use std::fmt::Debug;

use mappable_rc::Mrc;

use crate::ast::{Expr, Clause};
use crate::unwrap_or;
use crate::utils::iter::box_empty;
use crate::utils::{Side, Cache, mrc_derive, mrc_try_derive, to_mrc_slice};

use super::State;
use super::split_at_max_vec::split_at_max_vec;

/// Tuple with custom cloning logic
#[derive(Debug, Eq, PartialEq, Hash)]
pub struct CacheEntry<'a>(Mrc<[Expr]>, &'a SliceMatcherDnC);
impl<'a> Clone for CacheEntry<'a> {
  fn clone(&self) -> Self {
    let CacheEntry(mrc, matcher) = self;
    CacheEntry(Mrc::clone(mrc), matcher)
  }
}


/// Matcher that applies a pattern to a slice via divide-and-conquer
/// 
/// Upon construction, it selects the clause of highest priority, then
/// initializes its internal state for matching that clause and delegates
/// the left and right halves of the pattern to two submatchers.
/// 
/// Upon matching, it uses a cache to accelerate the process of executing
/// a pattern on the entire tree.
#[derive(Clone, Eq)]
pub struct SliceMatcherDnC {
  /// The entire pattern this will match
  pattern: Mrc<[Expr]>,
  /// The exact clause this can match
  clause: Mrc<Clause>,
  /// Matcher for the parts of the pattern right from us
  right_subm: Option<Box<SliceMatcherDnC>>,
  /// Matcher for the parts of the pattern left from us
  left_subm: Option<Box<SliceMatcherDnC>>,
  /// Matcher for the body of this clause if it has one.
  /// Must be Some if pattern is (Auto, Lambda or S)
  body_subm: Option<Box<SliceMatcherDnC>>,
  /// Matcher for the type of this expression if it has one (Auto usually does)
  /// Optional
  typ_subm: Option<Box<SliceMatcherDnC>>,
}

impl PartialEq for SliceMatcherDnC {
  fn eq(&self, other: &Self) -> bool {
    self.pattern == other.pattern
  }
}

impl std::hash::Hash for SliceMatcherDnC {
  fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
    self.pattern.hash(state);
  }
}

impl SliceMatcherDnC {
  /// If this is true, `clause`, `typ_subm`, `body_subm` and `clause_qual_name` are meaningless.
  /// If it's false, it's also false for both side matchers.
  pub fn clause_is_vectorial(&self) -> bool {
    matches!(self.clause.as_ref(), Clause::Placeh{vec: Some(..), ..})
  }
  /// If clause is a name, the qualified name this can match
  pub fn clause_qual_name(&self) -> Option<Mrc<[String]>> {
    if let Clause::Name { qualified, .. } = self.clause.as_ref() {Some(Mrc::clone(qualified))} else {None}
  }
  /// If clause is a Placeh, the key in the state the match will be stored at
  pub fn state_key(&self) -> Option<&String> {
    if let Clause::Placeh { key, .. } = self.clause.as_ref() {Some(key)} else {None}
  }
  pub fn own_max_size(&self, total: usize) -> Option<usize> {
    if !self.clause_is_vectorial() {
      if total == self.len() {Some(total)} else {None}
    } else {
      let margin = self.min(Side::Left) + self.min(Side::Right);
      if margin + self.own_min_size() <= total {Some(total - margin)} else {None}
    }
  }
  pub fn own_min_size(&self) -> usize {
    if let Clause::Placeh { vec: Some((_, nonzero)), .. } = self.clause.as_ref() {
      if *nonzero {1} else {0}
    } else {self.len()}
  }
  
  /// Enumerate all valid subdivisions based on the reported size constraints of self and
  /// the two subranges
  pub fn valid_subdivisions(&self,
    range: Mrc<[Expr]>
  ) -> impl Iterator<Item = (Mrc<[Expr]>, Mrc<[Expr]>, Mrc<[Expr]>)> {
    let own_max = unwrap_or!(self.own_max_size(range.len()); return box_empty());
    let own_min = self.own_min_size();
    let lmin = self.min(Side::Left);
    let _lmax = self.max(Side::Left, range.len());
    let rmin = self.min(Side::Right);
    let _rmax = self.max(Side::Right, range.len());
    let full_len = range.len();
    Box::new((own_min..=own_max).rev().flat_map(move |own_len| {
      let wiggle = full_len - lmin - rmin - own_len;
      let range = Mrc::clone(&range);
      (0..=wiggle).map(move |offset| {
        let first_break = lmin + offset;
        let second_break = first_break + own_len;
        let left = mrc_derive(&range, |p| &p[0..first_break]);
        let mid = mrc_derive(&range, |p| &p[first_break..second_break]);
        let right = mrc_derive(&range, |p| &p[second_break..]);
        (left, mid, right)
      })
    }))
  }

  pub fn new(pattern: Mrc<[Expr]>) -> Self {
    let (clause, left_subm, right_subm) = mrc_try_derive(&pattern, |p| {
      if p.len() == 1 {Some(&p[0].0)} else {None}
    }).map(|e| (e, None, None))
    .or_else(|| split_at_max_vec(Mrc::clone(&pattern)).map(|(left, _, right)| (
      mrc_derive(&pattern, |p| &p[left.len()].0),
      if !left.is_empty() {Some(Box::new(Self::new(left)))} else {None},
      if !right.is_empty() {Some(Box::new(Self::new(right)))} else {None}
    )))
    .unwrap_or_else(|| (
      mrc_derive(&pattern, |p| &p[0].0),
      None,
      Some(Box::new(Self::new(mrc_derive(&pattern, |p| &p[1..]))))
    ));
    Self {
      pattern, right_subm, left_subm,
      clause: Mrc::clone(&clause),
      body_subm: clause.body().map(|b| Box::new(Self::new(b))),
      typ_subm: clause.typ().map(|t| Box::new(Self::new(t)))
    }
  }

  /// The shortest slice this pattern can match
  fn len(&self) -> usize {
    if self.clause_is_vectorial() {
      self.min(Side::Left) + self.min(Side::Right) + self.own_min_size()
    } else {self.pattern.len()}
  }
  /// Pick a subpattern based on the parameter
  fn side(&self, side: Side) -> Option<&SliceMatcherDnC> {
    match side {
      Side::Left => &self.left_subm,
      Side::Right => &self.right_subm
    }.as_ref().map(|b| b.as_ref())
  }
  /// The shortest slice the given side can match
  fn min(&self, side: Side) -> usize {self.side(side).map_or(0, |right| right.len())}
  /// The longest slice the given side can match
  fn max(&self, side: Side, total: usize) -> usize {
    self.side(side).map_or(0, |m| if m.clause_is_vectorial() {
      total - self.min(side.opposite()) - self.own_min_size()
    } else {m.len()})
  }
  /// Take the smallest possible slice from the given side
  fn slice_min<'a>(&self, side: Side, range: &'a [Expr]) -> &'a [Expr] {
    side.slice(self.min(side), range)
  }

  /// Matches the body on a range
  /// # Panics
  /// when called on an instance that does not have a body (not Auto, Lambda or S)
  fn match_body<'a>(&'a self,
    range: Mrc<[Expr]>, cache: &Cache<CacheEntry<'a>, Option<State>>
  ) -> Option<State> {
    self.body_subm.as_ref()
      .expect("Missing body matcher")
      .match_range_cached(range, cache)
  }
  /// Matches the type and body on respective ranges
  /// # Panics
  /// when called on an instance that does not have a body (not Auto, Lambda or S)
  fn match_parts<'a>(&'a self,
    typ_range: Mrc<[Expr]>, body_range: Mrc<[Expr]>,
    cache: &Cache<CacheEntry<'a>, Option<State>>
  ) -> Option<State> {
    let typ_state = if let Some(typ) = &self.typ_subm {
      typ.match_range_cached(typ_range, cache)?
    } else {State::new()};
    let body_state = self.match_body(body_range, cache)?;
    typ_state + body_state
  }

  /// Match the specified side-submatcher on the specified range with the cache
  /// In absence of a side-submatcher empty ranges are matched to empty state
  fn apply_side_with_cache<'a>(&'a self,
    side: Side, range: Mrc<[Expr]>,
    cache: &Cache<CacheEntry<'a>, Option<State>>
  ) -> Option<State> {
    match &self.side(side) {
      None => {
        if !range.is_empty() {None}
        else {Some(State::new())}
      },
      Some(m) => cache.try_find(&CacheEntry(range, m)).map(|s| s.as_ref().to_owned())
    }
  }

  fn match_range_scalar_cached<'a>(&'a self,
    target: Mrc<[Expr]>,
    cache: &Cache<CacheEntry<'a>, Option<State>>
  ) -> Option<State> {
    let pos = self.min(Side::Left);
    if target.len() != self.pattern.len() {return None}
    let mut own_state = (
      self.apply_side_with_cache(Side::Left, mrc_derive(&target, |t| &t[0..pos]), cache)?
      + self.apply_side_with_cache(Side::Right, mrc_derive(&target, |t| &t[pos+1..]), cache)
    )?;
    match (self.clause.as_ref(), &target.as_ref()[pos].0) {
      (Clause::Literal(val), Clause::Literal(tgt)) => {
        if val == tgt {Some(own_state)} else {None}
      }
      (Clause::Placeh{key, vec: None}, tgt_clause) => {
        if let Some(real_key) = key.strip_prefix('_') {
          if let Clause::Name { local: Some(value), .. } = tgt_clause {
            own_state.insert_name(real_key, value)
          } else {None}
        } else {own_state.insert_scalar(&key, &target[pos])}
      }
      (Clause::S(c, _), Clause::S(c_tgt, body_range)) => {
        if c != c_tgt {return None}
        own_state + self.match_parts(to_mrc_slice(vec![]), Mrc::clone(body_range), cache)
      }
      (Clause::Name{qualified, ..}, Clause::Name{qualified: q_tgt, ..}) => {
        if qualified == q_tgt {Some(own_state)} else {None}
      }
      (Clause::Lambda(name, _, _), Clause::Lambda(name_tgt, typ_tgt, body_tgt)) => {
        // Primarily, the name works as a placeholder
        if let Some(state_key) = name.strip_prefix('$') {
          own_state = own_state.insert_name(state_key, name_tgt)?
        } else if name != name_tgt {return None}
        // ^ But if you're weird like that, it can also work as a constraint
        own_state + self.match_parts(Mrc::clone(typ_tgt), Mrc::clone(body_tgt), cache)
      }
      (Clause::Auto(name_opt, _, _), Clause::Auto(name_range, typ_range, body_range)) => {
        if let Some(name) = name_opt {
          // TODO: Enforce this at construction, on a type system level
          let state_key = name.strip_prefix('$')
            .expect("Auto patterns may only reference, never enforce the name");
          own_state = own_state.insert_name_opt(state_key, name_range.as_ref())?
        }
        own_state + self.match_parts(Mrc::clone(typ_range), Mrc::clone(body_range), cache)
      },
      _ => None
    }
  }

  /// Match the range with a vectorial _assuming we are a vectorial_
  fn match_range_vectorial_cached<'a>(&'a self,
    name: &str,
    target: Mrc<[Expr]>,
    cache: &Cache<CacheEntry<'a>, Option<State>>
  ) -> Option<State> {
    // Step through valid slicings based on reported size constraints in order
    // from longest own section to shortest and from left to right
    for (left, own, right) in self.valid_subdivisions(target) {
      return Some(unwrap_or!(
        self.apply_side_with_cache(Side::Left, left, cache)
        .and_then(|lres| lres + self.apply_side_with_cache(Side::Right, right, cache))
        .and_then(|side_res| side_res.insert_vec(name, own.as_ref()));
        continue
      ))
    }
    None
  }

  /// Try and match the specified range
  pub fn match_range_cached<'a>(&'a self,
    target: Mrc<[Expr]>,
    cache: &Cache<CacheEntry<'a>, Option<State>>
  ) -> Option<State> {
    if self.pattern.is_empty() {
      return if target.is_empty() {Some(State::new())} else {None}
    }
    if self.clause_is_vectorial() {
      let key = self.state_key().expect("Vectorial implies key");
      self.match_range_vectorial_cached(key, target, cache)
    } else {self.match_range_scalar_cached(target, cache)}
  }

  pub fn get_matcher_cache<'a>()
  -> Cache<'a, CacheEntry<'a>, Option<State>> {
    Cache::new(
      |CacheEntry(tgt, matcher), cache| {
        matcher.match_range_cached(tgt, cache)
      }
    )
  }

  pub fn match_range(&self, target: Mrc<[Expr]>) -> Option<State> {
    self.match_range_cached(target, &Self::get_matcher_cache())
  }
}

impl Debug for SliceMatcherDnC {
  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    f.debug_struct("Matcher")
      .field("clause", &self.clause)
      .field("vectorial", &self.clause_is_vectorial())
      .field("min", &self.len())
      .field("left", &self.left_subm)
      .field("right", &self.right_subm)
      .field("lmin", &self.min(Side::Left))
      .field("rmin", &self.min(Side::Right))
      .finish()
  }
}