Fix integer overflow when creating a large matrix (#81)

* Convert to long to avoid overflow when calculating the product m * n

* Add new test case for creating a large matrix without throwing an exception

Author: atraplet

src/main/java/com/ustermetrics/clarabel4j/Matrix.java

@@ -32,7 +32,7 @@ public record Matrix(int m, int n, long @NonNull [] colPtr, long @NonNull [] row
         checkArgument(nnz == rowVal.length, "length of data must be equal to the length of the row index");
         checkArgument(colPtr.length == n + 1,
                 "length of the column index must be equal to the number of columns plus one");
-        checkArgument(nnz <= m * n,
+        checkArgument(nnz <= (long) m * n,
                 "number of non-zero entries must be less equal than the number of rows times the number of columns");
         checkArgument(Arrays.stream(rowVal).allMatch(i -> 0 <= i && i < m),
                 "entries of the row index must be greater equal zero and less than the number of rows");

src/test/java/com/ustermetrics/clarabel4j/MatrixTest.java

@@ -3,6 +3,8 @@
 import lombok.val;
 import org.junit.jupiter.api.Test;
 
+import java.util.Arrays;
+
 import static org.junit.jupiter.api.Assertions.*;
 
 class MatrixTest {
@@ -125,4 +127,14 @@ void createMatrixWithUnorderedRowIndexWithinColumnThrowsException() {
         assertEquals("entries of the row index within each column must be strictly ordered", exception.getMessage());
     }
 
+    @Test
+    void createLargeMatrixDoesNotThrowException() {
+        val m = 46341;
+        val colPtr = new long[m + 1];
+        Arrays.fill(colPtr, 1, m + 1, 1);
+        assertDoesNotThrow(() ->
+                new Matrix(m, m, colPtr, new long[]{0}, new double[]{1.})
+        );
+    }
+
 }