class Bundler::Resolver

def resolve(reqs, activated)
  # If the requirements are empty, then we are in a success state. Aka, all
  # gem dependencies have been resolved.
  throw :success, activated if reqs.empty?
  debug { STDIN.gets ; print "\e[2J\e[f" ; "==== Iterating ====\n\n" }
  # Sort dependencies so that the ones that are easiest to resolve are first.
  # Easiest to resolve is defined by:
  #   1) Is this gem already activated?
  #   2) Do the version requirements include prereleased gems?
  #   3) Sort by number of gems available in the source.
  reqs = reqs.sort_by do |a|
    [ activated[a.name] ? 0 : 1,
      a.version_requirements.prerelease? ? 0 : 1,
      @errors[a.name]   ? 0 : 1,
      activated[a.name] ? 0 : search(a).size ]
  end
  debug { "Activated:\n" + activated.values.map { |a| "  #{a.name} (#{a.version})" }.join("\n") }
  debug { "Requirements:\n" + reqs.map { |r| "  #{r.name} (#{r.version_requirements})"}.join("\n") }
  activated = activated.dup
  # Pull off the first requirement so that we can resolve it
  current   = reqs.shift
  debug { "Attempting:\n  #{current.name} (#{current.version_requirements})"}
  # Check if the gem has already been activated, if it has, we will make sure
  # that the currently activated gem satisfies the requirement.
  if existing = activated[current.name]
    if current.version_requirements.satisfied_by?(existing.version)
      debug { "    * [SUCCESS] Already activated" }
      @errors.delete(existing.name)
      # Since the current requirement is satisfied, we can continue resolving
      # the remaining requirements.
      resolve(reqs, activated)
    else
      debug { "    * [FAIL] Already activated" }
      @errors[existing.name] = [existing, current]
      debug { current.required_by.map {|d| "      * #{d.name} (#{d.version_requirements})" }.join("\n") }
      # debug { "    * All current conflicts:\n" + @errors.keys.map { |c| "      - #{c}" }.join("\n") }
      # Since the current requirement conflicts with an activated gem, we need
      # to backtrack to the current requirement's parent and try another version
      # of it (maybe the current requirement won't be present anymore). If the
      # current requirement is a root level requirement, we need to jump back to
      # where the conflicting gem was activated.
      parent = current.required_by.last || existing.required_by.last
      # We track the spot where the current gem was activated because we need
      # to keep a list of every spot a failure happened.
      debug { "    -> Jumping to: #{parent.name}" }
      throw parent.name, existing.required_by.last.name
    end
  else
    # There are no activated gems for the current requirement, so we are going
    # to find all gems that match the current requirement and try them in decending
    # order. We also need to keep a set of all conflicts that happen while trying
    # this gem. This is so that if no versions work, we can figure out the best
    # place to backtrack to.
    conflicts = Set.new
    # Fetch all gem versions matching the requirement
    #
    # TODO: Warn / error when no matching versions are found.
    matching_versions = search(current)
    if matching_versions.empty?
      if current.required_by.empty?
        location = current.source ? current.source.to_s : "any of the sources"
        raise GemNotFound, "Could not find gem '#{current}' in #{location}"
      else
        @errors[current.name] = [nil, current]
      end
    end
    matching_versions.reverse_each do |spec|
      conflict = resolve_requirement(spec, current, reqs.dup, activated.dup)
      conflicts << conflict if conflict
    end
    # If the current requirement is a root level gem and we have conflicts, we
    # can figure out the best spot to backtrack to.
    if current.required_by.empty? && !conflicts.empty?
      # Check the current "catch" stack for the first one that is included in the
      # conflicts set. That is where the parent of the conflicting gem was required.
      # By jumping back to this spot, we can try other version of the parent of
      # the conflicting gem, hopefully finding a combination that activates correctly.
      @stack.reverse_each do |savepoint|
        if conflicts.include?(savepoint)
          debug { "    -> Jumping to: #{savepoint}" }
          throw savepoint
        end
      end
    end
  end
end