delete edge by clicking on either terminus

This wasn't my original plan; I wanted to detach/reattach edges, but
that seems more complicated now when I consider the possibility of
detaching the source of an edge.

I think I'm not going to bother with a mechanic for deleting nodes. Just
disconnect edges and repurpose them. That way the number of widges per
node doesn't get worse, and I also don't have to worry about undoing
deletion.

0006-on.mouse_press

@@ -24,9 +24,15 @@ on.mouse_press = function(x,y, mouse_button)
 	end
 	node = on_border(sx,sy)
 	if node then
-		Edge = {source=node, s={sx=sx, sy=sy}, e={sx=sx, sy=sy}}
+		local edge = on_edge(sx,sy)
+		if edge == nil then
+			Edge = {source=node, s={sx=sx, sy=sy}, e={sx=sx, sy=sy}}
+		else
+			detach_edge(edge)
+			A()
+		end
 		return
 	end
 	-- default
 	Pan = {sx=sx,sy=sy}
-end
+end

0018-on.update

@@ -24,6 +24,8 @@ on.update = function(dt)
 		set_mouse_cursor('crosshair')
 	elseif on_resize(sx, sy) then
 		set_mouse_cursor('sizewe')
+	elseif on_edge(sx,sy) then
+		set_mouse_cursor('no')
 	else
 		set_mouse_cursor('arrow')
 	end

0069-on_edge

@@ -0,0 +1,17 @@
+on_edge = function(sx,sy)
+	-- check if mouse is on the border of a node and also on an edge to another node.
+	-- reading from Surface; I'm going to regret this.
+	for _,shape in ipairs(Surface) do
+		if shape.type == 'line' then
+			if shape.keys then
+				-- it's an edge; assuming keys are in a specific, brittle order
+				if distance_sq(sx,sy, shape.data[1], shape.data[2]) < 5*5 then
+					return shape.keys
+				end
+				if distance_sq(sx,sy, shape.data[3], shape.data[4]) < 5*5 then
+					return shape.keys
+				end
+			end
+		end
+	end
+end

0070-detach_edge

@@ -0,0 +1,5 @@
+detach_edge = function(edge)
+	local src, dest = edge[1], edge[2]
+	table.remove_value(Nodes[src].outgoing_edges, dest)
+	table.remove_value(Nodes[dest].incoming_edges, src)
+end

0071-table.remove_value

@@ -0,0 +1,6 @@
+table.remove_value = function(h, val)
+	local i = table.find(h, val)
+	if i then
+		table.remove(h, i)
+	end
+end

0072-test_remove_value

@@ -0,0 +1,17 @@
+test_remove_value = function()
+	local h = {1,2,3}
+	table.remove_value(h, 1)
+	check_eq(h, {2,3}, "a")
+
+	local h = {1,2,3}
+	table.remove_value(h, 2)
+	check_eq(h, {1,3}, "b")
+
+	local h = {1,2,3}
+	table.remove_value(h, 3)
+	check_eq(h, {1,2}, "b")
+
+	local h = {1,2,3}
+	table.remove_value(h, 4)
+	check_eq(h, {1,2,3}, "b")
+end