158. 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.

**Example 1: **

Given buf = "abc"
read("abc", 1) // returns "a"
read("abc", 2); // returns "bc"
read("abc", 1); // returns ""

**Example 2: **

Given buf = "abc"
read("abc", 4) // returns "abc"
read("abc", 1); // returns ""

Solution

Buffer

添加一个缓冲区tmp[]。

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

public class Solution extends Reader4 {
    private char[] tmp = new char[4];
    private int pos = 0;
    private int count = 0;
    /**
     * @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 i = 0;

        while (i < n) {
            if (pos == 0) {     // tmp is eat up
                count = read4(tmp);
            }
            
            if (count == 0) {   // file ended
                break;
            }
            
            buf[i++] = tmp[pos++];
            
            if (pos == count) {
                pos = 0;
            }
        }
        
        return i;
    }
}