content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package accord.utils;
  
  import java.util.function.BiFunction;
  import java.util.function.Function;
  import java.util.function.IntPredicate;
  import java.util.function.IntSupplier;
  import java.util.function.IntUnaryOperator;
  import java.util.function.LongPredicate;
  import java.util.function.LongSupplier;
  import java.util.function.LongUnaryOperator;
  import java.util.function.Predicate;
  import java.util.function.Supplier;
  import java.util.function.ToIntFunction;
  import java.util.function.ToLongFunction;
  import java.util.stream.IntStream;
  import java.util.stream.LongStream;
  import java.util.stream.Stream;
  
  /**
   * A generator (gen for short) interface that produces values using a {@link RandomSource}.
   * <p>
   * This interface provides a way to generate random values of type {@code A} and transform them
   * using various functional operators like {@link #map(Function)}, {@link #filter(Predicate)}, and {@link #flatMap(Function)}.
   * <p>
   * Specialized versions exist for primitive types ({@link IntGen}, {@link LongGen}) to avoid boxing/unboxing overhead.
   *
   * @param <A> The type of values this generator produces
   */
  @SuppressWarnings("unused")
  public interface Gen<A>
  {
      /**
       * For cases where method handles isn't able to detect the proper type, this method acts as a cast
       * to inform the compiler of the desired type.
       */
      static <A> Gen<A> of(Gen<A> fn)
      {
          return fn;
      }
  
      /**
       * Core method of the {@link Gen} interface that produces a value of type {@code A} using the provided {@link RandomSource}.
       * <p>
       * The semantics of this method are implementation-specific. Some important considerations:
       * <ul>
       * <li>Some implementations may return null values</li>
       * <li>Implementations may return duplicate or identical values across calls</li>
       * <li>Values may contain or be backed by shared mutable state</li>
       * </ul>
       * <p>
       * Due to these implementation-specific behaviors, callers should avoid storing references to returned
       * values for extended periods unless the specific generator's safety guarantees are known.
       *
       * @param random The random source to use for generating values
       * @return A generated value of type {@code A}
       */
      A next(RandomSource random);
  
      default <B> Gen<B> map(Function<? super A, ? extends B> fn)
      {
          return r -> fn.apply(this.next(r));
      }
  
      default <B> Gen<B> map(BiFunction<RandomSource, ? super A, ? extends B> fn)
      {
          return r -> fn.apply(r, this.next(r));
      }
  
      default IntGen mapToInt(ToIntFunction<A> fn)
      {
          return r -> fn.applyAsInt(next(r));
      }
  
      default LongGen mapToLong(ToLongFunction<A> fn)
      {
          return r -> fn.applyAsLong(next(r));
      }
  
      default <B> Gen<B> flatMap(Function<? super A, Gen<? extends B>> mapper)
      {
          return rs -> mapper.apply(this.next(rs)).next(rs);
      }
  
      default <B> Gen<B> flatMap(BiFunction<RandomSource, ? super A, Gen<? extends B>> mapper)
      {
          return rs -> mapper.apply(rs, this.next(rs)).next(rs);
      }
  
      /**
       * Creates a new generator that only produces values matching the provided predicate.
       * <p>
       * <strong>Warning:</strong> This method is unbounded and will loop indefinitely if the predicate
       * never returns true for any generated value. Use with caution, especially with rare conditions.
       * <p>
       * For a bounded alternative that will stop after a specified number of attempts,
       * see {@link #filter(int, Object, Predicate)}.
       *
       * @param fn the predicate function that values must satisfy
       * @return a new generator that only produces values matching the predicate
       */
      default Gen<A> filter(Predicate<A> fn)
      {
          Gen<A> self = this;
          return r -> {
              A value;
              do {
                  value = self.next(r);
              }
              while (!fn.test(value));
              return value;
          };
      }
  
      /**
       * Creates a new bounded generator that only produces values matching the provided predicate.
       * <p>
       * Unlike the unbounded {@link #filter(Predicate)}, this method will make at most {@code maxAttempts}
       * attempts to generate a value that satisfies the predicate. If no matching value is found within
       * the specified number of attempts, it returns the provided {@code defaultValue} instead.
       *
       * @param maxAttempts the maximum number of generation attempts before falling back to the default value
       * @param defaultValue the value to return if no matching value is found within maxAttempts
       * @param fn the predicate function that values must satisfy
       * @return a new bounded generator that produces values matching the predicate or the default value
       * @throws IllegalArgumentException if maxAttempts is not positive
       */
      default Gen<A> filter(int maxAttempts, A defaultValue, Predicate<A> fn)
      {
          Invariants.requireArgument(maxAttempts > 0, "Max attempts must be positive; given %d", maxAttempts);
          Gen<A> self = this;
          return r -> {
              for (int i = 0; i < maxAttempts; i++)
              {
                  A v = self.next(r);
                  if (fn.test(v))
                      return v;
  
              }
              return defaultValue;
          };
      }
  
      default Supplier<A> asSupplier(RandomSource rs)
      {
          return () -> next(rs);
      }
  
      default Stream<A> asStream(RandomSource rs)
      {
          return Stream.generate(() -> next(rs));
      }
  
      interface Int2IntMapFunction
      {
          int applyAsInt(RandomSource rs, int value);
      }
  
      interface Int2LongMapFunction
      {
          long applyAsLong(RandomSource rs, int value);
      }
  
      interface Long2LongMapFunction
      {
          long applyAsLong(RandomSource rs, long value);
      }
  
      interface IntGen extends Gen<Integer>
      {
          int nextInt(RandomSource random);
  
          @Override
          default Integer next(RandomSource random)
          {
              return nextInt(random);
          }
  
          default IntGen mapAsInt(IntUnaryOperator fn)
          {
              return r -> fn.applyAsInt(nextInt(r));
          }
  
          default IntGen mapAsInt(Int2IntMapFunction fn)
          {
              return r -> fn.applyAsInt(r, nextInt(r));
          }
  
          default LongGen mapAsLong(Int2LongMapFunction fn)
          {
              return r -> fn.applyAsLong(r, nextInt(r));
          }
  
          default Gen.IntGen filterAsInt(IntPredicate fn)
          {
              return rs -> {
                  int value;
                  do
                  {
                      value = nextInt(rs);
                  }
                  while (!fn.test(value));
                  return value;
              };
          }
  
          @Override
          default Gen.IntGen filter(Predicate<Integer> fn)
          {
              return filterAsInt(fn::test);
          }
  
          default IntSupplier asIntSupplier(RandomSource rs)
          {
              return () -> nextInt(rs);
          }
  
          default IntStream asIntStream(RandomSource rs)
          {
              return IntStream.generate(() -> nextInt(rs));
          }
      }
  
      interface LongGen extends Gen<Long>
      {
          long nextLong(RandomSource random);
  
          @Override
          default Long next(RandomSource random)
          {
              return nextLong(random);
          }
  
          default LongGen mapAsLong(LongUnaryOperator fn)
          {
              return r -> fn.applyAsLong(nextLong(r));
          }
  
          default LongGen mapAsLong(Long2LongMapFunction fn)
          {
              return r -> fn.applyAsLong(r, nextLong(r));
          }
  
          default Gen.LongGen filterAsLong(LongPredicate fn)
          {
              return rs -> {
                  long value;
                  do
                  {
                      value = nextLong(rs);
                  }
                  while (!fn.test(value));
                  return value;
              };
          }
  
          @Override
          default Gen.LongGen filter(Predicate<Long> fn)
          {
              return filterAsLong(fn::test);
          }
  
          default LongSupplier asLongSupplier(RandomSource rs)
          {
              return () -> nextLong(rs);
          }
  
          default LongStream asLongStream(RandomSource rs)
          {
              return LongStream.generate(() -> nextLong(rs));
          }
      }
  }
  