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// Copyright 2000-2024 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.

package com.intellij.util.text;

import com.intellij.openapi.util.text.CharSequenceWithStringHash;
import com.intellij.openapi.util.text.StringUtilRt;
import com.intellij.openapi.util.text.Strings;
import org.jetbrains.annotations.NotNull;

/**
 * {@code CharSequenceSubSequence} allows to save time and memory in text processing code. It avoids
 * creation of a new char array on every {@code subSequence(int, int)} call in contrast to {@link String#subSequence(int, int)},
 * which actually creates a new {@link String} instance every time it's invoked.
 * 
 * The downside of using {@code CharSequenceSubSequence} is that it keeps reference to the original sequence, which may be large.
 * Therefore, results of text processing should always be stored as {@link String}, to allow garbage collection of the original sequence.
 * 
 * {@code CharSequenceSubSequence} implements `hashCode` and `equals` in such a way that it can be compared against {@link String} map keys
 * and set elements without creating a {@link String} instance. However, {@code CharSequenceSubSequence} should not be used
 * as a map key or set element, since it keeps reference to the original sequence and prevents its collection.
 **/
public class CharSequenceSubSequence implements CharSequence, CharArrayExternalizable, CharSequenceWithStringHash {
  private final CharSequence myChars;
  private final int myStart;
  private final int myEnd;

  public CharSequenceSubSequence(@NotNull CharSequence chars) {
    this(chars, 0, chars.length());
  }

  public CharSequenceSubSequence(@NotNull CharSequence chars, int start, int end) {
    if (start < 0 || end > chars.length() || start > end) {
      throw new IndexOutOfBoundsException("chars sequence.length:" + chars.length() +
        ", start:" + start +
        ", end:" + end);
    }
    myChars = chars;
    myStart = start;
    myEnd = end;
  }

  @Override
  public final int length() {
    return myEnd - myStart;
  }

  @Override
  public final char charAt(int index) {
    return myChars.charAt(index + myStart);
  }

  @Override
  public @NotNull CharSequence subSequence(int start, int end) {
    if (start == myStart && end == myEnd) return this;
    return new CharSequenceSubSequence(myChars, myStart + start, myStart + end);
  }

  @Override
  public @NotNull String toString() {
    if (myChars instanceof String) return ((String) myChars).substring(myStart, myEnd);
    return new String(CharArrayUtil.fromSequence(myChars, myStart, myEnd));
  }

  @NotNull
  CharSequence getBaseSequence() {
    return myChars;
  }

  @Override
  public void getChars(int start, int end, char @NotNull [] dest, int destPos) {
    assert end - start <= myEnd - myStart;
    CharArrayUtil.getChars(myChars, dest, start + myStart, destPos, end - start);
  }

  private transient int hash;

  @Override
  public int hashCode() {
    int h = hash;
    if (h == 0) {
      hash = h = Strings.stringHashCode(myChars, myStart, myEnd);
    }
    return h;
  }

  @Override
  public boolean equals(Object obj) {
    if (obj == this) return true;
    if (obj instanceof CharSequence) {
      return StringUtilRt.equal(this, (CharSequence)obj, true);
    }
    else {
      return false;
    }
  }
}