116 lines
4.1 KiB
Rust
116 lines
4.1 KiB
Rust
use crate::{user_preferences::ParseError, util::{self, pcg64, pcg64_seed, seed_with_date, seed_with_today}};
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use std::{collections::BTreeMap, hash::Hash};
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use std::fmt::Debug;
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use std::cmp::Eq;
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use time::Date;
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/// A list of pronouns and their associated weights, used for random selection
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///
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/// Weights are typically representative of a user's preference towards a pronoun. A pronoun with
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/// a weight of 10 is twice as likely to be selected as a pronoun with a weight of 5.
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///
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/// This struct is use to represent these weights before they are used to randomly select a
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/// pronoun. Additional methods are provided to perform this random selection on a weighted list,
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/// using as a seed both an arbitrary string of bytes and a Date.
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#[derive(Clone, PartialEq, Eq, Debug)]
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pub struct WeightedTable<Loot: Eq + Ord + Hash + Debug>(BTreeMap<Loot, u8>);
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impl<Loot: Eq + Ord + Hash + Copy + Debug> WeightedTable<Loot> {
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/// Attempt to generate a new table
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///
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/// Accepts an iterator of tuples. Each tupple represents one outcome. The left
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/// element of the tupple is the value that outcome produces (the table loot, if you
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/// will), and the right element of the tuple is the weight associated with that
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/// outcome.
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///
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/// Performs a safety check to prevent creating an empty roll table. Raises a parse
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/// error if the safety check fails. The original tabel can be accessed as a
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/// [`HashMap`] via the [`WeightedTable::weights()`] method
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pub fn new(table: impl IntoIterator<Item=(Loot, u8)>) -> Result<Self, ParseError> {
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let table: BTreeMap<Loot, u8> = table.into_iter()
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.filter(|(_, weight)| *weight > 0)
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.collect();
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if table.is_empty() {
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Err(ParseError::EmptyWeightedTable)
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} else {
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Ok(WeightedTable(table))
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}
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}
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/// Access the orginal table of weights underlying this table
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///
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/// Returns a [`HashMap`] mapping pronouns to their respective weights.
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pub fn weights(&self) -> &BTreeMap<Loot, u8> {
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&self.0
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}
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/// Produces a table of cumulative weights for the roll table
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///
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/// Each entry has the weight of itself as well as all of the previous entries. This
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/// is provided in a [`Vec`] ordered from lowest weight to highest. This table can be
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/// used to efficiently perform a custom roll.
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///
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/// Two values are returned: The table itself, and the highest value in the table
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/// (that is, the sum of all of the weights)
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pub fn rollable_table(&self) -> (Vec<(u64, Loot)>, u64) {
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let mut cumulative_weights: Vec<(u64, Loot)> = Vec::with_capacity(self.0.len());
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let mut weight_so_far = 0;
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for (pronoun, weight) in &self.0 {
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weight_so_far += *weight as u64;
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cumulative_weights.push((weight_so_far, *pronoun));
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}
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(cumulative_weights, weight_so_far)
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}
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/// A shorthand for calling [`WeightedTable::select_on_date()`] with today's date
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///
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/// The date is generated for the system's time and timezone
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pub fn select_today(&self, seed: &[u8]) -> Loot {
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let mut generator = util::INITIAL_STATE;
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seed_with_today(&mut generator);
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pcg64_seed(&mut generator, seed);
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self.select(&mut generator)
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}
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/// Randomly select a pronoun set for a given date and name.
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///
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/// Is a wrapper for calling [`WeightedTable::select`] with the given date mixed into the seed.
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pub fn select_on_date(&self, seed: &[u8], date: Date) -> Loot {
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let mut generator = util::INITIAL_STATE;
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seed_with_date(&mut generator, date);
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pcg64_seed(&mut generator, seed);
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self.select(&mut generator)
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}
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/// Randomly select a pronoun set from a given pre-seeded genenator
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///
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/// You're probably looking for [`select_today()`] or [`select_on_date()`]
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///
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/// This function is *pure*, and any randomness is produced internally using PRNG seeded with
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/// the given date and seed. That is to say, for any given seed, this table must always
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/// produce the same pronoun set.
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pub fn select(&self, generator: &mut u128) -> Loot {
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let (rollable_table, sum_weights) = self.rollable_table();
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let random = pcg64(generator) % sum_weights;
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rollable_table.iter()
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.filter(|(weight, _)| random < *weight)
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.next()
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.expect("A table was generated with zero entries. This should be impossible.")
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.1
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}
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}
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