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

//!

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

use crate::cache::{CacheResult, RewriteCache};

use crate::engine_zipper::{EngineZipper, NaiveZipper};

use crate::events::EventHandlers;

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

use crate::prelude::Rule;

use crate::rule::{RuleGroups, RuleSet, apply_into_update};

use crate::update::Update;

use rayon::prelude::*;

use tracing::{debug, error, info, instrument, trace};

use uniplate::Uniplate;

/// 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, C>

where

    T: Uniplate + Send + Sync,

    R: Rule<T, M> + Clone,

    C: RewriteCache<T>,

{

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

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

    pub(crate) rule_groups: RuleGroups<T, M, R>,

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

    pub(crate) cache: C,

    /// Whether to use parallel (Rayon) iteration when checking rules.

    pub(crate) parallel: bool,

    pub(crate) fixedpoint: bool,

    pub(crate) down_predicate: fn(&T) -> bool,

}

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

where

    T: Uniplate + Send + Sync,

    R: Rule<T, M> + Clone + Sync,

    M: Sync,

    C: RewriteCache<T>,

{

    #[instrument(skip(selector, parallel, subtree, meta, rules, event_handlers))]

        } else {

        }

        debug!("Applying Rule '{}'", rule.name());

        // Clear any stale cached hashes in the replacement subtree

        // Replace the current subtree, invalidating subtree node states

        // Mark all ancestors as dirty, insert ancestor mappings, and move back to the root

            debug!("Root transformed, clearing state.");

            // This must unfortunately throw all node states away,

            // since the `transform` command may redefine the whole tree

            }

        // Apply the initial rule

        debug!("Applying Rules in Fixed Point Mode");

        // No transform — apply locally

            }

                    debug!("Applying Rule '{}' till Fixed Point (local)", rule.name());

                        }

                }

            }

        }

        // Node stabilized — mark pass-through and go to root

        debug!("Applying Rule '{}' in Fast Mode (naive)", rule.name());

        // No transform — apply locally (no root traversal)

            }

        // Try rules from level 0 up to current level

                    debug!(

                        "Applying Rule '{}' in Fast Mode (naive, local)",

                    );

                        }

                }

            }

        }

        // Node stabilized — need to go to root and rebuild ancestors

        debug!("Applying Rule '{}'", rule.name());

            }

    /// 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 mut 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 mut 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

    /// ```

    // #[instrument(skip(self, tree, meta))]

    {

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

        );

        'main: loop {

            // Return here after every successful rule application

                    {

                        debug!("Checking Level {} with {} Rules", level, rules.len());

                                debug!(

                                    "Cache Hit - Nothing Applicable (clean through level {})",

                                    clean_level

                                );

                            }

                                debug!("Cache Hit");

                            }

                        };

                    }

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

                        }

                        None => {

                        }

                    }

                }

            }

            // All rules have been tried with no more changes

        }

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

    ///

    /// It is not recommended to use this function besides testing and for correctness.

    {

        );

        'main: loop {

                loop {

                    {

                                debug!("Cache Hit - Nothing Applicable");

                            }

                                debug!("Cache Hit");

                            }

                        };

                    }

                    debug!("Checking Level {} with {} Rules", level, rules.len());

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

                    );

                            );

                            // Stabilized — continue 'main to restart from root

                    } else {

                        debug!("Nothing Applicable");

                    }

                }

            }

        }

}