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    use super::dep_cache::RegistryQueryer;
    use super::errors::ActivateResult;
    use super::types::{ConflictMap, ConflictReason, FeaturesSet, ResolveOpts};
    use super::RequestedFeatures;
    use crate::core::{Dependency, PackageId, SourceId, Summary};
    use crate::util::interning::InternedString;
    use crate::util::Graph;
    use anyhow::format_err;
    use std::collections::HashMap;
    use std::num::NonZeroU64;
    use tracing::debug;

    // A `Context` is basically a bunch of local resolution information which is
    // kept around for all `BacktrackFrame` instances. As a result, this runs the
    // risk of being cloned *a lot* so we want to make this as cheap to clone as
    // possible.
    #[derive(Clone)]
    pub struct ResolverContext {
        pub age: ContextAge,
        pub activations: Activations,
        /// list the features that are activated for each package
        pub resolve_features: im_rc::HashMap<PackageId, FeaturesSet>,
        /// get the package that will be linking to a native library by its links attribute
        pub links: im_rc::HashMap<InternedString, PackageId>,

        /// a way to look up for a package in activations what packages required it
        /// and all of the exact deps that it fulfilled.
        pub parents: Graph<PackageId, im_rc::HashSet<Dependency>>,
    }

    /// When backtracking it can be useful to know how far back to go.
    /// The `ContextAge` of a `Context` is a monotonically increasing counter of the number
    /// of decisions made to get to this state.
    /// Several structures store the `ContextAge` when it was added,
    /// to be used in `find_candidate` for backtracking.
    pub type ContextAge = usize;

    /// Find the activated version of a crate based on the name, source, and semver compatibility.
    /// By storing this in a hash map we ensure that there is only one
    /// semver compatible version of each crate.
    /// This all so stores the `ContextAge`.
    pub type ActivationsKey = (InternedString, SourceId, SemverCompatibility);
    pub type Activations = im_rc::HashMap<ActivationsKey, (Summary, ContextAge)>;

    /// A type that represents when cargo treats two Versions as compatible.
    /// Versions `a` and `b` are compatible if their left-most nonzero digit is the
    /// same.
    #[derive(Clone, Copy, Eq, PartialEq, Hash, Debug, PartialOrd, Ord)]
    pub enum SemverCompatibility {
        Major(NonZeroU64),
        Minor(NonZeroU64),
        Patch(u64),
    }

    impl From<&semver::Version> for SemverCompatibility {
        fn from(ver: &semver::Version) -> Self {
            if let Some(m) = NonZeroU64::new(ver.major) {
                return SemverCompatibility::Major(m);
            }
            if let Some(m) = NonZeroU64::new(ver.minor) {
                return SemverCompatibility::Minor(m);
            }
            SemverCompatibility::Patch(ver.patch)
        }
    }

    impl PackageId {
        pub fn as_activations_key(self) -> ActivationsKey {
            (self.name(), self.source_id(), self.version().into())
        }
    }

    impl ResolverContext {
        pub fn new() -> ResolverContext {
            ResolverContext {
                age: 0,
                resolve_features: im_rc::HashMap::new(),
                links: im_rc::HashMap::new(),
                parents: Graph::new(),
                activations: im_rc::HashMap::new(),
            }
        }

        /// Activate this summary by inserting it into our list of known activations.
        ///
        /// The `parent` passed in here is the parent summary/dependency edge which
        /// cased `summary` to get activated. This may not be present for the root
        /// crate, for example.
        ///
        /// Returns `true` if this summary with the given features is already activated.
        pub fn flag_activated(
            &mut self,
            summary: &Summary,
            opts: &ResolveOpts,
            parent: Option<(&Summary, &Dependency)>,
        ) -> ActivateResult<bool> {
            let id = summary.package_id();
            let age: ContextAge = self.age;
            match self.activations.entry(id.as_activations_key()) {
                im_rc::hashmap::Entry::Occupied(o) => {
                    debug_assert_eq!(
                        &o.get().0,
                        summary,
                        "cargo does not allow two semver compatible versions"
                    );
                }
                im_rc::hashmap::Entry::Vacant(v) => {
                    if let Some(link) = summary.links() {
                        if self.links.insert(link, id).is_some() {
                            return Err(format_err!(
                                "Attempting to resolve a dependency with more than \
                                 one crate with links={}.\nThis will not build as \
                                 is. Consider rebuilding the .lock file.",
                                &*link
                            )
                            .into());
                        }
                    }
                    v.insert((summary.clone(), age));

                    // If we've got a parent dependency which activated us, *and*
                    // the dependency has a different source id listed than the
                    // `summary` itself, then things get interesting. This basically
                    // means that a `[patch]` was used to augment `dep.source_id()`
                    // with `summary`.
                    //
                    // In this scenario we want to consider the activation key, as
                    // viewed from the perspective of `dep.source_id()`, as being
                    // fulfilled. This means that we need to add a second entry in
                    // the activations map for the source that was patched, in
                    // addition to the source of the actual `summary` itself.
                    //
                    // Without this it would be possible to have both 1.0.0 and
                    // 1.1.0 "from crates.io" in a dependency graph if one of those
                    // versions came from a `[patch]` source.
                    if let Some((_, dep)) = parent {
                        if dep.source_id() != id.source_id() {
                            let key = (id.name(), dep.source_id(), id.version().into());
                            let prev = self.activations.insert(key, (summary.clone(), age));
                            if let Some((previous_summary, _)) = prev {
                                return Err(
                                    (previous_summary.package_id(), ConflictReason::Semver).into()
                                );
                            }
                        }
                    }

                    return Ok(false);
                }
            }
            debug!("checking if {} is already activated", summary.package_id());
            match &opts.features {
                // This returns `false` for CliFeatures just for simplicity. It
                // would take a bit of work to compare since they are not in the
                // same format as DepFeatures (and that may be expensive
                // performance-wise). Also, it should only occur once for a root
                // package. The only drawback is that it may re-activate a root
                // package again, which should only affect performance, but that
                // should be rare. Cycles should still be detected since those
                // will have `DepFeatures` edges.
                RequestedFeatures::CliFeatures(_) => Ok(false),
                RequestedFeatures::DepFeatures {
                    features,
                    uses_default_features,
                } => {
                    let has_default_feature = summary.features().contains_key("default");
                    Ok(match self.resolve_features.get(&id) {
                        Some(prev) => {
                            features.is_subset(prev)
                                && (!uses_default_features
                                    || prev.contains("default")
                                    || !has_default_feature)
                        }
                        None => features.is_empty() && (!uses_default_features || !has_default_feature),
                    })
                }
            }
        }

        /// If the package is active returns the `ContextAge` when it was added
        pub fn is_active(&self, id: PackageId) -> Option<ContextAge> {
            self.activations
                .get(&id.as_activations_key())
                .and_then(|(s, l)| if s.package_id() == id { Some(*l) } else { None })
        }

        /// Checks whether all of `parent` and the keys of `conflicting activations`
        /// are still active.
        /// If so returns the `ContextAge` when the newest one was added.
        pub fn is_conflicting(
            &self,
            parent: Option<PackageId>,
            conflicting_activations: &ConflictMap,
        ) -> Option<usize> {
            let mut max = 0;
            if let Some(parent) = parent {
                max = std::cmp::max(max, self.is_active(parent)?);
            }

            for id in conflicting_activations.keys() {
                max = std::cmp::max(max, self.is_active(*id)?);
            }
            Some(max)
        }

        pub fn resolve_replacements(
            &self,
            registry: &RegistryQueryer<'_>,
        ) -> HashMap<PackageId, PackageId> {
            self.activations
                .values()
                .filter_map(|(s, _)| registry.used_replacement_for(s.package_id()))
                .collect()
        }

        pub fn graph(&self) -> Graph<PackageId, std::collections::HashSet<Dependency>> {
            let mut graph: Graph<PackageId, std::collections::HashSet<Dependency>> = Graph::new();
            self.activations
                .values()
                .for_each(|(r, _)| graph.add(r.package_id()));
            for i in self.parents.iter() {
                graph.add(*i);
                for (o, e) in self.parents.edges(i) {
                    let old_link = graph.link(*o, *i);
                    assert!(old_link.is_empty());
                    *old_link = e.iter().cloned().collect();
                }
            }
            graph
        }
    }