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Lightweight REST for Java
There is a newer version: 4.9
/*
 * 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.
 */

package org.httprpc.kilo.io;

import java.io.IOException;
import java.io.Reader;
import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;

class PagedReader extends Reader {
    private Reader reader;
    private int pageSize;

    private int position = 0;
    private int count = 0;

    private boolean endOfFile = false;

    private List pages = new ArrayList<>();
    private Deque marks = new LinkedList<>();

    private static final int DEFAULT_PAGE_SIZE = 1024;
    private static final int EOF = -1;

    PagedReader(Reader reader) {
        this(reader, DEFAULT_PAGE_SIZE);
    }

    PagedReader(Reader reader, int pageSize) {
        this.reader = reader;
        this.pageSize = pageSize;
    }

    @Override
    public int read() throws IOException {
        int c;
        if (position < count) {
            c = pages.get(position / pageSize)[position % pageSize];

            position++;
        } else if (!endOfFile) {
            c = reader.read();

            if (c != EOF) {
                if (position / pageSize == pages.size()) {
                    pages.add(new char[pageSize]);
                }

                pages.get(pages.size() - 1)[position % pageSize] = (char)c;

                position++;
                count++;
            } else {
                endOfFile = true;
            }
        } else {
            c = EOF;
        }

        return c;
    }

    @Override
    public int read(char[] cbuf, int off, int len) throws IOException {
        var c = 0;
        var n = 0;

        for (var i = off; i < cbuf.length && n < len; i++) {
            c = read();

            if (c == EOF) {
                break;
            }

            cbuf[i] = (char)c;

            n++;
        }

        return c == EOF && n == 0 ? EOF : n;
    }

    @Override
    public boolean ready() throws IOException {
        return position < count || reader.ready();
    }

    @Override
    public void mark(int readAheadLimit) {
        marks.push(position);
    }

    @Override
    public boolean markSupported() {
        return true;
    }

    @Override
    public void reset() {
        if (marks.isEmpty()) {
            position = 0;
        } else {
            position = marks.pop();
        }
    }

    @Override
    public void close() throws IOException {
        reader.close();
    }
}