models.rb

class Mineral < ActiveRecord::Base
  has_many :ore_contents
  has_many :ores, :through => :ore_contents
end

class Ore < ActiveRecord::Base
  has_many :ore_contents
  has_many :minerals, :through => :ore_contents

  validates_presence_of :batch_size

  def add_mineral(name, amount)
    return unless mineral = Mineral.find_by_name(name)

    unless content = ore_contents.find_by_mineral_id(mineral.id)
      content = ore_contents.build(:mineral_id => mineral.id)
    end

    content.amount = amount
    content.save!
  end

  def batch_value
    ore_contents.map(&:value).reduce(&:+)
  end

  def refine(amount, value = false)
    batches = (amount / batch_size)
    refined = Hash.new

    ore_contents.dup.each do |content|
      refined[content.mineral.name] = batches * (value ? content.mineral.value : content.amount)
    end

    refined
  end
end

class OreContent < ActiveRecord::Base
  belongs_to :ore
  belongs_to :mineral

  validates_presence_of :amount

  def value
    mineral.price * amount
  end
end

ore_calculator.rb

class OreCalculator
  attr_accessor :stock

  def initialize(stock)
    @stock = stock
  end

  def calculate_amounts
    refined = Hash.new(0)

    stock.each do |ore_name, amount|
      ore = Ore.find_by_name(ore_name)
      next unless ore

      ore.refine(amount).each do |mineral, amount|
        refined[mineral] += amount
      end
    end

    refined
  end
end

ore_calculator_spec.rb

require 'spec_helper'
require 'ore_calculator'

describe OreCalculator do
  describe '#calculate_amounts' do
    context 'one exact batch of veldspar' do
      it 'should return a hash with tritanium' do
        amounts = { 'veldspar' => 333 }
        expected_result = { 'tritanium' => 1000 }
        described_class.new(amounts).calculate_amounts.should == expected_result
      end
    end

    context 'one brimming batch of veldspar' do
      it 'should discard the excess ore and return a hash with tritanium' do
        amounts = { 'veldspar' => 400 }
        expected_result = { 'tritanium' => 1000 }
        described_class.new(amounts).calculate_amounts.should == expected_result
      end
    end

    context 'one exact batch of veldspar and one of scordite' do
      it 'should return a hash with tritanium and pyerite' do
        amounts = { 'veldspar' => 333, 'scordite' => 333 }
        expected_result = { 'tritanium' => 1833, 'pyerite' => 416 }
        described_class.new(amounts).calculate_amounts.should == expected_result
      end
    end
  end
end

seeds.rb

Mineral.create(name: "tritanium", price: 3)
Mineral.create(name: "pyerite", price: 5)
Mineral.create(name: "mexallon", price: 30)
Mineral.create(name: "isogen", price: 65)
Mineral.create(name: "nocxium", price: 450)
Mineral.create(name: "zydrine", price: 750)
Mineral.create(name: "megacyte", price: 2700)
Mineral.create(name: "morphite", price: 3300)

Ore.create(name: "veldspar", batch_size: 333, unit_volume: 0.1) do |ore|
  ore.add_mineral('tritanium', 1000)
end
Ore.create(name: "scordite", batch_size: 333, unit_volume: 0.15) do |ore|
  ore.add_mineral("tritanium", 833)
  ore.add_mineral("pyerite", 416)
end
Ore.create(name: "plagioclase", batch_size: 333, unit_volume: 0.35) do |ore|
  ore.add_mineral("tritanium", 256)
  ore.add_mineral("pyerite", 512)
  ore.add_mineral("mexallon", 256)
end
Ore.create(name: "pyroxeres", batch_size: 333, unit_volume: 0.3) do |ore|
  ore.add_mineral("tritanium", 844)
  ore.add_mineral("pyerite", 59)
  ore.add_mineral("mexallon", 120)
  ore.add_mineral("nocxium", 11)
end
Ore.create(name: "omber", batch_size: 500, unit_volume: 0.6) do |ore|
  ore.add_mineral("tritanium", 307)
  ore.add_mineral("pyerite", 123)
  ore.add_mineral("isogen", 307)
end
Ore.create(name: "kernite", batch_size: 400, unit_volume: 1.2) do |ore|
  ore.add_mineral("tritanium", 386)
  ore.add_mineral("mexallon", 773)
  ore.add_mineral("isogen", 386)
end
Ore.create(name: "jaspet", batch_size: 500, unit_volume: 2) do |ore|
  ore.add_mineral("tritanium", 259)
  ore.add_mineral("pyerite", 259)
  ore.add_mineral("mexallon", 518)
  ore.add_mineral("nocxium", 259)
  ore.add_mineral("zydrine", 8)
end
Ore.create(name: "hemorphite", batch_size: 500, unit_volume: 3) do |ore|
  ore.add_mineral("tritanium", 212)
  ore.add_mineral("isogen", 212)
  ore.add_mineral("nocxium", 424)
  ore.add_mineral("zydrine", 28)
end
Ore.create(name: "hedbergite", batch_size: 500, unit_volume: 3) do |ore|
  ore.add_mineral("isogen", 708)
  ore.add_mineral("nocxium", 354)
  ore.add_mineral("zydrine", 32)
end
Ore.create(name: "gneiss", batch_size: 400, unit_volume: 5) do |ore|
  ore.add_mineral("tritanium", 171)
  ore.add_mineral("mexallon", 171)
  ore.add_mineral("isogen", 343)
  ore.add_mineral("zydrine", 250)
end
Ore.create(name: "dark ochre", batch_size: 400, unit_volume: 8) do |ore|
end
Ore.create(name: "spodumain", batch_size: 250, unit_volume: 16) do |ore|
  ore.add_mineral("tritanium", 700)
  ore.add_mineral("pyerite", 140)
  ore.add_mineral("megacyte", 140)
end
Ore.create(name: "crokite", batch_size: 250, unit_volume: 16) do |ore|
  ore.add_mineral("tritanium", 331)
  ore.add_mineral("nocxium", 331)
  ore.add_mineral("zydrine", 663)
end
Ore.create(name: "bistot", batch_size: 200, unit_volume: 16) do |ore|
  ore.add_mineral("pyerite", 170)
  ore.add_mineral("zydrine", 341)
  ore.add_mineral("megacyte", 170)
end
Ore.create(name: "arkonor", batch_size: 250, unit_volume: 16) do |ore|
  ore.add_mineral("tritanium", 300)
  ore.add_mineral("zydrine", 166)
  ore.add_mineral("megacyte", 333)
end
Ore.create(name: "mercoxit", batch_size: 250, unit_volume: 40) do |ore|
  ore.add_mineral("morphite", 530)
end