path: root/graph/src/lib.rs

#![warn(missing_docs)]
//! This crate implements a trait API for graphs that the crate "rep"
//! needs.
//!
//! Also a default implementation for the trait is provided, so that
//! by default no external crates are needed, whereas it is easy to
//! use external crates, if so derired.

use std::hash::Hash;

pub mod error;

pub mod adset;

pub use adset::ASGraph;

pub mod adlist;

pub use adlist::ALGraph;

pub mod labelled;

pub use labelled::DLGraph;

use error::Error;

/// The expected behaviour of an immutable graph.
pub trait Graph: Default {
    /// A type that iterates through the node indices.
    type Iter<'a>: Iterator<Item = usize> + 'a
    where
        Self: 'a;

    /// Return true if and only if the graph has no nodes.
    fn is_empty(&self) -> bool;

    /// Return the length of nodes in the graph.
    fn nodes_len(&self) -> usize;

    #[inline]
    /// Return the length of edges in the graph.
    ///
    /// This is optional.  Implementations need not support this.
    fn edges_len(&self) -> Option<usize> {
        None
    }

    #[inline]
    /// Return an iterator of nodes represented as unsigned integers.
    ///
    /// If a custom application needs to have labels on nodes, just
    /// associate the label to the node internally, and define an
    /// extension trait that allows querying those additional labels.
    ///
    /// This design choice is based on the idea that this library
    /// should be minimal and only provide the core of a graph.  As a
    /// label is not really core, it is not included here.
    fn nodes(&self) -> Box<dyn Iterator<Item = usize> + '_> {
        Box::new(0..self.nodes_len())
    }

    /// Return an iterator of edges going out of a node.
    ///
    /// Return an error if the node is not known to the graph.
    fn children_of(&self, node_id: usize) -> Result<Self::Iter<'_>, Error>;

    #[inline]
    /// Return an iterator of edges represented as pairs (FROM, TO).
    ///
    /// The default implementation iterates through the nodes and then
    /// iterates through their children.  If the implementation has a
    /// more efficient method, overwrite this method.
    fn edges(&self) -> Box<dyn Iterator<Item = (usize, usize)> + '_> {
        Box::new(self.nodes().flat_map(|node| {
            self.children_of(node)
                // If this node is invalid, this means the
                // implementation of `nodes` is wrong, so it is
                // appropriate to panic here.
                .unwrap()
                .map(move |child| (node, child))
        }))
    }

    #[inline]
    /// Return true if and only if the node is in the graph.
    fn has_node(&self, node_id: usize) -> bool {
        (0..self.nodes_len()).contains(&node_id)
    }

    /// Return the number of "children" of a node, or an error if the
    /// node is not a member of the graph.
    fn degree(&self, node_id: usize) -> Result<usize, Error>;

    /// Return a boolean indicating if the node has no children, or an
    /// error if the node is not a member of the graph.
    fn is_empty_node(&self, node_id: usize) -> Result<bool, Error>;

    /// Return true if and only if there is an edge from the source to
    /// the target.
    ///
    /// Return an error if either the source or the target is an
    /// invalid node.
    fn has_edge(&self, source: usize, target: usize) -> Result<bool, Error>;
}

/// A graph that can be extended, but not mutated, in the sense that
/// existing nodes and edges will not be modified nor removed, but new
/// nodes can be added.  The index of the new node will be returned.
///
/// Implementations can choose to keep a set of sets of edges, so that
/// new nodes will not have duplicate edge sets.  In this case, the
/// returned new node index is not necessarily equal to
/// self.nodes_len() - 1, and hence the return type is designed in
/// this way.
pub trait ExtGraph: Graph {
    /// Add a new node with `edges`.
    ///
    /// If an edge from `edges` points to a non-existent node, return
    /// an error.
    fn extend(&mut self, edges: impl IntoIterator<Item = usize>) -> Result<usize, Error>;
}

/// The type of labels should be comparable and hashable.
pub trait GraphLabel: Hash + Eq + PartialEq + Clone + Copy + Ord + PartialOrd {}

impl<T: Hash + Eq + PartialEq + Clone + Copy + Ord + PartialOrd> GraphLabel for T {}

/// A labelled graph is just a graph with labels associated to
/// vertices and / or edges.
///
/// This trait defines what the package needs out of a labelled graph.
///
/// Any implementation should be able to handle a set of types for
/// labels, so this trait is generic over the label type.
pub trait LabelGraph<T: GraphLabel>: Graph {
    /// A type that iterates through the node indices.
    type Iter<'a>: Iterator<Item = usize> + 'a
    where
        Self: 'a;

    /// A type that iterates through labels.
    type LabelIter<'a>: Iterator<Item = (&'a T, <Self as LabelGraph<T>>::Iter<'a>)> + 'a
    where
        Self: 'a,
        T: 'a;

    #[inline]
    /// Return the label of a vertex or an error if the node is
    /// invalid.
    ///
    /// The default implementation always returns None for a valid
    /// node.
    fn vertex_label(&self, node_id: usize) -> Result<Option<T>, Error> {
        if self.has_node(node_id) {
            Ok(None)
        } else {
            Err(Error::IndexOutOfBounds(node_id, self.nodes_len()))
        }
    }

    #[inline]
    /// Return the label of an edge or an error if some node is
    /// invalid.
    ///
    /// The default implementation always returns an empty vector for
    /// valid nodes.
    fn edge_label(&self, source: usize, target: usize) -> Result<Vec<T>, Error> {
        self.has_edge(source, target).map(|_| Vec::new())
    }

    /// Return an iterator of edges out of a node, whose associated
    /// label is as given.
    ///
    /// The efficiency of this method matters in implementations.
    fn find_children_with_label(
        &self,
        node_id: usize,
        label: &T,
    ) -> Result<<Self as LabelGraph<T>>::Iter<'_>, Error>;

    /// Return an iterator of labels of edges out of a node.
    ///
    /// The efficiency of this method matters in implementations.
    fn labels_of(&self, node_id: usize) -> Result<Self::LabelIter<'_>, Error>;
}

/// A labelled graph that can be extended, but not mutated, in the
/// sense that existing nodes and edges will not be modified nor
/// removed, but new nodes can be added.  The index of the new node
/// will be returned.
///
/// Implementations can choose to keep a set of sets of edges, so that
/// new nodes will not have duplicate edge sets.  In this case, the
/// returned new node index is not necessarily equal to
/// self.nodes_len() - 1, and hence the return type is designed in
/// this way.
pub trait LabelExtGraph<T: GraphLabel>: LabelGraph<T> {
    /// Add a new node with `edges`.
    ///
    /// If an edge from `edges` points to a non-existent node, return
    /// an error.
    fn extend(&mut self, edges: impl IntoIterator<Item = (T, usize)>) -> Result<usize, Error>;
}