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tdfec/td/fec/algebra/GaussianElimination.cpp

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+/*
+    This file is part of TON Blockchain Library.
+
+    TON Blockchain Library is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Lesser General Public License as published by
+    the Free Software Foundation, either version 2 of the License, or
+    (at your option) any later version.
+
+    TON Blockchain Library is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Lesser General Public License for more details.
+
+    You should have received a copy of the GNU Lesser General Public License
+    along with TON Blockchain Library.  If not, see <http://www.gnu.org/licenses/>.
+
+    Copyright 2017-2019 Telegram Systems LLP
+*/
+#include "td/fec/algebra/GaussianElimination.h"
+namespace td {
+Result<MatrixGF256> GaussianElimination::run(MatrixGF256 A, MatrixGF256 D) {
+  const size_t cols = A.cols();
+  const size_t rows = A.rows();
+
+  CHECK(cols <= rows);
+
+  std::vector<uint32> row_perm(rows);
+  for (uint32 i = 0; i < rows; i++) {
+    row_perm[i] = i;
+  }
+  for (size_t row = 0; row < cols; row++) {
+    size_t non_zero_row = row;
+    for (; non_zero_row < rows && A.get(row_perm[non_zero_row], row).is_zero(); non_zero_row++) {
+    }
+    if (non_zero_row == rows) {
+      return Status::Error("Non solvable");
+    }
+    if (non_zero_row != row) {
+      std::swap(row_perm[non_zero_row], row_perm[row]);
+    }
+    auto mul = A.get(row_perm[row], row).inverse();
+    A.row_multiply(row_perm[row], mul);
+    D.row_multiply(row_perm[row], mul);
+    CHECK(A.get(row_perm[row], row).value() == 1);
+    for (size_t zero_row = 0; zero_row < rows; zero_row++) {
+      if (zero_row == row) {
+        continue;
+      }
+      auto x = A.get(row_perm[zero_row], row);
+      if (!x.is_zero()) {
+        A.row_add_mul(row_perm[zero_row], row_perm[row], x);
+        D.row_add_mul(row_perm[zero_row], row_perm[row], x);
+      }
+    }
+  }
+
+  return D.apply_row_permutation(row_perm);
+}
+}  // namespace td