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Molinillo Architecture

At the highest level, Molinillo is a dependency resolution algorithm. You hand the Resolver a list of dependencies and a 'locking' DependencyGraph, and you get a resulting dependency graph out of that. In order to guarantee that the list of dependencies is properly resolved, however, an algorithm is required that is smarter than just walking the list of dependencies and activating each, and its own dependencies, in turn.

Backtracking

At the heart of Molinillo is a backtracking algorithm with forward checking. Essentially, the resolution process keeps track of two types of states (dependency and possibility) in a stack. If that stack is ever exhausted, resolution was impossible. New states are pushed onto the stack for every dependency, and every time a dependency is successfully 'activated' a new state is pushed onto the stack that represents that activation. This stack-based approach is used because backtracking (also known as unwinding) becomes as simple as popping a state off that stack.

Walkthrough

The client initializes a Resolver with a SpecificationProvider and UI
The client calls resolve with an array of user-requested dependencies and an optional 'locking' DependencyGraph
The Resolver creates a new Resolution with those four user-specified parameters and calls resolve on it
The Resolution creates an initial_state, which takes the user-requested dependencies and puts them into a DependencyState

In the process of creating the state, the SpecificationProvider is asked to sort the dependencies and return all the possibilities for the initial_requirement (taking into account whether the dependency is locked). These possibilities are then grouped into PossibilitySets, with each set representing a group of versions for the dependency which share the same sub-dependency requirements and are contiguous
A DependencyGraph is created that has all of these requirements point to root_vertices

The resolution process now enters its main loop, which continues as long as there is a current state to process, and the current state has requirements left to process
UI#indicate_progress is called to allow the client to report progress
If the current state is a DependencyState, we have it pop off a PossibilityState that encapsulates a PossibilitySet for that dependency
Process the topmost state on the stack
If there is a non-empty PossibilitySet for the state, attempt_to_activate it (jump to #11)
If there is no non-empty PossibilitySet for the state, create_conflict if the state is a PossibilityState, and then unwind_for_conflict

create_conflict builds a Conflict object, with details of all of the requirements for the given dependency, and adds it to a hash of conflicts stored on the state, indexed by the name of the dependency
unwind_for_conflict loops through all the conflicts on the state, looking for a state it can rewind to that might avoid that conflict. If no such state exists, it raises a VersionConflict error. Otherwise, it takes the most recent state with a chance to avoid the current conflicts and rewinds to it (go to #6)

Check if there is an existing vertex in the activated dependency graph for the dependency this state's requirement relates to
If there is no existing vertex in the activated dependency graph for the dependency this state's requirement relates to, activate_new_spec. This creates a new vertex in the activated dependency graph, with it's payload set to the possibility's PossibilitySet. It also pushes a new DependencyState, with the now-activated PossibilitySet's own dependencies. Go to #6
If there is an existing, activated vertex for the dependency, attempt_to_filter_existing_spec

This filters the contents of the existing vertex's PossibilitySet by the current state's requirement
If any possibilities remain within the PossibilitySet, it updates the activated vertex's payload with the new, filtered state and pushes a new DependencyState
If no possibilities remain within the PossibilitySet after filtering, or if the current state's PossibilitySet had a different set of sub-dependency requirements to the existing vertex's PossibilitySet, create_conflict and unwind_for_conflict, back to the last DependencyState that has a chance to not generate a conflict. Go to #6

Terminate with the topmost state's dependency graph when there are no more requirements left
For each vertex with a payload of allowable versions for this resolution (i.e., a PossibilitySet), pick a single specific version.

Optimal unwinding

For our backtracking algorithm to be efficient as well as correct, we need to unwind efficiently after a conflict is encountered. Unwind too far and we'll miss valid resolutions - once we unwind passed a DependencyState we can never get there again. Unwind too little and resolution will be extremely slow - we'll repeatedly hit the same conflict, processing many unnecessary iterations before getting to a branch that avoids it.

To unwind the optimal amount, we consider the current conflict, along with all the previous unwinds that have determined our current state.

First, consider the current conflict as follows:

Find the earliest (lowest index) set of requirements which combine to cause the conflict. Any non-binding requirements can be ignored, as removing them would not resolve the current conflict
For each binding requirement, find all the alternative possibilities that would relax the requirement:
- the requirement's DependencyState might have alternative possibilities that would satisfy all the other requirements
- the parent of the requirement might have alternative possibilities that would prevent the requirement existing
- the parent of the parent of the requirement might have alternative possibilities that would prevent the parent, and thus the requirement, from existing
- etc., etc.
Group all of the above possibilities into an array, and pick the one with the highest index (i.e., the smallest rewind) as our candidate rewind

Next, consider any previous unwinds that were not executed (because a different, smaller unwind was chosen instead):

Ignore any previously unused unwinds that would now unwind further than the highest index found in (1), if any
For the remaining unused unwinds, check whether the unwind has a chance of preventing us encountering the current conflict. For this to be the case, the unwind must have been rejected in favour of an unwind to one of the states in the current conflict's requirement tree
If any such unwinds exist, use the one with the highest index (smallest unwind) instead of the one found in (1) 3a. If no possible unwind was found in (1) and (2), raise a VersionConflict error as resolution is not possible. 3b. Filter the state that we're unwinding to, in order to remove any possibilities we know will result in a conflict. Consider all possible unwinds to the chosen state (there may be several, amassed from previous unused unwinds for different conflicts) when doing this filtering - only possibilities that will certainly result in all of those conflicts can be filtered out as having no chance of resolution

Update the list of unused unwinds:

Add all possible unwinds for the current conflict
Update the requirements_unwound_to_instead attribute on any considered unwind that was only rejected because it had a lower index than the chosen one
Remove all unwinds to a state greater than or equal to the chosen unwind

Go to #6 in the main loop

Specification Provider

The SpecificationProvider module forms the basis for the key integration point for a client library with Molinillo. Its methods convert the client's domain-specific model objects into concepts the resolver understands:

Nested dependencies
Names
Requirement satisfaction
Finding specifications (known internally as possibilities)
Sorting dependencies (for the sake of reasonable resolver performance)