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/// EXPERIMENTAL: Metadata for n-dimensional sparse arrays, aka "sparse tensors".
/// Arrow implementations in general are not required to implement this type

include "Tensor.fbs";

namespace org.apache.arrow.flatbuf;

/// ----------------------------------------------------------------------
/// EXPERIMENTAL: Data structures for sparse tensors

/// Coodinate (COO) format of sparse tensor index.
///
/// COO's index list are represented as a NxM matrix,
/// where N is the number of non-zero values,
/// and M is the number of dimensions of a sparse tensor.
///
/// indicesBuffer stores the location and size of the data of this indices
/// matrix.  The value type and the stride of the indices matrix is
/// specified in indicesType and indicesStrides fields.
///
/// For example, let X be a 2x3x4x5 tensor, and it has the following
/// 6 non-zero values:
///
///   X[0, 1, 2, 0] := 1
///   X[1, 1, 2, 3] := 2
///   X[0, 2, 1, 0] := 3
///   X[0, 1, 3, 0] := 4
///   X[0, 1, 2, 1] := 5
///   X[1, 2, 0, 4] := 6
///
/// In COO format, the index matrix of X is the following 4x6 matrix:
///
///   [[0, 0, 0, 0, 1, 1],
///    [1, 1, 1, 2, 1, 2],
///    [2, 2, 3, 1, 2, 0],
///    [0, 1, 0, 0, 3, 4]]
///
/// Note that the indices are sorted in lexicographical order.
table SparseTensorIndexCOO {
  /// The type of values in indicesBuffer
  indicesType: Int;

  /// Non-negative byte offsets to advance one value cell along each dimension
  indicesStrides: [long];

  /// The location and size of the indices matrix's data
  indicesBuffer: Buffer;
}

/// Compressed Sparse Row format, that is matrix-specific.
table SparseMatrixIndexCSR {
  /// The type of values in indptrBuffer
  indptrType: Int;

  /// indptrBuffer stores the location and size of indptr array that
  /// represents the range of the rows.
  /// The i-th row spans from indptr[i] to indptr[i+1] in the data.
  /// The length of this array is 1 + (the number of rows), and the type
  /// of index value is long.
  ///
  /// For example, let X be the following 6x4 matrix:
  ///
  ///   X := [[0, 1, 2, 0],
  ///         [0, 0, 3, 0],
  ///         [0, 4, 0, 5],
  ///         [0, 0, 0, 0],
  ///         [6, 0, 7, 8],
  ///         [0, 9, 0, 0]].
  ///
  /// The array of non-zero values in X is:
  ///
  ///   values(X) = [1, 2, 3, 4, 5, 6, 7, 8, 9].
  ///
  /// And the indptr of X is:
  ///
  ///   indptr(X) = [0, 2, 3, 5, 5, 8, 10].
  indptrBuffer: Buffer;

  /// The type of values in indicesBuffer
  indicesType: Int;

  /// indicesBuffer stores the location and size of the array that
  /// contains the column indices of the corresponding non-zero values.
  /// The type of index value is long.
  ///
  /// For example, the indices of the above X is:
  ///
  ///   indices(X) = [1, 2, 2, 1, 3, 0, 2, 3, 1].
  ///
  /// Note that the indices are sorted in lexicographical order for each row.
  indicesBuffer: Buffer;
}

union SparseTensorIndex {
  SparseTensorIndexCOO,
  SparseMatrixIndexCSR
}

table SparseTensor {
  /// The type of data contained in a value cell.
  /// Currently only fixed-width value types are supported,
  /// no strings or nested types.
  type: Type;

  /// The dimensions of the tensor, optionally named.
  shape: [TensorDim];

  /// The number of non-zero values in a sparse tensor.
  non_zero_length: long;

  /// Sparse tensor index
  sparseIndex: SparseTensorIndex;

  /// The location and size of the tensor's data
  data: Buffer;
}

root_type SparseTensor;