diff --git a/core/src/WorldBackup.cpp b/core/src/WorldBackup.cpp
new file mode 100644
index 0000000..1fab4c0
--- /dev/null
+++ b/core/src/WorldBackup.cpp
@@ -0,0 +1,156 @@
+#include "vf/WorldBackup.h"
+
+#include <algorithm>
+
+namespace vf {
+
+namespace {
+
+struct LiveChunk {
+    Vec3i coord;
+    uint32_t checksum = 0;
+    ByteBuffer payload;
+};
+
+std::size_t findLive(const std::vector<LiveChunk>& live, const Vec3i& coord) {
+    for (std::size_t i = 0; i < live.size(); ++i)
+        if (live[i].coord == coord) return i;
+    return live.size();
+}
+
+void insertLive(std::vector<LiveChunk>& live, const Vec3i& coord, uint32_t checksum,
+                const ByteBuffer& payload) {
+    std::size_t at = findLive(live, coord);
+    if (at != live.size()) {
+        live[at].checksum = checksum;
+        live[at].payload = payload;
+        return;
+    }
+    LiveChunk fresh;
+    fresh.coord = coord;
+    fresh.checksum = checksum;
+    fresh.payload = payload;
+    live.push_back(std::move(fresh));
+}
+
+void eraseLive(std::vector<LiveChunk>& live, const Vec3i& coord) {
+    std::size_t at = findLive(live, coord);
+    if (at != live.size()) live.erase(live.begin() + static_cast<std::ptrdiff_t>(at));
+}
+
+// Pulls every chunk of a container out as (coordinate, checksum, payload
+// bytes), in canonical order.
+bool sliceContainer(const ByteBuffer& save, SaveHeader& header, std::vector<LiveChunk>& out) {
+    if (!readSaveHeader(save.data(), save.size(), header)) return false;
+    if (header.magic != saveMagic || !isKnownSaveVersion(header.version)) return false;
+    if (header.sectorBytes == 0) return false;
+    std::vector<SaveTableEntry> entries;
+    if (!readSaveTable(save.data(), save.size(), header, entries)) return false;
+
+    out.clear();
+    out.reserve(entries.size());
+    for (const SaveTableEntry& entry : entries) {
+        std::size_t at = static_cast<std::size_t>(entry.sectorOffset) * header.sectorBytes;
+        if (at + entry.payloadBytes > save.size()) return false;
+        LiveChunk chunk;
+        chunk.coord = entry.coord;
+        chunk.payload.assign(save.begin() + static_cast<std::ptrdiff_t>(at),
+                             save.begin() + static_cast<std::ptrdiff_t>(at + entry.payloadBytes));
+        chunk.checksum = fnv1a32(chunk.payload.data(), chunk.payload.size());
+        out.push_back(std::move(chunk));
+    }
+    return true;
+}
+
+std::string idText(uint32_t id) { return "#" + std::to_string(id); }
+
+} // namespace
+
+const char* backupKindName(BackupKind kind) {
+    switch (kind) {
+        case BackupKind::Full: return "full";
+        case BackupKind::Incremental: return "incremental";
+    }
+    return "unknown";
+}
+
+std::size_t BackupEntry::storedBytes() const {
+    std::size_t total = 0;
+    for (const BackupChunk& chunk : stored) total += chunk.payload.size();
+    return total;
+}
+
+const BackupChunk* BackupEntry::findStored(const Vec3i& coord) const {
+    for (const BackupChunk& chunk : stored)
+        if (chunk.coord == coord) return &chunk;
+    return nullptr;
+}
+
+const BackupChunk* BackupEntry::findState(const Vec3i& coord) const {
+    for (const BackupChunk& chunk : state)
+        if (chunk.coord == coord) return &chunk;
+    return nullptr;
+}
+
+const BackupEntry* BackupIndex::find(uint32_t id) const {
+    for (const BackupEntry& entry : entries)
+        if (entry.id == id) return &entry;
+    return nullptr;
+}
+
+BackupEntry* BackupIndex::find(uint32_t id) {
+    for (BackupEntry& entry : entries)
+        if (entry.id == id) return &entry;
+    return nullptr;
+}
+
+uint32_t BackupIndex::latestId() const {
+    uint32_t best = 0;
+    for (const BackupEntry& entry : entries)
+        if (entry.id > best) best = entry.id;
+    return best;
+}
+
+std::size_t BackupIndex::storedBytes() const {
+    std::size_t total = 0;
+    for (const BackupEntry& entry : entries) total += entry.storedBytes();
+    return total;
+}
+
+std::vector<uint32_t> BackupIndex::chainTo(uint32_t id, bool& complete,
+                                           std::string& failure) const {
+    std::vector<uint32_t> walk;
+    complete = false;
+    failure.clear();
+
+    const BackupEntry* entry = find(id);
+    if (entry == nullptr) {
+        failure = "backup " + idText(id) + " is not in the index";
+        return walk;
+    }
+
+    std::size_t guard = entries.size() + 1;
+    while (guard-- > 0) {
+        walk.push_back(entry->id);
+        if (entry->kind == BackupKind::Full) {
+            std::reverse(walk.begin(), walk.end());
+            complete = true;
+            return walk;
+        }
+        const BackupEntry* parent = find(entry->parentId);
+        if (parent == nullptr) {
+            failure = "chain broken: incremental " + idText(entry->id) + " expects parent " +
+                      idText(entry->parentId) + ", which is not in the index";
+            walk.clear();
+            return walk;
+        }
+        entry = parent;
+    }
+    failure = "chain broken: the parent links from " + idText(id) + " never reach a full backup";
+    walk.clear();
+    return walk;
+}
+
+uint32_t BackupStore::capture(BackupKind kind, const ByteBuffer& save, uint64_t tick) {
+    failure.clear();
+