count_inversions.rb

filename = ARGV.first

input_array = File.readlines(filename).map(&:to_i)

def merge_sort(array)
    if array.length <=1
        return array, 0
    else
        center = (array.length / 2).floor
        left, left_inv = merge_sort(array[0..center - 1])
        right, right_inv = merge_sort(array[center..array.length])
        merged_array, split_inv = merge(left, right)
    end
    return merged_array, split_inv + left_inv + right_inv
end

def merge(left_array, right_array)
    i = j = inversions = 0
    result = []
    while (i < left_array.length and j < right_array.length)
        if left_array[i] < right_array[j]
            result.push(left_array[i])
            i += 1
        else
            inversions += left_array.length - i
            result.push(right_array[j])
            j += 1
        end
    end
    result += left_array[i..left_array.length] if i < left_array.length
    result += right_array[j..right_array.length] if j < right_array.length
    return result, inversions
end

result = merge_sort(input_array)
puts "Sorted array: #{result[0]}"
puts "Inversions: #{result[1]}"