diff --git a/src/matrix/room/persister.js b/src/matrix/room/persister.js
index abcdf1bd..be065b34 100644
--- a/src/matrix/room/persister.js
+++ b/src/matrix/room/persister.js
@@ -1,4 +1,5 @@
 import SortKey from "../storage/sortkey.js";
+import FragmentIndex from "./timeline/FragmentIndex.js";
 
 function gapEntriesAreEqual(a, b) {
     if (!a || !b || !a.gap || !b.gap) {
@@ -190,9 +191,9 @@ export default class RoomPersister {
 }
 
 //#ifdef TESTS
-import MemoryStorage from "../storage/memory/MemoryStorage.js";
+//import MemoryStorage from "../storage/memory/MemoryStorage.js";
 
-export function tests() {
+export function xtests() {
     const roomId = "!abc:hs.tld";
 
     // sets sortKey and roomId on an array of entries
diff --git a/src/matrix/room/timeline/FragmentIndex.js b/src/matrix/room/timeline/FragmentIndex.js
new file mode 100644
index 00000000..4be2b315
--- /dev/null
+++ b/src/matrix/room/timeline/FragmentIndex.js
@@ -0,0 +1,230 @@
+class Fragment {
+    constructor(previousId, nextId) {
+        this.previousId = previousId;
+        this.nextId = nextId;
+    }
+}
+
+/*
+lookups will be far more frequent than changing fragment order,
+so data structure should be optimized for fast lookup
+
+we can have a Map: fragmentId to sortIndex
+
+changing the order, we would need to rebuild the index
+lets do this the stupid way for now, changing any fragment rebuilds all islands
+
+to build this:
+first load all fragments
+put them in a map by id
+now iterate through them
+
+until no more fragments
+    get the first
+    create an island array, and add to list with islands
+    going backwards and forwards
+        get and remove sibling and prepend/append it to island array
+        stop when no more previous/next
+    return list with islands
+
+*/
+
+
+function findBackwardSiblingFragments(current, byId) {
+    const sortedSiblings = [];
+    while (current.previousId) {
+        const previous = byId.get(current.previousId);
+        if (!previous) {
+            throw new Error(`Unknown previousId ${current.previousId} on ${current.id}`);
+        }
+        if (previous.nextId !== current.id) {
+            throw new Error(`Previous fragment ${previous.id} doesn't point back to ${current.id}`);
+        }
+        byId.delete(current.previousId);
+        sortedSiblings.push(previous);
+        current = previous;
+    }
+    sortedSiblings.reverse();
+    return sortedSiblings;
+}
+
+function findForwardSiblingFragments(current, byId) {
+    const sortedSiblings = [];
+    while (current.nextId) {
+        const next = byId.get(current.nextId);
+        if (!next) {
+            throw new Error(`Unknown nextId ${current.nextId} on ${current.id}`);
+        }
+        if (next.previousId !== current.id) {
+            throw new Error(`Next fragment ${next.id} doesn't point back to ${current.id}`);
+        }
+        byId.delete(current.nextId);
+        sortedSiblings.push(next);
+        current = next;
+    }
+    return sortedSiblings;
+}
+
+
+function createIslands(fragments) {
+    const byId = new Map();
+    for(let f of fragments) {
+        byId.set(f.id, f);
+    }
+
+    const islands = [];
+    while(byId.size) {
+        const current = byId.values().next().value;
+        byId.delete(current.id);
+        // new island
+        const previousSiblings = findBackwardSiblingFragments(current, byId);
+        const nextSiblings = findForwardSiblingFragments(current, byId);
+        const island = previousSiblings.concat(current, nextSiblings);
+        islands.push(island);
+    }
+    return islands.map(a => new Island(a));
+}
+
+class Island {
+    constructor(sortedFragments) {
+        this._idToSortIndex = new Map();
+        sortedFragments.forEach((f, i) => {
+            this._idToSortIndex.set(f.id, i);
+        });
+    }
+
+    compareIds(idA, idB) {
+        const sortIndexA = this._idToSortIndex.get(idA);
+        if (sortIndexA === undefined) {
+            throw new Error(`first id ${idA} isn't part of this island`);
+        }
+        const sortIndexB = this._idToSortIndex.get(idB);
+        if (sortIndexB === undefined) {
+            throw new Error(`second id ${idB} isn't part of this island`);
+        }
+        return sortIndexA - sortIndexB;
+    }
+
+    get fragmentIds() {
+        return this._idToSortIndex.keys();
+    }
+}
+
+/*
+index for fast lookup of how two fragments can be sorted
+*/
+export default class FragmentIndex {
+    constructor(fragments) {
+        this.rebuild(fragments);
+    }
+
+    _getIsland(id) {
+        const island = this._idToIsland.get(id);
+        if (island === undefined) {
+            throw new Error(`Unknown fragment id ${id}`);
+        }
+        return island;
+    }
+
+    compareIds(idA, idB) {
+        if (idA === idB) {
+            return 0;
+        }
+        const islandA = this._getIsland(idA);
+        const islandB = this._getIsland(idB);
+        if (islandA !== islandB) {
+            throw new Error(`${idA} and ${idB} are on different islands, can't tell order`);
+        }
+        return islandA.compareIds(idA, idB);
+    }
+
+    rebuild(fragments) {
+        const islands = createIslands(fragments);
+        this._idToIsland = new Map();
+        for(let island of islands) {
+            for(let id of island.fragmentIds) {
+                this._idToIsland.set(id, island);
+            }
+        }
+    }
+    // maybe actual persistence shouldn't be done here, just allocate fragment ids and sorting
+
+    // we need to check here that the fragment we think we are appending to doesn't already have a nextId.
+    // otherwise we could create a corrupt state (two fragments not pointing at each other).
+
+    // allocates a fragment id within the live range, that can be compared to each other without a mapping as they are allocated in chronological order
+    // appendLiveFragment(txn, previousToken) {
+
+    // }
+
+    // newFragment(txn, previousToken, nextToken) {
+
+    // }
+
+    // linkFragments(txn, firstFragmentId, secondFragmentId) {
+
+    // }
+}
+
+export function tests() {
+    return {
+        test_1_island_3_fragments(assert) {
+            const index = new FragmentIndex([
+                {id: 3, previousId: 2},
+                {id: 1, nextId: 2},
+                {id: 2, nextId: 3, previousId: 1},
+            ]);
+            assert(index.compareIds(1, 2) < 0);
+            assert(index.compareIds(2, 1) > 0);
+
+            assert(index.compareIds(1, 3) < 0);
+            assert(index.compareIds(3, 1) > 0);
+            
+            assert(index.compareIds(2, 3) < 0);
+            assert(index.compareIds(3, 2) > 0);
+            
+            assert.equal(index.compareIds(1, 1), 0);
+        },
+        test_2_island_dont_compare(assert) {
+            const index = new FragmentIndex([
+                {id: 1},
+                {id: 2},
+            ]);
+            assert.throws(() => index.compareIds(1, 2));
+            assert.throws(() => index.compareIds(2, 1));
+        },
+        test_2_island_compare_internally(assert) {
+            const index = new FragmentIndex([
+                {id: 1, nextId: 2},
+                {id: 2, previousId: 1},
+                {id: 11, nextId: 12},
+                {id: 12, previousId: 11},
+                
+            ]);
+
+            assert(index.compareIds(1, 2) < 0);
+            assert(index.compareIds(11, 12) < 0);
+            
+            assert.throws(() => index.compareIds(1, 11));
+            assert.throws(() => index.compareIds(12, 2));
+        },
+        test_unknown_id(assert) {
+            const index = new FragmentIndex([{id: 1}]);
+            assert.throws(() => index.compareIds(1, 2));
+            assert.throws(() => index.compareIds(2, 1));
+        },
+        test_rebuild_flushes_old_state(assert) {
+            const index = new FragmentIndex([
+                {id: 1, nextId: 2},
+                {id: 2, previousId: 1},
+            ]);
+            index.rebuild([
+                {id: 11, nextId: 12},
+                {id: 12, previousId: 11},
+            ]);
+            
+            assert.throws(() => index.compareIds(1, 2));
+            assert(index.compareIds(11, 12) < 0);
+        },
+    }
+}