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Introduction

LeetCode 158 is an extension of LeetCode 157: Read N Characters Given Read4, but with the added complexity that the read function can be called multiple times. The objective is to efficiently read n characters from a file, using the read4 API, which reads up to 4 characters at a time, and handling multiple calls to read without resetting the reading process.

In this case, multiple calls to read are needed. The challenge is to manage the buffer between successive calls and keep track of how much has been read across all calls.

Problem Statement

We are given the function read4 (which reads 4 characters at a time), and our task is to implement a function read that reads up to n characters from a file, making multiple calls to read.

go

func read(buf []byte, n int) int

• buf: The buffer to store the characters read.
• n: The number of characters to read.

The function should return the number of characters actually read.

Helper Function:

The read4 function is predefined and reads up to 4 characters at a time from the file. Its signature is:

go

func read4(buf []byte) int

It reads up to 4 characters and returns the number of characters read.

Approach

Key Idea:

• The read function needs to handle multiple invocations where the characters read in previous calls must be remembered.
• Since read4 can only return up to 4 characters, we need to store the leftover characters from previous calls (if any) for use in subsequent calls.
• The goal is to keep track of a buffer where we store the remaining characters that were read by read4, and reuse them in the next call to read as needed.

Steps:

1. Maintain an internal buffer: Keep a buffer to store the characters read by read4 across multiple read calls.
2. Track current and total read positions: We maintain two pointers: one for the current position in the internal buffer and another for the total characters read.
3. On each read call: Continue reading from the internal buffer or from read4 if necessary, until we reach the required number of characters.

Go Implementation

go

// The read4 function is predefined for us to use.
// It reads up to 4 bytes from the file into the provided buffer and returns the number of bytes read.
func read4(buf []byte) int {
    // This is just a mock of the read4 function, in real usage it reads from file.
    return 0
}

// The internal buffer to store leftover characters across multiple read calls.
var buffer []byte
// The pointer to track where we are reading from in the buffer.
var ptr int = 0

// Read N characters from the file using read4, and maintain state across multiple calls.
func read(buf []byte, n int) int {
    totalRead := 0

    // While we need more characters to read and there are more characters left to return
    for totalRead < n {
        // If the buffer is empty, fill it by calling read4
        if ptr == 0 {
            count := read4(buffer)
            if count == 0 {
                break  // No more characters to read
            }
        }

        // Copy characters from the internal buffer to the output buffer
        for totalRead < n && ptr < len(buffer) {
            buf[totalRead] = buffer[ptr]
            totalRead++
            ptr++
        }

        // If we exhausted the internal buffer, reset it and move the pointer to 0.
        if ptr == len(buffer) {
            ptr = 0
        }
    }

    return totalRead
}

Explanation of the Code:

1. The read4 Function:

• This is a predefined function that simulates reading up to 4 characters at a time from a file. In a real implementation, it would interact with a file or stream to read data.

2. The read Function:

• Global buffer (buffer): This buffer stores the characters read in previous calls to read4.
• Pointer (ptr): Tracks the current position in the buffer. It ensures we don't read characters we've already processed.
• Main loop: The read function reads characters in chunks of up to 4 from read4. It fills the provided buf with the requested number of characters (n). If there are leftovers in buffer, they are reused across calls.

3. Handling Multiple Calls:

• The function keeps track of the position in the buffer (ptr). If the buffer is exhausted, it is refilled by calling read4.
• It returns the correct number of characters and handles the state between multiple calls.

Time and Space Complexity

OperationComplexityreadO(n)read4O(1)

• Time Complexity:
• The read function calls read4 multiple times, but each call to read4 is O(1). Since we are reading n characters, the overall time complexity is O(n).
• Space Complexity:
• We use a buffer to store characters between calls. The buffer can hold at most 4 characters, so the space complexity is O(1).

Example

Example 1:

go

buf := make([]byte, 5)
n := 5
result := read(buf, n)
fmt.Println(result)  // Output: 5
fmt.Println(string(buf)) // Output: "Hello" (Assuming 'read4' returns "Hell" and "o" across calls)

Explanation:

• We want to read 5 characters.
• In the first call to read, read4 might return "Hell" and the remaining characters, like "o", will be read in the next call.
• The final output is "Hello", and the result returned by read is 5.

Conclusion

LeetCode 158 asks us to efficiently read n characters from a file using the read4 function, handling multiple calls. By maintaining a buffer and tracking the position of characters read across multiple calls, we can reuse leftover characters and ensure efficient reading. The solution operates with O(n) time complexity and O(1) space complexity.