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

use super::{LookDirection, LookPolarity, Pattern, byte_range::ByteRange};

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

fn cartesian_product<T: Clone>(x: Vec<Vec<T>>) -> Vec<Vec<T>> {
    let mut result = vec![Vec::new()];

    for xs in x {
        let mut next = Vec::new();

        for prefix in &result {
            for item in &xs {
                let mut v = prefix.clone();
                v.push(item.clone());
                next.push(v);
            }
        }

        result = next;
    }

    result
}

#[cfg(test)]
mod product_tests {
    use super::cartesian_product;

    #[test]
    fn basic_case() {
        let x = vec![vec![1, 2], vec![10, 20]];

        let out = cartesian_product(x);

        assert_eq!(
            out,
            vec![vec![1, 10], vec![1, 20], vec![2, 10], vec![2, 20],]
        );
    }

    #[test]
    fn single_dimension() {
        let x = vec![vec![1, 2, 3]];

        let out = cartesian_product(x);

        assert_eq!(out, vec![vec![1], vec![2], vec![3],]);
    }

    #[test]
    fn empty_outer_vector() {
        let x: Vec<Vec<i32>> = vec![];

        let out: Vec<Vec<i32>> = cartesian_product(x);
        let r: Vec<Vec<i32>> = vec![vec![]];

        // One empty combination.
        assert_eq!(out, r);
    }

    #[test]
    fn empty_inner_vector() {
        let x = vec![vec![1, 2], vec![], vec![3, 4]];

        let out = cartesian_product(x);

        assert!(out.is_empty());
    }

    #[test]
    fn output_size_matches_product() {
        let x = vec![vec![1, 2, 3], vec![4, 5], vec![6, 7, 8, 9]];

        let out = cartesian_product(x);

        assert_eq!(out.len(), 3 * 2 * 4);
    }

    #[test]
    fn every_output_has_correct_length() {
        let x = vec![vec!['a', 'b'], vec!['x', 'y', 'z'], vec!['0']];

        let out = cartesian_product(x);

        assert!(out.iter().all(|v| v.len() == 3));
    }

    #[test]
    fn works_with_strings() {
        let x = vec![
            vec!["a".to_string(), "b".to_string()],
            vec!["x".to_string()],
        ];

        let out = cartesian_product(x);

        assert_eq!(
            out,
            vec![
                vec!["a".to_string(), "x".to_string()],
                vec!["b".to_string(), "x".to_string()],
            ]
        );
    }
}

#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
struct Thread {
    state: StateId,
    positives: Vec<Thread>,
    negatives: Vec<Thread>,
}

impl Thread {
    fn new_simple(state: StateId) -> Self {
        Self {
            state,
            positives: Vec::new(),
            negatives: Vec::new(),
        }
    }

    fn new(
        enfa: &ENFA,
        state: StateId,
        mut positives: Vec<Thread>,
        mut negatives: Vec<Thread>,
    ) -> Option<Self> {
        positives.sort();
        positives.dedup();
        positives.retain(|t| !t.accept(enfa));
        positives.shrink_to_fit();

        negatives.sort();
        negatives.dedup();
        negatives.shrink_to_fit();

        if negatives.iter().any(|t| t.accept(enfa)) {
            return None;
        }

        Some(Self {
            state,
            positives,
            negatives,
        })
    }

    fn accept(&self, enfa: &ENFA) -> bool {
        let pos = self.positives.iter().all(|t| t.accept(enfa));
        let neg = self.negatives.iter().all(|t| !t.accept(enfa));
        let this = match enfa.states[self.state].accept {
            Acceptance::Accept => true,
            Acceptance::Assertion => true,
            Acceptance::NotYet => false,
        };
        pos && neg && this
    }

    fn step_epsilon0(
        self,
        enfa: &ENFA,
        ret: &mut impl FnMut(Thread),
        visited: &mut [bool],
        new_assertions: Vec<Assertion>,
    ) {
        if visited[self.state] {
            return;
        }
        visited[self.state] = true;

        for t in enfa.states[self.state].trans.iter() {
            if t.consumes.is_some() {
                continue;
            }

            let mut new_assertions = new_assertions.clone();
            if let Some(assertion) = enfa.states[self.state].assert.as_ref() {
                new_assertions.push(assertion.clone());
            }

            Self {
                state: t.to,
                ..self.clone()
            }
            .step_epsilon0(enfa, ret, visited, new_assertions);
        }

        let Self {
            state,
            positives,
            negatives,
        } = self;
        let mut p = Vec::new();
        let mut n = Vec::new();
        for assertion in new_assertions {
            let threads = Self::new_simple(assertion.to).step_epsilon(enfa);
            let vec = match assertion.polarity {
                LookPolarity::Positive => &mut p,
                LookPolarity::Negative => &mut n,
            };
            vec.push(threads);
        }
        let p = cartesian_product(p);
        for mut p in p {
            let mut positives = positives.clone();
            let mut negatives = negatives.clone();
            positives.append(&mut p);
            negatives.append(&mut n.iter().flatten().cloned().collect());
            if let Some(thread) = Self::new(enfa, state, positives, negatives) {
                ret(thread);
            }
        }
    }

    fn step_epsilon(self, enfa: &ENFA) -> Vec<Self> {
        let mut vec = Vec::new();
        self.step_epsilon0(
            enfa,
            &mut |t| vec.push(t),
            &mut vec![false; enfa.states.len()],
            Vec::new(),
        );
        vec
    }

    fn step0(self, enfa: &ENFA, input: ByteRange, ret: &mut impl FnMut(Thread)) {
        let positives = self
            .positives
            .clone()
            .into_iter()
            .map(|t| t.step(enfa, input))
            .collect();
        let negatives: Vec<_> = self
            .negatives
            .into_iter()
            .flat_map(|t| t.step(enfa, input))
            .collect();
        let positives = cartesian_product(positives);
        let next_states: Vec<StateId> = enfa.states[self.state]
            .trans
            .iter()
            .filter_map(|t| {
                if let Some(ch) = t.consumes
                    && ch.overlaps(input)
                {
                    Some(t.to)
                } else {
                    None
                }
            })
            .collect();

        for s in next_states {
            for p in positives.clone() {
                if let Some(thread) = Self::new(enfa, s, p.clone(), negatives.clone()) {
                    thread.step_epsilon0(
                        enfa,
                        ret,
                        &mut vec![false; enfa.states.len()],
                        Vec::new(),
                    );
                }
            }
        }
    }

    fn step(self, enfa: &ENFA, input: ByteRange) -> Vec<Self> {
        let mut vec = Vec::new();
        self.step0(enfa, input, &mut |x| vec.push(x));
        vec
    }

    fn possible_transitions(&self, enfa: &ENFA, f: &mut impl FnMut(ByteRange)) {
        for t in enfa.states[self.state].trans.iter() {
            if let Some(ch) = t.consumes {
                f(ch);
            }
        }
        for t in self.positives.iter() {
            t.possible_transitions(enfa, f);
        }
        for t in self.negatives.iter() {
            t.possible_transitions(enfa, f);
        }
    }
}

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

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

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

        let accept = threads.iter().any(|t| t.accept(nfa));
        let mut hasher = DefaultHasher::new();
        threads.hash(&mut hasher);
        let hash = hasher.finish();

        Self {
            nfa,
            threads,
            accept,
            hash,
        }
    }

    /// all the chars that will make an interesting transition
    pub fn possible_transitions(&self) -> Vec<ByteRange> {
        let mut vec = Vec::new();
        for t in self.threads.iter() {
            t.possible_transitions(self.nfa, &mut |x| vec.push(x));
        }
        vec = ByteRange::split_to_disjoint(vec);
        vec.sort();
        vec.dedup();
        vec.shrink_to_fit();
        vec
    }

    pub fn transition(&self, ch: ByteRange) -> Self {
        let new_states = self
            .threads
            .iter()
            .flat_map(|t| t.clone().step(self.nfa, ch))
            .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 ENFA {
    fn shift(self, amt: usize) -> Vec<EState> {
        let mut s = self.states;

        for state in s.iter_mut() {
            state.remap(|i| i + amt);
            if state.accept == Acceptance::Accept {
                state.accept = Acceptance::NotYet;
            }
        }

        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 {
    fn looping(self) -> Self {
        let has_submatches = self.has_submatches;
        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, has_submatches }
    }

    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()],
                has_submatches: false,
            };
        }

        let mut has_submatches = false;
        let mut states: Vec<EState> = Vec::new();
        for nfa in nfas.into_iter() {
            has_submatches = has_submatches || nfa.has_submatches;
            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 = Acceptance::Accept;

        Self { states, has_submatches }
    }
}

impl ENFA {
    pub fn start_multi_state<'a>(&'a self) -> MultiState<'a> {
        let threads = Thread::new_simple(0).step_epsilon(self);
        MultiState::new(self, threads)
    }

    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 {
    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}: ")?;
            if let Some(a) = s.assert.as_ref() {
                match a.polarity {
                    LookPolarity::Positive => write!(f, "+{} ", a.to)?,
                    LookPolarity::Negative => write!(f, "-{} ", a.to)?,
                }
            }
            for t in s.trans.iter() {
                let k = t.to;
                if let Some(c) = t.consumes {
                    write!(f, "{c:?}=>{k} ")?;
                } else {
                    write!(f, "~>{k} ")?;
                }
            }
            match s.accept {
                Acceptance::Accept => write!(f, "accept")?,
                Acceptance::Assertion => write!(f, "assert")?,
                Acceptance::NotYet => {}
            }
            writeln!(f)?;
        }
        write!(f, "}}")
    }
}

pub type StateId = usize;

#[derive(Debug, Clone, Hash, PartialEq, Eq)]
pub struct Assertion {
    to: StateId,
    polarity: LookPolarity,
}

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

impl Transition {
    fn new(consumes: ByteRange, to: StateId) -> Self {
        Self {
            to,
            consumes: Some(consumes),
        }
    }

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

    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, Copy, Clone, PartialEq)]
pub enum Acceptance {
    Accept,
    Assertion,
    NotYet,
}

#[derive(Debug, Clone)]
pub struct EState {
    pub trans: HashSet<Transition>,
    pub assert: Option<Assertion>,
    pub accept: Acceptance,
}

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

    fn start() -> Self {
        Self {
            trans: HashSet::new(),
            assert: None,
            accept: Acceptance::NotYet,
        }
    }
    fn terminal() -> Self {
        Self {
            trans: HashSet::new(),
            assert: None,
            accept: Acceptance::Accept,
        }
    }

    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();
        if let Some(a) = self.assert.as_mut() {
            a.to = f(a.to);
        }
    }

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

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

impl TryFrom<Pattern> for ENFA {
    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)],
                        assert: None,
                        accept: Acceptance::NotYet,
                    },
                    EState::terminal(),
                ],
                has_submatches: false,
            },
            Pattern::CharacterClass(_) => {
                return Err(EnfaTranslationError::CharacterClassNotSupported);
            }
            Pattern::Alt(alts) => {
                let nfas: Vec<ENFA> = alts
                    .into_iter()
                    .map(Self::try_from)
                    .collect::<Result<_, _>>()?;
                let mut states = vec![EState::start()];
                let mut ends = vec![];
                let mut has_submatches = false;
                for nfa in nfas.into_iter() {
                    has_submatches = has_submatches || nfa.has_submatches;
                    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, has_submatches }
            }
            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()],
                has_submatches: false,
            },
            Pattern::Assertion(dir, polarity, pat) => {
                if dir == LookDirection::Behind {
                    return Err(EnfaTranslationError::AssertionsNotSupported);
                }
                let mut regex = Self::try_from(*pat)?;
                for s in regex.states.iter_mut() {
                    if s.accept == Acceptance::Accept {
                        s.accept = Acceptance::Assertion;
                    }
                }
                let mut regex = regex.shift(1);
                let mut states = Vec::with_capacity(regex.len() + 2);
                states.push(EState {
                    trans: set![Transition::epsilon(regex.len() + 1)],
                    assert: Some(Assertion { to: 1, polarity }),
                    accept: Acceptance::NotYet,
                });
                states.append(&mut regex);
                states.push(EState::terminal());
                Self { states, has_submatches: false, }
            }
            Pattern::Submatch(pat) => {
                let mut this = Self::try_from(*pat)?;
                this.has_submatches = true;
                this
            }
        })
    }
}