Changed most everything

src/utils/substack.rs

@@ -0,0 +1,43 @@
+
+/// Implement a FILO stack that lives on the regular call stack as a linked list.
+/// Mainly useful to detect loops in recursive algorithms where the recursion isn't
+/// deep enough to warrant a heap-allocated set
+#[derive(Debug, Clone, Copy)]
+pub struct Substack<'a, T> {
+    pub item: T,
+    pub prev: Option<&'a Self>
+}
+
+impl<'a, T> Substack<'a, T> {
+    pub fn item(&self) -> &T { &self.item }
+    pub fn prev(&self) -> Option<&'a Substack<'a, T>> { self.prev }
+
+    pub fn new(item: T) -> Self {
+        Self {
+            item,
+            prev: None
+        }
+    }
+    pub fn push(&'a self, item: T) -> Self {
+        Self {
+            item,
+            prev: Some(self)
+        }
+    }
+    pub fn iter(&'a self) -> SubstackIterator<'a, T> {
+        SubstackIterator { curr: Some(self) }
+    }
+}
+
+pub struct SubstackIterator<'a, T> {
+    curr: Option<&'a Substack<'a, T>>
+}
+
+impl<'a, T> Iterator for SubstackIterator<'a, T> {
+    type Item = &'a T;
+    fn next(&mut self) -> Option<&'a T> {
+        let Substack{ item, prev } = self.curr?;
+        self.curr = *prev;
+        Some(item)
+    }
+}