// SPDX-License-Identifier: Apache-2.0
// Copyright (c) 2020-2022 MariaDB Corporation Ab
package org.mariadb.r2dbc.authentication.standard.ed25519.math;

/**
 * Common interface for all $(b-1)$-bit encodings of elements of EdDSA finite fields.
 *
 * @author str4d
 */
public abstract class Encoding {

  protected Field f;

  public synchronized void setField(Field f) {
    if (this.f != null) {
      throw new IllegalStateException("already set");
    }
    this.f = f;
  }

  /**
   * Encode a FieldElement in its $(b-1)$-bit encoding.
   *
   * @param x the FieldElement to encode
   * @return the $(b-1)$-bit encoding of this FieldElement.
   */
  public abstract byte[] encode(FieldElement x);

  /**
   * Decode a FieldElement from its $(b-1)$-bit encoding. The highest bit is masked out.
   *
   * @param in the $(b-1)$-bit encoding of a FieldElement.
   * @return the FieldElement represented by 'val'.
   */
  public abstract FieldElement decode(byte[] in);

  /**
   * From the Ed25519 paper:<br>
   * $x$ is negative if the $(b-1)$-bit encoding of $x$ is lexicographically larger than the
   * $(b-1)$-bit encoding of -x. If $q$ is an odd prime and the encoding is the little-endian
   * representation of $\{0, 1,\dots, q-1\}$ then the negative elements of $F_q$ are $\{1, 3,
   * 5,\dots, q-2\}$.
   *
   * @param x the FieldElement to check
   * @return true if negative
   */
  public abstract boolean isNegative(FieldElement x);
}
