Star generation + simulation

# Generate 100 stars
n <- 100
# stars.m holds our galaxy
stars.m <- matrix(0, n, 5)

# Add a star to the galaxy.
genStar <- function(m) {
  # Try 20 times to find a new spot for a star.
  for (tries in 1:20) {
    # Pick a radius and angle.
    # Roughly, stars are a randomly-chosen integer distance from the center.
    # Larger radii have a larger chance of being chosen than smaller.
    radius <- 3+floor((1 - runif(1)^2) * 10)
    angle <- runif(1)*2*pi

    collision = FALSE
    for (i in 1:n) {
      # Check all current stars in the galaxy to make sure there aren't
      # any too close since it looks bad.
      if (m[i, 1] == radius & (abs(m[i, 2] - angle) < ((0.25*2*pi) / radius))) {
        # Too close to a star in the same ring, so try another.
        collision = TRUE
        break
      }
    }
    if (!collision) {
      # No collision, so we've found a new location for a star.
      return(c(radius, angle))
    }
 }
  return(c(0, 0))
}

for (i in 1:n) {
  # Assign radius/angle for the new star.
  stars.m[i, 1:2] = genStar(stars.m)

  # Calculate x, y positions.
  stars.m[i, 3] = stars.m[i, 1]*sin(stars.m[i, 2])
  stars.m[i, 4] = stars.m[i, 1]*cos(stars.m[i, 2])
  # Use initial angle to set the color.
  stars.m[i, 5] = 1+floor(stars.m[i, 2] * 3 / pi)
}

# Red, Yellow, Green, Teal, Blue, Purple
cols=c("#FF0000", "#FFFF00", "#00FF00", "#00FFFF", "#0000FF", "#FF00FF")

# Advance the galaxy by some time.
advance <- function(m, t) {
  for (i in 1:n) {
    # Stars closer to the center move faster.
    m[i, 2] = m[i, 2] + (t / m[i, 1])

    # Update x, y positions.
    m[i, 3] = m[i, 1]*sin(m[i, 2])
    m[i, 4] = m[i, 1]*cos(m[i, 2])
  }
  return(m)
}

# Plot the initial galaxy.
png(filename="tick_0.png")
par(bg = "black")
plot(stars.m[, 3:4], col=cols[stars.m[, 5]], main="Tick 0", col.main="white")
dev.off()

for (i in 1:15) {
  # Advance the galaxy in day-increments and plot.
  tick <- 24*i
  fn <- paste0("tick_", as.character(tick), ".png")


  png(filename=fn)
  stars.m = advance(stars.m, 1)
  par(bg = "black")
  plot(stars.m[, 3:4], col=cols[stars.m[, 5]], main=paste("Tick", tick), col.main="white")
  dev.off()
}