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The DSI utilities are a mishmash of classes accumulated during the last twenty years in projects developed at the DSI (Dipartimento di Scienze dell'Informazione, i.e., Information Sciences Department), now DI (Dipartimento di Informatica, i.e., Informatics Department), of the Universita` degli Studi di Milano.

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package it.unimi.dsi.util;

/*
 * DSI utilities
 *
 * Copyright (C) 2006-2020 Paolo Boldi
 *
 *  This library is free software; you can redistribute it and/or modify it
 *  under the terms of the GNU Lesser General Public License as published by the Free
 *  Software Foundation; either version 3 of the License, or (at your option)
 *  any later version.
 *
 *  This library is distributed in the hope that it will be useful, but
 *  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 *  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with this program; if not, see .
 *
 */

import java.util.Arrays;

/** A circular char buffer that can be used to implement a sliding
 *  window over a text. The buffer has a maximum size, fixed at construction time. As new characters
 *  are {@linkplain #add(char[], int, int) added}, the oldest ones are discarded if necessary. At all
 *  times, the number of available characters present in the buffer is {@link #size()} (it will never be larger
 *  than the buffer size). The characters present in the buffer are provided through the {@link #toCharArray(char[], int, int)}
 *  method and alike.
 */
public class CircularCharArrayBuffer {
	private static final boolean DEBUG = false;

	/** The buffer. */
	private final char[] buffer;
	/** The number of character available in the buffer. */
	private int len;
	/** The index of the first character available in the buffer:
	 *  hence the available characters are buffer[start], …,
	 *  buffer[(start+len+buffer.length-1)%buffer.length]
	 */
	private int start;

	/** Creates a new circular buffer.
	 *
	 * @param maxChar the buffer size.
	 */
	public CircularCharArrayBuffer(int maxChar) {
		buffer = new char[maxChar];
		if (DEBUG) Arrays.fill(buffer, ' ');
	}

	/** Adds the characters b[offset], … b[offset+length-1] to the
	 *  buffer (possibly just the last ones, and possibly discarding the oldest characters in the buffer).
	 *
	 * @param b the array whence characters are copied.
	 * @param offset the starting point.
	 * @param length the (maximum) number of characters to be copied.
	 */
	public void add(char[] b, int offset, int length) {
		if (buffer.length == 0) return;
		int toBeCopied = length;
		if (DEBUG) System.out.println("Called add(" + new String(b, offset, toBeCopied) + ")");
		if (toBeCopied > buffer.length)  {
			offset += toBeCopied - buffer.length;
			toBeCopied = buffer.length;
		}
		// Now length contains the number of bytes to be _really_ copied
		int toEnd = Math.min(toBeCopied, buffer.length - (start + len) % buffer.length);
		// toEnd is the number of bytes that are free from the first free position to the end of the buffer
		System.arraycopy(b, offset, buffer, (start + len) % buffer.length, toEnd);
		if (toBeCopied > toEnd) {
			// length - toEnd is the number of other characters remaining
			System.arraycopy(b, offset + toEnd, buffer, 0, toBeCopied - toEnd);
		}
		if (toBeCopied > buffer.length - len) {
			start = (start + toBeCopied + len) % buffer.length;
		}
		// After filling, length is just buffer.length
		len = Math.min(buffer.length, len + toBeCopied);

	}

	/** Copies the content of the buffer to an array.
	 *
	 * @param b the buffer where the content is to be copied.
	 * @param offset the starting index where copy should start from.
	 * @param length the maximum number of characters to be copied.
	 */
	public void toCharArray(char[] b, int offset, int length) {
		if (buffer.length == 0) return;
		int t = Math.min(length, len);
		int toEnd = Math.min(t, buffer.length - start);
		System.arraycopy(buffer, start, b, offset, toEnd);
		if (toEnd < t) System.arraycopy(buffer, 0, b, offset + toEnd, t - toEnd);
	}

	/** Copies the content of the buffer to an array.
	 *
	 * @param b the buffer where the content is to be copied.
	 */
	public void toCharArray(char[] b) {
		toCharArray(b, 0, b.length);
	}

	/** Returns a new array containing a copy of the buffer content.
	 *
	 * @return a copy of the buffer content.
	 */
	public char[] toCharArray() {
		char[] b = new char[size()];
		toCharArray(b);
		return b;
	}

	/** The number of characters present in the buffer.
	 *
	 * @return the number of characters present in the buffer.
	 */
	public int size() {
		return len;
	}

	/** Clears the buffer content, before reuse. */
	public void clear() {
		len = 0;
	}

}