intersect_all as an addon. seems ok! needs user testing

intersect_all.py

'''
BEGIN GPL LICENSE BLOCK

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.    See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

END GPL LICENCE BLOCK
'''

bl_info = {
    "name": "Edge tools : tinyCAD X ALL",
    "author": "zeffii",
    "version": (0, 0, 4),
    "blender": (2, 6, 7),
    "category": "Mesh",
    "location": "View3D > EditMode > (w) Specials",
    "warning": "Still under development",
    "wiki_url": "",
    "tracker_url": ""
}

import bpy
import bmesh
from mathutils import Vector
from mathutils.geometry import intersect_line_line as LineIntersect

import itertools
from collections import defaultdict

VTX_PRECISION = 1.0e-5 # Epsilon. or 1.0e-6 ..if you need  
VTX_DOUBLES_THRSHLD = 0.0001

''' helpers '''


def mDist(A, B):
    ''' returns distance between two given points '''
    return (A-B).length

def isPointOnEdge(point, A, B):
    ''' returns True / False if a point happens to lie on an edge '''
    eps = ((mDist(A, B) - mDist(point,B)) - mDist(A,point))
    if abs(eps) < VTX_PRECISION: return True
    return False

def CountPointOnEdges(point, edges):
    ''' returns the number of edges that a point lies on. '''
    v1, v2, v3, v4 = edges
    count = 0
    if(isPointOnEdge(point, v1, v2)): count+=1
    if(isPointOnEdge(point, v3, v4)): count+=1
    return count

def vertex_indices_from_edges_tuple(bm, edge):
    ''' find the vertex indices of a 2-tuple of edges '''
    k = lambda v, w: bm.edges[edge[v]].verts[w].index
    return [k(i>>1, i%2)for i in range(4)]

def vector_from_indices(bm, raw_vert_indices):
    return [bm.verts[i].co for i in raw_vert_indices]

def order_points(edge, point_list):
    ''' order these edges from distance to v1, then 
    sandwich the sorted list with v1, v2 '''
    
    v1, v2 = edge
    dist = lambda co: (v1-co).length 
    point_list = sorted(point_list, key=dist)
    return [v1] + point_list + [v2]

def has_shared_vertex(vert_indices):
    if len(set(vert_indices)) < len(vert_indices):
        return True

def remove_permutations_that_share_a_vertex(bm, permutations):
    ''' Get useful Permutations '''
    final_permutations = []
    for edges in permutations:
        raw_vert_indices = vertex_indices_from_edges_tuple(bm, edges)
        if has_shared_vertex(raw_vert_indices): continue
        
        # reaches this point if they do not share.
        final_permutations.append(edges)

    return final_permutations

def get_valid_permutations(bm, edge_indices):
    raw_permutations = itertools.permutations(edge_indices, 2)
    permutations = [result for result in raw_permutations if result[0] < result[1]]
    return remove_permutations_that_share_a_vertex(bm, permutations)

def can_skip(closest_points, vert_vectors):
    '''this checks if the intersection lies on both edges, returns True 
    when criteria are not met, and thus this point can be skipped'''
    if not closest_points: return True
    if not isinstance(closest_points[0].x, float): return True
    if CountPointOnEdges(closest_points[0], vert_vectors) < 2: return True

    # if this distance is larger than than VTX_PRECISION, we can skip it.
    if mDist(closest_points[0], closest_points[1]) > VTX_PRECISION: return True
    return False

def get_intersection_dictionary(bm, edge_indices):
    permutations = get_valid_permutations(bm, edge_indices)

    k = defaultdict(list)
    d = defaultdict(list)

    for edges in permutations:
        raw_vert_indices = vertex_indices_from_edges_tuple(bm, edges)
        vert_vectors = vector_from_indices(bm, raw_vert_indices)

        points = LineIntersect(*vert_vectors)

        # some can be skipped.    (NaN, None, not on both edges)
        if can_skip(points, vert_vectors): continue

        # reaches this point only when an intersection happens on both edges.
        [k[edge].append(points[0]) for edge in edges]

    # k will contain a dict of edge indices and points found on those edges.
    for edge_idx, unordered_points in k.items():
        tv1, tv2 = bm.edges[edge_idx].verts
        v1 = bm.verts[tv1.index].co
        v2 = bm.verts[tv2.index].co
        ordered_points = order_points((v1,v2), unordered_points)
        d[edge_idx].extend(ordered_points)

    return d

def update_mesh(obj, d):    
    ''' Make new geometry (delete old first) '''

    bpy.ops.mesh.delete(type='EDGE')
    bpy.ops.object.editmode_toggle()
    
    oe = obj.data.edges
    ov = obj.data.vertices
    vert_count = len(ov)
    edge_count = len(oe)
     
    for old_edge, point_list in d.items():
        num_points = len(point_list)
        num_edges_to_add = num_points-1
     
        for i in range(num_edges_to_add):
            oe.add(1)
            ov.add(2)
            ov[vert_count].co = point_list[i]
            ov[vert_count+1].co = point_list[i+1]
           
            oe[edge_count].vertices = [vert_count, vert_count+1]        
            vert_count = len(ov)
            edge_count = len(oe)
     
    # set edit mode        
    bpy.ops.object.editmode_toggle()
    bpy.ops.mesh.remove_doubles(threshold=VTX_DOUBLES_THRSHLD, use_unselected=False)

def unselect_nonintersecting(bm, d_edges, edge_indices):
    if len(edge_indices) > len(d_edges):
        reserved_edges = set(edge_indices) - set(d_edges)
        for edge in reserved_edges:
            bm.edges[edge].select = False
        print("unselected {}, non intersecting edges".format(reserved_edges))

class IntersectAllEdges(bpy.types.Operator):

    bl_idname = 'mesh.intersectall'
    bl_label = 'Edge tools : tinyCAD X ALL'
    # bl_options = {'REGISTER', 'UNDO'}

    @classmethod
    def poll(self, context):
        obj = context.active_object
        return obj != None and obj.type == 'MESH' and obj.mode == 'EDIT'

    def execute(self, context):
        # must force edge selection mode here
        bpy.context.tool_settings.mesh_select_mode = (False, True, False)

        obj = context.active_object
        if obj.mode == "EDIT":
            bm = bmesh.from_edit_mesh(obj.data)

            selected_edges = [edge for edge in bm.edges if edge.select]
            edge_indices = [i.index for i in selected_edges]

            d =  get_intersection_dictionary(bm, edge_indices)

            unselect_nonintersecting(bm, d.keys(), edge_indices)
            update_mesh(obj, d)
        else:
            print('must be in edit mode')    
            
        return {'FINISHED'}


def menu_func(self, context):
    self.layout.operator(IntersectAllEdges.bl_idname, text="Intersect selected edges")
 
def register():
    bpy.utils.register_module(__name__)
    bpy.types.VIEW3D_MT_edit_mesh_specials.append(menu_func)
 
def unregister():
    bpy.utils.unregister_module(__name__)
    bpy.types.VIEW3D_MT_edit_mesh_specials.remove(menu_func)
 
 
if __name__ == "__main__":
    register()