use crate::{user_preferences::ParseError, util::{self, pcg64, pcg64_seed, seed_with_date, seed_with_today}};

use std::{collections::BTreeMap, hash::Hash};
use std::fmt::Debug;
use std::cmp::Eq;

use time::Date;

/// A list of pronouns and their associated weights, used for random selection
///
/// Weights are typically representative of a user's preference towards a pronoun.  A pronoun with
/// a weight of 10 is twice as likely to be selected as a pronoun with a weight of 5.
///
/// This struct is use to represent these weights before they are used to randomly select a
/// pronoun.  Additional methods are provided to perform this random selection on a weighted list,
/// using as a seed both an arbitrary string of bytes and a Date.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct WeightedTable<Loot: Eq + Ord + Hash + Debug>(BTreeMap<Loot, u8>);

impl<Loot: Eq + Ord + Hash + Copy + Debug> WeightedTable<Loot> {

	/// Attempt to generate a new table
	///
	/// Accepts an iterator of tuples.  Each tupple represents one outcome.  The left
	/// element of the tupple is the value that outcome produces (the table loot, if you
	/// will), and the right element of the tuple is the weight associated with that
	/// outcome.
	///
	/// Performs a safety check to prevent creating an empty roll table.  Raises a parse
	/// error if the safety check fails.  The original tabel can be accessed as a
	/// [`HashMap`] via the [`WeightedTable::weights()`] method
	pub fn new(table: impl IntoIterator<Item=(Loot, u8)>) -> Result<Self, ParseError> {
		let table: BTreeMap<Loot, u8> = table.into_iter()
			.filter(|(_, weight)| *weight > 0)
			.collect();

		if table.is_empty() {
			Err(ParseError::EmptyWeightedTable)
		} else {
			Ok(WeightedTable(table))
		}
	}

	/// Access the orginal table of weights underlying this table
	///
	/// Returns a [`HashMap`] mapping pronouns to their respective weights.
	pub fn weights(&self) -> &BTreeMap<Loot, u8> {
		&self.0
	}

	/// Produces a table of cumulative weights for the roll table
	///
	/// Each entry has the weight of itself as well as all of the previous entries.  This
	/// is provided in a [`Vec`] ordered from lowest weight to highest.  This table can be
	/// used to efficiently perform a custom roll.
	///
	/// Two values are returned:  The table itself, and the highest value in the table
	/// (that is, the sum of all of the weights)
	pub fn rollable_table(&self) -> (Vec<(u64, Loot)>, u64) {
		let mut cumulative_weights: Vec<(u64, Loot)> = Vec::with_capacity(self.0.len());
		let mut weight_so_far = 0;
		for (pronoun, weight) in &self.0 {
			weight_so_far += *weight as u64;
			cumulative_weights.push((weight_so_far, *pronoun));
		}
		(cumulative_weights, weight_so_far)
	}

	/// A shorthand for calling [`WeightedTable::select_on_date()`] with today's date
	///
	/// The date is generated for the system's time and timezone
	pub fn select_today(&self, seed: &[u8]) -> Loot {
		let mut generator = util::INITIAL_STATE;

		seed_with_today(&mut generator);
		pcg64_seed(&mut generator, seed);

		self.select(&mut generator)
	}

	/// Randomly select a pronoun set for a given date and name.
	///
	/// Is a wrapper for calling [`WeightedTable::select`] with the given date mixed into the seed.
	pub fn select_on_date(&self, seed: &[u8], date: Date) -> Loot {
		let mut generator = util::INITIAL_STATE;

		seed_with_date(&mut generator, date);
		pcg64_seed(&mut generator, seed);

		self.select(&mut generator)
	}

	/// Randomly select a pronoun set from a given pre-seeded genenator
	///
	/// You're probably looking for [`select_today()`] or [`select_on_date()`]
	///
	/// This function is *pure*, and any randomness is produced internally using PRNG seeded with
	/// the given date and seed.  That is to say, for any given seed, this table must always
	/// produce the same pronoun set.
	pub fn select(&self, generator: &mut u128) -> Loot {
		let (rollable_table, sum_weights) = self.rollable_table();

		let random = pcg64(generator) % sum_weights;

		rollable_table.iter()
			.filter(|(weight, _)| random < *weight)
			.next()
			.expect("A table was generated with zero entries.  This should be impossible.")
			.1
	}

}