Read N Characters Given Read4 II - Call multiple times

Description

The API: int read4(char *buf) reads 4 characters at a time from a file.

The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file.

By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.

Note: The read function may be called multiple times.

Hint

Train of Thought

Code

//read being called only once
public int read(char[] buf, int n) {
  boolean eof = false;      // end of file flag
  int total = 0;            // total bytes have read
  char[] tmp = new char[4]; // temp buffer

  while (!eof && total < n) {
    int count = read4(tmp);

    // check if it's the end of the file
    eof = count < 4;

    // get the actual count
    count = Math.min(count, n - total);

    // copy from temp buffer to buf
    for (int i = 0; i < count; i++) 
      buf[total++] = tmp[i];
  }

  return total;
}

//read function may be called multiple times
//used buffer pointer (buffPtr) and buffer Counter (buffCnt) to store the data received in previous calls. In the while loop, if buffPtr reaches current buffCnt, it will be set as zero to be ready to read new data.

 private int buffPtr = 0;
    private int buffCnt = 0;
    private char[] buff = new char[4];
    public int read(char[] buf, int n) {
        int ptr = 0;
        while (ptr < n) {
            if (buffPtr == 0) {
                buffCnt = read4(buff);
            }
            if (buffCnt == 0) break;
            while (ptr < n && buffPtr < buffCnt) {
                buf[ptr++] = buff[buffPtr++];
            }
            if (buffPtr >= buffCnt) buffPtr = 0;
        }
        return ptr;
    }

Complexity