require "set"
require "solve/errors"
require_relative "solver/serializer"

module Solve
  class GecodeSolver
    class << self
      # The timeout (in seconds) to use when resolving graphs. Default is 10. This can be
      # configured by setting the SOLVE_TIMEOUT environment variable.
      #
      # @return [Integer]
      def timeout
        seconds = 30 unless seconds = ENV["SOLVE_TIMEOUT"]
        seconds.to_i * 1_000
      end

      # Attemp to load the dep_selector gem which this solver engine requires.
      def activate
        require "dep_selector"
      rescue LoadError => e
        raise Errors::EngineNotAvailable, "dep_selector is not installed, GecodeSolver cannot be used (#{e})"
      end
    end

    # Graph object with references to all known artifacts and dependency
    # constraints.
    #
    # @return [Solve::Graph]
    attr_reader :graph

    # @example Demands are Arrays of Arrays with an artifact name and optional constraint:
    #   [['nginx', '= 1.0.0'], ['mysql']]
    # @return [Array<String>, Array<Array<String, String>>] demands
    attr_reader :demands_array

    # @example Basic use:
    #   graph = Solve::Graph.new
    #   graph.artifacts("mysql", "1.2.0")
    #   demands = [["mysql"]]
    #   GecodeSolver.new(graph, demands)
    # @param [Solve::Graph] graph
    # @param [Array<String>, Array<Array<String, String>>] demands
    # @param [Hash] options - unused, only present for API compatibility with RubySolver
    def initialize(graph, demands, options = {})
      @ds_graph      = DepSelector::DependencyGraph.new
      @graph         = graph
      @demands_array = demands
      @timeout_ms    = self.class.timeout
    end

    # The problem demands given as Demand model objects
    # @return [Array<Solve::Demand>]
    def demands
      demands_array.map do |name, constraint|
        Demand.new(self, name, constraint)
      end
    end

    # @option options [Boolean] :sorted
    #   return the solution as a sorted list instead of a Hash
    #
    # @return [Hash, List] Returns a hash like { "Artifact Name" => "Version",... }
    #   unless the :sorted option is true, then it returns a list like [["Artifact Name", "Version],...]
    # @raise [Errors::NoSolutionError] when the demands cannot be met for the
    #   given graph.
    # @raise [Errors::UnsortableSolutionError] when the :sorted option is true
    #   and the demands have a solution, but the solution contains a cyclic
    #   dependency
    def resolve(options = {})
      solution = solve_demands(demands_as_constraints)

      unsorted_solution = solution.inject({}) do |stringified_soln, (name, version)|
        stringified_soln[name] = version.to_s
        stringified_soln
      end

      if options[:sorted]
        build_sorted_solution(unsorted_solution)
      else
        unsorted_solution
      end
    end

    private

      # DepSelector::DependencyGraph object representing the problem.
    attr_reader :ds_graph

      # Timeout in milliseconds. Hardcoded to 1s for now.
    attr_reader :timeout_ms

      # Runs the solver with the set of demands given. If any DepSelector
      # exceptions are raised, they are rescued and re-raised
    def solve_demands(demands_as_constraints)
      selector = DepSelector::Selector.new(ds_graph, (timeout_ms / 1000.0))
      selector.find_solution(demands_as_constraints, all_artifacts)
    rescue DepSelector::Exceptions::InvalidSolutionConstraints => e
      report_invalid_constraints_error(e)
    rescue DepSelector::Exceptions::NoSolutionExists => e
      report_no_solution_error(e)
    rescue DepSelector::Exceptions::TimeBoundExceeded
      # DepSelector timed out trying to find the solution. There may or may
      # not be a solution.
      raise Solve::Errors::NoSolutionError.new(
        "The dependency constraints could not be solved in the time allotted.")
    rescue DepSelector::Exceptions::TimeBoundExceededNoSolution
      # DepSelector determined there wasn't a solution to the problem, then
      # timed out trying to determine which constraints cause the conflict.
      raise Solve::Errors::NoSolutionCauseUnknown.new(
        "There is a dependency conflict, but the solver could not determine the precise cause in the time allotted.")
    end

      # Maps demands to corresponding DepSelector::SolutionConstraint objects.
    def demands_as_constraints
      @demands_as_constraints ||= demands_array.map do |demands_item|
        item_name, constraint_with_operator = demands_item
        version_constraint = Semverse::Constraint.new(constraint_with_operator)
        DepSelector::SolutionConstraint.new(ds_graph.package(item_name), version_constraint)
      end
    end

      # Maps all artifacts in the graph to DepSelector::Package objects. If not
      # already done, artifacts are added to the ds_graph as a necessary side effect.
    def all_artifacts
      return @all_artifacts if @all_artifacts
      populate_ds_graph!
      @all_artifacts
    end

      # Converts artifacts to DepSelector::Package objects and adds them to the
      # DepSelector graph. This should only be called once; use #all_artifacts
      # to safely get the set of all artifacts.
    def populate_ds_graph!
      @all_artifacts = Set.new

      graph.artifacts.each do |artifact|
        add_artifact_to_ds_graph(artifact)
        @all_artifacts << ds_graph.package(artifact.name)
      end
    end

    def add_artifact_to_ds_graph(artifact)
      package_version = ds_graph.package(artifact.name).add_version(artifact.version)
      artifact.dependencies.each do |dependency|
        dependency = DepSelector::Dependency.new(ds_graph.package(dependency.name), dependency.constraint)
        package_version.dependencies << dependency
      end
      package_version
    end

    def report_invalid_constraints_error(e)
      non_existent_cookbooks = e.non_existent_packages.inject([]) do |list, constraint|
        list << constraint.package.name
      end

      constrained_to_no_versions = e.constrained_to_no_versions.inject([]) do |list, constraint|
        list << [constraint.package.name, constraint.constraint.to_s]
      end

      raise Solve::Errors::NoSolutionError.new(
        "Required artifacts do not exist at the desired version",
        missing_artifacts: non_existent_cookbooks,
        constraints_excluding_all_artifacts: constrained_to_no_versions
      )
    end

    def report_no_solution_error(e)
      most_constrained_cookbooks = e.disabled_most_constrained_packages.inject([]) do |list, package|
        list << "#{package.name} = #{package.versions.first}"
      end

      non_existent_cookbooks = e.disabled_non_existent_packages.inject([]) do |list, package|
        list << package.name
      end

      raise Solve::Errors::NoSolutionError.new(
        e.message,
        unsatisfiable_demand: e.unsatisfiable_solution_constraint.to_s,
        missing_artifacts: non_existent_cookbooks,
        artifacts_with_no_satisfactory_version: most_constrained_cookbooks
      )
    end

    def build_sorted_solution(unsorted_solution)
      nodes = Hash.new
      unsorted_solution.each do |name, version|
        nodes[name] = @graph.artifact(name, version).dependencies.map(&:name)
      end

      # Modified from http://ruby-doc.org/stdlib-1.9.3/libdoc/tsort/rdoc/TSort.html
      class << nodes
        include TSort
        alias tsort_each_node each_key
        def tsort_each_child(node, &block)
          fetch(node).each(&block)
        end
      end
      begin
        sorted_names = nodes.tsort
      rescue TSort::Cyclic => e
        raise Solve::Errors::UnsortableSolutionError.new(e, unsorted_solution)
      end

      sorted_names.map do |artifact|
        [artifact, unsorted_solution[artifact]]
      end
    end
  end
end