dance858 / PSLP / 39772704903

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Hand parallel-row duals over by sign in postsolve (#90)

* Hand parallel-row duals over by sign in postsolve

When a parallel row's side has been moved onto row i, retrieve_lhs_change / retrieve_rhs_change only moved row i's dual back to the removed row if |a_i'x - new_side| <= FEAS_TOL in absolute terms. With an approximate reduced solution (e.g. PDLP) and large row activities that test fails although the row is tight in relative terms, and the dual stays on row i, where it can have the wrong sign. Decide by the sign of y_i instead: a positive y_i at an LHS change (negative at an RHS change) already says the moved side is the active one. Exact solutions are unchanged.

Adds test_parallel_rows_dual_approximate_solution, which fails without the change.

* postsolve: decide the parallel-row hand-over by sign alone

The residual test is a no-op once y_i has the right sign; drop it, and assert y_j retrieved rather than zero so the second side's record of the same row does not trip (new test). Rename PresolvedProblem to ProblemData after #89.

* postsolve: drop the comment on retrieve_lhs_change

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Co-authored-by: dance858 <danielcederberg1@gmail.com>

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1 existing line in 1 file now uncovered.

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