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Showing content from https://astrometrynet.readthedocs.io/en/latest/libkd.html below:

libkd documentation — Astrometry.net master documentation

kdtree_t* kdtree_build(kdtree_t* kd, void *data, int N, int D, int Nleaf, int treetype, unsigned int options);

Build a tree from an array of data, of size N*D*sizeof(data_item).

kd: NULL to allocate a new kdtree_t structure, or the address

of the structure in which to store the result.

data: your N x D-dimensional data, stored in N-major direction:

data[n*D + d] is the address of data item “n”, dimension “d”. If 3-dimensional data, eg, order is x0,y0,z0,x1,y1,z1,x2,y2,z2.

N: number of vectors

D: dimensionality of vectors

Nleaf: number of element in a kd-tree leaf node. Typical value

would be about 32.

treetype:

• if your data are doubles, KDTT_DOUBLE
• if your data are floats, KDTT_FLOAT

For fancier options, see kd_tree_types.

options: bitfield of kd_build_options values. Specify one of:

• KD_BUILD_BBOX: keep a full bounding-box at each node;
• KD_BUILD_SPLIT: just keep the split dimension and value at each node.

see kd_build_options for additional fancy stuff.

NOTE that this function will permute the contents of the data array!

When you’re done with your tree, be sure to kdtree_free() it.

Example:

double mydata[] = { 1,1, 2,2, 3,3, 4,4, 5,5, 6,6, 7,7, 8,8 };
int D = 2;
int N = sizeof(mydata) / (D * sizeof(double));
kdtree_t* kd = kdtree_build(NULL, mydata, N, D, 4, KDTT_DOUBLE, KD_BUILD_BBOX);
kdtree_print(kd);
kdtree_free(kd);

void kdtree_free(kdtree_t *kd);

Frees the given kd. By default, the kd->data is NOT freed. Set kd->free_data = 1 to free the data when kdtree_free() is called.

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