/*
 * Copyright contributors to the AERE Network.
 *
 * Licensed 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.
 *
 * SPDX-License-Identifier: Apache-2.0
 */
package org.hyperledger.besu.evm.precompile;

import org.hyperledger.besu.evm.frame.MessageFrame;
import org.hyperledger.besu.evm.gascalculator.GasCalculator;

import jakarta.validation.constraints.NotNull;
import org.apache.tuweni.bytes.Bytes;
import org.bouncycastle.crypto.digests.SHAKEDigest;

/**
 * AERE PQC precompile: SHAKE256 extendable-output function (FIPS 202).
 *
 * <p>Input layout: outLen (32 bytes, big-endian, capped at MAX_OUTPUT) || data (arbitrary length).
 * Output: exactly {@code outLen} bytes of SHAKE256(data).
 *
 * <p>SHAKE256 is the hashing bottleneck inside Falcon, ML-DSA and SLH-DSA; exposing it natively lets
 * on-chain PQC flows offload the hot path to audited Bouncy Castle rather than hand-rolled Solidity.
 */
public class SHAKE256PrecompiledContract extends AbstractPrecompiledContract {

  /** Upper bound on requested output length to keep gas/allocation bounded. */
  static final int MAX_OUTPUT = 1 << 16; // 65536 bytes

  private static final int BASE_GAS = 60;
  private static final int GAS_PER_WORD = 12;

  /**
   * Instantiates a new SHAKE256 precompiled contract.
   *
   * @param gasCalculator the gas calculator
   */
  SHAKE256PrecompiledContract(final GasCalculator gasCalculator) {
    super("AereSHAKE256", gasCalculator);
  }

  private static int outputLength(final Bytes input) {
    if (input.size() < 32) {
      return 0;
    }
    // Big-endian 32-byte length; only the low 4 bytes are honoured, then capped.
    long v = input.slice(28, 4).toLong() & 0xFFFFFFFFL;
    // If any of the high 28 bytes are non-zero the value is enormous; cap regardless.
    if (!input.slice(0, 28).isZero()) {
      return MAX_OUTPUT;
    }
    if (v > MAX_OUTPUT) {
      return MAX_OUTPUT;
    }
    return (int) v;
  }

  @Override
  public long gasRequirement(final Bytes input) {
    int outLen = outputLength(input);
    int dataLen = input.size() < 32 ? 0 : input.size() - 32;
    long words = ((long) dataLen + 31) / 32 + ((long) outLen + 31) / 32;
    return BASE_GAS + GAS_PER_WORD * words;
  }

  @NotNull
  @Override
  public PrecompileContractResult computePrecompile(
      final Bytes input, @NotNull final MessageFrame messageFrame) {
    if (input.size() < 32) {
      return PrecompileContractResult.success(Bytes.EMPTY);
    }
    final int outLen = outputLength(input);
    final byte[] data = input.slice(32).toArrayUnsafe();
    final SHAKEDigest digest = new SHAKEDigest(256);
    if (data.length > 0) {
      digest.update(data, 0, data.length);
    }
    final byte[] out = new byte[outLen];
    if (outLen > 0) {
      digest.doFinal(out, 0, outLen);
    }
    return PrecompileContractResult.success(Bytes.wrap(out));
  }
}