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37 #ifndef GMX_LINEARALGEBRA_SPARSEMATRIX_H
38 #define GMX_LINEARALGEBRA_SPARSEMATRIX_H
42 #include "gromacs/utility/basedefinitions.h"
43 #include "gromacs/utility/real.h"
45 typedef struct gmx_sparsematrix_entry
49 } gmx_sparsematrix_entry_t;
51 /*! \brief Sparse matrix storage format
53 * This structure specifies a storage format for a sparse matrix.
54 * The memory requirements are only proportional to the number
55 * of nonzero elements, and it provides a reasonably fast way to
56 * perform matrix-vector multiplications.
58 * The data format is very similar to a neighborlist. It is optimized
59 * for fast access, but it is difficult to add entries. If you are
60 * constructing a matrix you should either do it in exactly the order
61 * specified here, or use some other more flexible intermediate structure.
63 * The index array is of size nrow+1. All non-zero matrix elements
64 * on row i are stored in positions index[i] through index[i+1]-1 in
65 * the arrays column and value. The column array contains the column
66 * index for each entry, in ascending order, and the corresponding
67 * position in the value array contains the floating point matrix element.
69 * index[nrow] should be equal to the total number of elements stored.
71 * Thus, to find the value of matrix element [5,4] you should loop
72 * over positions index[5] to index[6]-1 in column until you either find
73 * the value 4, or a higher value (meaning the element was zero).
75 * It is fairly easy to construct the matrix on-the-fly if you can do
79 * If compressed_symmetric is set to TRUE, you should only store EITHER the upper OR
80 * lower triangle (and the diagonal), and the other half is assumed to be
81 * symmetric. Otherwise, if compressed_symmetric==FALSE, no symmetry is implied and all
82 * elements should be stored.
84 * The symmetry compression saves us a factor 2 both in storage and
85 * matrix multiplication CPU-time, which can be very useful for huge eigenproblems.
87 * If you are unsure, just set compressed_symmetric to FALSE and list all elements. If
88 * you enable it but still list all elements (both upper and lower triangle) you will be sorry...
90 * Internally, the sparse data is stored as a separate list for each row, where the list
91 * element is a structure with a column and (floating-point) data value. This makes it
92 * possible, although not completely transparent, to update values in random access order.
93 * The drawback is that the structure will allocate nrow memory regions.
94 * The matrix data could be stored in a single contiguous array with indices for each row,
95 * but then we could only insert elements at the end without copying the entire matrix.
99 * In other words: Not perfect, but it works.
101 typedef struct gmx_sparsematrix
103 gmx_bool compressed_symmetric; /**< Store half elements and assume symmetry. */
104 int nrow; /**< Number of rows in matrix */
105 int* ndata; /**< Number of entries on each row (list) */
106 int* nalloc; /**< Allocated entry list length for each row */
107 gmx_sparsematrix_entry_t** data; /**< data[i] is a list with entries on row i */
108 } gmx_sparsematrix_t;
111 /*! \brief Allocate a new sparse matrix structure
113 * The number of rows is used to allocate the index array entry. Obviously you
114 * can reallocate these later yourself if necessary - this is a
115 * convenience routine.
117 * By default, the compressed_symmetric flag in the structure will
118 * be FALSE. Set it to TRUE manually if you are only storing either the
119 * upper or lower half of the matrix.
121 gmx_sparsematrix_t* gmx_sparsematrix_init(int nrow);
124 /*! \brief Release all resources used by a sparse matrix structure
126 * All arrays in the structure will be freed, and the structure itself.
128 void gmx_sparsematrix_destroy(gmx_sparsematrix_t* A);
131 /*! \brief Print sparse matrix to a stream.
133 * Mainly used for debugging. Be warned that the real sparse matrices used
134 * in Gromacs runs can be HUGE (think 100,000 rows).
136 void gmx_sparsematrix_print(FILE* stream, gmx_sparsematrix_t* A);
138 /* Adds value at row,col. If the value did not exist
139 * previously it is added, otherwise it is incremented with difference.
141 * The column sort order might change, so you need to run fix_sparsematrix
142 * once you are done changing the matrix.
144 real gmx_sparsematrix_value(gmx_sparsematrix_t* A, int row, int col);
147 /* Adds value at row,col. If the value did not exist
148 * previously it is added, otherwise it is incremented with difference.
150 * The column sort order might change, so you need to run fix_sparsematrix
151 * once you are done changing the matrix.
153 void gmx_sparsematrix_increment_value(gmx_sparsematrix_t* A, int row, int col, real difference);
156 /*! \brief Sort elements in each column and remove zeros.
158 * Sparse matrix access is faster when the elements are stored in
159 * increasing column order in each row. In some cases previously non-zero
160 * elements will be zero after adding more data, and this routine also removes
161 * those entries to reduce the storage requirements.
163 * It never hurts to run this routine if you have been updating the matrix...
165 void gmx_sparsematrix_compress(gmx_sparsematrix_t* A);
168 /*! \brief Sparse matrix vector multiplication
170 * Calculate y = A * x for a sparse matrix A.
172 void gmx_sparsematrix_vector_multiply(gmx_sparsematrix_t* A, real* x, real* y);