Skip to content

Avoid fixing zero-cost singleton columns to infinite bounds in HPresolve::dualFixing #3046

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
jajhall merged 7 commits into from
Jun 14, 2026
Merged

Avoid fixing zero-cost singleton columns to infinite bounds in HPresolve::dualFixing #3046

Changes from all commits
Commits
Show all changes
7 commits
Select commit Hold shift + click to select a range
5cb9754
Handle zero-cost singletons
fwesselm Jun 2, 2026
781638a
Another copy-and-paste issue, argggh
fwesselm Jun 2, 2026
4c74399
Simplify
fwesselm Jun 2, 2026
790bd5e
Try to revert changes to comments
fwesselm Jun 2, 2026
a3f9e80
Chaser
fwesselm Jun 2, 2026
7662aad
Add missing return
fwesselm Jun 2, 2026
2de9763
Fix comment
fwesselm Jun 3, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
31 changes: 26 additions & 5 deletions highs/presolve/HPresolve.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -3359,6 +3359,23 @@ HPresolve::Result HPresolve::singletonCol(HighsPostsolveStack& postsolve_stack,
return Result::kOk;
}

auto zeroCostSingleton = [&](HighsInt col, HighsInt row, double val,
HighsInt direction) {
// try to relax constraint by fixing zero-cost singleton to a bound
if (model->col_cost_[col] != 0.0 || isRanged(row)) return Result::kOk;
if (direction * val > 0 && model->col_lower_[col] != -kHighsInf)
HPRESOLVE_CHECKED_CALL(fixColToLower(postsolve_stack, col));
else if (direction * val < 0 && model->col_upper_[col] != kHighsInf)
HPRESOLVE_CHECKED_CALL(fixColToUpper(postsolve_stack, col));
return Result::kOk;
};

// handle zero-cost singleton
HPRESOLVE_CHECKED_CALL(zeroCostSingleton(
col, row, colCoef,
model->row_upper_[row] != kHighsInf ? HighsInt{1} : HighsInt{-1}));
if (colDeleted[col]) return Result::kOk;

// detect strong / weak domination
HPRESOLVE_CHECKED_CALL(detectDominatedCol(postsolve_stack, col, false));
if (colDeleted[col]) return Result::kOk;
Expand Down Expand Up @@ -3409,7 +3426,6 @@ HPresolve::Result HPresolve::singletonCol(HighsPostsolveStack& postsolve_stack,
return checkLimits(postsolve_stack);
}

// todo: check for zero cost singleton and remove
return Result::kOk;
}

Expand Down Expand Up @@ -4916,10 +4932,15 @@ HPresolve::Result HPresolve::dualFixing(HighsPostsolveStack& postsolve_stack,
// check if variable can be fixed
if (numDownLocks == 0 || numUpLocks == 0) {
// fix variable
if (numDownLocks == 0)
HPRESOLVE_CHECKED_CALL(fixColToLower(postsolve_stack, col));
else
HPRESOLVE_CHECKED_CALL(fixColToUpper(postsolve_stack, col));
if (numDownLocks == 0) {
// avoid fixing zero-cost column to an infinite bound
if (model->col_cost_[col] != 0.0 || model->col_lower_[col] != -kHighsInf)
HPRESOLVE_CHECKED_CALL(fixColToLower(postsolve_stack, col));
} else {
// avoid fixing zero-cost column to an infinite bound
if (model->col_cost_[col] != 0.0 || model->col_upper_[col] != kHighsInf)
HPRESOLVE_CHECKED_CALL(fixColToUpper(postsolve_stack, col));
}
} else {
bool hasSingleDownLock = numDownLocks == 1 && downLockRow != -1;
bool hasSingleUpLock = numUpLocks == 1 && upLockRow != -1;
Expand Down
Loading