Read N characters Given

Problem Description

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

Examples

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.

Note:

• Consider that you cannot manipulate the file directly, the file is only accesible 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.

Algorithm

1. Initialization:

   • buf4: A temporary buffer that stores the characters read by read4.
   • i4: Index for iterating over buf4.
   • n4: Number of characters actually read by read4.
   • i: Index for iterating over the destination buffer buf.

2. Reading Characters:

   • The loop continues until i (number of characters written to buf) reaches n (the requested number of characters).
   • If all characters in buf4 are consumed (i4 == n4), read4 is called again to refill buf4 and reset i4.
   • If read4 returns 0 (n4 == 0), it means the end of the file has been reached, and the function returns the number of characters read so far.
   • Characters from buf4 are copied to buf one by one, and both indices (i and i4) are incremented accordingly.

3. Return:

   • The function returns the total number of characters read, which is the value of i.

Code Implementation

C++

class Solution {
 public:
  /**
   * @param buf Destination buffer
   * @param n   Number of characters to read
   * @return    The number of actual characters read
   */
  int read(char* buf, int n) {
    char* buf4 = new char[4];
    int i4 = 0;  // buf4's index
    int n4 = 0;  // buf4's size
    int i = 0;   // buf's index

    while (i < n) {
      if (i4 == n4) {      // All the characters in the buf4 are consumed.
        i4 = 0;            // Reset the buf4's index.
        n4 = read4(buf4);  // Read <= 4 characters from the file to the buf4.
        if (n4 == 0)       // Reach the EOF.
          return i;
      }
      buf[i++] = buf4[i4++];
    }

    return i;
  }
};

Python

class Solution:
    def __init__(self):
        self.buf4 = [''] * 4  # Temporary buffer to store characters read by read4
        self.i4 = 0           # Index for iterating over buf4
        self.n4 = 0           # Number of characters actually read by read4

    def read4(self, buf4):
        # This method is provided in the parent class or environment.
        pass

    def read(self, buf, n):
        i = 0  # Index for the destination buffer buf

        while i < n:
            if self.i4 == self.n4:
                self.n4 = self.read4(self.buf4)
                self.i4 = 0
                if self.n4 == 0:
                    break

            while i < n and self.i4 < self.n4:
                buf[i] = self.buf4[self.i4]
                i += 1
                self.i4 += 1

        return i

Java

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

public class Solution extends Reader4 {
    private char[] buf4 = new char[4];
    private int i4 = 0;
    private int n4 = 0;

    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    public int read(char[] buf, int n) {
        int i = 0;

        while (i < n) {
            if (i4 == n4) {
                n4 = read4(buf4);
                i4 = 0;
                if (n4 == 0) {
                    break;
                }
            }

            while (i < n && i4 < n4) {
                buf[i++] = buf4[i4++];
            }
        }

        return i;
    }
}

JavaScript

/**
 * Definition for read4 API
 * read4 = function(buf4) {
 *     // Reads 4 characters at a time and writes to buf4
 *     // Returns the number of characters actually read
 * }
 */

var Solution = function () {
  this.buf4 = new Array(4);
  this.i4 = 0;
  this.n4 = 0;
};

/**
 * @param {character[]} buf Destination buffer
 * @param {number} n Number of characters to read
 * @return {number} The number of actual characters read
 */
Solution.prototype.read = function (buf, n) {
  let i = 0;

  while (i < n) {
    if (this.i4 === this.n4) {
      this.n4 = read4(this.buf4);
      this.i4 = 0;
      if (this.n4 === 0) {
        break;
      }
    }

    while (i < n && this.i4 < this.n4) {
      buf[i++] = this.buf4[this.i4++];
    }
  }

  return i;
};