private int[] FindBestTrail(int[][] dists, int numCities)
{
random = new Random(0);
var ants = InitAnts(numCities);
// initialize ants to random trails
int[] bestTrail = BestTrail(ants, dists);
// determine the best initial trail
double bestLength = Length(bestTrail, dists);
// the length of the best trail
Pheromone pheromone = new Pheromone(numCities);
int time = 0;
while (time < maxTime)
{
UpdateAnts(ants, pheromone, dists);
UpdatePheromones(pheromone, ants, dists);
int[] currBestTrail = BestTrail(ants, dists);
double currBestLength = Length(currBestTrail, dists);
if (currBestLength < bestLength)
{
bestLength = currBestLength;
bestTrail = currBestTrail;
//Console.WriteLine("New best length of " + bestLength.ToString("F1") + " found at " + time);
}
time += 1;
}
return(bestTrail);
}
private void UpdateAnts(Ant[] ants, Pheromone pheromone, int[][] dists)
{
int numCities = pheromone.Size;
for (int k = 0; k <= ants.Length - 1; k++)
{
int start = isStartPointFixed ? startPoint : random.Next(0, numCities);
int[] newTrail = isEndPointFixed ? BuildTrail(start, endPoint, pheromone, dists) : BuildTrail(start, pheromone, dists);
ants[k].Trail = newTrail;
}
}
private double[] MoveProbs(int cityX, bool[] visited, Pheromone pheromone, int[][] dists)
{
// for ant, located at nodeX, with visited[], return the prob of moving to each city
int numCities = pheromone.Size;
double[] taueta = new double[numCities];
// inclues cityX and visited cities
double sum = 0.0;
// sum of all tauetas
// i is the adjacent city
for (int i = 0; i <= taueta.Length - 1; i++)
{
if (i == cityX)
{
taueta[i] = 0.0;
// prob of moving to self is 0
}
else if (visited[i])
{
taueta[i] = 0.0;
// prob of moving to a visited city is 0
}
else
{
taueta[i] = Math.Pow(pheromone.Get(cityX, i), alpha) * Math.Pow((1.0 / Distance(cityX, i, dists)), beta);
// could be huge when pheromone[][] is big
if (taueta[i] < 0.0001)
{
taueta[i] = 0.0001;
}
else if (taueta[i] > (double.MaxValue / (numCities * 100)))
{
taueta[i] = double.MaxValue / (numCities * 100);
}
}
sum += taueta[i];
}
double[] probs = new double[numCities];
for (int i = 0; i <= probs.Length - 1; i++)
{
probs[i] = taueta[i] / sum;
// big trouble if sum = 0.0
}
return(probs);
}
private int[] BuildTrail(int start, Pheromone pheromone, int[][] dists)
{
int numCities = pheromone.Size;
int[] trail = new int[numCities];
bool[] visited = new bool[numCities];
trail[0] = start;
visited[start] = true;
for (int i = 0; i <= numCities - 2; i++)
{
int cityX = trail[i];
int next = NextCity(cityX, visited, pheromone, dists);
trail[i + 1] = next;
visited[next] = true;
}
return(trail);
}
private void UpdatePheromones(Pheromone pheromone, Ant[] ants, int[][] dists)
{
for (int i = 0; i <= pheromone.Size - 1; i++)
{
for (int j = i + 1; j <= pheromone.Size - 1; j++)
{
for (int k = 0; k <= ants.Length - 1; k++)
{
double length = Length(ants[k].Trail, dists);
// length of ant k trail
double decrease = (1.0 - rho) * pheromone.Get(i, j);
double increase = 0.0;
if (EdgeInTrail(i, j, ants[k].Trail))
{
increase = (Q / length);
}
pheromone.Set(i, j, decrease + increase);
}
}
}
}
private int NextCity(int cityX, bool[] visited, Pheromone pheromone, int[][] dists)
{
// for ant (with visited[]), at nodeX, what is next node in trail?
double[] probs = MoveProbs(cityX, visited, pheromone, dists);
double[] cumul = new double[probs.Length + 1];
for (int i = 0; i <= probs.Length - 1; i++)
{
cumul[i + 1] = cumul[i] + probs[i];
// consider setting cumul[cuml.Length-1] to 1.00
}
double p = random.NextDouble();
for (int i = 0; i <= cumul.Length - 2; i++)
{
if (p >= cumul[i] && p < cumul[i + 1])
{
return(i);
}
}
throw new Exception("Failure to return valid city in NextCity");
}
