# the graph
graph = {}
graph["start"] = {}
graph["start"]["a"] = 6
graph["start"]["b"] = 2

graph["a"] = {}
graph["a"]["fin"] = 1

graph["b"] = {}
graph["b"]["a"] = 3
graph["b"]["fin"] = 5

graph["fin"] = {}

# the costs table
costs = {}
costs["a"] = 6
costs["b"] = 2
costs["fin"] = Float::INFINITY

# the parents table
parents = {}
parents["a"] = "start"
parents["b"] = "start"
parents["fin"] = nil

@processed = []

def find_lowest_cost_node(costs)
  lowest_cost = Float::INFINITY
  lowest_cost_node = nil
  # Go through each node.
  costs.each do |node, cost|
    # If it's the lowest cost so far and hasn't been processed yet...
    if cost < lowest_cost && !@processed.member?(node)
      # ... set it as the new lowest-cost node.
      lowest_cost = cost
      lowest_cost_node = node
    end
  end
  lowest_cost_node
end

# Find the lowest-cost node that you haven't processed yet.
node = find_lowest_cost_node(costs)
# If you've processed all the nodes, this while loop is done.
until node.nil?
  cost = costs[node]
  # Go through all the neighbors of this node.
  neighbors = graph[node]
  neighbors.keys.each do |n|
    new_cost = cost + neighbors[n]
    # If it's cheaper to get to this neighbor by going through this node...
    if costs[n] > new_cost
      # ... update the cost for this node.
      costs[n] = new_cost
      # This node becomes the new parent for this neighbor.
      parents[n] = node
    end
  end
  # Mark the node as processed.
  @processed << node
  # Find the next node to process, and loop.
  node = find_lowest_cost_node(costs)
end

puts "Cost from the start to each node:"
puts costs
