nested method testing in ruby with rspec

i have a piece of code in which i have an input file named Small_Input.txt and in that input file i have a lot of tests. for each test case my code is trying to find whether there exists three no whose sum is equal to a given number. for this i have to write tests using rspec i tried to write but as i am new to this one and didn't get any good article about how to test nested methods. here is my code

class Solution
#main
def has_sum? (input_file_name)
    #input_file_name = gets.strip
    input_file = File.exist?(input_file_name) ? IO.readlines(input_file_name) : nil
    #binding.pry
    output_file_name = (input_file_name[0] == 'S' || input_file_name[0] == 's') ? "Small_Output.txt" : "Big_Output.txt"
    File.truncate(output_file_name, 0) if File.exist?(output_file_name)

    if input_file.nil? || input_file.size() < 1
        puts("Error there is no data in input file or file doesn't exists")
        return -1
    else
        solve_for_given_input_file(input_file)
    end
end

def solve_for_given_input_file(input_file)
    line_no = 0
    test_cases = input_file[line_no].to_i
    line_no += 1

    for i in 1..test_cases
        number_of_ele = input_file[line_no].to_i
        line_no += 1

        array = input_file[line_no].split(' ').map(&:to_i)
        line_no += 1

        target = input_file[line_no].to_i
        line_no += 1
        k = 0
        k = k + solve(input_file_name, number_of_ele, array, target)
    end
end

def solve(input_file_name, number_of_ele, array, target)
    array.sort!
    u = 0
    solution = nil
    for i in 0..number_of_ele
        ptr1 = i + 1
        ptr2 = number_of_ele - 1
        while ptr1 < ptr2 do
            sum = array[i] + array[ptr1] + array[ptr2]
            if(sum == target)
                solution = update_solution(solution, array[i], array[ptr1], array[ptr2])
            end

            if(sum > target) 
                ptr2 -= 1
            else
                ptr1 += 1
            end
        end
    end
    output_file_name = (input_file_name[0] == 'S' || input_file_name[0] == 's') ? "Small_Output.txt" : "Big_Output.txt"
    output_file = File.new(output_file_name, "a+")
    if(solution.nil?) 
        output_file.write("Solution Doesn't exists\n")
    else
        output_file.write(solution[0].to_s + " " + solution[1].to_s + " " + solution[2].to_s + "\n")
        u=1
    end
    output_file.close
    return u
end

def update_solution(solution, value1, value2, value3)
    if solution.nil? || (solution[0] > value1)
        solution = Array.new(3)
        solution[0] = value1
        solution[1] = value2
        solution[2] = value3 
    elsif solution[1] > value2 || (solution[1] == value2 && solution[2] > value3)
        solution[0] = value1
        solution[1] = value2
        solution[2] = value3
    end
    return solution
end
end

and i have written some test which may be wrong or not written properly require 'problem'

describe Solution do   
  it "should detect when file does not exist" do 
     sa = Solution.new 
     test_string = 'spec/no_file.txt' 
     expect(sa.has_sum? test_string).to be -1 
  end 

  it "should detect when a string contains sum" do 
     sa = Solution.new 
     test_string = 'spec/Small_Input.txt' 
     expect(sa.has_sum? test_string).to be true 
  end 

  it "should detect when a string doesn't contain sum" do 
     sa = Solution.new 
     test_string = 'spec/Small_Input.txt'  
     expect(sa.has_sum? test_string).to be false
  end 
  end

as i am john snow(who knows nothing) in rspec any help would be appreciated.