# LeetCode 157. Read N Characters Given Read4

Given a `file` and assume that you can only read the file using a given method `read4`, implement a method to read `n` characters.

**Method read4:** 

The API `read4` reads **four consecutive characters** from `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

buf4[] is a destination, not a source. The results from read4 will be copied to buf4[].
```

Below is a high-level example of how `read4` works:![](https://assets.leetcode.com/uploads/2020/07/01/157_example.png)

```
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
```

**Method read:**

By using the `read4` method, implement the method read that reads `n` characters from `file` and store it in the buffer array `buf`. Consider that you cannot manipulate `file` directly.

The return value is the number of actual characters read.

**Definition of read:** 

```
    Parameters:	char[] buf, int n
    Returns:	int

buf[] is a destination, not a source. You will need to write the results to buf[].
```

**Note:**

* Consider that you cannot manipulate the file directly. The file is only accessible for `read4` but not for `read`.
* The `read` function will only be called once for each test case.
* You may assume the destination buffer array, `buf`, is guaranteed to have enough space for storing `n`characters.

**Example 1:**

```
Input: file = "abc", n = 4
Output: 3
Explanation: After calling your read method, buf should contain "abc". We read a total of 3 characters from the file, so return 3.
Note that "abc" is the file's content, not buf. buf is the destination buffer that you will have to write the results to.
```

**Example 2:**

```
Input: file = "abcde", n = 5
Output: 5
Explanation: After calling your read method, buf should contain "abcde". We read a total of 5 characters from the file, so return 5.
```

**Example 3:**

```
Input: file = "abcdABCD1234", n = 12
Output: 12
Explanation: After calling your read method, buf should contain "abcdABCD1234". We read a total of 12 characters from the file, so return 12.
```

**Example 4:**

```
Input: file = "leetcode", n = 5
Output: 5
Explanation: After calling your read method, buf should contain "leetc". We read a total of 5 characters from the file, so return 5.
```

**Constraints:**

* `1 <= file.length <= 500`
* `file` consist of English letters and digits.
* `1 <= n <= 1000`

## Solution

[English Version in Youtube](https://youtu.be/bziuNha1nXw)

[中文版解答Youtube Link](https://youtu.be/Zw2UzFsMfZI)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1rX4y137FB/)

```
/**
 * The read4 API is defined in the parent class Reader4.
 *     int read4(char *buf4);
 */

class Solution {
public:
    int read(char *buf, int n) {
        int ind = 0;
        while (ind < n) {
            char cache[4];
            int cache_cnt = read4(cache);
            if (cache_cnt == 0) {
                break;
            }
            int cache_ind = 0;
            while (cache_cnt > 0 && ind < n) {
                buf[ind] = cache[cache_ind];
                cache_ind++;
                ind++;
                cache_cnt--;
            }
        }
        return ind;
    }
};
```


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