sample test method

main.rb

def solution(x, a)
    return 0 if a.empty? || !x.is_a?(Integer)
    
    indexes = matched_indexes(a, x)
    not_matched = a.size - indexes.size
    k = -1
    return k if indexes.size <= 1 || not_matched.odd?
    indexes.each do |ind, count|
        if ind > count + not_matched / 2
            k = ind - not_matched / 2
            break
        end
    end
    k
end

def matched_indexes(a, x)
    result = {}
    a.each_with_index do |item, index|
        result[index] = result.keys.size + 1 if item == x
    end
    result
end

def test(result, compare_result)
  if result == compare_result
    print '.' #success
  else
    print '-' #fail
  end
end

a = []
x = 0
k = 0
test(solution(x, a), k)

a = []
x = -1
k = 0
test(solution(x, a), k)


a = [1, 2, 5, 5, 2, 3, 5]
x = 9
k = -1
test(solution(x, a), k)


a = [1, 2, 5, 5, 2, 3, 5]
x = 2
k = -1
test(solution(x, a), k)


a = [5, 5, 1, 2, 2, 3, 5]
x = 5
k = 4
test(solution(x, a), k)

a = (1..99_999).to_a + [5]
x = 5
k = 50_000
test(solution(x, a), k)


a = (1..99_999).to_a
x = 5
k = -1
test(solution(x, a), k)