function form_outline_d(form_n, x, y){

  // square
  if (form_n == 0) {
    return "M" + (x) + "," + (y) + "l10,0 l0,10 l-10,0 z"
  }

  // triangle
  if (form_n == 1) {
    return "M" + (x) + "," + (y + 10) + "l5,-10 l5,10 z"
  }

  // circle
  if (form_n == 2) {
    return "M" + (x) + "," + (y + 5) + "a 5 5 0 1 0 0.01 0 z"
  }

}


function start_inventory_animation() {

    // vis config variables

    var svg = d3.select("#inventory-cycle")

    var speed = 0.00015 // ticks per millisecond

    var outer_r = 210
    var inner_r = 160

    var center_x = 289
    var center_y_main = 380
    var center_y_in = -30

    // mapping functions

    var coord_x_in = function(r,a) { return center_x + r * Math.cos(a) }
    var coord_y_in = function(r,a) { return center_y_in+ r * Math.sin(a) }

    var coord_x_main = function(r,a) { return center_x + r * Math.cos(a) }
    var coord_y_main = function(r,a) { return center_y_main + r * Math.sin(a) }

    var angle_in = function(t, t1, t2) {
      var rel_t
      if (t2 == t1) { rel_t = 1 }
      else { rel_t = (t - t1) / (t2 - t1) }
      var a = (180.0 - 90.0 * rel_t) * (Math.PI/180.0)
      return a
    }

    var angle_main = function(t, t1, t2, e1, e2) {
      // compute angle in circle for a dot,
      // given its previous and next events, their timestamps, and the current time
      var rel_t
      if (t2 == t1) { rel_t = 1 }
      else { rel_t = (t - t1) / (t2 - t1) }

      if (e2 == 1) {
        return (90.0 + 180.0 * rel_t) * (Math.PI/180.0)
      }

      if (e2 == 2) {
        return (-90.0 + 135.0 * rel_t) * (Math.PI/180.0)
      }

      if (e2 == 3) {
        return (-90.0 + 135.0 + 45.0 * rel_t) * (Math.PI/180.0)
      }

    }

    // internal state variables
    var particle_count = 0
    var current_events = []

    d3.timer(function(t_ms) {
        var t = t_ms * speed

        // *** SIMULATION ***

        // check for expired events since last t
        // if previous event was #1, then move to #2 with random speed
        // if previous event was #2, then move to either #3 or #4 with some probability and random speed
        // if previous event was #3, then move to either #4 or #1 with some probability and random speed
        // if previous event was #4, then exit
        var removals = []
        current_events.forEach(function(d,i){
          if (d.t2 < t) {
            d.t1 = d.t2
            d.e1 = d.e2
            d.t2 = d.t1 + 0.3 + Math.random() * 0.4
            if (d.e1 == 1) {
              d.e2 = 2
            } else if (d.e1 == 2) {
              if (Math.random() < 0.5) { d.e2 = 4; d.t2 = d.t1 + 0.5 * ( 0.3 + Math.random() * 0.4 ) }
              else { d.e2 = 3 }
            } else if (d.e1 == 3) {
              if (Math.random() < 0.5) { d.e2 = 4; d.t2 = d.t1 + 0.5 * ( 0.3 + Math.random() * 0.4 ) }
              else { d.e2 = 1 }
            } else {
              removals.push(i)
            }
          }
        })

        for (var i=removals.length - 1; i>-1; i--){
          current_events.splice(removals[i],1)
        }

        // add some new particles, with some probability and random speed
        for (var i=0; i<2; i++) {
          if (Math.random() < 0.3) {
            particle_count += 1
            current_events.push({
                'particle_id': particle_count,
                'rad': inner_r + (outer_r - inner_r) * Math.random(),
                'sil': Math.floor(Math.random() * 3),
                't1': t,
                't2': t + 0.5 * ( 0.3 + Math.random() * 0.4 ),
                'e1': 0,
                'e2': 1
            })
          }
        }

        // *** UPDATE SVG ANIMATION ***

        var inventory_items = svg.selectAll(".inventory-cycle-silhouette")
                          .data(current_events, function(d) { return d.particle_id })

        inventory_items.enter().append("path")
          .attr("class", "inventory-cycle-silhouette")
          .attr("fill", "#fff")
          .attr("stroke", "#000")
          .attr("stroke-width", 0.5)
          .attr("opacity", 1)
          .attr("d", function(d) { return form_outline_d(d.sil, 290, 0) })

        inventory_items
          .attr("d", function(d){

              var tx, ty

              if (d.e1 == 0) {
                var new_rad = inner_r + (outer_r - d.rad)
                if (coord_y_in(new_rad, angle_in(t, d.t1, d.t2, d.e1, d.e2)) > center_y_main - outer_r + new_rad) {
                  tx = coord_x_in(new_rad, angle_in(t, d.t1, d.t2, d.e1, d.e2))
                } else {
                  tx = -90 + d.rad * 2
                }
                ty = coord_y_in(new_rad, angle_in(t, d.t1, d.t2, d.e1, d.e2))
              } else if (d.e2 == 4) {
                tx = 280
                ty = 550
              } else {
                tx = coord_x_main(d.rad, angle_main(t, d.t1, d.t2, d.e1, d.e2))
                ty = coord_y_main(d.rad, angle_main(t, d.t1, d.t2, d.e1, d.e2))
              }

              return form_outline_d(d.sil, tx, ty)

            })

        inventory_items.exit().remove()


    })

}