# 157 Read N Characters Given Read4

Given an API: int read4(char *buf) 一次從某檔案裡讀4個字元，回傳int表示讀取的字元數，當檔案讀到最後時剩不到4個字元，就會只讀剩下的字元數。

寫一個int read(char[] buf, int n)，從檔案裡讀n個字元存進buf裡面，若不到n個字元檔案就結束就只讀僅有的內容

• idea: 題意有點難懂，有偷瞄一下別人解法才理解。當檔案還沒讀完，而且讀的字數total不超過n，就一直用read4讀取，並將當迴圈讀的字數存進total，最後回傳total

• pseudocode:

          while file not finish, and total < n,
              read4 into tmp, 
              add 4 to total.
              if total read after read4 < n
                  put into buffer
              else
                  remainder = n-(total-4)
                  total = n
                  put remainder char into buf 
  
          return total
  

• code:

  /* 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
       */
      public int read(char[] buf, int n) {
          int total = 0;
          int idx = 0;
          boolean eof = false;
  
          while(!eof && total<n){
              char[] tmp = new char[4];
              int count = read4(tmp);
  
              if(count < 4) eof = true;
  
              total+=count;
              if(total < n){
                  for(int i=0; i<4; i++){
                      buf[idx++] = tmp[i];
                  }
              }else{
                  int remainder = n-(total-4);
                  total = n;
                  for(int i=0; i<remainder; i++){
                      buf[idx++] = tmp[i];
                  }
              }
          }
  
          return total;
      }
  }