Question

Formatted question description: https://leetcode.ca/all/158.html

 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.

 Note:
 The read function will only be called once for each test case.

Algorithm

Two variables readPos and writePos are used to record the position of reading and writing, i starts looping from 0 to n,

• If the read and write positions are the same at this time, call the read4 function, assign the result to writePos, and set readPos to zero,
• If writePos is zero, indicating that there is nothing in buf, return the current coordinate i. Then use the readPos position of the built-in buff variable to overwrite the i position of the input string buf,
• If the traversal is completed, return n

Code

Java

public class Read_N_Characters_Given_Read4 {
    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) {
            boolean isEof = false;
            int charsRead = 0;
            char[] buf4 = new char[4];

            while (!isEof && charsRead < n) {
                int size = read4(buf4);
                if (size < 4) {
                    isEof = true;
                }

                if (charsRead + size > n) {
                    size = n - charsRead;
                }

                /*
                        arraycopy(Object src,  int  srcPos,
                                  Object dest, int destPos,
                                  int length);
                 */
                // if last iteration and size==0, then copy length==0, not over-copy
                System.arraycopy(buf4, 0, buf, charsRead, size);
                charsRead += size;
            }

            return charsRead;
        }

    }

    private class Reader4 {
        public int read4(char[] buf4) {
            return 1; // stub
        }
    }
}

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