How do I select from a given set of values, rounding up?

Question

I've got a ruby array of hashes with two keys, 'tier' and 'price'. For a given price, I want to return the tier.

This is simple enough with exact matching, but how do I ensure I always have a match by rounding my input value up to the next value in the array?

For example, given the array below, if I have a value of '499', I want to return '10'.

tiers = [
    { tier: 0, price: 0 },
    { tier: 1, price: 50 },
    { tier: 2, price: 100 },
    { tier: 3, price: 150 },
    { tier: 4, price: 200 },
    { tier: 5, price: 250 },
    { tier: 6, price: 300 },
    { tier: 7, price: 350 },
    { tier: 8, price: 400 },
    { tier: 9, price: 450 },
    { tier: 10, price: 500 },
    { tier: 11, price: 550 },
    { tier: 12, price: 600 },
    { tier: 13, price: 650 },
    { tier: 14, price: 700 },
    { tier: 15, price: 750 },
    { tier: 16, price: 800 },
    { tier: 17, price: 850 },
    { tier: 18, price: 880 },
    { tier: 19, price: 950 },
    { tier: 20, price: 1000 }
]

I can get an exact match with tiers.detect { |tier| tier[:price] == "500"}[:tier], but this will return an error if I don't have a match. I could increment my input value until I return a match but that seems very inefficient.

I considered rounding my input value, but you'll notice that the increment isn't always the same (from tier 17 to 18 the price only increases by 30).

Answer 1

You could enumerate all tiers in pairs and evaluate your price against the pair. Something like this:

def detect_tier(price)
  tiers.each_cons(2) do |t1, t2|
    next if price < t1[:price] # too low
    next if price > t2[:price] # too high

    return t1[:tier] if price == t1[:price] # matches tier price exactly
    return t2[:tier] # "round up"
  end
end

detect_tier(10) # => 1
detect_tier(100) # => 2
detect_tier(499) # => 10