diff --git a/src/storage.rs b/src/storage.rs
index 312a47b..4e31e7f 100644
--- a/src/storage.rs
+++ b/src/storage.rs
@@ -1,22 +1,95 @@
-use std::cmp::Ordering;
-use std::path::Path;
+use std::collections::HashMap;
+use std::collections::HashSet;
 
 #[derive(Copy, Clone, Debug, PartialOrd, PartialEq)]
 pub struct Version(pub u64);
 
-struct Index;
+impl Version {
+    fn next(self) -> Self {
+        Version(self.0 + 1)
+    }
+}
 
-impl Index {
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+struct Hash([u8; 32]);
+
+#[derive(Clone)]
+struct IndexHashEntry {
+    source_paths: HashSet<String>,
+    storage_path: String,
+}
+
+#[derive(Clone)]
+struct IndexPathEntry {
+    hash: Hash,
+    version: Version,
+    storage_path: String,
+}
+
+struct Index<'a> {
+    file_hashes: HashMap<Hash, IndexHashEntry>,
+    file_paths: HashMap<&'a str, IndexPathEntry>,
+}
+
+impl<'a> Index<'a> {
     fn new() -> Self {
-        Self {}
+        Self {
+            file_hashes: HashMap::new(),
+            file_paths: HashMap::new()
+        }
     }
 
-    fn store(&mut self, path: &str, hash: &[u8]) -> (Version, String) {
-        (Version(0), "".to_string())
+    fn store(&mut self, source_path: &'a str, hash: Hash) -> (Version, String) {
+        let path_entry = {
+            self.file_paths.get(source_path).map_or_else(
+                || IndexPathEntry {
+                    hash,
+                    version: Version(0),
+                    storage_path: format!("{:X?}", hash.0),
+                },
+                |old_entry| {
+                    if old_entry.hash == hash {
+                        old_entry.clone() // FIXME optimise
+                    } else {
+                        IndexPathEntry {
+                            hash,
+                            version: old_entry.version.next(),
+                            storage_path: old_entry.storage_path.clone(),
+                        }
+                    }
+                },
+            )
+        };
+
+        self.file_paths.insert(source_path, path_entry.clone());
+
+        let hash_entry = {
+            self.file_hashes.get(&hash).map_or_else(
+                || {
+                    let mut source_paths = HashSet::new();
+                    source_paths.insert(source_path.to_string());
+                    IndexHashEntry {
+                        source_paths,
+                        storage_path: format!("{:X?}", hash.0),
+                    }
+                },
+                |old_entry| {
+                    let mut source_paths = old_entry.source_paths.clone();
+                    source_paths.insert(source_path.to_string());
+                    IndexHashEntry {
+                        source_paths,
+                        storage_path: old_entry.storage_path.clone()
+                    }
+                },
+            )
+        };
+
+        self.file_hashes.insert(hash, hash_entry);
+        (path_entry.version, path_entry.storage_path.to_string())
     }
 
-    fn version(&self, hash: &[u8]) -> Version {
-        Version(0)
+    fn latest_version_for_path(&self, path: &str) -> Option<Version> {
+        self.file_paths.get(path).map_or(None, |entry| Some(entry.version))
     }
 }
 
@@ -25,14 +98,16 @@ mod should {
 
     use super::*;
 
+    const SOME_HASH: Hash = Hash([1; 32]);
+    const SOME_OTHER_HASH: Hash = Hash([2; 32]);
+
     #[test]
     fn support_file_versions() {
         let mut index = Index::new();
-        let (v1, _) = index.store("/some/path", "some hash".as_bytes());
-        let (v2, _) = index.store("/some/path", "some other hash".as_bytes());
+        let (v1, _) = index.store("/some/path", SOME_HASH);
+        let (v2, _) = index.store("/some/path", SOME_OTHER_HASH);
 
-        assert_eq!(v1, index.version("some hash".as_bytes()));
-        assert_eq!(v2, index.version("some other hash".as_bytes()));
+        assert_eq!(v2, index.latest_version_for_path("/some/path").unwrap());
 
         assert!(v2 > v1);
     }
@@ -40,11 +115,11 @@ mod should {
     #[test]
     fn support_deduplication() {
         let mut index = Index::new();
-        let (_, storage_path1) = index.store("/some/path", "same hash".as_bytes());
-        let (_, storage_path2) = index.store("/some/path", "same hash".as_bytes());
-        let (_, storage_path3) = index.store("/some/other/path", "same hash".as_bytes());
+        let (_, storage_path1) = index.store("/some/path", SOME_HASH);
+        let (_, storage_path2) = index.store("/some/path", SOME_HASH);
+        let (_, storage_path3) = index.store("/some/other/path", SOME_HASH);
 
         assert_eq!(storage_path1, storage_path2);
-        assert_ne!(storage_path1, storage_path3);
+        assert_eq!(storage_path1, storage_path3);
     }
 }