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Improve comments in mju_cholFactorNNZ, clarify that this function reads from the upper triangle of the source matrix.

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PiperOrigin-RevId: 686113654
Change-Id: I029f9279d177f4cb9ff60396a26d64821aee3a6c
This commit is contained in:
Yuval Tassa
2024-10-15 08:25:21 -07:00
committed by Copybara-Service
parent 096853e192
commit 0ffaeb4d56
2 changed files with 26 additions and 27 deletions
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src/engine/engine_util_sparse.c
+14 -11
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@@ -857,8 +857,9 @@ void mju_sqrMatTDSparse(mjtNum* res, const mjtNum* mat, const mjtNum* matT,
mj_freeStack(d);
}
// compute row non-zeros of reverse-Cholesky factor L, return total
// compute row non-zeros of reverse-Cholesky factor L, return total non-zeros
// based on ldl_symbolic from 'Algorithm 8xx: a concise sparse Cholesky factorization package'
// note: reads pattern from upper triangle
int mju_cholFactorNNZ(int* L_rownnz, const int* rownnz, const int* rowadr, const int* colind,
int n, mjData* d) {
mj_markStack(d);
@@ -870,18 +871,21 @@ int mju_cholFactorNNZ(int* L_rownnz, const int* rownnz, const int* rowadr, const
parent[r] = -1;
flag[r] = r;
L_rownnz[r] = 1; // start with 1 for diagonal
// loop over non-zero columns
int start = rowadr[r];
int end = start + rownnz[r];
// loop over non-zero columns
for (int p = start; p < end; p++) {
int i = colind[p];
for (int c = start; c < end; c++) {
int i = colind[c];
if (i > r) {
// follow path from i to root of elimination tree, stop at flagged node
// traverse from i to ancestor, stop when row is flagged
while (flag[i] != r) {
// find parent of i if not yet determined
// if not yet set, set parent to current row
if (parent[i] == -1) {
parent[i] = r;
}
// increment non-zeros, flag row i, advance to parent
L_rownnz[i]++;
flag[i] = r;
i = parent[i];
@@ -892,12 +896,11 @@ int mju_cholFactorNNZ(int* L_rownnz, const int* rownnz, const int* rowadr, const
mj_freeStack(d);
// accumulate sum
int sum = 0;
// sum up all row non-zeros
int nnz = 0;
for (int r = 0; r < n; r++) {
sum += L_rownnz[r];
nnz += L_rownnz[r];
}
// return total non-zeros
return sum;
return nnz;
}