use anyhow::bail;
use rand::{Rng, RngExt};
use std::convert::Infallible;
use std::error::Error;
use std::fmt::{Write, format};
use thiserror::Error;

pub type ExpBox = Box<Expression>;

#[derive(Debug, PartialEq, Clone)]
pub enum Expression {
    Const(f64),
    Dice(DiceFormula),
    Neg(ExpBox),
    Add(ExpBox, ExpBox),
    Sub(ExpBox, ExpBox),
    Mul(ExpBox, ExpBox),
    Div(ExpBox, ExpBox),
}

#[derive(Debug, PartialEq, Clone)]
pub struct DiceFormula {
    pub(crate) count: Option<ExpBox>,
    pub(crate) size: ExpBox,
    pub(crate) kh: Option<ExpBox>,
    pub(crate) kl: Option<ExpBox>,
    pub(crate) dh: Option<ExpBox>,
    pub(crate) dl: Option<ExpBox>,
    pub(crate) x: Option<Vec<CompareFragment>>,
}

#[derive(Debug, PartialEq, Clone)]
pub enum CompareFragment {
    Eq(ExpBox),
    Gt(ExpBox),
    Ge(ExpBox),
    Lt(ExpBox),
    Le(ExpBox),
}

const DICE_POOL_LIMIT: usize = 10_000;

impl Expression {
    pub fn collect_evaluation(&self, mut rng: impl Rng) -> anyhow::Result<String> {
        let mut witness = DiscordMdWitness::default();
        let total = self.evaluate(&mut rng, &mut witness, 100)?;
        let result_text = witness.buffer;
        Ok(format!("**{total}** = {result_text}"))
    }

    pub fn average(&self, rng: &mut impl Rng) -> anyhow::Result<f64> {
        if let Ok(avg) = self.avg() {
            return Ok(avg);
        }
        let mut average = self.sample(rng)?;
        for idx in 1..3000 {
            average = (average * idx as f64 + self.sample(rng)?) / (idx as f64 + 1f64);
        }
        Ok(average)
    }

    fn evaluate<W, E>(
        &self,
        rng: &mut impl Rng,
        w: &mut W,
        outer_precedence: i64,
    ) -> anyhow::Result<f64>
    where
        W: Witness<Error = E>,
        E: Error + Send + Sync + 'static,
    {
        use Expression::*;
        let precedence = self.precedence();
        let needs_parens = precedence > outer_precedence;
        if needs_parens {
            write!(w, "(")?;
        }
        let result = match self {
            Const(x) => {
                write!(w, "{}", x)?;
                *x
            }
            Dice(DiceFormula {
                count: count_node,
                size: size_node,
                kh,
                kl,
                dh,
                dl,
                x,
            }) => {
                let count = if let Some(count_node) = count_node {
                    count_node.evaluate(rng, w, precedence)? as usize
                } else {
                    1
                };
                if count > DICE_POOL_LIMIT {
                    bail!("Too many dice.")
                };
                write!(w, "d")?;
                let size = size_node.evaluate(rng, w, precedence)? as i64;
                if size < 1 {
                    bail!("Invalid die size.")
                }

                let mut rolls = (0..count)
                    .map(|_| rng.random_range(1..=size))
                    .collect::<Vec<_>>();

                // Exploding dice are early, they add rolls
                if let Some(fragments) = x {
                    write!(w, "x")?;
                    if size < 2 {
                        bail!("Infinite explosion.")
                    }
                    let mut comparers = Vec::with_capacity(fragments.len());
                    for fragment in fragments {
                        comparers.push(fragment.comparer(rng, w)?);
                    }
                    for i in 0..DICE_POOL_LIMIT {
                        if i >= rolls.len() {
                            break;
                        }
                        if comparers.len() == 0 {
                            // Explode on max size dice
                            if rolls[i] == size {
                                rolls.push(rng.random_range(1..=size));
                            }
                        } else {
                            // Explode based on comparers provided
                            for comparer in comparers.iter() {
                                if comparer(rolls[i] as f64) {
                                    rolls.push(rng.random_range(1..=size));
                                    break;
                                }
                            }
                        }
                    }
                }

                // Advantage/disadvantage roll filters
                let kh = if let Some(child) = kh {
                    write!(w, "kh")?;
                    Some(child.evaluate(rng, w, precedence)? as usize)
                } else {
                    None
                };
                let kl = if let Some(child) = kl {
                    write!(w, "kl")?;
                    Some(child.evaluate(rng, w, precedence)? as usize)
                } else {
                    None
                };
                let dh = if let Some(child) = dh {
                    write!(w, "dh")?;
                    Some(child.evaluate(rng, w, precedence)? as usize)
                } else {
                    None
                };
                let dl = if let Some(child) = dl {
                    write!(w, "dl")?;
                    Some(child.evaluate(rng, w, precedence)? as usize)
                } else {
                    None
                };

                // Don't sort the rolls directly, to preserve their original order for other logic
                // instead, `ranks` contains the indices where they would be in the array,
                // if it was sorted.
                let mut sorted_rolls = (0..rolls.len()).collect::<Vec<_>>();
                sorted_rolls.sort_by_key(|idx| rolls[*idx]);
                let mut ranks = vec![0usize; sorted_rolls.len()];
                for (rank, roll_idx) in sorted_rolls.into_iter().enumerate() {
                    ranks[roll_idx] = rank;
                }
                let mut result_set = Vec::with_capacity(rolls.len());

                write!(w, " ")?;
                let mut w_set = w.witness_set()?;

                for idx in 0..rolls.len() {
                    let mut skip = false;
                    let roll = rolls[idx];
                    let rank = ranks[idx];
                    if let Some(kh) = kh {
                        if rolls.len() - 1 - rank >= kh {
                            skip = true;
                        }
                    }
                    if let Some(dh) = dh {
                        if rolls.len() - 1 - rank < dh {
                            skip = true;
                        }
                    }
                    if let Some(kl) = kl {
                        if rank >= kl {
                            skip = true;
                        }
                    }
                    if let Some(dl) = dl {
                        if rank < dl {
                            skip = true;
                        }
                    }
                    if !skip {
                        result_set.push(roll as f64);
                    }
                    w_set.witness_roll(DiceRoll {
                        size: size as usize,
                        roll: roll as usize,
                        is_admitted: !skip,
                        did_proliferate: false,
                        is_from_proliferation: idx >= count,
                    })?;
                }
                w_set.end()?;

                result_set.iter().sum()
            }
            Neg(inner_node) => {
                write!(w, "-")?;
                let inner = inner_node.evaluate(rng, w, precedence)?;
                -inner
            }
            Add(lhs_node, rhs_node) => {
                let lhs = lhs_node.evaluate(rng, w, precedence)?;
                write!(w, " + ")?;
                let rhs = rhs_node.evaluate(rng, w, precedence)?;
                lhs + rhs
            }
            Sub(lhs_node, rhs_node) => {
                let lhs = lhs_node.evaluate(rng, w, precedence)?;
                write!(w, " - ")?;
                let rhs = rhs_node.evaluate(rng, w, precedence)?;
                lhs - rhs
            }
            Mul(lhs_node, rhs_node) => {
                let lhs = lhs_node.evaluate(rng, w, precedence)?;
                write!(w, " × ")?;
                let rhs = rhs_node.evaluate(rng, w, precedence)?;
                lhs * rhs
            }
            Div(lhs_node, rhs_node) => {
                let lhs = lhs_node.evaluate(rng, w, precedence)?;
                write!(w, " ÷ ")?;
                let rhs = rhs_node.evaluate(rng, w, precedence)?;
                lhs / rhs
            }
        };
        if needs_parens {
            write!(w, ")")?;
        }
        Ok(result)
    }

    fn sample(&self, rng: &mut impl Rng) -> anyhow::Result<f64> {
        self.evaluate(rng, &mut std::io::sink(), 100)
    }

    fn avg(&self) -> anyhow::Result<f64> {
        let node = self;
        let result = match node {
            Expression::Const(x) => *x,
            Expression::Dice(DiceFormula {
                count,
                size,
                kh,
                kl,
                dh,
                dl,
                x,
            }) => {
                let count = if let Some(count) = count {
                    count.avg()? as usize
                } else {
                    1
                };
                if count > 1_000_000 {
                    bail!("Too many dice.")
                };
                let size = size.avg()? as i64;
                if size < 1 {
                    bail!("Invalid die size.")
                }

                if let Some(_) = x {
                    bail!("Not implemented yet");
                }
                if let Some(_) = kh {
                    bail!("Not implemented yet");
                };
                if let Some(_) = kl {
                    bail!("Not implemented yet");
                };
                if let Some(_) = dh {
                    bail!("Not implemented yet");
                };
                if let Some(_) = dl {
                    bail!("Not implemented yet");
                };

                count as f64 * (size as f64 + 1f64) * 0.5
            }
            Expression::Neg(child) => -child.avg()?,
            Expression::Add(lhs, rhs) => lhs.avg()? + rhs.avg()?,
            Expression::Sub(lhs, rhs) => lhs.avg()? - rhs.avg()?,
            Expression::Mul(lhs, rhs) => lhs.avg()? * rhs.avg()?,
            Expression::Div(lhs, rhs) => lhs.avg()? / rhs.avg()?,
        };
        Ok(result)
    }
}

impl CompareFragment {
    fn comparer<W, E>(
        &self,
        rng: &mut impl Rng,
        witness: &mut W,
    ) -> anyhow::Result<Box<dyn Fn(f64) -> bool>>
    where
        W: Witness<Error = E>,
        E: Error + Send + Sync + 'static,
    {
        Ok(match self {
            CompareFragment::Eq(inner) => {
                write!(witness, "=")?;
                let comp = inner.evaluate(rng, witness, 2)?;
                Box::new(move |value| value == comp)
            }
            CompareFragment::Gt(inner) => {
                write!(witness, ">")?;
                let comp = inner.evaluate(rng, witness, 2)?;
                Box::new(move |value| value > comp)
            }
            CompareFragment::Ge(inner) => {
                write!(witness, ">=")?;
                let comp = inner.evaluate(rng, witness, 2)?;
                Box::new(move |value| value >= comp)
            }
            CompareFragment::Lt(inner) => {
                write!(witness, "<")?;
                let comp = inner.evaluate(rng, witness, 2)?;
                Box::new(move |value| value < comp)
            }
            CompareFragment::Le(inner) => {
                write!(witness, "<=")?;
                let comp = inner.evaluate(rng, witness, 2)?;
                Box::new(move |value| value <= comp)
            }
        })
    }
}

impl Expression {
    fn precedence(&self) -> i64 {
        use Expression::*;
        match self {
            Const(_) => 1,
            Dice { .. } => 2,
            Neg(_) => 5,
            Mul(_, _) | Div(_, _) => 7,
            Add(_, _) | Sub(_, _) => 8,
        }
    }
}

trait Witness {
    type Ok;
    type Error: Error;
    type WitnessSet<'a>: WitnessSet<Ok = Self::Ok, Error = Self::Error>
    where
        Self: 'a;

    fn witness_source_text(&mut self, text: &str) -> Result<Self::Ok, Self::Error>;
    fn witness_set(&mut self) -> Result<Self::WitnessSet<'_>, Self::Error>;
    fn write_fmt(&mut self, args: std::fmt::Arguments<'_>) -> Result<Self::Ok, Self::Error>
    where
        Self: Sized,
    {
        self.witness_source_text(&format(args))
    }
}

trait WitnessSet {
    type Ok;
    type Error: Error;

    fn witness_roll(&mut self, dice: DiceRoll) -> Result<Self::Ok, Self::Error>;
    fn end(self) -> Result<Self::Ok, Self::Error>;
}

#[derive(Debug)]
struct DiceRoll {
    size: usize,
    roll: usize,
    is_admitted: bool,
    did_proliferate: bool,
    is_from_proliferation: bool,
}

impl Default for DiceRoll {
    fn default() -> Self {
        Self {
            size: 1,
            roll: 1,
            is_admitted: true,
            did_proliferate: false,
            is_from_proliferation: false,
        }
    }
}

struct DiscordMdWitness {
    buffer: String,
    dice_written: usize,
}
struct DiscordMdWitnessSet<'a> {
    parent: &'a mut DiscordMdWitness,
    index: usize,
    dice_elided: bool,
}

#[derive(Error, Debug)]
enum DiscordMdWitnessError {
    #[error("formatting failed")]
    Format(#[from] std::fmt::Error),
}

impl Default for DiscordMdWitness {
    fn default() -> Self {
        Self {
            buffer: String::new(),
            dice_written: 0,
        }
    }
}

impl Witness for DiscordMdWitness {
    type Ok = ();
    type Error = DiscordMdWitnessError;
    type WitnessSet<'a> = DiscordMdWitnessSet<'a>;

    fn witness_source_text(&mut self, text: &str) -> Result<Self::Ok, Self::Error> {
        self.buffer.push_str(text);
        Ok(())
    }

    fn witness_set(&mut self) -> Result<Self::WitnessSet<'_>, Self::Error> {
        write!(self.buffer, "‹ ")?;
        Ok(DiscordMdWitnessSet {
            parent: self,
            index: 0,
            dice_elided: false,
        })
    }
}

impl<'a> WitnessSet for DiscordMdWitnessSet<'a> {
    type Ok = ();
    type Error = DiscordMdWitnessError;

    fn witness_roll(&mut self, dice: DiceRoll) -> Result<Self::Ok, Self::Error> {
        if self.parent.dice_written >= 100 {
            if !self.dice_elided {
                write!(self.parent.buffer, "…")?;
                self.dice_elided = true;
            }
            return Ok(());
        }

        let is_extreme = dice.size >= 4 && (dice.roll == 1 || dice.roll == dice.size);
        if self.index > 0 {
            write!(self.parent.buffer, " ")?;
        }
        if !dice.is_admitted {
            write!(self.parent.buffer, "~~")?;
        }
        if dice.is_from_proliferation {
            write!(self.parent.buffer, "*")?;
        }
        if is_extreme || dice.did_proliferate {
            write!(self.parent.buffer, "**")?;
        }

        write!(self.parent.buffer, "`{}`", dice.roll)?;

        if is_extreme || dice.did_proliferate {
            write!(self.parent.buffer, "**")?;
        }
        if dice.is_from_proliferation {
            write!(self.parent.buffer, "*")?;
        }
        if !dice.is_admitted {
            write!(self.parent.buffer, "~~")?;
        }
        self.index += 1;
        self.parent.dice_written += 1;
        Ok(())
    }

    fn end(self) -> Result<Self::Ok, Self::Error> {
        write!(self.parent.buffer, " ›")?;
        Ok(())
    }
}

impl Witness for std::io::Sink {
    type Ok = ();
    type Error = Infallible;
    type WitnessSet<'a>
        = &'a mut Self
    where
        Self: 'a;

    fn witness_source_text(&mut self, _text: &str) -> Result<Self::Ok, Self::Error> {
        Ok(())
    }

    fn witness_set(&mut self) -> Result<Self::WitnessSet<'_>, Self::Error> {
        Ok(self)
    }
}

impl WitnessSet for &mut std::io::Sink {
    type Ok = ();
    type Error = Infallible;

    fn witness_roll(&mut self, _dice: DiceRoll) -> Result<Self::Ok, Self::Error> {
        Ok(())
    }

    fn end(self) -> Result<Self::Ok, Self::Error> {
        Ok(())
    }
}