https://stackoverflow.com/a/78438689/1271826

SparseMultiply.swift

var valuesA: [Float] = [2, 0.5, 9, 1, 18]
let rowsA = Int32(valuesA.count)
let columnsA = Int32(valuesA.count)
var rowIndicesA = Array(0 ..< rowsA)
var columnStarts = Array(0 ... valuesA.count)

var valuesB: [Float] = [3,   5,   8,   7,   2,
                        1,   4,   9,   1,   3,
                        2,   7,   3,   1,   2,
                        5,   4,   9,   1,   6,
                        4,   2,   9,   6,   7]
let count = valuesB.count

valuesA.withUnsafeMutableBufferPointer { aPointer in
    valuesB.withUnsafeMutableBufferPointer { bPointer in
        rowIndicesA.withUnsafeMutableBufferPointer { rowsPointer in
            columnStarts.withUnsafeMutableBufferPointer { columnsPointer in
                var a = SparseMatrix_Float(
                    structure: SparseMatrixStructure(
                        rowCount: rowsA,
                        columnCount: columnsA,
                        columnStarts: columnsPointer.baseAddress!,
                        rowIndices: rowsPointer.baseAddress!,
                        attributes: SparseAttributes_t(),
                        blockSize: 1
                    ),
                    data: aPointer.baseAddress!
                )

                let results = Array<Float>(unsafeUninitializedCapacity: count) {
                    resultsPointer,
                    initializedCount in
                    let b = DenseMatrix_Float(
                        rowCount: rowsA,
                        columnCount: columnsA,
                        columnStride: rowsA,
                        attributes: SparseAttributes_t(
                            transpose: true,
                            triangle: SparseLowerTriangle,
                            kind: SparseSymmetric,
                            _reserved: 0,
                            _allocatedBySparse: false
                        ),
                        data: bPointer.baseAddress!
                    )
                    let c = DenseMatrix_Float(
                        rowCount: rowsA,
                        columnCount: columnsA,
                        columnStride: rowsA,
                        attributes: SparseAttributes_t(
                            transpose: true,
                            triangle: SparseLowerTriangle,
                            kind: SparseSymmetric,
                            _reserved: 0,
                            _allocatedBySparse: false
                        ),
                        data: resultsPointer.baseAddress!
                    )

                    SparseMultiply(a, b, c)
                    initializedCount = count
                }

                // Print the result matrix

                let formatter = NumberFormatter()
                formatter.minimumFractionDigits = 1
                formatter.formatWidth = 5
                for row in 0..<rowsA {
                    for column in 0..<columnsA {
                        let value = results[Int(row * columnsA + column)]
                        print(formatter.string(for: value)!, terminator: " ")
                    }
                    print()
                }
            }
        }
    }
}