problem: find all pairs of numbers in a range of 1..n that add up to n

total_pairs.rb

# $ time ruby total_pairs.rb 
# there are 999 pairs that create 2000 between 1 and 2000
# real	0m0.129s
# user	0m0.074s
# sys	0m0.046s


max_size = 2000
numbers = (1..max_size).to_a
numbers.delete_at(-1) unless numbers.size.even?

split_size = numbers.size / 2
above = numbers[..split_size - 1]
below = numbers[split_size..].reverse!
above.delete_at(-1)
below.delete_at(0)
# the remaining split arrays are the valid combinations that can result in the numbers total
# so it's just the array size to get the count, or [above[i], below[i]] to get the pairs
print "there are #{below.size} pairs that create #{numbers.size} between #{numbers.first} and #{max_size}"