Unix tail command

Implement UNIX tail command. You are given the filename and the number of lines to print from the end of the file.

Solution

• Unix does allow random file access, so treat it like an array and read chars from the end of file moving towards the start.
• When the desired number of lines are read, stop the pointer there and print from that point until end of file.

Code

def tail(filename, num_lines)
    return nil if filename.nil? or num_lines == 0

    file = File.open(filename)

    lines_read = 0
    i = 0
    # Find (end-i)th line
    while i < file.size
        c = file_ptr.read_char(file.end - i)
        lines_read += 1 if c == '\n'
        break if lines_read > num_lines
        i += 1
    end

    # Print last num_lines
    while i < file.size
        puts file_ptr.read_char(i)
        i += 1
    end
end

Time complexity

• Time O(n), when n is the num of lines requested
• Space O(1), no extra space is used to store temp lines