add depsolver directly to the system

This is sub-optimal but I suspect rcl_depsolver is going to migrate
away from depsolver in any case.

1 files changed, 714 insertions, 0 deletions

diff --git a/src/rcl_depsolver.erl b/src/rcl_depsolver.erl
new file mode 100644
index 0000000..4d61d6c
--- /dev/null
+++ b/src/rcl_depsolver.erl
@@ -0,0 +1,714 @@
+%% -*- erlang-indent-level: 4; indent-tabs-mode: nil; fill-column: 80 -*-
+%% ex: ts=4 sx=4 et
+%%
+%% Copyright 2012 Opscode, Inc. All Rights Reserved.
+%%
+%% This file is provided to you under the Apache License,
+%% Version 2.0 (the "License"); you may not use this file
+%% except in compliance with the License.  You may obtain
+%% a copy of the License at
+%%
+%%   http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing,
+%% software distributed under the License is distributed on an
+%% "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+%% KIND, either express or implied.  See the License for the
+%% specific language governing permissions and limitations
+%% under the License.
+%%
+%% @author Eric Merritt <[email protected]>
+%%
+%%%-------------------------------------------------------------------
+%%% @doc
+%%% This is a dependency constraint solver. You add your 'world' to the
+%%% solver. That is the packages that exist, their versions and their
+%%% dependencies. Then give the system a set of targets and ask it to solve.
+%%%
+%%%  Lets say our world looks as follows
+%%%
+%%%      app1 that has versions "0.1"
+%%%        depends on app3 any version greater then "0.2"
+%%%       "0.2" with no dependencies
+%%%       "0.3" with no dependencies
+%%%
+%%%      app2 that has versions "0.1" with no dependencies
+%%%       "0.2" that depends on app3 exactly "0.3"
+%%%       "0.3" with no dependencies
+%%%
+%%%      app3 that has versions
+%%%       "0.1", "0.2" and "0.3" all with no dependencies
+%%%
+%%% we can add this world to the system all at once as follows
+%%%
+%%%      Graph0 = rcl_depsolver:new_graph(),
+%%%      Graph1 = rcl_depsolver:add_packages(
+%%%             [{app1, [{"0.1", [{app2, "0.2"},
+%%%                               {app3, "0.2", '>='}]},
+%%%                               {"0.2", []},
+%%%                               {"0.3", []}]},
+%%%              {app2, [{"0.1", []},
+%%%                       {"0.2",[{app3, "0.3"}]},
+%%%                       {"0.3", []}]},
+%%%              {app3, [{"0.1", []},
+%%%                      {"0.2", []},
+%%%                      {"0.3", []}]}]).
+%%%
+%%% We can also build it up incrementally using the other add_package and
+%%% add_package_version functions.
+%%%
+%%% Finally, once we have built up the graph we can ask rcl_depsolver to solve the
+%%% dependency constraints. That is to give us a list of valid dependencies by
+%%% using the solve function. Lets say we want the app3 version "0.3" and all of
+%%% its resolved dependencies. We could call solve as follows.
+%%%
+%%%    rcl_depsolver:solve(Graph1, [{app3, "0.3"}]).
+%%%
+%%% That will give us the completely resolved dependencies including app3
+%%% itself. Lets be a little more flexible. Lets ask for a graph that is rooted
+%%% in anything greater then or equal to app3 "0.3". We could do that by
+%%%
+%%%    rcl_depsolver:solve(Graph1, [{app3, "0.3", '>='}]).
+%%%
+%%% Of course, you can specify any number of goals at the top level.
+%%% @end
+%%%-------------------------------------------------------------------
+-module(rcl_depsolver).
+
+%% Public Api
+-export([format_error/1,
+         format_roots/1,
+         format_culprits/1,
+         format_constraint/1,
+         format_version/1,
+         new_graph/0,
+         solve/2,
+         add_packages/2,
+         add_package/3,
+         add_package_version/3,
+         add_package_version/4,
+         parse_version/1,
+         filter_packages/2]).
+
+%% Internally Exported API. This should *not* be used outside of the rcl_depsolver
+%% application. You have been warned.
+-export([dep_pkg/1,
+         filter_package/2,
+         primitive_solve/3]).
+
+-export_type([t/0,
+              pkg/0,
+              constraint_op/0,
+              pkg_name/0,
+              vsn/0,
+              constraint/0,
+              dependency_set/0]).
+
+-export_type([dep_graph/0, constraints/0,
+              ordered_constraints/0, fail_info/0,
+              fail_detail/0]).
+%%============================================================================
+%% type
+%%============================================================================
+-type dep_graph() :: gb_tree().
+-opaque t() :: {?MODULE, dep_graph()}.
+-type pkg() :: {pkg_name(), vsn()}.
+-type pkg_name() :: binary() | atom().
+-type raw_vsn() :: ec_semver:any_version().
+
+-type vsn() :: 'NO_VSN'
+             | ec_semver:semver().
+
+-type constraint_op() ::
+        '=' | gte | '>=' | lte | '<='
+      | gt | '>' | lt | '<' | pes | '~>' | between.
+
+-type raw_constraint() :: pkg_name()
+                        | {pkg_name(), raw_vsn()}
+                        | {pkg_name(), raw_vsn(), constraint_op()}
+                        | {pkg_name(), raw_vsn(), vsn(), between}.
+
+-type constraint() :: pkg_name()
+                    | {pkg_name(), vsn()}
+                    | {pkg_name(), vsn(), constraint_op()}
+                    | {pkg_name(), vsn(), vsn(), between}.
+
+
+-type vsn_constraint() :: {raw_vsn(), [raw_constraint()]}.
+-type dependency_set() :: {pkg_name(), [vsn_constraint()]}.
+
+%% Internal Types
+-type constraints() :: [constraint()].
+-type ordered_constraints() :: [{pkg_name(), constraints()}].
+-type fail_info() :: {[pkg()], ordered_constraints()}.
+-type fail_detail() :: {fail, [fail_info()]}.
+-type version_checker() :: fun((vsn()) -> fail_detail() | vsn()).
+
+%%============================================================================
+%% API
+%%============================================================================
+%% @doc create a new empty dependency graph
+-spec new_graph() -> t().
+new_graph() ->
+    {?MODULE, gb_trees:empty()}.
+
+%% @doc add a complete set of list of packages to the graph. Where the package
+%% consists of the name and a list of versions and dependencies.
+%%
+%% ``` rcl_depsolver:add_packages(Graph,
+%%               [{app1, [{"0.1", [{app2, "0.2"},
+%%                                 {app3, "0.2", '>='}]},
+%%                                 {"0.2", []},
+%%                                 {"0.3", []}]},
+%%                 {app2, [{"0.1", []},
+%%                         {"0.2",[{app3, "0.3"}]},
+%%                         {"0.3", []}]},
+%%                 {app3, [{"0.1", []},
+%%                         {"0.2", []},
+%%                         {"0.3", []}]}])
+%% '''
+-spec add_packages(t(),[dependency_set()]) -> t().
+add_packages(Dom0, Info)
+  when is_list(Info) ->
+    lists:foldl(fun({Pkg, VsnInfo}, Dom1) ->
+                        add_package(Dom1, Pkg, VsnInfo)
+                end, Dom0, Info).
+
+%% @doc add a single package to the graph, where it consists of a package name
+%% and its versions and thier dependencies.
+%%  ```rcl_depsolver:add_package(Graph, app1, [{"0.1", [{app2, "0.2"},
+%%                                              {app3, "0.2", '>='}]},
+%%                                              {"0.2", []},
+%%                                              {"0.3", []}]}]).
+%% '''
+-spec add_package(t(),pkg_name(),[vsn_constraint()]) -> t().
+add_package(State, Pkg, Versions)
+  when is_list(Versions) ->
+    lists:foldl(fun({Vsn, Constraints}, Dom1) ->
+                        add_package_version(Dom1, Pkg, Vsn, Constraints);
+                   (Version, Dom1) ->
+                        add_package_version(Dom1, Pkg, Version, [])
+                end, State, Versions).
+
+%% @doc add a set of dependencies to a specific package and version.
+%% and its versions and thier dependencies.
+%%  ```rcl_depsolver:add_package(Graph, app1, "0.1", [{app2, "0.2"},
+%%                                              {app3, "0.2", '>='}]},
+%%                                              {"0.2", []},
+%%                                              {"0.3", []}]).
+%% '''
+-spec add_package_version(t(), pkg_name(), raw_vsn(), [raw_constraint()]) -> t().
+add_package_version({?MODULE, Dom0}, RawPkg, RawVsn, RawPkgConstraints) ->
+    Pkg = fix_pkg(RawPkg),
+    Vsn = parse_version(RawVsn),
+    %% Incoming constraints are raw
+    %% and need to be fixed
+    PkgConstraints = [fix_con(PkgConstraint) ||
+                         PkgConstraint <- RawPkgConstraints],
+    Info2 =
+        case gb_trees:lookup(Pkg, Dom0) of
+            {value, Info0} ->
+                case lists:keytake(Vsn, 1, Info0) of
+                    {value, {Vsn, Constraints}, Info1} ->
+                        [{Vsn, join_constraints(Constraints,
+                                                PkgConstraints)}
+                         | Info1];
+                    false ->
+                        [{Vsn,  PkgConstraints}  | Info0]
+                end;
+            none ->
+                [{Vsn, PkgConstraints}]
+        end,
+    {?MODULE, gb_trees:enter(Pkg, Info2, Dom0)}.
+
+%% @doc add a package and version to the dependency graph with no dependency
+%% constraints, dependency constraints can always be added after the fact.
+%%
+%% ```rcl_depsolver:add_package_version(Graph, app1, "0.1").'''
+-spec add_package_version(t(),pkg_name(),raw_vsn()) -> t().
+add_package_version(State, Pkg, Vsn) ->
+    add_package_version(State, Pkg, Vsn, []).
+
+%% @doc Given a set of goals (in the form of constrains) find a set of packages
+%% and versions that satisfy all constraints. If no solution can be found then
+%% an exception is thrown.
+%% ``` rcl_depsolver:solve(State, [{app1, "0.1", '>='}]).'''
+-spec solve(t(),[constraint()]) -> {ok, [pkg()]} | {error, term()}.
+solve({?MODULE, DepGraph0}, RawGoals)
+  when erlang:length(RawGoals) > 0 ->
+    Goals = [fix_con(Goal) || Goal <- RawGoals],
+    case trim_unreachable_packages(DepGraph0, Goals) of
+        Error = {error, _} ->
+            Error;
+        DepGraph1 ->
+            case primitive_solve(DepGraph1, Goals, no_path) of
+                {fail, _} ->
+                    [FirstCons | Rest] = Goals,
+                    {error, rcl_depsolver_culprit:search(DepGraph1, [FirstCons], Rest)};
+                Solution ->
+                    {ok, Solution}
+            end
+    end.
+
+%% Parse a string version into a tuple based version
+-spec parse_version(raw_vsn() | vsn()) -> vsn().
+parse_version(RawVsn)
+  when erlang:is_list(RawVsn);
+       erlang:is_binary(RawVsn) ->
+    ec_semver:parse(RawVsn);
+parse_version(Vsn)
+  when erlang:is_tuple(Vsn) ->
+    Vsn.
+
+%% @doc given a list of package name version pairs, and a list of constraints
+%% return every member of that list that matches all constraints.
+-spec filter_packages([{pkg_name(), raw_vsn()}], [raw_constraint()]) ->
+                             {ok, [{pkg_name(), raw_vsn()}]}
+                                 | {error, Reason::term()}.
+filter_packages(PVPairs, RawConstraints) ->
+    Constraints = [fix_con(Constraint) || Constraint <- RawConstraints],
+    case check_constraints(Constraints) of
+        ok ->
+            {ok, [PVPair || PVPair <- PVPairs,
+                            filter_pvpair_by_constraint(fix_con(PVPair), Constraints)]};
+        Error ->
+            Error
+    end.
+
+%% @doc Produce a full error message for a given error condition.  This includes
+%% details of the paths taken to resolve the dependencies and shows which dependencies
+%% could not be satisfied
+-spec format_error({error, {unreachable_package, list()} |
+                           {invalid_constraints, [constraint()]} |
+                           list()}) -> iolist().
+format_error(Error) ->
+    rcl_depsolver_culprit:format_error(Error).
+
+%% @doc Return a formatted list of roots of the dependency trees which
+%% could not be satisified. These may also have versions attached.
+%% Example:
+%%
+%%    ```(foo = 1.2.0), bar```
+%%
+-spec format_roots([constraints()]) -> iolist().
+format_roots(Roots) ->
+    rcl_depsolver_culprit:format_roots(Roots).
+
+
+%% @doc Return a formatted list of the culprit depenedencies which led to
+%% the dependencies not being satisfied. Example:
+%%
+%%     ```(foo = 1.2.0) -> (bar > 2.0.0)```
+-spec format_culprits([{[constraint()], [constraint()]}]) -> iolist().
+format_culprits(Culprits) ->
+    rcl_depsolver_culprit:format_culprits(Culprits).
+
+%% @doc A formatted version tuple
+-spec format_version(vsn()) -> iolist().
+format_version(Version) ->
+    rcl_depsolver_culprit:format_version(Version).
+
+%% @doc A formatted constraint tuple
+-spec format_constraint(constraint()) -> iolist().
+format_constraint(Constraint) ->
+    rcl_depsolver_culprit:format_constraint(Constraint).
+
+%%====================================================================
+%% Internal Functions
+%%====================================================================
+-spec check_constraints(constraints()) ->
+                               ok | {error, {invalid_constraints, [term()]}}.
+check_constraints(Constraints) ->
+    PossibleInvalids =
+        lists:foldl(fun(Constraint, InvalidConstraints) ->
+                            case is_valid_constraint(Constraint) of
+                                true ->
+                                    InvalidConstraints;
+                                false ->
+                                [Constraint | InvalidConstraints]
+                            end
+                    end, [], Constraints),
+    case PossibleInvalids of
+        [] ->
+            ok;
+        _ ->
+        {error, {invalid_constraints, PossibleInvalids}}
+    end.
+
+
+-spec filter_pvpair_by_constraint({pkg_name(), vsn()}, [constraint()]) ->
+                                         boolean().
+filter_pvpair_by_constraint(PVPair, Constraints) ->
+    lists:all(fun(Constraint) ->
+                      filter_package(PVPair, Constraint)
+              end, Constraints).
+
+-spec filter_package({pkg_name(), vsn()}, constraint()) ->
+                            boolean().
+filter_package({PkgName, Vsn}, C = {PkgName, _}) ->
+    is_version_within_constraint(Vsn, C);
+filter_package({PkgName, Vsn}, C = {PkgName, _, _}) ->
+    is_version_within_constraint(Vsn, C);
+filter_package({PkgName, Vsn}, C = {PkgName, _, _, _}) ->
+    is_version_within_constraint(Vsn, C);
+filter_package(_, _) ->
+    %% If its not explicitly excluded its included
+    true.
+
+%% @doc
+%% fix the package name. If its a list turn it into a binary otherwise leave it as an atom
+fix_pkg(Pkg) when is_list(Pkg) ->
+    erlang:list_to_binary(Pkg);
+fix_pkg(Pkg) when is_binary(Pkg); is_atom(Pkg) ->
+    Pkg.
+
+%% @doc
+%% fix package. Take a package with a possible invalid version and fix it.
+-spec fix_con(raw_constraint()) -> constraint().
+fix_con({Pkg, Vsn}) ->
+    {fix_pkg(Pkg), parse_version(Vsn)};
+fix_con({Pkg, Vsn, CI}) ->
+    {fix_pkg(Pkg), parse_version(Vsn), CI};
+fix_con({Pkg, Vsn1, Vsn2, CI}) ->
+    {fix_pkg(Pkg), parse_version(Vsn1),
+     parse_version(Vsn2), CI};
+fix_con(Pkg) ->
+    fix_pkg(Pkg).
+
+
+%% @doc given two lists of constraints join them in such a way that no
+%% constraint is duplicated but the over all order of the constraints is
+%% preserved. Order drives priority in this solver and is important for that
+%% reason.
+-spec join_constraints([constraint()], [constraint()]) ->
+                              [constraint()].
+join_constraints(NewConstraints, ExistingConstraints) ->
+    ECSet = sets:from_list(ExistingConstraints),
+    FilteredNewConstraints = [NC || NC <- NewConstraints,
+                                    not sets:is_element(NC, ECSet)],
+    ExistingConstraints ++ FilteredNewConstraints.
+
+
+%% @doc constraints is an associated list keeping track of all the constraints
+%% that have been placed on a package
+-spec new_constraints() -> constraints().
+new_constraints() ->
+    [].
+
+%% @doc Given a dep graph and a set of goals this either solves the problem or
+%% fails. This is basically the root solver of the system, the main difference
+%% from the exported solve/2 function is the fact that this does not do the
+%% culprit search.
+-spec primitive_solve(dep_graph(),[constraint()], term()) ->
+                             [pkg()] | fail_detail().
+primitive_solve(State, PackageList, PathInd)
+  when erlang:length(PackageList) > 0 ->
+    Constraints = lists:foldl(fun(Info, Acc) ->
+                                      add_constraint('_GOAL_', 'NO_VSN', Acc, Info)
+                              end, new_constraints(), PackageList),
+
+    Pkgs = lists:map(fun dep_pkg/1, PackageList),
+    all_pkgs(State, [], Pkgs, Constraints, PathInd).
+
+%% @doc
+%% given a Pkg | {Pkg, Vsn} | {Pkg, Vsn, Constraint} return Pkg
+-spec dep_pkg(constraint()) -> pkg_name().
+dep_pkg({Pkg, _Vsn}) ->
+    Pkg;
+dep_pkg({Pkg, _Vsn, _}) ->
+    Pkg;
+dep_pkg({Pkg, _Vsn1, _Vsn2, _}) ->
+    Pkg;
+dep_pkg(Pkg) when is_atom(Pkg) orelse is_binary(Pkg) ->
+    Pkg.
+
+-spec is_valid_constraint(constraint()) -> boolean().
+is_valid_constraint(Pkg) when is_atom(Pkg) orelse is_binary(Pkg) ->
+    true;
+is_valid_constraint({_Pkg, Vsn}) when is_tuple(Vsn) ->
+    true;
+is_valid_constraint({_Pkg, Vsn, '='}) when is_tuple(Vsn) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, gte}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, '>='}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, lte}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, '<='}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, gt}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, '>'}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, lt}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, '<'}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, pes}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn, '~>'}) ->
+    true;
+is_valid_constraint({_Pkg, _LVsn1, _LVsn2, between}) ->
+    true;
+is_valid_constraint(_InvalidConstraint) ->
+    false.
+
+-spec add_constraint(pkg_name(), vsn(), [constraint()],constraint()) -> ordered_constraints().
+add_constraint(SrcPkg, SrcVsn, PkgsConstraints, PkgConstraint) ->
+    case is_valid_constraint(PkgConstraint) of
+        true -> ok;
+        false -> erlang:throw({invalid_constraint, PkgConstraint})
+    end,
+    PkgName = dep_pkg(PkgConstraint),
+    Constraints1 =
+        case lists:keysearch(PkgName, 1, PkgsConstraints) of
+            false ->
+                [];
+            {value, {PkgName, Constraints0}} ->
+                Constraints0
+        end,
+    [{PkgName, [{PkgConstraint, {SrcPkg, SrcVsn}} | Constraints1]} |
+     lists:keydelete(PkgName, 1, PkgsConstraints)].
+
+%% @doc
+%% Extend the currently active constraints correctly for the given constraints.
+-spec extend_constraints(pkg_name(), vsn(), constraints(),constraints()) -> [{pkg_name(), constraints()}].
+extend_constraints(SrcPkg, SrcVsn, ExistingConstraints0, NewConstraints) ->
+    lists:foldl(fun (Constraint, ExistingConstraints1) ->
+                        add_constraint(SrcPkg, SrcVsn, ExistingConstraints1, Constraint)
+                end,
+                ExistingConstraints0, [{SrcPkg, SrcVsn} | NewConstraints]).
+
+-spec is_version_within_constraint(vsn(),constraint()) -> boolean().
+is_version_within_constraint(_Vsn, Pkg) when is_atom(Pkg) orelse is_binary(Pkg) ->
+    true;
+is_version_within_constraint(Vsn, {_Pkg, NVsn}) ->
+    ec_semver:eql(Vsn, NVsn);
+is_version_within_constraint(Vsn, {_Pkg, NVsn, '='}) ->
+    ec_semver:eql(Vsn, NVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, gte})  ->
+    ec_semver:gte(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, '>='}) ->
+    ec_semver:gte(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, lte}) ->
+    ec_semver:lte(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, '<='}) ->
+    ec_semver:lte(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, gt}) ->
+    ec_semver:gt(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, '>'}) ->
+    ec_semver:gt(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, lt}) ->
+    ec_semver:lt(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, '<'}) ->
+    ec_semver:lt(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, 'pes'}) ->
+    ec_semver:pes(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn, '~>'}) ->
+    ec_semver:pes(Vsn, LVsn);
+is_version_within_constraint(Vsn, {_Pkg, LVsn1, LVsn2, between}) ->
+    ec_semver:between(LVsn1, LVsn2, Vsn);
+is_version_within_constraint(_Vsn, _Pkg) ->
+    false.
+
+%% @doc
+%% Get the currently active constraints that relate to the specified package
+-spec get_constraints([{pkg_name(), constraints()}],pkg_name()) -> constraints().
+get_constraints(PkgsConstraints, PkgName) ->
+    case lists:keysearch(PkgName, 1, PkgsConstraints) of
+        false ->
+            [];
+        {value, {PkgName, Constraints}} ->
+            Constraints
+    end.
+
+%% @doc
+%% Given a package name get the list of all versions available for that package.
+-spec get_versions(dep_graph(),pkg_name()) -> [vsn()].
+get_versions(DepGraph, PkgName) ->
+    case gb_trees:lookup(PkgName, DepGraph) of
+        none ->
+            [];
+        {value, AllVsns} ->
+            [Vsn || {Vsn, _} <- AllVsns]
+    end.
+
+%% @doc
+%% make sure a given name/vsn meets all current constraints
+-spec valid_version(pkg_name(),vsn(),constraints()) -> boolean().
+valid_version(PkgName, Vsn, PkgConstraints) ->
+    lists:all(fun ({L, _ConstraintSrc}) ->
+                      is_version_within_constraint(Vsn, L)
+              end,
+              get_constraints(PkgConstraints, PkgName)).
+
+%% @doc
+%% Given a Package Name and a set of constraints get a list of package
+%% versions that meet all constraints.
+-spec constrained_package_versions(dep_graph(),pkg_name(),constraints()) ->
+                                          [vsn()].
+constrained_package_versions(State, PkgName, PkgConstraints) ->
+    Versions = get_versions(State, PkgName),
+    [Vsn || Vsn <- Versions, valid_version(PkgName, Vsn, PkgConstraints)].
+
+%% Given a list of constraints filter said list such that only fail (for things
+%% that do not match a package and pkg are returned. Since at the end only pkg()
+%% we should have a pure list of packages.
+-spec filter_package_constraints([constraint()]) -> fail | pkg().
+filter_package_constraints([]) ->
+    fail;
+filter_package_constraints([PkgCon | PkgConstraints]) ->
+    case PkgCon of
+        {Pkg, _} when is_atom(Pkg); is_binary(Pkg) ->
+            filter_package_constraints(PkgConstraints);
+        {{_Pkg1, _Vsn} = PV, _} ->
+            PV;
+        {{_Pkg2, _Vsn, _R}, _} ->
+            filter_package_constraints(PkgConstraints);
+        {{_Pkg2, _Vsn1, _Vsn2, _R}, _} ->
+            filter_package_constraints(PkgConstraints)
+    end.
+
+%% @doc all_pkgs is one of the set of mutually recursive functions (all_pkgs and
+%% pkgs) that serve to walk the solution space of dependency.
+-spec all_pkgs(dep_graph(),[pkg()],[pkg_name()],[{pkg_name(), constraints()}], term()) ->
+                      fail_detail() | [pkg()].
+all_pkgs(_State, Visited, [], Constraints, _PathInd) ->
+    PkgVsns =
+        [filter_package_constraints(PkgConstraints)
+         || {_, PkgConstraints} <- Constraints],
+
+    %% PkgVsns should be a list of pkg() where all the constraints are correctly
+    %% met. If not we fail the solution. If so we return those pkg()s
+    case lists:all(fun({Pkg, Vsn}) ->
+                           lists:all(fun({Constraint, _}) ->
+                                             is_version_within_constraint(Vsn, Constraint)
+                                     end,  get_constraints(Constraints, Pkg))
+                   end, PkgVsns) of
+        true ->
+            PkgVsns;
+        false ->
+            {fail, [{Visited, Constraints}]}
+    end;
+all_pkgs(State, Visited, [PkgName | PkgNames], Constraints, PathInd)
+  when is_atom(PkgName); is_binary(PkgName) ->
+    case lists:keymember(PkgName, 1, Visited) of
+        true ->
+            all_pkgs(State, Visited, PkgNames, Constraints, PathInd);
+        false ->
+            pkgs(State, Visited, PkgName, Constraints, PkgNames, PathInd)
+    end.
+
+%% @doc this is the key graph walker. Set of constraints it walks forward into
+%% the solution space searching for a path that solves all dependencies.
+-spec pkgs(dep_graph(),[pkg()], pkg_name(), [{pkg_name(), constraints()}],
+           [pkg_name()], term()) -> fail_detail() | [pkg()].
+pkgs(DepGraph, Visited, Pkg, Constraints, OtherPkgs, PathInd) ->
+    F = fun (Vsn) ->
+                Deps = get_dep_constraints(DepGraph, Pkg, Vsn),
+                UConstraints = extend_constraints(Pkg, Vsn, Constraints, Deps),
+                DepPkgs =[dep_pkg(Dep) || Dep <- Deps],
+                NewVisited = [{Pkg, Vsn} | Visited],
+                Res = all_pkgs(DepGraph, NewVisited, DepPkgs ++ OtherPkgs, UConstraints, PathInd),
+                Res
+
+        end,
+    case constrained_package_versions(DepGraph, Pkg, Constraints) of
+        [] ->
+            {fail, [{Visited, Constraints}]};
+        Res ->
+            lists_some(F, Res, PathInd)
+    end.
+
+
+%% @doc This gathers the dependency constraints for a given package vsn from the
+%% dependency graph.
+-spec get_dep_constraints(dep_graph(), pkg_name(), vsn()) -> [constraint()].
+get_dep_constraints(DepGraph, PkgName, Vsn) ->
+    {Vsn, Constraints} = lists:keyfind(Vsn, 1,
+                                       gb_trees:get(PkgName, DepGraph)),
+    Constraints.
+
+
+lists_some(F, Res, PathInd) ->
+    lists_some(F, Res, [], PathInd).
+
+-spec lists_some(version_checker(), [vsn()], term(), term()) -> vsn() | fail_detail().
+%% @doc lists_some is the root of the system the actual backtracing search that
+%% makes the dep solver posible. It a takes a function that checks whether the
+%% 'problem' has been solved and an fail indicator. As long as the evaluator
+%% returns the fail indicator processing continues. If the evaluator returns
+%% anything but the fail indicator that indicates success.
+lists_some(_, [], FailPaths, _PathInd) ->
+    {fail, FailPaths};
+lists_some(F, [H | T], FailPaths, PathInd) ->
+    case F(H) of
+        {fail, FailPath} ->
+            case PathInd of
+                keep_paths ->
+                    lists_some(F, T, [FailPath | FailPaths], PathInd);
+                _ ->
+                    lists_some(F, T, [], PathInd)
+            end;
+        Res ->
+            Res
+    end.
+
+%% @doc given a graph and a set of top level goals return a graph that contains
+%% only those top level packages and those packages that might be required by
+%% those packages.
+-spec trim_unreachable_packages(dep_graph(), [constraint()]) ->
+                                       dep_graph() | {error, term()}.
+trim_unreachable_packages(State, Goals) ->
+    {_, NewState0} = new_graph(),
+    lists:foldl(fun(_Pkg, Error={error, _}) ->
+                        Error;
+                   (Pkg, NewState1) ->
+                        PkgName = dep_pkg(Pkg),
+                        find_reachable_packages(State, NewState1, PkgName)
+                end, NewState0, Goals).
+
+%% @doc given a list of versions and the constraints for that version rewrite
+%% the new graph to reflect the requirements of those versions.
+-spec rewrite_vsns(dep_graph(), dep_graph(), [{vsn(), [constraint()]}]) ->
+                          dep_graph() | {error, term()}.
+rewrite_vsns(ExistingGraph, NewGraph0, Info) ->
+    lists:foldl(fun(_, Error={error, _}) ->
+                        Error;
+                   ({_Vsn, Constraints}, NewGraph1) ->
+                        lists:foldl(fun(_DepPkg, Error={error, _}) ->
+                                            Error;
+                                       (DepPkg, NewGraph2) ->
+                                            DepPkgName = dep_pkg(DepPkg),
+                                            find_reachable_packages(ExistingGraph,
+                                                                    NewGraph2,
+                                                                    DepPkgName)
+                                    end, NewGraph1, Constraints)
+                end, NewGraph0, Info).
+
+%% @doc Rewrite the existing dep graph removing anything that is not reachable
+%% required by the goals or any of its potential dependencies.
+-spec find_reachable_packages(dep_graph(), dep_graph(), pkg_name()) ->
+                                     dep_graph() | {error, term()}.
+find_reachable_packages(_ExistingGraph, Error={error, _}, _PkgName) ->
+    Error;
+find_reachable_packages(ExistingGraph, NewGraph0, PkgName) ->
+    case contains_package_version(NewGraph0, PkgName) of
+        true ->
+            NewGraph0;
+        false ->
+            case gb_trees:lookup(PkgName, ExistingGraph) of
+                {value, Info} ->
+                    NewGraph1 = gb_trees:insert(PkgName, Info, NewGraph0),
+                    rewrite_vsns(ExistingGraph, NewGraph1, Info);
+                none ->
+                    {error, {unreachable_package, PkgName}}
+            end
+    end.
+
+%% @doc
+%%  Checks to see if a package name has been defined in the dependency graph
+-spec contains_package_version(dep_graph(), pkg_name()) -> boolean().
+contains_package_version(Dom0, PkgName) ->
+    gb_trees:is_defined(PkgName, Dom0).