/*
About the used algorithms look at:
http://cgafaq.info/wiki/Simple_Polygon_Orientation
*/

size(8cm, false);

real signedArea(pair [] pt)
{
  // Return the signed area of a simple (NON CROSSED) polygon of vertex "pt"
  // Retourne l'aire algébrique d'un polygone NON CROIÉ
  pair [] pt_= copy(pt);
  real n=pt.length,
    sa=0;
  pt_.push(pt_[0]);
  pt_.push(pt_[1]);
  
  for (int i=1; i<=n; ++i) sa +=pt_[i].x * (pt_[i+1].y - pt_[i-1].y);
  return sa/2;
}

bool counterclockwise(pair [] pt)
{
  // Return "true" if the polygon (SIMPLE CURVE i.e. NON CROSSED)
  // of vertex "pt" is counterclockwise
  // Retourne "true" si le polygone (NON CROISÉ) de sommets "pt"
  // est dans le sens des aiguilles d'une montre
  return (signedArea(pt) > 0);
}

pair [] reverse(pair [] pt)
{
  pair [] pt_=copy(pt);
  int begin=0, end=pt.length-1;
  while (begin<end)
    {
      pair temp=pt_[begin];
      pt_[begin]=pt_[end];
      pt_[end]=temp;
      ++begin; --end;
    }
  return pt_;
}

pair [] counterclockdirected(pair [] pt)
{
  if (counterclockwise(pt)) return pt; else return reverse(pt);
}

path polygon(pair [] pt)
{
  int l=pt.length;
  guide opath;
  for (int i=0; i<l; ++i)
    {
      opath = opath--pt[i];
    }
  return opath;
}

pair [] pg = {(0,0), (1,0), (1,1), (2,2), (-1,1)};
draw(polygon(pg)--cycle,
     Arrow(Relative(.1)), BeginBar);
draw(polygon(counterclockdirected(reverse(pg)))--cycle,
     Arrow(position=Relative(.2), FillDraw(red)), BeginBar);

    
size(6cm);
bool counterclockwise(path g, int n=10^3)
{
  // Return "true" if "g" (SIMPLE CURVE i.e. NON CROSSED) is counterclockwise
  // Retounre "true" si "g" (NON CROISÉ) est dans le sens contraire des aiguilles d'une montre
  if (!cyclic(g) || length(g)==0) abort("The function 'clocksize' needs cyclic path.");
  pair [] pt;
  real l=length(g),
    step=1/n,
    t=0,
    sa=0;
  do {
    sa +=point(g,t).x * (point(g,t+step).y - point(g,t-step).y);
    t+=step;
  } while (t<l);
  return (sgn(sa) > 0);
}

path counterclockdirected(path g,int n=10^3)
{
  // Return "g" (SIMPLE CURVE i.e. NON CROSSED) counterclockwise
  // Retourne "g" (NON CROISÉ) dans le sens des aiguilles d'une montre
  if (counterclockwise(g,n)) return g; else return reverse(g);
}

path p=unitcircle;
draw(counterclockdirected(reverse(p)),
     Arrow(Relative(.1)), BeginBar);
draw(counterclockdirected(p),
     Arrow(position=Relative(.2),FillDraw(red)), BeginBar);

    
// Use of the native windingnumber Asymptote routine
// this is the most fast and robust.
// Utilisation du nombre d'enroulement avec la routine native windingnumber d'Asymptote
// c'est plus rapide et plus robuste.
/*
Explanations are here: http://mathworld.wolfram.com/ContourWindingNumber.html
*/

size(6cm,0, false);
bool counterclockwise(path g, pair z) {return windingnumber(g,z) > 0;}

path counterclockdirected(path g,pair z)
{
  if (counterclockwise(g,z)) return g; else return reverse(g);
}

pair z=(1,0);
dot(z);
path p=(0,0){N}..(4,0){N}..cycle;
draw(counterclockdirected(reverse(p),z),Arrow(Relative(.1)), BeginBar);
draw(counterclockdirected(p,z),Arrow(position=Relative(.2),FillDraw(red)), BeginBar);

pair z=(3,-2);
dot(z);
path p=(4,-2){N}..(0,-2){N}..cycle;
draw(counterclockdirected(reverse(p),z),Arrow(Relative(.1)), BeginBar);
draw(counterclockdirected(p,z),Arrow(position=Relative(.2),FillDraw(red)), BeginBar);

pair z=(1,-4.5);
dot(z);
path p=yscale(.75)*((0,-6){N}..(2,-6){S}..(0,-6){N}..(4,-6){S}..cycle);
draw(counterclockdirected(reverse(p),z),Arrow(Relative(.1)), BeginBar);
draw(counterclockdirected(p,z),Arrow(position=Relative(.2),FillDraw(red)), BeginBar);

pair z=(3,-8);
dot(z);
path p=shift((0,-3.5))*p;
draw(counterclockdirected(reverse(p),z),Arrow(Relative(.1)), BeginBar);
draw(counterclockdirected(p,z),Arrow(position=Relative(.2),FillDraw(red)), BeginBar);

    
// Use of the windingnumber works also for CROSSED curves

size(8cm,10cm,false);

import math;

bool counterclockwise(path g)
{
  // Return "true" if "g" is counterclockwise
  // Retounre "true" si "g" est dans le sens contraire des aiguilles d'une montre
  return (windingnumber(g,inside(g)) > 0);
}

path counterclockdirected(path g)
{
  if (counterclockwise(g)) return g; else return reverse(g);
}

guide p=randompath(30)..cycle;
draw(counterclockdirected(reverse(p)),Arrow(10bp,Relative(0.025)), BeginBar);
draw(counterclockdirected(p),Arrow(10bp,FillDraw(red),Relative(.05)), BeginBar);