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.

3 cases:

file is smaller than n,

file is larger than or equal to n.

// Forward declaration of the read4 API.
// malloc in read4 function?
int read4(char *buf);

class Solution {
public:
    /**
     * @param buf Destination buffer
     * @param n   Maximum number of characters to read
     * @return    The number of characters read
     */
    int read(char *buf, int n) {
        char * p = buf;
        while(n > 0){
            int num = read4(p);
            if(num == 0) break; // EOF
            if(n < num){
                // read more than needed
                // may overflow.
                p = p + n;
                break;
            }
            else{
                n -= num;
                p = p + num;
            }
        }
        return p - buf; 
    }
};

Read N Characters Given Read4 II – Call multiple times

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.

Use a circle buffer to store the overread characters.

Be aware of variable name shadows!

// Forward declaration of the read4 API.
int read4(char *buf);

class Solution {
public:
    /**
     * @param buf Destination buffer
     * @param n   Maximum number of characters to read
     * @return    The number of characters read
     */
    char buffer[3]; // this name should not be the same as buf, cause it will be shadowed by the parameter of read.
    int size = 0; // not needed actually.
    int front = 0;
    int back = 0;
    int read(char *buf, int n) {
        char * p = buf;
        int i = 0;
        // fetch buffer
        while(n > 0 && size > 0){
            *p = buffer[front++ % 3]; // may overflow
            size--;
            n--;
            p++;
        }
        
        while(n > 0){
            int num = read4(p);
            if(num == 0) break;
            // store exceeded chars to buffer
            if(num > n){
                while(n--){
                    num--;
                    p++;
                }
                // p moves to the first exceeded char
                i = 0;
                while(i < num){
                    buffer[back++ % 3] = *(p+i);
                    size++;
                    i++;
                }
                break;
            }
            else{
                p = p + num;
                n = n - num;
            }
        }
        return p - buf;
    }
};