1 files changed, 124 insertions, 0 deletions
diff --git a/src/km-init-kmeans.c b/src/km-init-kmeans.c
new file mode 100644
index 0000000..f0c1b5f
--- /dev/null
+++ b/src/km-init-kmeans.c
@@ -0,0 +1,124 @@
+#include <stdbool.h> // bool
+#include <string.h> // memset()
+#include <float.h> // FLT_MAX
+#include "util.h"
+#include "km.h"
+
+// sum the squared distance of every row in set from this point
+static inline float
+sum_distance_squared(
+  const float * const floats,
+  const km_set_t * const set
+) {
+  float r = 0;
+
+  // sum squared distances
+  for (size_t i = 0; i < set->num_rows; i++) {
+    const float * const vals = km_set_get_row(set, i);
+    r += distance_squared(set->shape.num_floats, floats, vals);
+  }
+
+  // return result
+  return r;
+}
+
+static const float *
+get_random_row(
+  const km_set_t * const set,
+  km_rand_t * const rs
+) {
+  // get random offset
+  size_t ofs = 0;
+  if (!km_rand_get_sizes(rs, 1, &ofs)) {
+    die("km_rand_fill_sizes()");
+  }
+
+  return km_set_get_row(set, ofs % set->num_rows);
+}
+
+// init a cluster set of num_clusters by picking the first cluster from
+// the set of points at random
+// random initial points from the set
+bool
+km_init_kmeans(
+  km_set_t * const cs,
+  const size_t num_clusters,
+  const km_set_t * const set,
+  km_rand_t * const rs
+) {
+  const size_t num_floats = set->shape.num_floats,
+               stride = sizeof(float) * num_floats;
+  
+  // row values (filled below)
+  float sums[num_clusters];
+  float floats[num_floats * num_clusters];
+
+  // pick first row randomly
+  {
+    // get random row
+    const float * const vals = get_random_row(set, rs);
+
+    // copy row values to floats buffer
+    memcpy(floats, vals, stride);
+
+    // copy row values to floats buffer
+    sums[0] = sum_distance_squared(floats, set);
+  }
+
+  for (size_t i = 1; i < num_clusters;) {
+    // get random row
+    const float * const vals = get_random_row(set, rs);
+
+    // calculate squared distance to nearest cluster
+    size_t ofs = 0;
+    float min_d2 = FLT_MAX;
+    for (size_t j = 0; j < i; j++) {
+      const float d2 = distance_squared(num_floats, vals, floats + j * stride);
+      if (d2 < min_d2) {
+        ofs = j;
+        min_d2 = d2;
+      }
+    }
+
+    // get a random floating point value
+    float rand_val = 0;
+    if (!km_rand_get_floats(rs, 1, &rand_val)) {
+      // return failure
+      return false;
+    }
+
+    // check random value
+    if (rand_val / sums[ofs] < min_d2) {
+      // get destination
+      float * const dst = floats + i * num_floats;
+
+      // copy row values
+      memcpy(dst, vals, stride);
+
+      // calculate total cluster distance
+      sums[i] = sum_distance_squared(dst, set);
+
+      // increment cluster count
+      i++;
+    }
+  }
+
+  // FIXME: should probably be heap-allocated
+  int ints[num_clusters];
+  memset(ints, 0, sizeof(ints));
+
+  // init cluster shape
+  const km_shape_t shape = {
+    .num_floats = num_floats,
+    .num_ints = 1,
+  };
+
+  // init cluster set
+  if (!km_set_init(cs, &shape, num_clusters)) {
+    // return failure
+    return false;
+  }
+
+  // add data, return result
+  return km_set_push(cs, num_clusters, floats, ints);
+}