From 3530038ca45117de4b4ee8284f00d6bae3e6bc32 Mon Sep 17 00:00:00 2001
From: Emi Simpson <emi@alchemi.dev>
Date: Mon, 25 Oct 2021 12:16:57 -0400
Subject: [PATCH] Expose some more randomization utilities

---
 src/util.rs           | 42 +++++++++++++++++++++++++++++++++
 src/weighted_table.rs | 54 +++++++++++++++----------------------------
 2 files changed, 61 insertions(+), 35 deletions(-)

diff --git a/src/util.rs b/src/util.rs
index 2e93d75..8b30918 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -1,3 +1,21 @@
+use time::{Date, Month, OffsetDateTime};
+
+/// The start of the COVID-19 lockdowns
+///
+/// This is used as an epoch in order to convert from a given date to an integer seed.  This is
+/// specified as part of the algorithm for randomly selecting from a weighted list.
+pub const COVID_EPOCH: Date = match Date::from_calendar_date(2020, Month::January, 26) {
+	Ok(d) => d,
+	Err(_) => Date::MIN, // This never runs, but we can't unwrap, so this is what we're stuck with
+};
+
+/// A state to use as an initial state before seeding
+///
+/// Think of this as a ::new() method for a pcg64 generator.  Subsiquent calls should be
+/// made to the [`pcg64_seed()`] methods or the [`seed_with_date()`]/[`seed_with_today()`]
+/// methods
+pub const INITIAL_STATE: u128 = 1312_1312_1312;
+
 pub fn pcg64(state: &mut u128) -> u64 {
 	let mut x = *state;
 	pcg64_iterstate(state);
@@ -18,3 +36,27 @@ pub fn pcg64_seed(state: &mut u128, bytes: &[u8]) {
     }
 	pcg64_iterstate(state);
 }
+
+/// Seed the generator with a given [`Date`] object
+pub fn seed_with_date(state: &mut u128, date: Date) {
+	pcg64_seed(
+		state,
+		&(
+			(date - COVID_EPOCH)
+			.whole_days()
+			as u32
+		).to_le_bytes()
+	)
+}
+
+/// Seed the generator with the [`Date`] object representing today's date
+///
+/// Uses the system's local time, or falls back to UTC
+pub fn seed_with_today(state: &mut u128) {
+	seed_with_date(
+		state,
+		OffsetDateTime::now_local()
+			.unwrap_or_else(|_| OffsetDateTime::now_utc())
+			.date()
+	)
+}
diff --git a/src/weighted_table.rs b/src/weighted_table.rs
index dd03535..55a730a 100644
--- a/src/weighted_table.rs
+++ b/src/weighted_table.rs
@@ -1,22 +1,10 @@
-use crate::{
-	user_preferences::ParseError,
-	util::{pcg64, pcg64_seed},
-};
+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, Month, OffsetDateTime};
-
-/// The start of the COVID-19 lockdowns
-///
-/// This is used as an epoch in order to convert from a given date to an integer seed.  This is
-/// specified as part of the algorithm for randomly selecting from a weighted list.
-pub const COVID_EPOCH: Date = match Date::from_calendar_date(2020, Month::January, 26) {
-	Ok(d) => d,
-	Err(_) => Date::MIN, // This never runs, but we can't unwrap, so this is what we're stuck with
-};
+use time::Date;
 
 /// A list of pronouns and their associated weights, used for random selection
 ///
@@ -82,41 +70,37 @@ impl<Loot: Eq + Ord + Hash + Copy + Debug> WeightedTable<Loot> {
 	///
 	/// The date is generated for the system's time and timezone
 	pub fn select_today(&self, seed: &[u8]) -> Loot {
-		self.select_on_date(
-			seed,
-			OffsetDateTime::now_local()
-				.unwrap_or_else(|_| OffsetDateTime::now_utc())
-				.date()
-		)
+		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 new_seed: Vec<u8> = Vec::with_capacity(seed.len() + 4);
-		new_seed.extend(
-			(
-				(date - COVID_EPOCH)
-				.whole_days()
-				as u32
-			).to_le_bytes()
-		);
-		new_seed.extend(seed);
-		self.select(&new_seed)
+		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 for a given seed
+	/// 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, seed: &[u8]) -> Loot {
+	pub fn select(&self, generator: &mut u128) -> Loot {
 		let (rollable_table, sum_weights) = self.rollable_table();
 
-		let mut generator: u128 = 131213121312;
-		pcg64_seed(&mut generator, seed);
-		let random = pcg64(&mut generator) % sum_weights;
+		let random = pcg64(generator) % sum_weights;
 
 		rollable_table.iter()
 			.filter(|(weight, _)| random < *weight)