1 files changed, 105 insertions, 0 deletions
diff --git a/lib/rubygems/resolver/api_specification.rb b/lib/rubygems/resolver/api_specification.rb
new file mode 100644
index 0000000000..ccfd6fe084
--- /dev/null
+++ b/lib/rubygems/resolver/api_specification.rb
@@ -0,0 +1,105 @@
+# frozen_string_literal: true
+
+##
+# Represents a specification retrieved via the Compact Index API.
+#
+# This is used to avoid loading the full Specification object when all we need
+# is the name, version, and dependencies.
+
+class Gem::Resolver::APISpecification < Gem::Resolver::Specification
+  ##
+  # We assume that all instances of this class are immutable;
+  # so avoid duplicated generation for performance.
+  @@cache = {}
+  def self.new(set, api_data)
+    cache_key = [set, api_data]
+    cache = @@cache[cache_key]
+    return cache if cache
+    @@cache[cache_key] = super
+  end
+
+  ##
+  # Creates an APISpecification for the given +set+ from the Compact Index API
+  # +api_data+.
+  #
+  # See https://guides.rubygems.org/rubygems-org-compact-index-api for the
+  # format of the +api_data+.
+
+  def initialize(set, api_data)
+    super()
+
+    @set = set
+    @name = api_data[:name]
+    @version = Gem::Version.new(api_data[:number]).freeze
+    @platform = Gem::Platform.new(api_data[:platform]).freeze
+    @original_platform = api_data[:platform].freeze
+    @dependencies = api_data[:dependencies].map do |name, ver|
+      Gem::Dependency.new(name, ver.split(/\s*,\s*/)).freeze
+    end.freeze
+    @required_ruby_version = Gem::Requirement.new(api_data.dig(:requirements, :ruby)).freeze
+    @required_rubygems_version = Gem::Requirement.new(api_data.dig(:requirements, :rubygems)).freeze
+  end
+
+  def ==(other) # :nodoc:
+    self.class === other &&
+      @set          == other.set &&
+      @name         == other.name &&
+      @version      == other.version &&
+      @platform     == other.platform
+  end
+
+  def hash
+    @set.hash ^ @name.hash ^ @version.hash ^ @platform.hash
+  end
+
+  def fetch_development_dependencies # :nodoc:
+    spec = source.fetch_spec Gem::NameTuple.new @name, @version, @platform
+
+    @dependencies = spec.dependencies
+  end
+
+  def installable_platform? # :nodoc:
+    Gem::Platform.match_gem? @platform, @name
+  end
+
+  def pretty_print(q) # :nodoc:
+    q.group 2, "[APISpecification", "]" do
+      q.breakable
+      q.text "name: #{name}"
+
+      q.breakable
+      q.text "version: #{version}"
+
+      q.breakable
+      q.text "platform: #{platform}"
+
+      q.breakable
+      q.text "dependencies:"
+      q.breakable
+      q.pp @dependencies
+
+      q.breakable
+      q.text "set uri: #{@set.dep_uri}"
+    end
+  end
+
+  ##
+  # Fetches a Gem::Specification for this APISpecification.
+
+  def spec # :nodoc:
+    @spec ||=
+      begin
+        tuple = Gem::NameTuple.new @name, @version, @platform
+        source.fetch_spec tuple
+      rescue Gem::RemoteFetcher::FetchError
+        raise if @original_platform == @platform
+
+        tuple = Gem::NameTuple.new @name, @version, @original_platform
+        source.fetch_spec tuple
+      end
+  end
+
+  def source # :nodoc:
+    @set.source
+  end
+end