//! Perform gradual rule-based transformations on trees.

//!

//! See the [`morph`](Engine::morph) for more information.

use crate::events::EventHandlers;

use crate::helpers::{SelectorFn, one_or_select};

use crate::prelude::Rule;

use crate::rule::apply_into_update;

use paste::paste;

use uniplate::{Uniplate, tagged_zipper::TaggedZipper};

/// An engine for exhaustively transforming trees with user-defined rules.

///

/// See the [`morph`](Engine::morph) method for more information.

pub struct Engine<T, M, R>

where

    T: Uniplate,

    R: Rule<T, M>,

{

    pub(crate) event_handlers: EventHandlers<T, M>,

    /// A collection of groups of equally-prioritised rules.

    pub(crate) rule_groups: Vec<Vec<R>>,

    pub(crate) selector: SelectorFn<T, M, R>,

}

impl<T, M, R> Engine<T, M, R>

where

    T: Uniplate,

    R: Rule<T, M>,

{

    /// Exhaustively rewrites a tree using user-defined rule groups.

    ///

    /// Rewriting is complete when all rules have been attempted with no change. Rules may be organised

    /// into groups to control the order in which they are attempted.

    ///

    /// # Rule Groups

    /// If all rules are treated equally, those which apply higher in the tree will take precedence

    /// because of the left-most outer-most traversal order of the engine.

    ///

    /// This can cause problems if a rule which should ideally be applied early (e.g. evaluating

    /// constant expressions) is left until later.

    ///

    /// To solve this, rules are organised into different collections or "groups".

    /// The engine will apply rules in an earlier group to the entire tree before trying later groups.

    /// That is, no rule is attempted if a rule in an earlier group is applicable to any part of the

    /// tree.

    ///

    /// # Selector Functions

    ///

    /// If multiple rules in the same group are applicable to an expression, the user-defined

    /// selector function is used to choose one. This function is given an iterator over pairs of

    /// rules and the engine-created [`Update`] values which contain their modifications to the tree.

    ///

    /// Some useful selector functions are available in the [`helpers`](crate::helpers) module. One

    /// common function is [`select_first`](crate::helpers::select_first), which simply returns the

    /// first applicable rule.

    ///

    /// # Event Handlers

    ///

    /// The engine provides events for more fine-tuned control over rewriting behaviour. Events have mutable

    /// access to the current metadata.

    ///

    /// The engine will call the provided handlers for the "enter" and

    /// "exits" events as it enters and exits subtrees while traversing, respectively.

    ///

    /// The "enter" event is triggered first on the root, and then whenever the engine moves down

    /// into a subtree. As a result, when a node is passed to rules, all nodes above it will have

    /// been passed to handlers for this event, in ascending order of their proximity to the root.

    ///

    /// The "exit" event is triggered when the engine leaves a subtree.

    /// All nodes passed to "enter" event handlers (except the root) will also be passed to "exit"

    /// event handlers in reverse order.

    ///

    /// In effect, before a node is passed to rules, all nodes in the path from the root (including the

    /// current node) will have been passed to the "enter" event in order. No nodes are skipped.

    ///

    /// Event handling is useful when, for example, using a symbol table to keep track of variable definitions.

    /// When entering a scope where a variable is defined, one can place the variable and its value into the table.

    /// That stack can then be used for quick value lookup while inside the scope. When leaving the scope the

    /// variable can be removed from the table.

    ///

    /// # Optimizations

    ///

    /// To optimize the morph function, we use a dirty/clean approach. Since whether a rule applies

    /// is purely a function of a node and its children, rules should only be checked once unless a node

    /// or one of its children has been changed.

    ///

    /// # Example

    /// ```rust

    /// use tree_morph::{prelude::*, helpers::select_panic};

    /// use uniplate::Uniplate;

    ///

    ///

    /// // A simple language of multiplied and squared constant expressions

    /// #[derive(Debug, Clone, PartialEq, Eq, Uniplate)]

    /// #[uniplate()]

    /// enum Expr {

    ///     Val(i32),

    ///     Mul(Box<Expr>, Box<Expr>),

    ///     Sqr(Box<Expr>),

    /// }

    ///

    /// // a * b ~> (value of) a * b, where 'a' and 'b' are literal values

    /// fn rule_eval_mul(cmds: &mut Commands<Expr, i32>, subtree: &Expr, meta: &i32) -> Option<Expr> {

    ///     cmds.mut_meta(Box::new(|m: &mut i32| *m += 1));

    ///

    ///     if let Expr::Mul(a, b) = subtree {

    ///         if let (Expr::Val(a_v), Expr::Val(b_v)) = (a.as_ref(), b.as_ref()) {

    ///             return Some(Expr::Val(a_v * b_v));

    ///         }

    ///     }

    ///     None

    /// }

    ///

    /// // e ^ 2 ~> e * e, where e is an expression

    /// // If this rule is applied before the sub-expression is fully evaluated, duplicate work

    /// // will be done on the resulting two identical sub-expressions.

    /// fn rule_expand_sqr(cmds: &mut Commands<Expr, i32>, subtree: &Expr, meta: &i32) -> Option<Expr> {

    ///     cmds.mut_meta(Box::new(|m: &mut i32| *m += 1));

    ///

    ///     if let Expr::Sqr(expr) = subtree {

    ///         return Some(Expr::Mul(

    ///             Box::new(*expr.clone()),

    ///             Box::new(*expr.clone())

    ///         ));

    ///     }

    ///     None

    /// }

    ///

    /// // (1 * 2) ^ 2

    /// let expr = Expr::Sqr(

    ///     Box::new(Expr::Mul(

    ///         Box::new(Expr::Val(1)),

    ///         Box::new(Expr::Val(2))

    ///     ))

    /// );

    ///

    /// // Try with both rules in the same group, keeping track of the number of rule applications

    /// let engine = EngineBuilder::new()

    ///     .set_selector(select_panic)

    ///     .add_rule_group(rule_fns![rule_eval_mul, rule_expand_sqr])

    ///     .build();

    /// let (result, num_applications) = engine.morph(expr.clone(), 0);

    /// assert_eq!(result, Expr::Val(4));

    /// assert_eq!(num_applications, 4); // The `Sqr` is expanded first, causing duplicate work

    ///

    /// // Move the evaluation rule to an earlier group

    /// let engine = EngineBuilder::new()

    ///     .set_selector(select_panic)

    ///     .add_rule_group(rule_fns![rule_eval_mul])

    ///     .add_rule_group(rule_fns![rule_expand_sqr])

    ///     .build();

    /// let (result, num_applications) = engine.morph(expr.clone(), 0);

    /// assert_eq!(result, Expr::Val(4));

    /// assert_eq!(num_applications, 3); // Now the sub-expression (1 * 2) is evaluated first

    /// ```

    {

        // Owns the tree/meta and is consumed to get them back at the end

        'main: loop {

            // Return here after every successful rule application

                // Try each rule group in the whole tree

                    // Choose one transformation from all applicable rules at this level

                    };

                        // Replace the current subtree, invalidating subtree node states

                        // Mark all ancestors as dirty and move back to the root

                }

            }

            // All rules have been tried with no more changes

        }

}

#[derive(Debug, Clone)]

struct EngineNodeState {

    /// Rule groups with lower indices have already been applied without change.

    /// For a level `n`, a state is 'dirty' if and only if `n >= dirty_from`.

    dirty_from: usize,

}

impl EngineNodeState {

    /// Marks the state as dirty for anything >= `level`.

    /// For a level `n`, a state is "dirty" if and only if `n >= dirty_from`.

    /// That is, all rules groups before `n` have been applied without change.

}

impl EngineNodeState {

}

macro_rules! movement_fns {

    (

        directions: [$($dir:ident),*]

    ) => {

        paste! {

        }

    };

}

/// A Zipper with optimisations for tree transformation.

struct EngineZipper<'events, T: Uniplate, M> {

    inner: TaggedZipper<T, EngineNodeState, fn(&T) -> EngineNodeState>,

    event_handlers: &'events EventHandlers<T, M>,

    meta: M,

}

impl<'events, T: Uniplate, M> EngineZipper<'events, T, M> {

    /// Go to the next node in the tree which is dirty for the given level.

    /// That node may be the current one if it is dirty.

    /// If no such node exists, go to the root and return `None`.

                // go right until we find a dirty child, if it exists.

                loop {

                        // all children are clean

                }

                // Neither this node, nor any of its children are dirty

                // Go right then up until we find a dirty node or reach the root

                loop {

                        // Reached the root without finding a dirty node

                }

    // We never move left in the tree

    movement_fns! { directions: [up, down, right] }

    /// Mark the current focus as visited at the given level.

    /// Calling `go_next_dirty` with the same level will no longer yield this node.

    /// Mark ancestors as dirty for all levels, and return to the root

}

impl<T: Uniplate, M> From<EngineZipper<'_, T, M>> for (T, M) {

}