use crate::*;

use log::{info, error};

type PushFn = fn(&mut String, &SourceFile);


/// Resolve undeclared symbols in a source unit with definitions from other units.
pub struct Resolver {
    /// Definitions from all included source units.
    pub definitions: Vec<TrackedSymbol>,
    /// All resolved references in all included source units.
    pub resolved: Vec<TrackedSymbol>,
    /// All unresolved references in all included source units.
    pub unresolved: Vec<TrackedSymbol>,
    /// All redefined (duplicate) definitions in all included source units.
    /// Points to the 'definitions' index of the existing definition.
    pub redefinitions: Vec<(TrackedSymbol, usize)>,
    /// All included source units.
    pub source_units: Vec<HeirarchicalSourceUnit>,
    /// The 'source_units' indices of the root source units.
    pub root_unit_ids: Vec<usize>,
    /// Source units that can be included later to resolve symbols.
    pub library_source_units: Vec<SourceUnit>,
}


impl Resolver {
    pub fn new(source_unit: SourceUnit) -> Self {
        let mut new = Self {
            definitions: Vec::new(),
            resolved: Vec::new(),
            unresolved: Vec::new(),
            redefinitions: Vec::new(),
            source_units: Vec::new(),
            root_unit_ids: Vec::new(),
            library_source_units: Vec::new(),
        };
        new.include_source_unit(source_unit, None);
        return new;
    }

    pub fn include_source_unit(&mut self, mut source_unit: SourceUnit, parent_id: Option<usize>) {
        let source_id = self.source_units.len();
        use std::mem::take;

        info!("Including source file at {:?}", source_unit.main.path);
        self.include_symbols(take(&mut source_unit.main.symbols), source_id, SourceRole::Main);
        if let Some(head) = &mut source_unit.head {
            self.include_symbols(take(&mut head.symbols), source_id, SourceRole::Head); }
        if let Some(tail) = &mut source_unit.tail {
            self.include_symbols(take(&mut tail.symbols), source_id, SourceRole::Tail); }

        match parent_id {
            Some(parent_id) => match self.source_units.get_mut(parent_id) {
                Some(parent) => parent.child_ids.push(source_id),
                None => error!("Could not find parent (#{parent_id}) of source unit #{source_id}"),
            }
            None => self.root_unit_ids.push(source_id),
        }
        self.source_units.push(
            HeirarchicalSourceUnit {
                source_unit,
                child_ids: Vec::new(),
                parent_ids: Vec::new(),
            }
        );
    }

    fn include_symbols(&mut self, symbols: Vec<Symbol>, source_id: usize, source_role: SourceRole) {
        for symbol in symbols {
            match symbol.role {
                SymbolRole::Definition(_) => {
                    // Check if the symbol has already been defined.
                    let equal = |d: &TrackedSymbol| { &d.symbol == &symbol };
                    if let Some(original) = self.definitions.iter().position(equal) {
                        let definition = TrackedSymbol { symbol, source_id, source_role };
                        let redefinition = (definition, original);
                        self.redefinitions.push(redefinition);
                    } else {
                        // Resolve all unresolved references that match this symbol.
                        let defines = |r: &mut TrackedSymbol| symbol.defines(&r.symbol);
                        for symbol in self.unresolved.extract_if(defines) {
                            self.resolved.push(symbol);
                        }
                        let definition = TrackedSymbol { symbol, source_id, source_role };
                        self.definitions.push(definition);
                    }
                }
                SymbolRole::Reference => {
                    let reference = TrackedSymbol { symbol, source_id, source_role };
                    match self.definitions.iter().any(|d| d.symbol.defines(&reference.symbol)) {
                        true => self.resolved.push(reference),
                        false => self.unresolved.push(reference),
                    }
                }
            }
        }
    }

    /// Add a set of source units that might contain definitions for unresolved symbols.
    pub fn add_library_source_units(&mut self, mut source_units: Vec<SourceUnit>) {
        self.library_source_units.append(&mut source_units);
    }

    /// Attempt to resolve unresolved references with library source units.
    pub fn resolve(&mut self) {
        // Repeatedly test if each library source unit resolves an unresolved
        // symbol, breaking the loop when no new resolutions are found.
        'outer: loop {
            for (i, source_unit) in self.library_source_units.iter().enumerate() {
                if let Some(id) = self.unit_resolved_by_unit(&source_unit) {
                    let source_unit = self.library_source_units.remove(i);
                    self.include_source_unit(source_unit, Some(id));
                    continue 'outer;
                }
            }
            break;
        }
    }

    /// Returns true if all references in all included source units have been
    /// resolved.
    pub fn error(&self) -> Option<ResolverError> {
        match self.unresolved.is_empty() {
            true => None,
            false => Some(ResolverError { resolver: self })
        }
    }

    /// Return a type that can print the structure of the source tree.
    pub fn hierarchy(&self) -> SourceHierarchy {
        SourceHierarchy { resolver: self }
    }

    /// Return the ID of a source unit that contains an unresolved reference
    /// to a symbol defined by this unit.
    fn unit_resolved_by_unit(&self, source_unit: &SourceUnit) -> Option<usize> {
        if let Some(id) = self.unit_resolved_by_symbol(&source_unit.main.symbols) {
            return Some(id);
        }
        if let Some(head) = &source_unit.head {
            if let Some(id) = self.unit_resolved_by_symbol(&head.symbols) {
                return Some(id);
            }
        }
        if let Some(tail) = &source_unit.tail {
            if let Some(id) = self.unit_resolved_by_symbol(&tail.symbols) {
                return Some(id);
            }
        }
        return None;
    }

    /// Returns the ID of a source unit that contains an unresolved reference
    /// to a symbol defined by one of these symbols.
    fn unit_resolved_by_symbol(&self, symbols: &[Symbol]) -> Option<usize> {
        for symbol in symbols {
            if let SymbolRole::Definition(_) = symbol.role {
                for unresolved in &self.unresolved {
                    if unresolved.symbol.name == symbol.name {
                        return Some(unresolved.source_id);
                    }
                }
            }
        }
        return None;
    }

    /// Populate the .parent_ids field of every source unit. The parents of
    /// each source unit are the units that define a symbol referenced by the
    /// unit, where the definition type is MustPrecedeReference.
    pub fn calculate_hierarchy(&mut self) {
        // Clear the .parent_ids field of every source unit.
        for source_unit in &mut self.source_units {
            source_unit.parent_ids.clear();
        }
        // Populate the .parent_ids field of every source unit.
        for reference in &self.resolved {
            let predicate = |d: &&TrackedSymbol| d.symbol.name == reference.symbol.name;
            if let Some(definition) = self.definitions.iter().find(predicate) {
                // A unit cannot be its own parent.
                let is_self = reference.source_id == definition.source_id;
                let must_precede = SymbolRole::Definition(DefinitionType::MustPrecedeReference);
                if  is_self || definition.symbol.role != must_precede { continue; }
                let referencing_unit = &mut self.source_units[reference.source_id];
                referencing_unit.parent_ids.push(definition.source_id);
            };
        }
    }

    /// Concatenate all included source units into one string.
    /// If the source unit dependency graph contains a cycle, the IDs of the
    /// source units involved in the cycle will be returned.
    pub fn get_merged_source_code(&self, push: PushFn) -> Result<String, MergeError> {
        // The ID of each source unit will come after the IDs of all
        // parents of that unit.
        let head_order = {
            let mut included_source_ids = Vec::new();
            let mut remaining_source_ids = (0..self.source_units.len()).collect::<Vec<_>>();

            'restart: while !remaining_source_ids.is_empty() {
                // Iterate over source units, most-recently-included first.
                'next: for (i, id) in remaining_source_ids.iter().enumerate() {
                    let unit = &self.source_units[*id];
                    for parent_id in &unit.parent_ids {
                        // Skip this unit if a parent hasn't yet been included.
                        if !included_source_ids.contains(parent_id) {
                            continue 'next;
                        }
                    }
                    // Include this unit, then check remaining units from the start.
                    included_source_ids.push(*id);
                    remaining_source_ids.remove(i);
                    continue 'restart;
                }
                // All remaining source units depend on at least one remaining
                // source unit, indicating a dependency cycle.
                return Err(MergeError {
                    resolver: self,
                    cyclic_unit_ids: remaining_source_ids,
                });
            }
            included_source_ids
        };
        let mut source_code = String::new();

        // Push head source code in calculated parent-preceding order.
        for id in &head_order {
            let source_unit = &self.source_units[*id];
            if let Some(head) = &source_unit.source_unit.head {
                push(&mut source_code, head);
            }
        }
        // Push main source code in source-added order.
        // The root unit will be pushed first.
        for source_unit in self.source_units.iter() {
            let main = &source_unit.source_unit.main;
            push(&mut source_code, &main);
        }
        // Push tail source code in reverse source-added order.
        // The root unit will be pushed last.
        for source_unit in self.source_units.iter().rev() {
            if let Some(tail) = &source_unit.source_unit.tail {
                push(&mut source_code, tail);
            }
        }
        return Ok(source_code);
    }
}


/// A source unit tracked with pointers to parents and dependents.
pub struct HeirarchicalSourceUnit {
    pub source_unit: SourceUnit,
    /// Pointers to source units that resolve references this unit.
    pub child_ids: Vec<usize>,
    /// Pointers to source units that must be included before this unit.
    pub parent_ids: Vec<usize>,
}

pub struct TrackedSymbol {
    pub symbol: Symbol,
    pub source_id: usize,
    pub source_role: SourceRole,
}

impl TrackedSymbol {
    pub fn context<'a>(&'a self, resolver: &'a Resolver) -> Context<'a> {
        let source_unit = &resolver.source_units[self.source_id].source_unit;
        let source_code = match self.source_role {
            SourceRole::Main => source_unit.main.source_code.as_str(),
            SourceRole::Head => match &source_unit.head {
                Some(head) => head.source_code.as_str(),
                None => unreachable!("Failed to find source code of head file"),
            }
            SourceRole::Tail => match &source_unit.tail {
                Some(tail) => tail.source_code.as_str(),
                None => unreachable!("Failed to find source code of tail file"),
            }
        };
        Context { source_code, source: &self.symbol.source }
    }
}

impl PartialEq for TrackedSymbol {
    fn eq(&self, other: &TrackedSymbol) -> bool {
        self.symbol.name.eq(&other.symbol.name)
    }
}

#[derive(Clone, Copy, Debug)]
pub enum SourceRole {
    Main,
    Head,
    Tail,
}