Re: [NTG-context] How can I compare picture variables in metapost?

6 May 2009


      On Tue, 5 May 2009, Aditya Mahajan wrote:
...
Both Taco's and my solutions can be adapted so that you do not randomize the 
radius. (Taco's solution will also work for arbitrary object that can then be 
rotated by a random amount).
Here is another idea. Ask metapost to test for intersection, but code the 
rest of the logic in lua. Below is a proof of concept code for 
communicating with metapost.

\startluacode
   circle = circle or {}

   function circle.path(x,y,r, name)
     local path = "path " .. name .. "; \n" ..
                   name .. "= fullcircle scaled " .. 2*r ..
                  " shifted (" .. x .. "," .. y ..") ; \n"
     return path
   end

   function circle.intersect(x1,y1,r1,x2,y2,r2)
     local mpx = metapost.format("metafun")
     local c = circle.path(x1,y1,r1, "c")
     local d = circle.path(x2,y2,r2, "d")
     local intersect = "pair n; n := c intersectiontimes d ; \n"
     local message   = "if (xpart n) < 0 : \n" ..
                       "   message(\"**false**\") ; \n" ..
                       "else : \n" ..
                       "   message(\"**true**\") ; \n" ..
                       " endif \n"
     local file = c .. d .. intersect .. message
     -- print(file)
     local result = mpx:execute(file)
     local log    = result.log
     if result.status < 2 then
       print("Metapost run successful *****************")
       circle.show_result(x1,y1,r1,x2,y2,r2,circle.parse(log))
     else
       print("Metapost run unsuccessful *****************")
     end
     print(log)
   end

   function circle.parse(log)
     local space = lpeg.S(" \t\n")
     local yes   = lpeg.C("**true**")
     local no    = lpeg.C("**false**")
     local result = (yes + no) / circle.result
     local parser = space^0 * result * space^0
     return parser:match(log)
   end

   function circle.result(str)
     return str == "**true**"
   end

   function circle.show_result(x1,y1,r1,x2,y2,r2,flag)
     local tprint = function(s) tex.sprint(tex.ctxcatcodes,s) end
     local result = "Circles  (" .. x1 .. "," .. y1 .. "):" .. r1 ..
                       " and  (" .. x2 .. "," .. y2 .. "):" .. r2 .. " "
     if flag then
       result = result .. " intersect. "
     else
       result = result .. " do not intersect. "
     end
     tprint (result)
   end


\stopluacode

\starttext
\startluacode
   circle.intersect(1,1,1,2,2,1.5)
   circle.intersect(1,1,1,3,3,0.5)
\stopluacode
\stoptext