157. Read N Characters Given Read4 I & II

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 only once.

 1 public class Solution extends Reader4 {
 2     char[] buffer = new char[4];
 3     
 4     public int read(char[] buf, int n) {
 5         int index = 0;
 6         int startIndex = 0;
 7         int bufferSize = read4(buffer);;
 8         while (index < n) {
 9             if (startIndex < bufferSize) {
10                 buf[index++] = buffer[startIndex++];
11             } else {
12                 bufferSize = read4(buffer);
13                 startIndex = 0;
14                 if (bufferSize == 0) {
15                     break;
16                 }
17             }
18             
19         }
20         return index;
21     }
22 }

Read N Characters Given Read4 II - Call multiple times

Given a file and assume that you can only read the file using a given method read4, implement a method read to read n characters. Your method read may be called multiple times.

 

Method read4:

The API read4 reads 4 consecutive characters from the file, then writes those characters into the buffer array buf4.

The return value is the number of actual characters read.

Note that read4() has its own file pointer, much like FILE *fp in C.

Definition of read4:

    Parameter:  char[] buf4
    Returns:    int

Note: buf4[] is destination not source, the results from read4 will be copied to buf4[]

Below is a high level example of how read4 works:

File file("abcde"); // File is "abcde", initially file pointer (fp) points to 'a'
char[] buf4 = new char[4]; // Create buffer with enough space to store characters
read4(buf4); // read4 returns 4. Now buf4 = "abcd", fp points to 'e'
read4(buf4); // read4 returns 1. Now buf4 = "e", fp points to end of file
read4(buf4); // read4 returns 0. Now buf4 = "", fp points to end of file

分析：

 1 public class Solution extends Reader4 {
 2     char[] buffer = new char[4];
 3     int bufferSize = 0, prevIndex = 0;
 4 
 5     public int read(char[] buf, int n) {
 6         int index = 0;
 7         while (index < n) {
 8             if (prevIndex < bufferSize) {
 9                 buf[index++] = buffer[prevIndex++];
10             } else {
11                 bufferSize = read4(buffer);
12                 prevIndex = 0;
13                 if (bufferSize == 0) { break; }
14             }
15         }
16         return index;
17     }
18 }