use anyhow::Result;
use rand::seq::SliceRandom;
use rand::Rng;
use regex::Regex;
use thiserror::Error;

#[cfg(test)]
mod lib_test;

#[derive(Debug, PartialEq, Clone)]
enum DiceFilter {
    DropLowest(usize),
    DropHighest(usize),
    KeepLowest(usize),
    KeepHighest(usize),
}

#[derive(Debug, PartialEq, Clone)]
enum Segment {
    DiceRoll {
        op: char,
        count: i32,
        size: i32,
        filter: Option<DiceFilter>,
    },
    Modifier {
        op: char,
        amount: i32,
    },
}

#[derive(Error, Debug, PartialEq)]
enum SegmentError {
    #[error("No dice size was given")]
    SizeIsMissing,
    #[error("Invalid filter operator: {op}")]
    InvalidFilterOperator { op: String },
    #[error("Incomplete filter")]
    IncompleteFilter,
}

fn construct_dice_filter(op: &str, amount: usize) -> Result<DiceFilter> {
    match op.to_lowercase().as_str() {
        "d" | "dl" => Ok(DiceFilter::DropLowest(amount)),
        "dh" => Ok(DiceFilter::DropHighest(amount)),
        "k" | "kh" => Ok(DiceFilter::KeepHighest(amount)),
        "kl" => Ok(DiceFilter::KeepLowest(amount)),
        _ => Err(SegmentError::InvalidFilterOperator { op: op.to_owned() }.into()),
    }
}

fn construct_dice_segment(cap: regex::Captures<'_>) -> Result<Segment> {
    let op = cap
        .name("op")
        .and_then(|i| i.as_str().chars().next())
        .unwrap_or('+');
    let modifier = cap.name("mod").map(|i| i.as_str().parse()).transpose()?;

    if let Some(amount) = modifier {
        return Ok(Segment::Modifier { op, amount });
    }

    let count: i32 = cap
        .name("count")
        .map(|i| i.as_str().parse())
        .transpose()?
        .unwrap_or(1);
    let size: i32 = cap
        .name("size")
        .map(|i| i.as_str().parse())
        .transpose()?
        .ok_or(SegmentError::SizeIsMissing)?;
    let filter_amount = cap
        .name("filter")
        .map(|i| i.as_str().parse::<usize>())
        .transpose()?;
    let filter_op = cap.name("filter_op").map(|op| op.as_str());
    /*
    dbg!(filter_amount);
    dbg!(filter_op);
    */
    if filter_amount.is_some() != filter_op.is_some() {
        return Err(SegmentError::IncompleteFilter.into());
    }
    let filter = filter_amount
        .and_then(|amount| (filter_op.map(|op| construct_dice_filter(op, amount))))
        .transpose()?;

    Ok(Segment::DiceRoll {
        op,
        count,
        size,
        filter,
    })
}

fn parse_dice_segments(cmd: &str) -> Result<Vec<Segment>> {
    let regex = Regex::new(
        r#"(?x)
    (?P<op>[+\-/*])?
    \s*
    (?:
      (?:
        (?P<count>\d+)? # count (optional)
        d
        (?P<size>\d+) # dice size
        (?P<filter_op>[dk][hl]?)?
        (?P<filter>\d+)?
      ) | (?:
        (?P<mod>\d+)
      )
    )
  "#,
    )
    .expect("Failed to compile regex");

    regex
        .captures_iter(cmd)
        .map(construct_dice_segment)
        .collect()
}

#[derive(Debug, PartialEq, Clone)]
struct RollWithModifier {
    diceroll: Option<Segment>,
    modifiers: Vec<Segment>,
}

fn group_modifiers_to_dicerolls(segments: &[Segment]) -> Vec<RollWithModifier> {
    let mut results = Vec::new();

    let mut current_diceroll: Option<Segment> = None;
    let mut current_modifiers = Vec::new();

    for segment in segments {
        if let Segment::DiceRoll { .. } = segment {
            if current_diceroll.is_some() || !current_modifiers.is_empty() {
                let with_modifier = RollWithModifier {
                    diceroll: current_diceroll.clone(),
                    modifiers: current_modifiers.clone(),
                };

                current_modifiers.clear();

                results.push(with_modifier);
            }

            current_diceroll = Some(segment.clone());
        } else if let Segment::Modifier { .. } = segment {
            current_modifiers.push(segment.clone());
        }
    }

    if current_diceroll.is_some() || !current_modifiers.is_empty() {
        let with_modifier = RollWithModifier {
            diceroll: current_diceroll,
            modifiers: current_modifiers.clone(),
        };

        current_modifiers.clear();

        results.push(with_modifier);
    }

    results
}

#[derive(Debug, PartialEq, Clone)]
struct Roll {
    operator: char,
    results: Vec<i32>,
    total: i32,
}

#[derive(Error, Debug, PartialEq, Clone)]
enum RollError {
    #[error("Roll failed. Empty RollWithModifier")]
    EmptyRollWithModifier,
    #[error("Roll failed. Invalid filter operator '{0}'")]
    InvalidFilterOperator(char),
    #[error("Roll failed. Invalid segment operator '{0}'")]
    InvalidOperator(char),
}

fn roll_dice_segments<R: Rng>(
    rwms: &[RollWithModifier],
    mut rng: R,
) -> Result<Vec<Roll>, RollError> {
    rwms.iter()
        .map(|rwm| {
            let mut results = Vec::new();
            let mut total = 0;

            // store first operator
            let operator = if let Some(Segment::DiceRoll { op, .. }) = rwm.diceroll {
                op
            } else if let Some(Segment::Modifier { op, .. }) = rwm.modifiers.iter().next() {
                *op
            } else {
                return Err(RollError::EmptyRollWithModifier);
            };

            if let Some(Segment::DiceRoll {
                count,
                size,
                filter,
                ..
            }) = &rwm.diceroll
            {
                results = (0..*count).map(|_| rng.gen_range(1, size + 1)).collect();

                results.sort();
                if let Some(filter) = filter {
                    match filter {
                        DiceFilter::DropLowest(n) => {
                            results = results.into_iter().skip(*n).collect();
                        }
                        DiceFilter::DropHighest(n) => {
                            let len = results.len();
                            results = results.into_iter().take(len - n).collect();
                        }
                        DiceFilter::KeepLowest(n) => {
                            results = results.into_iter().take(*n).collect();
                        }
                        DiceFilter::KeepHighest(n) => {
                            let len = results.len();
                            results = results.into_iter().skip(len - n).collect();
                        }
                    }
                }
                results.shuffle(&mut rng);

                total = results.iter().sum();
            }

            for segment in &rwm.modifiers {
                if let Segment::Modifier { op, amount } = segment {
                    match op {
                        '+' => {
                            total += amount;
                        }
                        '-' => {
                            total -= amount;
                        }
                        '/' => {
                            total /= amount;
                        }
                        '*' => {
                            total *= amount;
                        }
                        _ => {
                            return Err(RollError::InvalidFilterOperator(*op));
                        }
                    }
                }
            }

            Ok(Roll {
                operator,
                results,
                total,
            })
        })
        .collect::<Result<Vec<_>, _>>()
}

#[derive(Debug, PartialEq, Clone)]
struct RollSet {
    total: i32,
    rolls: Vec<(Roll, Vec<Segment>)>,
}

fn roll_dice<R: Rng>(s: &str, mut rng: R) -> Result<RollSet> {
    let segments = parse_dice_segments(s)?;

    let groups = group_modifiers_to_dicerolls(&segments);

    let rolls = roll_dice_segments(&groups, &mut rng)?;

    let total = rolls.iter().try_fold(0, |acc, roll| match roll.operator {
        '+' => Ok(acc + roll.total),
        '-' => Ok(acc - roll.total),
        '*' => Ok(acc * roll.total),
        '/' => Ok(acc / roll.total),
        _ => Err(RollError::InvalidOperator(roll.operator)),
    })?;

    let rolls_with_modifiers = rolls
        .into_iter()
        .zip(groups.into_iter().map(|grp| grp.modifiers))
        .collect();

    Ok(RollSet {
        total,
        rolls: rolls_with_modifiers,
    })
}