use crate::tile::Operator;
use enum_map::EnumMap;
use rand::Rng;

type DeckSize = u16;
type DigitDeck = [DeckSize; 19];

/// When a deck is empty, new tiles cannot be retrieved.
pub type EmptyDeckError = ();

/// A entire deck of tiles.
#[derive(Debug, Clone, Default, PartialEq)]
pub struct Deck {
    /// The digits and their current count.
    digits: DigitDeck,
    /// The operators and their current count.
    operators: EnumMap<Operator, DeckSize>,
}

impl Deck {
    pub fn new_complete() -> Self {
        let mut deck = Self::default();
        deck.add_operator_times(Operator::Add, 10);
        deck.add_operator_times(Operator::Subtract, 10);
        deck.add_operator_times(Operator::Multiply, 6);
        deck.add_operator_times(Operator::Divide, 4);
        for digit in -9..0 {
            deck.add_digit_times(digit, 2);
        }
        for digit in 0..=9 {
            deck.add_digit_times(digit, 8);
        }
        deck
    }

    /// Adds a single digit to the deck.
    pub fn add_digit(&mut self, digit: i8) {
        self.add_digit_times(digit, 1)
    }

    /// Adds a digit multiple times to the deck.
    pub fn add_digit_times(&mut self, digit: i8, times: DeckSize) {
        self.digits[Deck::digit_index(digit)] += times;
    }

    /// Adds a single operator to the deck.
    pub fn add_operator(&mut self, operator: Operator) {
        self.add_operator_times(operator, 1)
    }

    /// Adds an operator multiple times to the deck.
    pub fn add_operator_times(&mut self, operator: Operator, times: DeckSize) {
        self.operators[operator] += times;
    }

    /// Gets the index of a digit in the digit deck.
    fn digit_index(digit: i8) -> usize {
        (digit + 9) as usize
    }
}

/// A deck of tiles that can be chosen at random.
#[derive(Debug, Clone, Default)]
pub struct RngDeck {
    deck: Deck,
    rng: rand::rngs::ThreadRng,
}

impl RngDeck {
    pub fn new_complete() -> Self {
        Self {
            deck: Deck::new_complete(),
            rng: rand::thread_rng(),
        }
    }

    /// Gets a random tile from the deck and remove it from the deck.
    pub fn rand_digit(&mut self) -> Option<i8> {
        let sum = self.deck.digits.iter().sum();
        Self::select_rng(&mut self.rng, sum, self.deck.digits.iter_mut().enumerate())
            .map(|n| n as i8 - 9)
    }

    /// Gets a random operator from the deck and remove it from the deck.
    pub fn rand_operator(&mut self) -> Option<Operator> {
        let sum = self.deck.operators.values().sum();
        Self::select_rng(&mut self.rng, sum, self.deck.operators.iter_mut())
    }

    /// Selects a random item from an iterator of (item, count) pairs.
    /// The count is decremented by one if the item is selected.
    fn select_rng<'a, T>(
        rng: &mut rand::rngs::ThreadRng,
        sum: DeckSize,
        it: impl Iterator<Item = (T, &'a mut DeckSize)>,
    ) -> Option<T> {
        if sum == 0 {
            return None;
        }
        let mut threshold = rng.gen_range(1..=sum);
        for (item, count) in it {
            threshold = threshold.saturating_sub(*count);
            if threshold == 0 {
                *count -= 1;
                return Some(item);
            }
        }
        unreachable!()
    }
}

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

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn default_is_empty() {
        let mut deck = RngDeck::default();
        assert_eq!(deck.rand_digit(), None);
        assert_eq!(deck.rand_operator(), None);
    }

    #[test]
    fn one_digit() {
        let mut deck = RngDeck::default();
        deck.deck.add_digit(1);
        assert_eq!(deck.rand_digit(), Some(1));
        assert_eq!(deck.rand_operator(), None);
        assert_eq!(deck.rand_digit(), None);
    }

    #[test]
    fn one_operator() {
        let mut deck = RngDeck::default();
        deck.deck.add_operator(Operator::Multiply);
        assert_eq!(deck.rand_digit(), None);
        assert_eq!(deck.rand_operator(), Some(Operator::Multiply));
        assert_eq!(deck.rand_operator(), None);
    }

    #[test]
    fn respect_proportion() {
        let mut deck = RngDeck::default();
        deck.deck.add_digit_times(-4, 2);
        deck.deck.add_digit_times(3, 7);
        deck.deck.add_digit_times(7, 3);

        let mut actual = DigitDeck::default();
        while let Some(digit) = deck.rand_digit() {
            actual[Deck::digit_index(digit)] += 1;
        }

        let mut excepted = DigitDeck::default();
        excepted[Deck::digit_index(-4)] = 2;
        excepted[Deck::digit_index(3)] = 7;
        excepted[Deck::digit_index(7)] = 3;

        assert_eq!(actual, excepted);
    }
}