StackGenVis/frontend/node_modules/cdt2d/lib/monotone.js

'use strict'

var bsearch = require('binary-search-bounds')
var orient = require('robust-orientation')[3]

var EVENT_POINT = 0
var EVENT_END   = 1
var EVENT_START = 2

module.exports = monotoneTriangulate

//A partial convex hull fragment, made of two unimonotone polygons
function PartialHull(a, b, idx, lowerIds, upperIds) {
  this.a = a
  this.b = b
  this.idx = idx
  this.lowerIds = lowerIds
  this.upperIds = upperIds
}

//An event in the sweep line procedure
function Event(a, b, type, idx) {
  this.a    = a
  this.b    = b
  this.type = type
  this.idx  = idx
}

//This is used to compare events for the sweep line procedure
// Points are:
//  1. sorted lexicographically
//  2. sorted by type  (point < end < start)
//  3. segments sorted by winding order
//  4. sorted by index
function compareEvent(a, b) {
  var d =
    (a.a[0] - b.a[0]) ||
    (a.a[1] - b.a[1]) ||
    (a.type - b.type)
  if(d) { return d }
  if(a.type !== EVENT_POINT) {
    d = orient(a.a, a.b, b.b)
    if(d) { return d }
  }
  return a.idx - b.idx
}

function testPoint(hull, p) {
  return orient(hull.a, hull.b, p)
}

function addPoint(cells, hulls, points, p, idx) {
  var lo = bsearch.lt(hulls, p, testPoint)
  var hi = bsearch.gt(hulls, p, testPoint)
  for(var i=lo; i<hi; ++i) {
    var hull = hulls[i]

    //Insert p into lower hull
    var lowerIds = hull.lowerIds
    var m = lowerIds.length
    while(m > 1 && orient(
        points[lowerIds[m-2]],
        points[lowerIds[m-1]],
        p) > 0) {
      cells.push(
        [lowerIds[m-1],
         lowerIds[m-2],
         idx])
      m -= 1
    }
    lowerIds.length = m
    lowerIds.push(idx)

    //Insert p into upper hull
    var upperIds = hull.upperIds
    var m = upperIds.length
    while(m > 1 && orient(
        points[upperIds[m-2]],
        points[upperIds[m-1]],
        p) < 0) {
      cells.push(
        [upperIds[m-2],
         upperIds[m-1],
         idx])
      m -= 1
    }
    upperIds.length = m
    upperIds.push(idx)
  }
}

function findSplit(hull, edge) {
  var d
  if(hull.a[0] < edge.a[0]) {
    d = orient(hull.a, hull.b, edge.a)
  } else {
    d = orient(edge.b, edge.a, hull.a)
  }
  if(d) { return d }
  if(edge.b[0] < hull.b[0]) {
    d = orient(hull.a, hull.b, edge.b)
  } else {
    d = orient(edge.b, edge.a, hull.b)
  }
  return d || hull.idx - edge.idx
}

function splitHulls(hulls, points, event) {
  var splitIdx = bsearch.le(hulls, event, findSplit)
  var hull = hulls[splitIdx]
  var upperIds = hull.upperIds
  var x = upperIds[upperIds.length-1]
  hull.upperIds = [x]
  hulls.splice(splitIdx+1, 0,
    new PartialHull(event.a, event.b, event.idx, [x], upperIds))
}


function mergeHulls(hulls, points, event) {
  //Swap pointers for merge search
  var tmp = event.a
  event.a = event.b
  event.b = tmp
  var mergeIdx = bsearch.eq(hulls, event, findSplit)
  var upper = hulls[mergeIdx]
  var lower = hulls[mergeIdx-1]
  lower.upperIds = upper.upperIds
  hulls.splice(mergeIdx, 1)
}


function monotoneTriangulate(points, edges) {

  var numPoints = points.length
  var numEdges = edges.length

  var events = []

  //Create point events
  for(var i=0; i<numPoints; ++i) {
    events.push(new Event(
      points[i],
      null,
      EVENT_POINT,
      i))
  }

  //Create edge events
  for(var i=0; i<numEdges; ++i) {
    var e = edges[i]
    var a = points[e[0]]
    var b = points[e[1]]
    if(a[0] < b[0]) {
      events.push(
        new Event(a, b, EVENT_START, i),
        new Event(b, a, EVENT_END, i))
    } else if(a[0] > b[0]) {
      events.push(
        new Event(b, a, EVENT_START, i),
        new Event(a, b, EVENT_END, i))
    }
  }

  //Sort events
  events.sort(compareEvent)

  //Initialize hull
  var minX = events[0].a[0] - (1 + Math.abs(events[0].a[0])) * Math.pow(2, -52)
  var hull = [ new PartialHull([minX, 1], [minX, 0], -1, [], [], [], []) ]

  //Process events in order
  var cells = []
  for(var i=0, numEvents=events.length; i<numEvents; ++i) {
    var event = events[i]
    var type = event.type
    if(type === EVENT_POINT) {
      addPoint(cells, hull, points, event.a, event.idx)
    } else if(type === EVENT_START) {
      splitHulls(hull, points, event)
    } else {
      mergeHulls(hull, points, event)
    }
  }

  //Return triangulation
  return cells
}