LeetCode 158: Read N Characters Given Read4 II

/* The read4 API is defined in the parent class Reader4.
      int read4(char[] buf); */

public class Solution extends Reader4 {
    /**
     * @param buf Destination buffer
     * @param n   Maximum number of characters to read
     * @return    The number of characters read
     */
    private int prevStart = 0;
    private int prevEnd = 0;
    private char[] buffered = new char[4];
    public int read(char[] buf, int n) {
        int currentStart = 0;
        while (currentStart < n) {
            if (prevStart == 0) {
                prevEnd = read4(buffered);
            }
            if (prevEnd == 0) {
                break;
            }
            while (currentStart < n && prevStart < prevEnd) {
                buf[currentStart++] = buffered[prevStart++];
            }
            if (prevStart >= prevEnd) {
                prevStart = 0;
            }
        }
        return currentStart;
    }
}