// From the Checker Framework, but replace uses of "org.checkerframework.checker.nullness" by
// "daikon.tools.nullness" and comment out nullness annotations and their import statement.
package daikon.tools.nullness;

import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
import org.checkerframework.checker.nullness.qual.NonNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Utility class for the Nullness Checker.
 *
 * <p>To avoid the need to write the NullnessUtil class name, do:
 *
 * <pre>import static daikon.tools.nullness.NullnessUtil.castNonNull;</pre>
 *
 * or
 *
 * <pre>import static daikon.tools.nullness.NullnessUtil.*;</pre>
 *
 * <p><b>Run-time Dependency</b>
 *
 * <p>Please note that using this class introduces a run-time dependency. This means that you need
 * to distribute (or link to) Daikon, along with your binaries.
 *
 * <p>To eliminate this dependency, you can simply copy this class into your own project.
 */
@SuppressWarnings({
  "nullness", // Nullness utilities are trusted regarding nullness.
  "cast" // Casts look redundant if Nullness Checker is not run.
})
public final class NullnessUtil {

  private NullnessUtil() {
    throw new AssertionError("shouldn't be instantiated");
  }

  /**
   * A method that suppresses warnings from the Nullness Checker.
   *
   * <p>The method takes a possibly-null reference, unsafely casts it to have the @NonNull type
   * qualifier, and returns it. The Nullness Checker considers both the return value, and also the
   * argument, to be non-null after the method call. Therefore, the {@code castNonNull} method can
   * be used either as a cast expression or as a statement. The Nullness Checker issues no warnings
   * in any of the following code:
   *
   * <pre>{@code
   *  // one way to use as a cast:
   * {@literal @}NonNull String s = castNonNull(possiblyNull1);
   *
   *  // another way to use as a cast:
   *  castNonNull(possiblyNull2).toString();
   *
   *  // one way to use as a statement:
   *  castNonNull(possiblyNull3);
   *  possiblyNull3.toString();`
   * }</pre>
   *
   * The {@code castNonNull} method is intended to be used in situations where the programmer
   * definitively knows that a given reference is not null, but the type system is unable to make
   * this deduction. It is not intended for defensive programming, in which a programmer cannot
   * prove that the value is not null but wishes to have an earlier indication if it is. See the
   * Checker Framework Manual for further discussion.
   *
   * <p>The method throws {@link AssertionError} if Java assertions are enabled and the argument is
   * {@code null}. If the exception is ever thrown, then that indicates that the programmer misused
   * the method by using it in a circumstance where its argument can be null.
   *
   * @param ref a reference of @Nullable type
   * @return the argument, casted to have the type qualifier @NonNull
   */
  public static @EnsuresNonNull("#1") <T extends @Nullable Object> @NonNull T castNonNull(T ref) {
    assert ref != null : "Misuse of castNonNull: called with a null argument";
    return (@NonNull T) ref;
  }

  /**
   * Like castNonNull, but whereas that method only checks and casts the reference itself, this
   * traverses all levels of the argument array. The array is recursively checked to ensure that all
   * elements at every array level are non-null.
   *
   * @param <T> the type of array elements
   * @param arr an array that contains no null elements at any level
   * @return the argument, with each array level casted to {@code @NonNull}
   * @see #castNonNull(Object)
   */
  public static @EnsuresNonNull("#1") <T extends @Nullable Object>
      @NonNull T @NonNull [] castNonNullDeep(T @Nullable [] arr) {
    return (@NonNull T[]) castNonNullDeepArray(arr);
  }

  /**
   * Like castNonNull, but whereas that method only checks and casts the reference itself, this
   * traverses all levels of the argument array. The array is recursively checked to ensure that all
   * elements at every array level are non-null.
   *
   * @param <T> the type of array elements
   * @param arr an array that contains no null elements at any level
   * @return the argument, with each array level casted to {@code @NonNull}
   * @see #castNonNull(Object)
   */
  public static <T extends @Nullable Object> @NonNull T @NonNull [][] castNonNullDeep(
      T @Nullable [] @Nullable [] arr) {
    return (@NonNull T[][]) castNonNullDeepArray(arr);
  }

  /**
   * Like castNonNull, but whereas that method only checks and casts the reference itself, this
   * traverses all levels of the argument array. The array is recursively checked to ensure that all
   * elements at every array level are non-null.
   *
   * @param <T> the type of array elements
   * @param arr an array that contains no null elements at any level
   * @return the argument, with each array level casted to {@code @NonNull}
   * @see #castNonNull(Object)
   */
  public static <T extends @Nullable Object> @NonNull T @NonNull [][][] castNonNullDeep(
      T @Nullable [] @Nullable [] @Nullable [] arr) {
    return (@NonNull T[][][]) castNonNullDeepArray(arr);
  }

  /**
   * Like castNonNull, but whereas that method only checks and casts the reference itself, this
   * traverses all levels of the argument array. The array is recursively checked to ensure that all
   * elements at every array level are non-null.
   *
   * @param <T> the type of array elements
   * @param arr an array that contains no null elements at any level
   * @return the argument, with each array level casted to {@code @NonNull}
   * @see #castNonNull(Object)
   */
  public static @EnsuresNonNull("#1") <T extends @Nullable Object>
      @NonNull T @NonNull [][][][] castNonNullDeep(
          T @Nullable [] @Nullable [] @Nullable [] @Nullable [] arr) {
    return (@NonNull T[][][][]) castNonNullDeepArray(arr);
  }

  /**
   * Like castNonNull, but whereas that method only checks and casts the reference itself, this
   * traverses all levels of the argument array. The array is recursively checked to ensure that all
   * elements at every array level are non-null.
   *
   * @param <T> the type of array elements
   * @param arr an array that contains no null elements at any level
   * @return the argument, with each array level casted to {@code @NonNull}
   * @see #castNonNull(Object)
   */
  public static @EnsuresNonNull("#1") <T extends @Nullable Object>
      @NonNull T @NonNull [][][][][] castNonNullDeep(
          T @Nullable [] @Nullable [] @Nullable [] @Nullable [] @Nullable [] arr) {
    return (@NonNull T[][][][][]) castNonNullDeepArray(arr);
  }

  /**
   * Does the work for the {@code castNonNullDeep} family of overloads. Throws an exception if any
   * level of the array contains a null element.
   *
   * @param <T> the type of array elements
   * @param arr an array that contains no null elements at any level
   * @return the argument, with each array level casted to {@code @NonNull}
   */
  private static <T extends @Nullable Object> @NonNull T @NonNull [] castNonNullDeepArray(
      T @Nullable [] arr) {
    assert arr != null : "Misuse of castNonNullDeepArray: called with a null array argument";
    for (int i = 0; i < arr.length; ++i) {
      assert arr[i] != null : "Misuse of castNonNull: called with a null array element";
      castNonNullDeepIfArray(arr[i]);
    }
    return (@NonNull T[]) arr;
  }

  /**
   * If the argument is an array, calls {@link #castNonNullDeepArray}.
   *
   * @param ref a value that might be an array, and if so should be null at all levels
   */
  private static void castNonNullDeepIfArray(Object ref) {
    assert ref != null : "Misuse of castNonNullIfArray: called with a null argument";
    Class<?> comp = ref.getClass().getComponentType();
    if (comp != null) {
      // comp is non-null for arrays, otherwise null.
      if (comp.isPrimitive()) {
        // Nothing to do for arrays of primitive type: primitives are
        // never null.
      } else {
        castNonNullDeepArray((Object[]) ref);
      }
    }
  }
}