Improve comments for TransposeForCompressedRowSparseStructure.
Also make the code a little more safe in its assumptions around
zeroed out arrays.
Change-Id: I5a3588162da496fae2b31a4bf8adef1a02b85ec6
diff --git a/internal/ceres/compressed_row_sparse_matrix.cc b/internal/ceres/compressed_row_sparse_matrix.cc
index 40cdd10..d0c10b2 100644
--- a/internal/ceres/compressed_row_sparse_matrix.cc
+++ b/internal/ceres/compressed_row_sparse_matrix.cc
@@ -77,25 +77,40 @@
int* transpose_rows,
int* transpose_cols,
double* transpose_values) {
+ // Explicitly zero out transpose_rows.
+ std::fill(transpose_rows, transpose_rows + num_nonzeros, 0.0);
+
+ // Count the number of entries in each column of the original matrix
+ // and assign to transpose_rows[col + 1].
for (int idx = 0; idx < num_nonzeros; ++idx) {
++transpose_rows[cols[idx] + 1];
}
+ // Compute the starting position for each row in the transpose by
+ // computing the cumulative sum of the entries of transpose_rows.
for (int i = 1; i < num_cols + 1; ++i) {
transpose_rows[i] += transpose_rows[i - 1];
}
+ // Populate transpose_cols and (optionally) transpose_values by
+ // walking the entries of the source matrices. For each entry that
+ // is added, the value of transpose_row is incremented allowing us
+ // to keep track of where the next entry for that row should go.
+ //
+ // As a result transpose_row is shifted to the left by one entry.
for (int r = 0; r < num_rows; ++r) {
for (int idx = rows[r]; idx < rows[r + 1]; ++idx) {
const int c = cols[idx];
const int transpose_idx = transpose_rows[c]++;
transpose_cols[transpose_idx] = r;
- if (values) {
+ if (values != NULL && transpose_values != NULL) {
transpose_values[transpose_idx] = values[idx];
}
}
}
+ // This loop undoes the left shift to transpose_rows introduced by
+ // the previous loop.
for (int i = num_cols - 1; i > 0; --i) {
transpose_rows[i] = transpose_rows[i - 1];
}
@@ -475,9 +490,7 @@
vector<int>& transpose_crsb_cols = *transpose->mutable_crsb_cols();
transpose_crsb_rows.resize(col_blocks_.size() + 1);
- std::fill(transpose_crsb_rows.begin(), transpose_crsb_rows.end(), 0);
transpose_crsb_cols.resize(crsb_cols_.size());
-
TransposeForCompressedRowSparseStructure(row_blocks().size(),
col_blocks().size(),
crsb_cols().size(),
