Use Rc for better deduplication

This massively reduces memory usage in the number_to_setver example.

examples/number_to_setver.rs

@@ -1,5 +1,6 @@
 use setver::SetVersion;
 use std::env::args;
+use std::rc::Rc;
 
 fn main() {
 	let mut args = args();
@@ -11,14 +12,14 @@ fn main() {
 		// This is exactly how natural numbers (including zero) work in set theory.
 		// Some dynamic programming makes this ~ 1/2 O(n^2).
 		let mut previous_number_setvers = Vec::with_capacity(number);
-		previous_number_setvers.push(SetVersion::default());
+		previous_number_setvers.push(Rc::new(SetVersion::default()));
 
 		for n in 1..=number {
 			let mut n_version = SetVersion::default();
-			for child in previous_number_setvers.iter().take(n) {
-				n_version.add_child_version(child.clone());
+			for i in 0..n {
+				n_version.add_child_version(previous_number_setvers[i].clone());
 			}
-			previous_number_setvers.push(n_version);
+			previous_number_setvers.push(Rc::new(n_version));
 		}
 
 		let number_version = previous_number_setvers.last().unwrap();

src/lib.rs

@@ -4,6 +4,7 @@
 
 use std::collections::BTreeSet;
 use std::fmt::Display;
+use std::rc::Rc;
 use std::str::FromStr;
 
 /// A SetVer version specification.
@@ -13,7 +14,7 @@ use std::str::FromStr;
 #[derive(Eq, PartialEq, Clone, Debug, Ord, PartialOrd, Default)]
 pub struct SetVersion {
 	/// Making this an ordered set guarantees that all iterations are performed in order, which gives some nice guarantees for faster implementations.
-	versions: BTreeSet<SetVersion>,
+	versions: BTreeSet<Rc<SetVersion>>,
 }
 
 impl SetVersion {
@@ -58,7 +59,7 @@ impl SetVersion {
 	}
 
 	/// Adds the given version as a child version. This is useful when constructing a parent version for one or many previous child versions.
-	pub fn add_child_version(&mut self, child: SetVersion) -> &mut Self {
+	pub fn add_child_version(&mut self, child: Rc<SetVersion>) -> &mut Self {
 		self.versions.insert(child);
 		self
 	}
@@ -254,8 +255,8 @@ impl FromStr for SetVersion {
 
 		let versions = inner_sets
 			.iter()
-			.map(|string_set| string_set.parse())
-			.collect::<Result<BTreeSet<SetVersion>, SetVerParseError>>()?;
+			.map(|string_set| string_set.parse::<SetVersion>().map(Rc::new))
+			.collect::<Result<BTreeSet<Rc<SetVersion>>, SetVerParseError>>()?;
 		if versions.len() < inner_sets.len() {
 			return Err(SetVerParseError::NonUniqueElements);
 		}