diff --git a/src/hashmaps/mod.rs b/src/hashmaps/mod.rs
new file mode 100644
index 0000000..6266272
--- /dev/null
+++ b/src/hashmaps/mod.rs
@@ -0,0 +1,94 @@
+use std::hash::{BuildHasher, Hash};
+
+pub mod simple_open_addressing;
+
+trait HashMapFamily {
+    type Map<K, V, S>: HashMap<K, V, S>;
+}
+
+trait HashMap<K, V, S>: IntoIterator<Item = (K, V)> {
+    fn with_hasher(state: S) -> Self;
+
+    fn len(&self) -> usize;
+
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    fn get(&self, key: &K) -> Option<&V>
+    where
+        K: Eq + Hash,
+        S: BuildHasher;
+
+    fn insert(&mut self, key: K, value: V) -> Option<V>
+    where
+        K: Eq + Hash,
+        S: BuildHasher;
+}
+
+#[cfg(test)]
+mod tests {
+    use std::hash::{BuildHasher, BuildHasherDefault, Hasher, RandomState};
+
+    use super::{HashMap, HashMapFamily};
+
+    #[derive(Default)]
+    struct CollidingHasher;
+    impl Hasher for CollidingHasher {
+        fn finish(&self) -> u64 {
+            0
+        }
+        fn write(&mut self, _bytes: &[u8]) {}
+    }
+
+    pub(super) fn run_tests<M>()
+    where
+        M: HashMapFamily,
+    {
+        let mk_str = || M::Map::<&str, &str, _>::with_hasher(RandomState::new());
+
+        let m = mk_str();
+        assert_eq!(m.get(&"uwu"), None);
+        assert_eq!(m.get(&"uwu"), None);
+
+        let mut m = mk_str();
+        m.insert("hello", "world");
+        assert_eq!(m.get(&"hello"), Some(&"world"));
+        assert_eq!(m.len(), 1);
+        m.insert("aaa", "yes");
+        assert_eq!(m.get(&"hello"), Some(&"world"));
+        assert_eq!(m.get(&"aaa"), Some(&"yes"));
+        assert_eq!(m.len(), 2);
+
+        let mut m = mk_str();
+        m.insert("hello", "world");
+        assert_eq!(m.get(&"hello"), Some(&"world"));
+        assert_eq!(m.len(), 1);
+        m.insert("hello", "no");
+        assert_eq!(m.get(&"hello"), Some(&"no"));
+        assert_eq!(m.len(), 1);
+
+        for count in [1, 10, 100, 1000, 10_000, 100_000] {
+            test_many::<M, _>(count, RandomState::new());
+        }
+        test_many::<M, _>(5000, BuildHasherDefault::<CollidingHasher>::default());
+    }
+
+    fn test_many<M: HashMapFamily, H: BuildHasher>(count: usize, h: H) {
+        let mut m = M::Map::with_hasher(h);
+
+        for i in 0..count {
+            m.insert(i, i);
+        }
+
+        let mut found = vec![false; count];
+        for (k, v) in m.into_iter() {
+            assert_eq!(k, v);
+            assert!(!found[k], "duplicate element");
+            found[k] = true;
+        }
+        for (i, found) in found.iter().enumerate() {
+            assert!(found, "element {i} was lost");
+        }
+    }
+}
diff --git a/src/hashmap.rs b/src/hashmaps/simple_open_addressing.rs
similarity index 51%
rename from src/hashmap.rs
rename to src/hashmaps/simple_open_addressing.rs
index 098d528..a23a8d7 100644
--- a/src/hashmap.rs
+++ b/src/hashmaps/simple_open_addressing.rs
@@ -1,3 +1,4 @@
+use super::{HashMap, HashMapFamily};
 use std::{
     hash::{BuildHasher, Hash, RandomState},
     vec,
@@ -5,20 +6,44 @@ use std::{
 
 type Entry<K, V> = Option<(K, V)>;
 
-pub struct HashMap<K, V, S = RandomState> {
+pub struct SimpleOAHashMap<K, V, S = RandomState> {
     buckets: Vec<Entry<K, V>>,
     filled: usize,
     s: S,
 }
 
-impl<K: Eq + Hash, V> HashMap<K, V, RandomState> {
+impl<K: Eq + Hash, V> SimpleOAHashMap<K, V, RandomState> {
     pub fn new() -> Self {
         Self::with_hasher(RandomState::new())
     }
 }
 
-impl<K: Eq + Hash, V, S: BuildHasher> HashMap<K, V, S> {
-    pub fn with_hasher(state: S) -> Self {
+impl<K: Eq + Hash, V, S: BuildHasher> SimpleOAHashMap<K, V, S> {
+    fn bucket_of_elem(&self, key: &K) -> usize {
+        assert_ne!(self.buckets.len(), 0, "cannot compute bucket of empty map");
+        let hash = self.s.hash_one(&key) as usize;
+        hash % self.buckets.len()
+    }
+
+    fn grow(&mut self) {
+        let len = self.buckets.len();
+        let new = if len == 0 { 8 } else { len * 2 };
+        let old = IntoIter::new(std::mem::take(&mut self.buckets));
+        let new_buckets = (0..new).map(|_| None).collect();
+        self.buckets = new_buckets;
+        self.extend(old);
+    }
+}
+
+impl<K: Eq + Hash, V, S: BuildHasher> Extend<(K, V)> for SimpleOAHashMap<K, V, S> {
+    fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
+        iter.into_iter()
+            .for_each(|(key, value)| drop(self.insert(key, value)));
+    }
+}
+
+impl<K, V, S> super::HashMap<K, V, S> for SimpleOAHashMap<K, V, S> {
+    fn with_hasher(state: S) -> Self {
         Self {
             buckets: Vec::new(),
             filled: 0,
@@ -26,15 +51,19 @@ impl<K: Eq + Hash, V, S: BuildHasher> HashMap<K, V, S> {
         }
     }
 
-    pub fn len(&self) -> usize {
+    fn len(&self) -> usize {
         self.filled
     }
 
-    pub fn is_empty(&self) -> bool {
-        self.buckets.len() == 0
+    fn is_empty(&self) -> bool {
+        self.len() == 0
     }
 
-    pub fn get(&self, key: &K) -> Option<&V> {
+    fn get(&self, key: &K) -> Option<&V>
+    where
+        K: Eq + Hash,
+        S: BuildHasher,
+    {
         if self.is_empty() {
             return None;
         }
@@ -52,7 +81,11 @@ impl<K: Eq + Hash, V, S: BuildHasher> HashMap<K, V, S> {
         }
     }
 
-    pub fn insert(&mut self, key: K, value: V) {
+    fn insert(&mut self, key: K, value: V) -> Option<V>
+    where
+        K: Eq + Hash,
+        S: BuildHasher,
+    {
         if self.filled >= self.buckets.len() {
             self.grow();
         }
@@ -65,35 +98,13 @@ impl<K: Eq + Hash, V, S: BuildHasher> HashMap<K, V, S> {
                 if bucket.is_none() {
                     self.filled += 1;
                 }
-                *bucket = Some((key, value));
-                return;
+                let before = std::mem::replace(bucket, Some((key, value)));
+                return before.map(|(_, v)| v);
             } else {
                 self.grow();
             }
         }
     }
-
-    fn bucket_of_elem(&self, key: &K) -> usize {
-        assert_ne!(self.buckets.len(), 0, "cannot compute bucket of empty map");
-        let hash = self.s.hash_one(&key) as usize;
-        hash % self.buckets.len()
-    }
-
-    fn grow(&mut self) {
-        let len = self.buckets.len();
-        let new = if len == 0 { 8 } else { len * 2 };
-        let old = IntoIter::new(std::mem::take(&mut self.buckets));
-        let new_buckets = (0..new).map(|_| None).collect();
-        self.buckets = new_buckets;
-        self.extend(old);
-    }
-}
-
-impl<K: Eq + Hash, V, S: BuildHasher> Extend<(K, V)> for HashMap<K, V, S> {
-    fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
-        iter.into_iter()
-            .for_each(|(key, value)| self.insert(key, value));
-    }
 }
 
 pub struct IntoIter<K, V> {
@@ -116,7 +127,7 @@ impl<K, V> Iterator for IntoIter<K, V> {
     }
 }
 
-impl<K, V, S> IntoIterator for HashMap<K, V, S> {
+impl<K, V, S> IntoIterator for SimpleOAHashMap<K, V, S> {
     type Item = (K, V);
 
     type IntoIter = IntoIter<K, V>;
@@ -126,78 +137,15 @@ impl<K, V, S> IntoIterator for HashMap<K, V, S> {
     }
 }
 
+pub struct SimpleOAHashMapFamily;
+impl HashMapFamily for SimpleOAHashMapFamily {
+    type Map<K, V, S> = SimpleOAHashMap<K, V, S>;
+}
+
 #[cfg(test)]
 mod tests {
-    use std::hash::{BuildHasher, BuildHasherDefault, Hasher, RandomState};
-
-    use super::HashMap;
-
     #[test]
-    fn get_empty() {
-        let m = HashMap::<&str, ()>::new();
-        assert_eq!(m.get(&"uwu"), None);
-        assert_eq!(m.get(&"uwu"), None);
-    }
-
-    #[test]
-    fn insert() {
-        let mut m = HashMap::new();
-        m.insert("hello", "world");
-        assert_eq!(m.get(&"hello"), Some(&"world"));
-        assert_eq!(m.len(), 1);
-        m.insert("aaa", "yes");
-        assert_eq!(m.get(&"hello"), Some(&"world"));
-        assert_eq!(m.get(&"aaa"), Some(&"yes"));
-        assert_eq!(m.len(), 2);
-    }
-
-    #[test]
-    fn overriding() {
-        let mut m = HashMap::new();
-        m.insert("hello", "world");
-        assert_eq!(m.get(&"hello"), Some(&"world"));
-        assert_eq!(m.len(), 1);
-        m.insert("hello", "no");
-        assert_eq!(m.get(&"hello"), Some(&"no"));
-        assert_eq!(m.len(), 1);
-    }
-
-    #[derive(Default)]
-    struct CollidingHasher;
-    impl Hasher for CollidingHasher {
-        fn finish(&self) -> u64 {
-            0
-        }
-        fn write(&mut self, _bytes: &[u8]) {}
-    }
-
-    fn test_many<H: BuildHasher>(count: usize, h: H) {
-        let mut m = HashMap::with_hasher(h);
-
-        for i in 0..count {
-            m.insert(i, i);
-        }
-
-        let mut found = vec![false; count];
-        for (k, v) in m.into_iter() {
-            assert_eq!(k, v);
-            assert!(!found[k], "duplicate element");
-            found[k] = true;
-        }
-        for (i, found) in found.iter().enumerate() {
-            assert!(found, "element {i} was lost");
-        }
-    }
-
-    #[test]
-    fn many_elements() {
-        for count in [1, 10, 100, 1000, 10_000, 100_000] {
-            test_many(count, RandomState::new());
-        }
-    }
-
-    #[test]
-    fn many_many_collisions() {
-        test_many(5000, BuildHasherDefault::<CollidingHasher>::default());
+    fn do_tests() {
+        crate::hashmaps::tests::run_tests::<super::SimpleOAHashMapFamily>();
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 4a66f44..53bd25b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,7 +5,7 @@
 #![feature(strict_provenance)]
 
 pub mod cfg_match;
-pub mod hashmap;
+pub mod hashmaps;
 pub mod innocent_linked_list;
 pub mod scratch;
 pub mod sendsync;