=true 
//            width=.3,closed=true); 
// Example(2D): But if you use rounding with offset then you need `check_valid=true` when `r` is big enough.  It works without the validity check as long as the offset shape retains a some of the straight edges at the star tip, but once the shape shrinks smaller than that, it fails.  There is no simple way to get a correct result for the case with `r=10`, because as in the previous example, it will fail if you turn on validity checks.  
//   star = star(5, r=22, ir=13);
//   color("green")
//     stroke(offset(star, r=-8, closed=true,check_valid=false), width=.1, closed=true);
//   color("red")
//     stroke(offset(star, r=-10, closed=true,check_valid=false), width=.1, closed=true);
// Example(2D,NoAxes): The extra triangles in this example show that the validity check cannot be skipped
//   ellipse = scale([20,4], p=circle(r=1,$fn=64));
//   stroke(ellipse, closed=true, width=0.3);
//   stroke(offset(ellipse, r=-3, check_valid=false, closed=true),
//          width=0.3, closed=true);
// Example(2D,NoAxes): The triangles are removed by the validity check
//   ellipse = scale([20,4], p=circle(r=1,$fn=64));
//   stroke(ellipse, closed=true, width=0.3);
//   stroke(offset(ellipse, r=-3, check_valid=true, closed=true),
//          width=0.3, closed=true);
// Example(2D): Open path.  The path moves from left to right and the positive offset shifts to the left of the initial red path.
//   sinpath = 2*[for(theta=[-180:5:180]) [theta/4,45*sin(theta)]];
//   #stroke(sinpath, width=2);
//   stroke(offset(sinpath, r=17.5),width=2);
// Example(2D,NoAxes): Region
//   rgn = difference(circle(d=100),
//                    union(square([20,40], center=true),
//                          square([40,20], center=true)));
//   #linear_extrude(height=1.1) stroke(rgn, width=1);
//   region(offset(rgn, r=-5));
// Example(2D,NoAxes): Using `same_length=true` to align the original curve to the offset.  Note that lots of points map to the corner at the top.
//   closed=false;
//   path = [for(angle=[0:5:180]) 10*[angle/100,2*sin(angle)]];
//   opath = offset(path, delta=-3,same_length=true,closed=closed);
//   stroke(path,closed=closed,width=.3);
//   stroke(opath,closed=closed,width=.3);
//   color("red") for(i=idx(path)) stroke([path[i],opath[i]],width=.3);

function offset(
    path, r=undef, delta=undef, chamfer=false,
    closed=false, check_valid=true,
    quality=1, return_faces=false, firstface_index=0,
    flip_faces=false, same_length=false
) =
    assert(!(same_length && return_faces), "Cannot combine return_faces with same_length")
    is_region(path)?
        assert(!return_faces, "return_faces not supported for regions.")
        let(
            ofsregs = [for(R=region_parts(path))
                difference([for(i=idx(R)) offset(R[i], r=u_mul(i>0?-1:1,r), delta=u_mul(i>0?-1:1,delta),
                                      chamfer=chamfer, check_valid=check_valid, quality=quality,closed=true)])]
        )
        union(ofsregs)
    :
    let(rcount = num_defined([r,delta]))
    assert(rcount==1,"Must define exactly one of 'delta' and 'r'")
    assert(!same_length || (is_def(delta) && !chamfer), "Must specify delta, with chamfer=false, when same_length=true")
    assert(is_path(path), "Input must be a path or region")
    let(
        chamfer = is_def(r) ? false : chamfer,
        quality = max(0,round(quality)),
        flip_dir = closed && !is_polygon_clockwise(path)? -1 : 1,
        d = flip_dir * (is_def(r) ? r : delta),
//        shiftsegs = [for(i=[0:len(path)-1]) _shift_segment(select(path,i,i+1), d)],
        shiftsegs = [for(i=[0:len(path)-2]) _shift_segment([path[i],path[i+1]], d),
                     if (closed) _shift_segment([last(path),path[0]],d)
                     else [path[0],path[1]]  // dummy segment, not used
                    ],
        // good segments are ones where no point on the segment is less than distance d from any point on the path
        good = check_valid ? _good_segments(path, abs(d), shiftsegs, closed, quality) : repeat(true,len(shiftsegs)),
        goodsegs = bselect(shiftsegs, good),
        goodpath = bselect(path,good)
    )
    assert(len(goodsegs)-(!closed && select(good,-1)?1:0)>0,"Offset of path is degenerate")
    let(
        // Extend the shifted segments to their intersection points
        sharpcorners = [for(i=[0:len(goodsegs)-1]) _segment_extension(select(goodsegs,i-1), select(goodsegs,i))],
        // If some segments are parallel then the extended segments are undefined.  This case is not handled
        // Note if !closed the last corner doesn't matter, so exclude it
        parallelcheck =
            (len(sharpcorners)==2 && !closed) ||
            all_defined(closed? sharpcorners : select(sharpcorners, 1,-2))
    )
    assert(parallelcheck, "Path contains a segment that reverses direction (180 deg turn)")
    let(
        // This is a boolean array that indicates whether a corner is an outside or inside corner
        // For outside corners, the newcorner is an extension (angle 0), for inside corners, it turns backward
        // If either side turns back it is an inside corner---must check both.
        // Outside corners can get rounded (if r is specified and there is space to round them)
        outsidecorner = len(sharpcorners)==2 ? [false,false]
           :
            [for(i=[0:len(goodsegs)-1])
                let(prevseg=select(goodsegs,i-1))
                (i==0 || i==len(goodsegs)-1) && !closed ? false  // In open case first entry is bogus
               :  
                (goodsegs[i][1]-goodsegs[i][0]) * (goodsegs[i][0]-sharpcorners[i]) > 0
                 && (prevseg[1]-prevseg[0]) * (sharpcorners[i]-prevseg[1]) > 0
            ],
        steps = is_def(delta) ? [] : [
            for(i=[0:len(goodsegs)-1])  
                r==0 ? 0
                // if path is open but first and last entries match value is not used, but
                // computation below gives error, so special case handle it
              : i==len(goodsegs)-1 && !closed && approx(goodpath[i],goodsegs[i][0]) ? 0 
                // floor is important here to ensure we don't generate extra segments when nearly straight paths expand outward
              : 1+floor(segs(r)*vector_angle(   
                                             select(goodsegs,i-1)[1]-goodpath[i],
                                             goodsegs[i][0]-goodpath[i])
                        /360)
        ],
        // If rounding is true then newcorners replaces sharpcorners with rounded arcs where needed
        // Otherwise it's the same as sharpcorners
        // If rounding is on then newcorners[i] will be the point list that replaces goodpath[i] and newcorners later
        // gets flattened.  If rounding is off then we set it to [sharpcorners] so we can later flatten it and get
        // plain sharpcorners back.
        newcorners = is_def(delta) && !chamfer ? [sharpcorners]
            : [for(i=[0:len(goodsegs)-1])
                  (!chamfer && steps[i] <=1)  // Don't round if steps is smaller than 2
                  || !outsidecorner[i]        // Don't round inside corners
                  || (!closed && (i==0 || i==len(goodsegs)-1))  // Don't round ends of an open path
                ? [sharpcorners[i]]
                : chamfer ? _offset_chamfer(
                                  goodpath[i], [
                                      select(goodsegs,i-1)[1],
                                      sharpcorners[i],
                                      goodsegs[i][0]
                                  ], d
                              )     
                : // rounded case
                  arc(cp=goodpath[i],
                      points=[
                          select(goodsegs,i-1)[1],
                          goodsegs[i][0]
                      ],
                      n=steps[i])
              ],
        pointcount = (is_def(delta) && !chamfer)?
            repeat(1,len(sharpcorners)) :
            [for(i=[0:len(goodsegs)-1]) len(newcorners[i])],
        start = [goodsegs[0][0]],
        end = [goodsegs[len(goodsegs)-2][1]],
        edges =  closed?
            flatten(newcorners) :
            concat(start,slice(flatten(newcorners),1,-2),end),
        faces = !return_faces? [] :
            _makefaces(
                flip_faces, firstface_index, good,
                pointcount, closed
            ),
        final_edges = same_length ? select(edges, [0,each list_head (cumsum([for(g=good) g?1:0]))])
                                  : edges
    ) return_faces? [edges,faces] : final_edges;



/// Internal Function: _filter_region_parts()
///
/// splits region1 into subpaths where either it touches itself or crosses region2.  Classifies all of the
/// subpaths as described below and keeps the ones listed in keep1.  A similar process is performed for region2.
/// All of the kept subpaths are assembled into polygons and returned as a lst.
/// .
/// The four types of subpath from the region are defined relative to the second region:
///    "O" - the subpath is outside the second region
///    "I" - the subpath is in the second region's interior
///    "S" - the subpath is on the 2nd region's border and the two regions interiors are on the same side of the subpath
///    "U" - the subpath is on the 2nd region's border and the two regions meet at the subpath from opposite sides
/// You specify which type of subpaths to keep with a string of the desired types such as "OS".  
function _filter_region_parts(region1, region2, keep, eps=EPSILON) = 
    // We have to compute common vertices between paths in the region because
    // they can be places where the path must be cut, even though they aren't
    // found my the split_path function.  
    let(
        subpaths = split_region_at_region_crossings(region1,region2,eps=eps),
        regions=[force_region(region1),
                 force_region(region2)]
    )        
    _assemble_path_fragments(
        [for(i=[0:1])
           let(
               keepS = search("S",keep[i])!=[],
               keepU = search("U",keep[i])!=[],        
               keepoutside = search("O",keep[i]) !=[],
               keepinside = search("I",keep[i]) !=[],
               all_subpaths = flatten(subpaths[i])
           )
           for (subpath = all_subpaths)
               let(
                   midpt = mean([subpath[0], subpath[1]]),
                   rel = point_in_region(midpt,regions[1-i],eps=eps),
                   keepthis = rel<0 ? keepoutside
                            : rel>0 ? keepinside
                            : !(keepS || keepU) ? false
                            : let(
                                  sidept = midpt + 0.01*line_normal(subpath[0],subpath[1]),
                                  rel1 = point_in_region(sidept,regions[0],eps=eps)>0,
                                  rel2 = point_in_region(sidept,regions[1],eps=eps)>0
                              )
                              rel1==rel2 ? keepS : keepU
               )
               if (keepthis) subpath
        ]
    );


function _list_three(a,b,c) =
   is_undef(b) ? a : 
   [
     a,
     if (is_def(b)) b,
     if (is_def(c)) c
   ];



// Function&Module: union()
// Usage:
//   union() CHILDREN;
//   region = union(regions);
//   region = union(REGION1,REGION2);
//   region = union(REGION1,REGION2,REGION3);
// Description:
//   When called as a function and given a list of regions or 2D polygons,
//   returns the union of all given regions and polygons.  Result is a single region.
//   When called as the built-in module, makes the union of the given children.
// Arguments:
//   regions = List of regions to union.
// Example(2D):
//   shape1 = move([-8,-8,0], p=circle(d=50));
//   shape2 = move([ 8, 8,0], p=circle(d=50));
//   color("green") region(union(shape1,shape2));
//   for (shape = [shape1,shape2]) color("red") stroke(shape, width=0.5, closed=true);
function union(regions=[],b=undef,c=undef,eps=EPSILON) =
    let(regions=_list_three(regions,b,c))
    len(regions)==0? [] :
    len(regions)==1? regions[0] :
    let(regions=[for (r=regions) is_path(r)? [r] : r])
    union([
           _filter_region_parts(regions[0],regions[1],["OS", "O"], eps=eps),           
           for (i=[2:1:len(regions)-1]) regions[i]
          ],
          eps=eps
    );


// Function&Module: difference()
// Usage:
//   difference() CHILDREN;
//   region = difference(regions);
//   region = difference(REGION1,REGION2);
//   region = difference(REGION1,REGION2,REGION3);
// Description:
//   When called as a function, and given a list of regions or 2D polygons, 
//   takes the first region or polygon and differences away all other regions/polygons from it.  The resulting
//   region is returned.
//   When called as the built-in module, makes the set difference of the given children.
// Arguments:
//   regions = List of regions or polygons to difference.
// Example(2D):
//   shape1 = move([-8,-8,0], p=circle(d=50));
//   shape2 = move([ 8, 8,0], p=circle(d=50));
//   for (shape = [shape1,shape2]) color("red") stroke(shape, width=0.5, closed=true);
//   color("green") region(difference(shape1,shape2));
function difference(regions=[],b=undef,c=undef,eps=EPSILON) =
     let(regions = _list_three(regions,b,c))
     len(regions)==0? []
   : len(regions)==1? regions[0]
   : regions[0]==[] ? []
   : let(regions=[for (r=regions) is_path(r)? [r] : r])
     difference([
                 _filter_region_parts(regions[0],regions[1],["OU", "I"], eps=eps),                
                 for (i=[2:1:len(regions)-1]) regions[i]
                ],
                eps=eps
     );


// Function&Module: intersection()
// Usage:
//   intersection() CHILDREN;
//   region = intersection(regions);
//   region = intersection(REGION1,REGION2);
//   region = intersection(REGION1,REGION2,REGION3);
// Description:
//   When called as a function, and given a list of regions or polygons returns the
//   intersection of all given regions.  Result is a single region.
//   When called as the built-in module, makes the intersection of all the given children.
// Arguments:
//   regions = List of regions to intersect.
// Example(2D):
//   shape1 = move([-8,-8,0], p=circle(d=50));
//   shape2 = move([ 8, 8,0], p=circle(d=50));
//   for (shape = [shape1,shape2]) color("red") stroke(shape, width=0.5, closed=true);
//   color("green") region(intersection(shape1,shape2));
function intersection(regions=[],b=undef,c=undef,eps=EPSILON) =
     let(regions = _list_three(regions,b,c))
     len(regions)==0 ? []
   : len(regions)==1? regions[0]
   : regions[0]==[] || regions[1]==[] ? []   
   : intersection([
                   _filter_region_parts(regions[0],regions[1],["IS","I"],eps=eps),                       
                   for (i=[2:1:len(regions)-1]) regions[i]
                  ],
                  eps=eps
     );



// Function&Module: exclusive_or()
// Usage:
//   exclusive_or() CHILDREN;
//   region = exclusive_or(regions);
//   region = exclusive_or(REGION1,REGION2);
//   region = exclusive_or(REGION1,REGION2,REGION3);
// Description:
//   When called as a function and given a list of regions or 2D polygons, 
//   returns the exclusive_or of all given regions.  Result is a single region.
//   When called as a module, performs a boolean exclusive-or of up to 10 children.  Note that when
//   the input regions cross each other the exclusive-or operator will produce shapes that
//   meet at corners (non-simple regions), which do not render in CGAL.  
// Arguments:
//   regions = List of regions or polygons to exclusive_or
// Example(2D): As Function.  A linear_sweep of this shape fails to render in CGAL.  
//   shape1 = move([-8,-8,0], p=circle(d=50));
//   shape2 = move([ 8, 8,0], p=circle(d=50));
//   for (shape = [shape1,shape2])
//       color("red") stroke(shape, width=0.5, closed=true);
//   color("green") region(exclusive_or(shape1,shape2));
// Example(2D): As Module.  A linear_extrude() of the resulting geometry fails to render in CGAL.  
//   exclusive_or() {
//       square(40,center=false);
//       circle(d=40);
//   }
function exclusive_or(regions=[],b=undef,c=undef,eps=EPSILON) =
     let(regions = _list_three(regions,b,c))
     len(regions)==0? []
   : len(regions)==1? force_region(regions[0])
   : regions[0]==[] ? exclusive_or(list_tail(regions))
   : regions[1]==[] ? exclusive_or(list_remove(regions,1))
   : exclusive_or([
                   _filter_region_parts(regions[0],regions[1],["IO","IO"],eps=eps),                  
                   for (i=[2:1:len(regions)-1]) regions[i]
                  ],
                  eps=eps
     );


module exclusive_or() {
    if ($children==1) {
        children();
    } else if ($children==2) {
        difference() {
            children(0);
            children(1);
        }
        difference() {
            children(1);
            children(0);
        }
    } else if ($children==3) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
            }
            children(2);
        }
    } else if ($children==4) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
            }
            exclusive_or() {
                children(2);
                children(3);
            }
        }
    } else if ($children==5) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
                children(2);
                children(3);
            }
            children(4);
        }
    } else if ($children==6) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
                children(2);
                children(3);
            }
            children(4);
            children(5);
        }
    } else if ($children==7) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
                children(2);
                children(3);
            }
            children(4);
            children(5);
            children(6);
        }
    } else if ($children==8) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
                children(2);
                children(3);
            }
            exclusive_or() {
                children(4);
                children(5);
                children(6);
                children(7);
            }
        }
    } else if ($children==9) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
                children(2);
                children(3);
            }
            exclusive_or() {
                children(4);
                children(5);
                children(6);
                children(7);
            }
            children(8);
        }
    } else if ($children==10) {
        exclusive_or() {
            exclusive_or() {
                children(0);
                children(1);
                children(2);
                children(3);
            }
            exclusive_or() {
                children(4);
                children(5);
                children(6);
                children(7);
            }
            children(8);
            children(9);
        }
    } else {
        assert($children<=10, "exclusive_or() can only handle up to 10 children.");
    }
}


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