scripts/builtin/knnGraph.dml

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# Builtin for k nearest neighbor graph construction
#
# INPUT:
# --------------------------
# X        ---
# k        ---
# --------------------------
#
# OUTPUT:
# ------------------------
# graph  ---
# ------------------------

m_knnGraph = function(Matrix[double] X, integer k) return (Matrix[double] graph) {
  distances = dist(X);
  graph = matrix(0, rows=nrow(distances), cols=ncol(distances));
  ksmall = matrix(0, rows=nrow(distances), cols=1)
  for (row in 1:nrow(distances)) {
    referent = kthSmallest(distances[row], k + 1);
    ksmall[row] = referent
  }
  graph = distances <= ksmall
  # # assign zero to diagonal elements 
  diagonal = diag(matrix(1, rows=nrow(distances), cols=1)) == 0
  graph = graph * diagonal
}

# # # TODO vectorize the below function
kthSmallest = function(Matrix[double] array, integer k)
return (integer res) {
  left = 1;
  right = ncol(array);
  found = FALSE;

  while ((left <= right) & !found) {
    pivot = as.scalar(array[1,right]);
    i = (left - 1);
    j = left;
    while (j < right) {
      if (as.scalar(array[1,j]) <= pivot) {
        i = i + 1;
        temp = as.scalar(array[1,i]);
        array[1,i] = array[1,j];
        array[1,j] = temp;
      }
      j = j + 1;
    }

    temp = as.scalar(array[1,i + 1]);
    array[1,i + 1] = array[1,right];
    array[1,right] = temp;

    pivot = i + 1;

    if(pivot == k) {
      res = as.scalar(array[1,pivot]);
      found = TRUE;
    }
    else if (pivot > k)
      right = pivot - 1;
    else 
      left = pivot + 1; 
  }
  if (!found) 
    res = -1; 
}