use std::{
    collections::HashSet,
    hash::{DefaultHasher, Hash, Hasher},
};

pub trait Stage : Clone + std::fmt::Debug + std::hash::Hash + Eq {
    type Consume: Clone + std::fmt::Debug + std::hash::Hash + Eq;
}

#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
pub struct Epsilon;
impl Stage for Epsilon {
    type Consume = Option<ByteRange>;
}

#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
pub struct Resolved;
impl Stage for Resolved {
    type Consume = ByteRange;
}

use super::Pattern;
use super::byte_range::ByteRange;

/// NFA with epsilon transitions
#[derive(Clone)]
#[allow(clippy::upper_case_acronyms)]
pub struct ENFA<S: Stage> {
    pub states: Vec<EState<S>>,
}

#[derive(Clone)]
pub struct MultiState<'a> {
    nfa: &'a ENFA<Resolved>,
    states: Vec<StateId>,
    accept: bool,
    hash: u64,
}

impl<'a> PartialEq for MultiState<'a> {
    fn eq(&self, other: &Self) -> bool {
        (self.nfa as *const ENFA<Resolved> as u64) == (other.nfa as *const ENFA<Resolved> as u64)
            && self.states == other.states
            && self.accept == other.accept
            && self.hash == other.hash
    }
}
impl<'a> Eq for MultiState<'a> {}

impl<'a> MultiState<'a> {
    pub fn new(nfa: &'a ENFA<Resolved>, mut states: Vec<StateId>) -> Self {
        states.sort();
        states.dedup();
        states.shrink_to_fit();

        let accept = states.iter().any(|&x| nfa.states[x].accept);
        let mut hasher = DefaultHasher::new();
        states.hash(&mut hasher);
        let hash = hasher.finish();

        Self {
            nfa,
            states,
            accept,
            hash,
        }
    }

    /// all the chars that will make an interesting transition
    pub fn possible_transitions(&self) -> Vec<ByteRange> {
        let mut vec: Vec<_> = self
            .states
            .iter()
            .flat_map(|&i| self.nfa.states[i].trans.iter().map(|x| x.consumes))
            .collect();
        vec = ByteRange::non_overlapping(vec);
        vec.sort();
        vec.dedup();
        vec.shrink_to_fit();
        vec
    }

    pub fn transition(&self, ch: ByteRange) -> Self {
        let new_states = self
            .states
            .iter()
            .flat_map(|&s| {
                self.nfa.states[s].trans.iter().filter_map(|t| {
                    if t.consumes.overlaps(ch) {
                        Some(t.to)
                    } else {
                        None
                    }
                })
            })
            .collect();

        Self::new(self.nfa, new_states)
    }

    pub fn accept(&self) -> bool {
        self.accept
    }
}

impl<'a> Hash for MultiState<'a> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.hash.hash(state)
    }
}

macro_rules! set {
    () => {
        std::collections::HashSet::new()
    };
    ( $( $x:expr ),* ) => {{
        let mut set = std::collections::HashSet::new();
        $(
            set.insert($x);
        )*
        set
    }};
}

impl<S: Stage> ENFA<S> {
    fn shift(self, amt: usize) -> Vec<EState<S>> {
        let mut s = self.states;

        for state in s.iter_mut() {
            state.remap(|i| i + amt);
            state.accept = false;
        }

        s
    }

    pub fn remove_unreachable(&mut self) {
        let mut used = vec![false; self.states.len()];
        used[0] = true;
        for s in self.states.iter() {
            for i in s.reachable_states() {
                used[i] = true;
            }
        }
        let mut remap = vec![0; self.states.len()];
        let mut shift = 0;
        for i in 0..self.states.len() {
            if used[i] {
                remap[i] = i - shift;
            } else {
                shift += 1;
            }
        }
        for i in (0..self.states.len()).rev() {
            if !used[i] {
                self.states.remove(i);
            }
        }
        for s in self.states.iter_mut() {
            s.remap(|i| remap[i]);
        }
    }
}

impl ENFA<Epsilon> {
    fn epsilon_dfs(&self, i: StateId, visited: &mut [bool]) {
        if visited[i] {
            return;
        }
        visited[i] = true;
        for t in self.states[i].trans.iter() {
            if t.is_epsilon() {
                self.epsilon_dfs(t.to, visited);
            }
        }
    }

    pub fn resolve_epsilon(self) -> ENFA<Resolved> {
        // state X --> { state Y, Z, W which get inlined into X }
        let includes: Vec<Vec<StateId>> = (0..self.states.len())
            .map(|i| {
                let mut reach = vec![false; self.states.len()];
                self.epsilon_dfs(i, &mut reach);
                reach
                    .into_iter()
                    .enumerate()
                    .filter_map(|(x, r)| if r { Some(x) } else { None })
                    .collect()
            })
            .collect();

        // states without epsilon transitions
        let mut states: Vec<EState<Resolved>> = self
            .states
            .into_iter()
            .map(EState::remove_epsilon)
            .collect();

        // inline real transitions
        for i in 0..states.len() {
            for &k in includes[i].iter() {
                let new = states[k].trans.clone();
                states[i].trans.extend(new);
                if states[k].accept {
                    states[i].accept = true;
                }
            }
        }

        ENFA { states }
    }

    fn looping(self) -> Self {
        let mut states = vec![EState::start()];
        states.append(&mut self.shift(1));
        let len = states.len();
        states[0].set_epsilon_transitions([Transition::epsilon(1), Transition::epsilon(len)]);
        states[len - 1].set_epsilon_transitions([Transition::epsilon(0), Transition::epsilon(len)]);
        states.push(EState::terminal());
        Self { states }
    }

    fn repeat(self, times: usize) -> Self {
        let reps = vec![self; times];
        Self::concat(reps)
    }

    /// between 0 and x repetitions
    fn optx(self, x: usize) -> Self {
        let len = self.states.len();
        let mut repped = self.repeat(x);
        assert_eq!(repped.states.len(), x * len);
        for i in 1..=x {
            repped.states[0]
                .trans
                .insert(Transition::epsilon(i * len - 1));
        }
        repped
    }

    fn concat(nfas: Vec<Self>) -> Self {
        if nfas.is_empty() {
            return Self {
                states: vec![EState::terminal()],
            };
        }

        let mut states: Vec<EState<Epsilon>> = Vec::new();
        for nfa in nfas.into_iter() {
            let len = states.len();
            let mut ns = nfa.shift(len);
            if let Some(n) = states.last_mut() {
                n.trans.retain(|t| t.consumes.is_some());
                n.trans.insert(Transition::epsilon(len));
            }
            states.append(&mut ns);
        }

        let len = states.len();
        states[len - 1].accept = true;

        Self { states }
    }
}

impl ENFA<Resolved> {
    pub fn start_multi_state<'a>(&'a self) -> MultiState<'a> {
        MultiState::new(self, vec![0])
    }

    pub fn void_multi_state<'a>(&'a self) -> MultiState<'a> {
        MultiState::new(self, vec![])
    }

    pub fn all_multi_states<'a>(&'a self) -> HashSet<MultiState<'a>> {
        let mut states = set![self.start_multi_state()];
        let mut q = vec![self.start_multi_state()];

        while let Some(state) = q.pop() {
            let chars = state.possible_transitions();

            for chr in chars {
                let new = state.transition(chr);

                if !states.contains(&new) {
                    states.insert(new.clone());
                    q.push(new);
                }
            }
        }

        states
    }
}

impl std::fmt::Debug for ENFA<Epsilon> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        writeln!(f, "NFA {{")?;
        for (i, s) in self.states.iter().enumerate() {
            write!(f, "  {i}: ")?;
            for t in s.trans.iter() {
                let k = t.to;
                if let Some(c) = t.consumes {
                    write!(f, "{c:?}=>{k} ")?;
                } else {
                    write!(f, "~>{k} ")?;
                }
            }
            if s.accept {
                write!(f, "accept")?;
            }
            writeln!(f)?;
        }
        write!(f, "}}")
    }
}

pub type StateId = usize;

#[derive(Debug, Clone, Hash, PartialEq, Eq)]
pub struct Transition<S: Stage> {
    to: StateId,
    consumes: S::Consume,
}

impl Transition<Epsilon> {
    fn new(consumes: ByteRange, to: StateId) -> Self {
        let consumes = Some(consumes);
        Self { to, consumes }
    }

    fn epsilon(to: StateId) -> Self {
        Self { to, consumes: None }
    }

    pub fn is_epsilon(&self) -> bool {
        self.consumes.is_none()
    }

    fn non_epsilon(self) -> Option<Transition<Resolved>> {
        let consumes = self.consumes?;
        let to = self.to;
        Some(Transition { consumes, to })
    }
}

impl<S: Stage> Transition<S> {
    fn remap(&mut self, mut f: impl FnMut(StateId) -> StateId) {
        self.to = f(self.to);
    }

    fn reachable_states(&self) -> impl Iterator<Item = StateId> {
        [self.to].into_iter()
    }
}

#[derive(Debug, Clone)]
pub struct EState<S: Stage> {
    pub trans: HashSet<Transition<S>>,
    pub accept: bool,
}

impl EState<Epsilon> {
    fn remove_epsilon(self) -> EState<Resolved> {
        let trans = self
            .trans
            .into_iter()
            .filter_map(Transition::non_epsilon)
            .collect();
        let accept = self.accept;
        EState { trans, accept }
    }

    fn set_epsilon_transitions(&mut self, trans: impl IntoIterator<Item = Transition<Epsilon>>) {
        self.trans.retain(|t| t.consumes.is_some());
        for transition in trans.into_iter() {
            assert!(transition.consumes.is_none());
            self.trans.insert(transition);
        }
    }
}

impl<S: Stage> EState<S> {
    fn start() -> Self {
        Self {
            trans: HashSet::new(),
            accept: false,
        }
    }
    fn terminal() -> Self {
        Self {
            trans: HashSet::new(),
            accept: true,
        }
    }

    fn remap(&mut self, mut f: impl FnMut(StateId) -> StateId) {
        self.trans = self
            .trans
            .iter()
            .cloned()
            .map(|mut t| {
                t.remap(&mut f);
                t
            })
            .collect();
    }

    fn reachable_states(&self) -> impl Iterator<Item = StateId> {
        self.trans.iter().flat_map(|t| t.reachable_states())
    }
}

#[derive(Debug)]
pub enum EnfaTranslationError {
    AssertionsNotSupported,
}

impl TryFrom<Pattern> for ENFA<Epsilon> {
    type Error = EnfaTranslationError;

    fn try_from(value: Pattern) -> Result<Self, Self::Error> {
        Ok(match value {
            Pattern::Byte(c) => Self::try_from(Pattern::Range(c, c))?,
            Pattern::Range(c1, c2) => Self {
                states: vec![
                    EState {
                        trans: set![Transition::new(ByteRange::new_range(c1, c2), 1)],
                        accept: false,
                    },
                    EState::terminal(),
                ],
            },
            Pattern::Alt(alts) => {
                let nfas: Vec<ENFA<Epsilon>> = alts
                    .into_iter()
                    .map(Self::try_from)
                    .collect::<Result<_, _>>()?;
                let mut states = vec![EState::start()];
                let mut ends = vec![];
                for nfa in nfas.into_iter() {
                    let len = states.len();
                    states[0].trans.insert(Transition::epsilon(len));
                    states.append(&mut (nfa.shift(len)));
                    ends.push(states.len() - 1);
                }
                states.push(EState::terminal());
                for end in ends.into_iter() {
                    let last = states.len() - 1;
                    states[end].trans.insert(Transition::epsilon(last));
                }
                Self { states }
            }
            Pattern::Concat(seq) => {
                let nfas: Vec<Self> = seq
                    .into_iter()
                    .map(Self::try_from)
                    .collect::<Result<_, _>>()?;
                Self::concat(nfas)
            }
            Pattern::Rep(regex, min, None) => {
                let nfa = ENFA::try_from(*regex)?;
                let base = nfa.clone().repeat(min as usize);
                let tail = nfa.looping();
                Self::concat(vec![base, tail])
            }
            Pattern::Rep(regex, min, Some(max)) => {
                assert!(min < max);
                let nfa = Self::try_from(*regex)?;
                let base = nfa.clone().repeat(min as usize);
                let tail = nfa.optx((max - min) as usize);
                Self::concat(vec![base, tail])
            }
            Pattern::Nothing => Self {
                states: vec![EState::terminal()],
            },
            Pattern::Assertion(..) => {
                return Err(EnfaTranslationError::AssertionsNotSupported);
            }
        })
    }
}