Read N Characters Given Read4 - Solution & Explanation

Problem Overview: You are given an API read4(char[] buf4) that reads up to 4 characters from a file. The task is to implement read(char[] buf, int n) that copies at most n characters into the destination buffer. You can only access the file through read4, so your implementation must repeatedly call it and manage how many characters are copied.

Approach 1: Character-by-Character Wrapper (O(n) time, O(1) space)

A straightforward idea is to call read4 and copy characters into the output buffer one at a time until n characters are written. Each call fills a temporary array of size 4. You iterate through the returned characters and stop once either n characters are copied or the file ends. This approach works but performs unnecessary per‑character checks, which makes the implementation slightly less clean.

Approach 2: Chunk Copy Using read4 (O(n) time, O(1) space)

The optimal approach treats read4 as a chunk reader. Each iteration calls read4(buf4) to read up to four characters. Then copy min(count, n - totalRead) characters from buf4 into the destination buffer. Stop when either totalRead == n or read4 returns fewer than 4 characters, which signals end of file. This solution directly simulates how buffered file reads work and minimizes control overhead.

This problem is mainly about careful pointer/index management and correctly handling partial reads. You maintain a running index for the destination buffer and only copy the portion you actually need. No additional data structures are required.

Recommended for interviews: The chunk-based simulation using repeated read4 calls is what interviewers expect. It demonstrates control over buffer boundaries, loop termination conditions, and edge cases like early EOF. The brute wrapper idea shows understanding, but the optimized chunk copy is cleaner and closer to how real file I/O systems work. This problem commonly appears in discussions about simulation patterns and buffer handling with arrays such as array manipulation in constrained APIs, especially in interactive style problems.