class Bundler::Resolver

def self.resolve(requirements, index, source_requirements = {})
  resolver = new(index, source_requirements)
  result = catch(:success) do
    resolver.resolve(requirements, {})
    output = resolver.errors.inject("") do |o, (conflict, (origin, requirement))|
      if origin
        o << "  Conflict on: #{conflict.inspect}:\n"
        o << "    * #{conflict} (#{origin.version}) activated by #{origin.required_by.first}\n"
        o << "    * #{requirement} required by #{requirement.required_by.first}\n"
      else
        o << "  #{requirement} not found in any of the sources\n"
        o << "      required by #{requirement.required_by.first}\n"
      end
      o << "    All possible versions of origin requirements conflict."
    end
    raise VersionConflict, "No compatible versions could be found for required dependencies:\n  #{output}"
    nil
  end
  if result
    # Order gems in order of dependencies. Every gem's dependency is at
    # a smaller index in the array.
    ordered = []
    result.values.each do |spec1|
      index = nil
      place = ordered.detect do |spec2|
        spec1.dependencies.any? { |d| d.name == spec2.name }
      end
      place ?
        ordered.insert(ordered.index(place), spec1) :
        ordered << spec1
    end
    ordered.reverse
  end
end

def debug
  puts yield if defined?($debug) && $debug
end

def initialize(index, source_requirements)
  @errors = {}
  @stack  = []
  @index  = index
  @source_requirements = source_requirements
end

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

def resolve_requirement(spec, requirement, reqs, activated)
  # We are going to try activating the spec. We need to keep track of stack of
  # requirements that got us to the point of activating this gem.
  spec.required_by.replace requirement.required_by
  spec.required_by << requirement
  activated[spec.name] = spec
  debug { "  Activating: #{spec.name} (#{spec.version})" }
  debug { spec.required_by.map { |d| "    * #{d.name} (#{d.version_requirements})" }.join("\n") }
  # Now, we have to loop through all child dependencies and add them to our
  # array of requirements.
  debug { "    Dependencies"}
  spec.dependencies.each do |dep|
    next if dep.type == :development
    debug { "    * #{dep.name} (#{dep.version_requirements})" }
    dep.required_by.replace(requirement.required_by)
    dep.required_by << requirement
    reqs << dep
  end
  # We create a savepoint and mark it by the name of the requirement that caused
  # the gem to be activated. If the activated gem ever conflicts, we are able to
  # jump back to this point and try another version of the gem.
  length = @stack.length
  @stack << requirement.name
  retval = catch(requirement.name) do
    resolve(reqs, activated)
  end
  # Since we're doing a lot of throw / catches. A push does not necessarily match
  # up to a pop. So, we simply slice the stack back to what it was before the catch
  # block.
  @stack.slice!(length..-1)
  retval
end

def search(dep)
  index = @source_requirements[dep.name] || @index
  index.search(dep)
end