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Fix the O(N^2) bug of passColName and passRowName #1782

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Merged
jajhall merged 1 commit into from
May 30, 2024
Merged

Fix the O(N^2) bug of passColName and passRowName #1782

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2 changes: 2 additions & 0 deletions src/lp_data/HStruct.h
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Original file line number Diff line number Diff line change
Expand Up @@ -117,6 +117,8 @@ struct HighsNameHash {
std::unordered_map<std::string, int> name2index;
void form(const std::vector<std::string>& name);
bool hasDuplicate(const std::vector<std::string>& name);
void update(int index, const std::string& old_name,
const std::string& new_name);
void clear();
};

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6 changes: 4 additions & 2 deletions src/lp_data/Highs.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -591,8 +591,9 @@ HighsStatus Highs::passColName(const HighsInt col, const std::string& name) {
return HighsStatus::kError;
}
this->model_.lp_.col_names_.resize(num_col);
this->model_.lp_.col_hash_.update(col, this->model_.lp_.col_names_[col],
name);
this->model_.lp_.col_names_[col] = name;
this->model_.lp_.col_hash_.clear();
return HighsStatus::kOk;
}

Expand All @@ -611,8 +612,9 @@ HighsStatus Highs::passRowName(const HighsInt row, const std::string& name) {
return HighsStatus::kError;
}
this->model_.lp_.row_names_.resize(num_row);
this->model_.lp_.row_hash_.update(row, this->model_.lp_.row_names_[row],
name);
this->model_.lp_.row_names_[row] = name;
this->model_.lp_.row_hash_.clear();
return HighsStatus::kOk;
}

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22 changes: 17 additions & 5 deletions src/lp_data/HighsLp.cpp
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Expand Up @@ -594,14 +594,13 @@ void HighsNameHash::form(const std::vector<std::string>& name) {
size_t num_name = name.size();
this->clear();
for (size_t index = 0; index < num_name; index++) {
const bool duplicate = !this->name2index.emplace(name[index], index).second;
auto emplace_result = this->name2index.emplace(name[index], index);
const bool duplicate = !emplace_result.second;
if (duplicate) {
// Find the original and mark it as duplicate
auto search = this->name2index.find(name[index]);
assert(search != this->name2index.end());
auto& search = emplace_result.first;
assert(int(search->second) < int(this->name2index.size()));
this->name2index.erase(search);
this->name2index.insert({name[index], kHashIsDuplicate});
search->second = kHashIsDuplicate;
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@jajhall jajhall May 30, 2024 •
edited
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By deleting

this->name2index.erase(search);
this->name2index.insert({name[index], kHashIsDuplicate});

and only setting

search->second = kHashIsDuplicate;

(which has no effect since search is immediately destroyed) the property of marking duplicate names is lost. hence errors due to the same name being given to different columns are not trapped.

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@metab0t metab0t May 30, 2024

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emplace will only insert when there is no duplication. When there is duplication, search points to the duplicated key and value, and setting the value will influence the original map.

This is an example: https://godbolt.org/z/YsYYrKrPb

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@jajhall jajhall May 30, 2024 •
edited
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Thanks, it did cross my mind that there was more to setting

search->second = kHashIsDuplicate;

than I gave you credit for, sorry.

As I said, I'm not fluent in the use of std::unordered_map :-)

Just spotted the & in

auto& search = emplace_result.first;

Now I see why setting

search->second = kHashIsDuplicate;

achieves the desired result

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@metab0t metab0t May 30, 2024

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Never mind. I also learned about the meaning of return values of emplace today. It is a clever choice to use emplace to detect duplication.

}
}
}
Expand All @@ -618,4 +617,17 @@ bool HighsNameHash::hasDuplicate(const std::vector<std::string>& name) {
return has_duplicate;
}

void HighsNameHash::update(int index, const std::string& old_name,
const std::string& new_name) {
this->name2index.erase(old_name);
auto emplace_result = this->name2index.emplace(new_name, index);
const bool duplicate = !emplace_result.second;
if (duplicate) {
// Find the original and mark it as duplicate
auto& search = emplace_result.first;
assert(int(search->second) < int(this->name2index.size()));
search->second = kHashIsDuplicate;
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@jajhall jajhall May 30, 2024

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Ditto

}
}

void HighsNameHash::clear() { this->name2index.clear(); }