// Tags each row with the index of its nearest centroid. The old tags are used
// for optimization.
func tag(vecs, means [][]float64, oldTags []int) []int {
if len(means) == 0 {
panic("Cannot tag on 0 centroids.")
}
// Create a distance matrix of means from one another.
meansd := make([][]float64, len(means))
for i := range meansd {
meansd[i] = make([]float64, len(means))
for j := range means {
meansd[i][j] = vectors.L2(means[i], means[j])
}
}
tags := make([]int, len(vecs))
// Go over vectors.
for i := range vecs {
// Find nearest centroid.
tags[i] = oldTags[i]
d := vectors.L2(means[oldTags[i]], vecs[i])
for j := 0; j < len(means); j++ {
// Use triangle inequality to skip means that are too distant.
if j == tags[i] || meansd[j][tags[i]] >= 2*d {
continue
}
dj := vectors.L2(means[j], vecs[i])
if dj < d {
d = dj
tags[i] = j
}
}
}
return tags
}
// Calculates the average squared-distance of elements from their assigned
// means.
func MeanSquaredError(vecs, means [][]float64, tags []int) float64 {
if len(tags) != len(vecs) {
panic(fmt.Sprintf("Non-matching lengths of matrix and tags: %d, %d",
len(vecs), len(tags)))
}
if len(vecs) == 0 {
return 0
}
d := 0.0
for i := range tags {
dist := vectors.L2(means[tags[i]], vecs[i])
d += dist * dist
}
return d / float64(len(vecs))
}
// Picks the initial means with the K-means++ algorithm.
func initialMeans(vecs [][]float64, k int) [][]float64 {
result := make([][]float64, k)
perm := rand.Perm(len(vecs))
numTrials := 2 + int(math.Log(float64(k)))
probs := make([]float64, len(vecs)) // Probability of each vector.
nearest := make([]int, len(vecs)) // Index of nearest mean to each vector.
distance := make([]float64, len(vecs)) // Distance to nearest mean.
mdistance := make([][]float64, k) // Distance between means.
for i := range mdistance {
mdistance[i] = make([]float64, k)
}
// Pick each mean.
for i := range result {
result[i] = make([]float64, len(vecs[0]))
// First mean is first vector.
if i == 0 {
copy(result[0], vecs[perm[0]])
for _, j := range perm {
distance[j] = vectors.L2(vecs[j], result[0])
}
continue
}
// Find next mean.
bestCandidate := -1
bestImprovement := -math.MaxFloat64
for t := 0; t < numTrials; t++ { // Make a few attempts.
sum := 0.0
for _, j := range perm {
probs[j] = distance[j] * distance[j]
sum += probs[j]
}
// Pick element with probability relative to d^2.
r := rand.Float64() * sum
newMean := 0
for r > probs[newMean] {
r -= probs[newMean]
newMean++
}
copy(result[i], vecs[newMean])
// Update distances from new mean to other means.
for j := range mdistance[:i] {
mdistance[j][i] = vectors.L2(result[i], result[j])
mdistance[i][j] = mdistance[j][i]
}
// Check improvement.
newImprovement := 0.0
for j := range vecs {
if mdistance[i][nearest[j]] < 2*distance[j] {
d := vectors.L2(vecs[j], result[i])
d = math.Min(distance[j], d)
newImprovement += distance[j] - d
}
}
if newImprovement > bestImprovement {
bestCandidate = newMean
bestImprovement = newImprovement
}
}
copy(result[i], vecs[bestCandidate])
// Update distances.
for j := range mdistance[:i] { // From new mean to other means.
mdistance[j][i] = vectors.L2(result[i], result[j])
mdistance[i][j] = mdistance[j][i]
}
for j := range vecs { // From vecs to nearest means.
if mdistance[i][nearest[j]] < 2*distance[j] {
d := vectors.L2(vecs[j], result[i])
if d < distance[j] {
distance[j] = math.Min(distance[j], d)
nearest[j] = i
}
}
}
}
return result
}
