157. Read N Characters Given Read4

157. Read N Characters Given Read4




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.




public int read(char[] buf, int n) {
  // buf‘s size is n 
  // buf is empty right now
  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) {
    // tmp is populated with things after we called read4 on it 
    // the read4 gives you two things, one is the number of actual elements 
    // another one is to populate the tmp with the actual elements 
    int count = read4(tmp);
    
    
    // check if it‘s the end of the file
    if(count < 4){
      eof = true;
    }
    
    // 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;
}