#![warn(missing_docs)]
//! This module implements the type of items for the chain-rule
//! machine.
//!
//! More specifically, it implements the iten derivation forests for
//! the machine.

// TODO: Implement an enumeration for Parent or Segment, perhaps
// called PaSe.

// TODO: Move functions for handling forests, currently contained
// within the method derive to this module, and make them aware of the
// enumeration PaSe.

// TODO: Implement the Item trait from semirings for the item
// derivation forest, and then we can easily execute items later on.

use graph::{error::Error as GError, GraphLabel, LabelGraph, Parent, ParentsGraph};

use core::borrow::Borrow;

/// A parent or a segment.
///
/// A parent is a node with an edge index, which represents a certain
/// edge.
///
/// A segment represents every edge from the root node to the single
/// terminating node.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub enum PaSe {
    /// An edge from a node, as the n-th child.
    Parent(usize, usize),
    /// A segment from a root to the single terminating node.
    Segment(usize, usize),
}

impl From<Parent> for PaSe {
    fn from(value: Parent) -> Self {
        Self::Parent(value.node(), value.edge())
    }
}

impl Default for PaSe {
    fn default() -> Self {
        Self::Segment(0, 0)
    }
}

impl core::fmt::Display for PaSe {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Parent(node, edge) => write!(f, "the {edge}-th edge from {node}"),
            Self::Segment(root, leaf) => write!(f, "a segment from {root} to {leaf}"),
        }
    }
}

impl PaSe {
    fn parent(self) -> Option<Parent> {
        if let Self::Parent(node, edge) = self {
            Some(Parent::new(node, edge))
        } else {
            None
        }
    }

    fn segment(self) -> Option<(usize, usize)> {
        if let Self::Segment(root, leaf) = self {
            Some((root, leaf))
        } else {
            None
        }
    }
}

/// An internal type that indicates the status of a node as either a
/// packed, cloned, or plain node.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Default)]
enum ForestLabelType {
    Packed,
    #[default]
    Plain,
    Cloned(usize),
}

impl core::fmt::Display for ForestLabelType {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            Self::Packed => write!(f, "packed"),
            Self::Plain => write!(f, "plain"),
            Self::Cloned(index) => write!(f, "the {index}-th clone"),
        }
    }
}

/// A type that encodes the properties demanded by a forest.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub struct ForestLabel<T: GraphLabel> {
    label: T,
    status: ForestLabelType,
}

impl<T: GraphLabel> core::fmt::Display for ForestLabel<T> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        writeln!(f, "label: {}, status: {}", self.label, self.status)
    }
}

/// The type of erros for converting forest labels.
#[non_exhaustive]
#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
pub enum ForestLabelError {
    /// Try to pack a cloned node.
    PackClone,
    /// Try to clone a packed node.
    ClonePack,
}

impl core::fmt::Display for ForestLabelError {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            Self::PackClone => write!(f, "cannot pack a cloned node"),
            Self::ClonePack => write!(f, "cannot clone a packed node"),
        }
    }
}

impl std::error::Error for ForestLabelError {}

impl<T: GraphLabel> ForestLabel<T> {
    /// Retrieve the label.
    pub fn label(&self) -> T {
        self.label
    }

    /// Return true if and only if this node is packed.
    pub fn is_packed(&self) -> bool {
        matches!(self.status, ForestLabelType::Packed)
    }

    /// Retrieve the optional clone index.
    pub fn clone_index(&self) -> Option<usize> {
        match self.status {
            ForestLabelType::Cloned(index) => Some(index),
            _ => None,
        }
    }

    /// Try to clone a node.
    pub fn clone<F>(self, forest: &F) -> Result<Self, ForestLabelError>
    where
        F: Forest<T>,
    {
        if self.is_packed() {
            Err(ForestLabelError::ClonePack)
        } else {
            let clone_index = if let Some(old_index) = self.clone_index() {
                let mut new_index: usize = old_index + 1;

                let mut new_label = Self {
                    status: ForestLabelType::Cloned(new_index),
                    ..self
                };

                while forest.query_label(new_label).is_some() {
                    new_index += 1;
                    new_label = Self {
                        status: ForestLabelType::Cloned(new_index),
                        ..self
                    };
                }

                new_index
            } else {
                0
            };

            Ok(Self {
                status: ForestLabelType::Cloned(clone_index),
                ..self
            })
        }
    }

    /// Try to pack a node.
    pub fn pack(self) -> Result<Self, ForestLabelError> {
        if self.clone_index().is_some() {
            Err(ForestLabelError::PackClone)
        } else {
            let new_label = Self {
                status: ForestLabelType::Packed,
                ..self
            };

            Ok(new_label)
        }
    }
}

impl<T: GraphLabel> From<T> for ForestLabel<T> {
    fn from(label: T) -> Self {
        let status = ForestLabelType::default();
        Self { label, status }
    }
}

/// The expected behaviours of an item derivation forest.
///
/// Note that it requires only a subset of the functionalities of
/// labelled graphs.
pub trait Forest<T: GraphLabel>: ParentsGraph + LabelGraph<ForestLabel<T>> {
    /// The type of errors for operations on the forest.
    type Error: std::error::Error + From<GError> + From<ForestLabelError>;

    /// Return the root of the forest.
    ///
    /// A forest without a root is regarded as empty.
    fn root(&self) -> Option<usize>;

    /// Construct a forest consisting of one leaf node with the given
    /// label.
    fn new_leaf(label: T) -> Self;

    /// Detect if the fragment is a prefix of the sub-forest rooted at
    /// `node_id`.
    fn is_prefix<F>(&self, node_id: usize, fragment: F) -> Result<bool, Self::Error>
    where
        F: Borrow<Self>;

    /// Extend the forest by adjoining another forest at a given node.
    fn plant<F>(&mut self, node_id: usize, fragment: F, planted: bool) -> Result<(), Self::Error>
    where
        F: Borrow<Self>;

    /// Clone a node by making a new node and making all the nodes
    /// that previously pointed to the old node now point to the new
    /// node, and the new node points to the old node.  Return the
    /// index of the new node.  In addition, if the old node had
    /// already been cloned, just return the index of its
    /// representative.
    ///
    /// The labels of the representing node and of the cloned node
    /// will be handled automatically.
    fn clone_node(&mut self, node_id: usize) -> Result<usize, Self::Error>;
}

pub mod default;

pub mod genins;

pub use genins::generate_fragment;