Class: Kdtree

Inherits:

Object

• Object
• Kdtree

Defined in:

ext/kdtree/kdtree.c

Instance Method Summary

• #initialize(arg) ⇒ Object constructor

  Returns a new Kdtree.

• #nearest(x, y) ⇒ Object

  Finds the point closest to x, y and returns the id for that point.

• #nearestk(x, y, k) ⇒ Array

  Finds the k points closest to x, y.

• #persist(io) ⇒ Object

  Writes the tree out to io so you can quickly load it later with Kdtree.new.

• #to_s ⇒ String

  A string that tells you a bit about the tree.

Constructor Details

#new(points) ⇒ Object #new(io) ⇒ Object

Returns a new Kdtree. To construct a tree, pass an array of points. Each point should be an array of the form [x, y, id], where x and y are floats and id is an integer. The id is arbitrary and will be returned to you whenever you search with nearest or nearestk.

# create a new tree
points = []
points << [47.6, -122.3, 1] # Seattle
points << [40.7, -74.0, 2]  # New York
kd = Kdtree.new(points)

Alternately, you can pass in an IO object containing a persisted kdtree. This makes it possible to build the tree in advance, persist it, and start it up quickly later. See persist for more information.

# File 'ext/kdtree/kdtree.c', line 98

static VALUE kdtree_initialize(VALUE kdtree, VALUE arg)
{
    KDTREEP;

    if (TYPE(arg) == T_ARRAY) {
        // init from array of pints
        VALUE points = arg;
        int i;
        kdtreep->len = RARRAY_LEN(points);
        kdtreep->nodes = ALLOC_N(struct kdtree_node, kdtreep->len);

        for (i = 0; i < RARRAY_LEN(points); ++i) {
            struct kdtree_node *n = kdtreep->nodes + i;

            VALUE ptr = rb_ary_entry(points, i);
            VALUE v = rb_check_array_type(ptr);
            if (NIL_P(v) || RARRAY_LEN(v) != 3) {
                continue;
            }
            n->x = NUM2DBL(rb_ary_entry(v, 0));
            n->y = NUM2DBL(rb_ary_entry(v, 1));
            n->id = NUM2INT(rb_ary_entry(v, 2));
        }

        // now build the tree
        kdtreep->root = kdtree_build(kdtreep, 0, kdtreep->len, 0);
    } else if (rb_respond_to(arg, rb_intern("read"))) {
        VALUE io = arg;
        char buf[4];
        if (rb_respond_to(io, id_binmode)) {
            rb_funcall(io, id_binmode, 0);
        }

        // check magic
        read_all(io, buf, 4);
        if (memcmp(KDTREE_MAGIC, buf, 4) != 0) {
            rb_raise(rb_eRuntimeError, "wrong magic number in kdtree file");
        }

        // read start of the struct
        read_all(io, kdtreep, sizeof(struct kdtree_data) - sizeof(struct kdtree_node *));

        // read the nodes
        kdtreep->nodes = ALLOC_N(struct kdtree_node, kdtreep->len);
        read_all(io, kdtreep->nodes, sizeof(struct kdtree_node) * kdtreep->len);
    } else {
        rb_raise(rb_eTypeError, "array or IO required to init Kdtree");
    }

    return kdtree;
}

Instance Method Details

#nearest(x, y) ⇒ Object

Finds the point closest to x, y and returns the id for that point. Returns -1 if the tree is empty.

points = []
points << [47.6, -122.3, 1] # Seattle
points << [40.7, -74.0, 2]  # New York
kd = Kdtree.new(points)

# which city is closest to Portland?
kd.nearest(45.5, -122.8) #=> 1
# which city is closest to Boston?
kd.nearest(42.4, -71.1)  #=> 2

# File 'ext/kdtree/kdtree.c', line 201

static VALUE kdtree_nearest(VALUE kdtree, VALUE x, VALUE y)
{
    int n_index;
    float n_dist;
    KDTREEP;

    n_index = -1;
    n_dist = INT_MAX;

    kdtree_nearest0(kdtreep, kdtreep->root, NUM2DBL(x), NUM2DBL(y), 0, &n_index, &n_dist);
    if (n_index == -1) {
        return -1;
    }
    return INT2NUM((kdtreep->nodes + n_index)->id);
}

#nearestk(x, y, k) ⇒ Array

Finds the k points closest to x, y. Returns an array of ids, sorted by distance. Returns an empty array if the tree is empty. Note that k is capped at 255.

points = []
points << [47.6, -122.3, 1] # Seattle
points << [45.5, -122.8, 2] # Portland
points << [40.7, -74.0,  3] # New York
kd = Kdtree.new(points)

# which two cities are closest to San Francisco?
kd.nearestk(34.1, -118.2, 2) #=> [2, 1]

Returns:

• (Array)

# File 'ext/kdtree/kdtree.c', line 283

static VALUE kdtree_nearestk(VALUE kdtree, VALUE x, VALUE y, VALUE k)
{
    // note I leave an extra slot here at the end because of the way our binary insert works
    kresult k_list[MAX_K + 1];
    int k_len = 0;
    float k_dist = INT_MAX;
    int ki = NUM2INT(k);
    VALUE ary;
    int i;
    KDTREEP;

    if (ki < 1) {
        ki = 1;
    } else if (ki > MAX_K) {
        ki = MAX_K;
    }
    kdtree_nearestk0(kdtreep, kdtreep->root, NUM2DBL(x), NUM2DBL(y), ki, 0, k_list, &k_len, &k_dist);

    // convert result to ruby array
    ary = rb_ary_new();
    for (i = 0; i < k_len; ++i) {
        rb_ary_push(ary, INT2NUM(kdtreep->nodes[k_list[i].index].id));
    }
    return ary;
}

#persist(io) ⇒ Object

Writes the tree out to io so you can quickly load it later with Kdtree.new. This avoids the startup cost of initializing a tree. Apart from a small header, the size of the file is proportional to the number of points, requiring 20 bytes per point.

This file is NOT PORTABLE across different architectures due to endian issues.

points = []
points << [47.6, -122.3, 1] # Seattle
points << [45.5, -122.8, 2] # Portland
points << [40.7, -74.0,  3] # New York
kd = Kdtree.new(points)

# persist the tree to disk
File.open("treefile", "w") { |f| kd.persist(f) }

...

# later, read the tree from disk
kd2 = File.open("treefile") { |f| Kdtree.new(f) }

# File 'ext/kdtree/kdtree.c', line 407

static VALUE kdtree_persist(VALUE kdtree, VALUE io)
{
    VALUE str;
    KDTREEP;

    if (!rb_respond_to(io, rb_intern("write"))) {
        rb_raise(rb_eTypeError, "instance of IO needed");
    }
    if (rb_respond_to(io, id_binmode)) {
        rb_funcall(io, id_binmode, 0);
    }

    write_all(io, KDTREE_MAGIC, 4);
    write_all(io, kdtreep, sizeof(struct kdtree_data) - sizeof(struct kdtree_node *));
    write_all(io, kdtreep->nodes, sizeof(struct kdtree_node) * kdtreep->len);
    return io;
}

#to_s ⇒ String

A string that tells you a bit about the tree.

Returns:

• (String)

# File 'ext/kdtree/kdtree.c', line 431

static VALUE kdtree_to_s(VALUE kdtree)
{
    char buf[256];
    KDTREEP;

    sprintf(buf, "#<%s:%p nodes=%d>", rb_obj_classname(kdtree), (void*)kdtree, kdtreep->len);
    return rb_str_new(buf, strlen(buf));
}