class Solve::Solver

def resolve
  trace("Attempting to find a solution")
  seed_demand_dependencies
  while unbound_variable = variable_table.first_unbound
    possible_values_for_unbound = possible_values_for(unbound_variable)
    trace("Searching for a value for #{unbound_variable.artifact}")
    trace("Constraints are")
    constraint_table.constraints_on_artifact(unbound_variable.artifact).each do |constraint|
      trace("\t#{constraint}")
    end
    trace("Possible values are #{possible_values_for_unbound}")
    while possible_value = possible_values_for_unbound.shift
      possible_artifact = graph.get_artifact(unbound_variable.artifact, possible_value.version)
      possible_dependencies = possible_artifact.dependencies
      all_ok = possible_dependencies.all? { |dependency| can_add_new_constraint?(dependency) }
      if all_ok
        trace("Attempting to use #{possible_artifact}")
        add_dependencies(possible_dependencies, possible_artifact) 
        unbound_variable.bind(possible_value)
        break
      end
    end
    unless unbound_variable.bound?
      trace("Could not find an acceptable value for #{unbound_variable.artifact}")
      backtrack(unbound_variable) 
    end
  end
  solution = {}.tap do |solution|
    variable_table.rows.each do |variable|
      solution[variable.artifact] = variable.value.version.to_s
    end
  end
  trace("Found Solution")
  trace(solution)
  solution
end