class Gem::DependencyResolver

Given a set of Gem::Dependency objects as needed and a way to query the set of available specs via set, calculates a set of ActivationRequest objects which indicate all the specs that should be activated to meet the all the requirements.



Contains all the conflicts encountered while doing resolution

Public Class Methods

for_current_gems(needed) click to toggle source

Provide a DependencyResolver that queries only against the already installed gems.

# File lib/rubygems/dependency_resolver.rb, line 270
def self.for_current_gems(needed)
  new needed,
new(needed, click to toggle source

Create DependencyResolver object which will resolve the tree starting with needed Depedency objects.

set is an object that provides where to look for specifications to satisify the Dependencies. This defaults to IndexSet, which will query

# File lib/rubygems/dependency_resolver.rb, line 260
def initialize(needed,
  @set = set || # Allow nil to mean IndexSet
  @needed = needed

  @conflicts = nil

Public Instance Methods

requests(s, act) click to toggle source
# File lib/rubygems/dependency_resolver.rb, line 437
def requests(s, act)
  reqs = []
  s.dependencies.each do |d|
    next unless d.type == :runtime
    reqs <<, act)


resolve() click to toggle source

Proceed with resolution! Returns an array of ActivationRequest objects.

# File lib/rubygems/dependency_resolver.rb, line 281
def resolve
  @conflicts = []

  needed = { |n|, nil) }

  res = resolve_for needed, []

  if res.kind_of? DependencyConflict

resolve_for(needed, specs) click to toggle source

The meat of the algorithm. Given needed DependencyRequest objects and specs being a list to ActivationRequest, calculate a new list of ActivationRequest objects.

# File lib/rubygems/dependency_resolver.rb, line 453
def resolve_for(needed, specs)
  until needed.empty?
    dep = needed.shift

    # If there is already a spec activated for the requested name...
    if existing = specs.find { |s| == }

      # then we're done since this new dep matches the
      # existing spec.
      next if dep.matches_spec? existing

      # There is a conflict! We return the conflict
      # object which will be seen by the caller and be
      # handled at the right level.

      # If the existing activation indicates that there
      # are other possibles for it, then issue the conflict
      # on the dep for the activation itself. Otherwise, issue
      # it on the requester's request itself.
      if existing.others_possible?
        conflict =, existing)
        depreq = existing.request.requester.request
        conflict =, existing, dep)
      @conflicts << conflict

      return conflict

    # Get a list of all specs that satisfy dep
    possible = @set.find_all(dep)

    case possible.size
    when 0
      # If there are none, then our work here is done.
    when 1
      # If there is one, then we just add it to specs
      # and process the specs dependencies by adding
      # them to needed.

      spec = possible.first
      act =, dep, false)

      specs << act

      # Put the deps for at the beginning of needed
      # rather than the end to match the depth first
      # searching done by the multiple case code below.
      # This keeps the error messages consistent.
      needed = requests(spec, act) + needed
      # There are multiple specs for this dep. This is
      # the case that this class is built to handle.

      # Sort them so that we try the highest versions
      # first.
      possible = possible.sort_by { |s| s.version }

      # We track the conflicts seen so that we can report them
      # to help the user figure out how to fix the situation.
      conflicts = []

      # To figure out which to pick, we keep resolving
      # given each one being activated and if there isn't
      # a conflict, we know we've found a full set.
      # We use an until loop rather than #reverse_each
      # to keep the stack short since we're using a recursive
      # algorithm.
      until possible.empty?
        s = possible.pop

        # Recursively call #resolve_for with this spec
        # and add it's dependencies into the picture...

        act =, dep)

        try = requests(s, act) + needed

        res = resolve_for(try, specs + [act])

        # While trying to resolve these dependencies, there may
        # be a conflict!

        if res.kind_of? DependencyConflict
          # The conflict might be created not by this invocation
          # but rather one up the stack, so if we can't attempt
          # to resolve this conflict (conflict isn't with the spec +s+)
          # then just return it so the caller can try to sort it out.
          return res unless res.for_spec? s

          # Otherwise, this is a conflict that we can attempt to fix
          conflicts << [s, res]

          # Optimization:
          # Because the conflict indicates the dependency that trigger
          # it, we can prune possible based on this new information.
          # This cuts down on the number of iterations needed.
          possible.delete_if { |x| !res.dependency.matches_spec? x }
          # No conflict, return the specs
          return res

      # We tried all possibles and nothing worked, so we let the user
      # know and include as much information about the problem since
      # the user is going to have to take action to fix this.
      raise, conflicts)