[LeetCode] 158. Read N Characters Given Read4 II - Call multiple

Problem

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

public class Solution extends Reader4 {
    /**
     * @param buf Destination buffer
     * @param n   Maximum number of characters to read
     * @return    The number of characters read
     */
    int count = 0;  //count the length of data retrieved by read4()
    int index = 0;  //record the position of data consumed by read()
    char[] data = new char[4];
    public int read(char[] buf, int n) {
        int i = 0;
        while (i < n) {
            //get new data with read4 api
            if (index == 0) count = read4(data);
            //if no new data, break
            if (count == 0) break;
            //consume data
            while (i < n && index < count) {
                buf[i++] = data[index++];
            }
            //all existing data consumed, restart with index = 0
            if (index >= count) index = 0; 
        }
        //return the length of data read
        return i;
    }
}