diff --git a/src/lib.zig b/src/lib.zig
index ecfeade..969bab6 100644
--- a/src/lib.zig
+++ b/src/lib.zig
@@ -1,10 +1,26 @@
 const std = @import("std");
+const builtin = @import("builtin");
 const testing = std.testing;
 
-export fn add(a: i32, b: i32) i32 {
-    return a + b;
+pub fn OnceCell(comptime T: type) type {
+    if (builtin.single_threaded) {
+        return @import("./singlethread.zig").OnceCell(T);
+    } else {
+        return @import("./multithread.zig").OnceCell(T);
+    }
 }
 
-test "basic add functionality" {
-    try testing.expect(add(3, 7) == 10);
+// pub fn Lazy(comptime T: type, comptime f: fn () T) type {
+//     _ = f;
+//     return struct {
+//         cell: OnceCell(T),
+//     };
+// }
+
+test {
+    // Import All files
+    _ = @import("./singlethread.zig");
+    _ = @import("./multithread.zig");
+
+    std.testing.refAllDecls(@This());
 }
diff --git a/src/multithread.zig b/src/multithread.zig
new file mode 100644
index 0000000..3280935
--- /dev/null
+++ b/src/multithread.zig
@@ -0,0 +1,252 @@
+const std = @import("std");
+const testing = std.testing;
+
+pub fn Lazy(comptime T: type, comptime f: fn () T) type {
+    return struct {
+        cell: OnceCell(T),
+
+        const Self = @This();
+
+        pub fn init() Self {
+            return Self{ .cell = OnceCell(T).init() };
+        }
+
+        pub fn get(self: *Self) *T {
+            return self.cell.getOrInit(f);
+        }
+
+        pub fn getConst(self: *Self) *const T {
+            return self.cell.getOrInit(f);
+        }
+    };
+}
+
+pub fn OnceCell(comptime T: type) type {
+    return struct {
+        cell: T = undefined,
+        mutex: std.Thread.Mutex = std.Thread.Mutex{},
+        done: bool = false,
+
+        const Self = @This();
+
+        pub fn init() Self {
+            return Self{};
+        }
+
+        pub fn get(self: *Self) ?*T {
+            if (self.isInitialize()) {
+                return &self.cell;
+            }
+            return null;
+        }
+
+        pub fn getUnchecked(self: *Self) *T {
+            std.debug.assert(self.isInitialize());
+
+            return &self.cell;
+        }
+
+        pub fn getOrInit(self: *Self, comptime f: fn () T) *T {
+            // Fast path check
+            if (self.get()) |value| {
+                return value;
+            }
+
+            self.initialize(f);
+            std.debug.assert(self.isInitialize());
+
+            return self.getUnchecked();
+        }
+
+        // --------------------------------------------------------------------------------
+        //                                   Core API
+        // --------------------------------------------------------------------------------
+
+        fn isInitialize(self: Self) bool {
+            return @atomicLoad(bool, &self.done, .Acquire);
+        }
+
+        fn initialize(self: *Self, comptime f: fn () T) void {
+            @setCold(true);
+
+            self.mutex.lock();
+            defer self.mutex.unlock();
+
+            // The first thread to acquire the mutex gets to run the initializer
+
+            if (!self.done) {
+                self.cell = f();
+                defer @atomicStore(bool, &self.done, true, .Release);
+            }
+        }
+    };
+}
+
+// --------------------------------------------------------------------------------
+//                                   Testing
+// --------------------------------------------------------------------------------
+
+const allocator = testing.allocator;
+fn return_1() i32 {
+    return 1;
+}
+
+fn return_2() i32 {
+    return 2;
+}
+
+fn returnMap() std.StringHashMap(i32) {
+    var map = std.StringHashMap(i32).init(allocator);
+    map.put("b", 2) catch @panic("unable to put b");
+    return map;
+}
+
+var globalMap = OnceCell(std.StringHashMap(i32)).init();
+
+test "test global map" {
+    _ = globalMap.getOrInit(returnMap);
+    var r1 = globalMap.get().?;
+    try r1.*.put("a", 1);
+
+    try testing.expect(r1.*.get("b") != null);
+    try testing.expect(r1.*.get("b").? == 2);
+
+    // must be same hashmap
+    _ = globalMap.getOrInit(returnMap);
+    var r2 = globalMap.get().?;
+
+    try testing.expect(r2.*.get("a") != null);
+    try testing.expect(r2.*.get("a").? == 1);
+
+    defer r2.*.deinit();
+}
+
+test "test assume init" {
+    var cell1 = OnceCell(i32).init();
+    const r1 = cell1.getOrInit(return_1);
+    const r2 = cell1.getUnchecked();
+
+    try testing.expect(r1.* == 1);
+    try testing.expect(r2.* == 1);
+}
+
+test "test cell multi init" {
+    var cell1 = OnceCell(i32).init();
+    var cell2 = OnceCell(i32).init();
+
+    const r1 = cell1.getOrInit(return_1);
+    const r2 = cell1.getOrInit(return_1);
+    const r3 = cell1.getOrInit(return_1);
+
+    try testing.expect(r1.* == 1);
+    try testing.expectEqual(r1, r2);
+    try testing.expectEqual(r2, r3);
+
+    const a1 = cell2.getOrInit(return_2);
+    const a2 = cell2.getOrInit(return_2);
+    const a3 = cell2.getOrInit(return_2);
+
+    try testing.expect(a1.* == 2);
+    try testing.expectEqual(a1, a2);
+    try testing.expectEqual(a2, a3);
+}
+
+var shared: i32 = 0;
+
+fn incrShared() i32 {
+    shared += 1;
+    return shared;
+}
+
+var cell3 = OnceCell(i32).init();
+
+test "test multithread shared value" {
+    var threads: [10]std.Thread = undefined;
+    defer for (threads) |handle| handle.join();
+
+    for (&threads) |*handle| {
+        handle.* = try std.Thread.spawn(.{}, struct {
+            fn thread_fn(x: u8) void {
+                _ = x;
+                _ = cell3.getOrInit(incrShared);
+            }
+        }.thread_fn, .{0});
+    }
+
+    try testing.expectEqual(@as(i32, 1), shared);
+}
+
+var LazyMap = Lazy(std.StringHashMap(i32), returnMap).init();
+
+test "test lazy" {
+    var map = LazyMap.get();
+    defer map.*.deinit();
+    try map.*.put("c", 3);
+
+    var map2 = LazyMap.get();
+
+    try testing.expect(map2.*.get("c") != null);
+    try testing.expect(map2.*.get("c").? == 3);
+}
+
+const MutexStringHashMap = struct {
+    mutex: std.Thread.Mutex,
+    str_map: std.StringHashMap(i32),
+
+    const Self = @This();
+    pub fn init() Self {
+        return Self{
+            .mutex = std.Thread.Mutex{},
+            .str_map = std.StringHashMap(i32).init(allocator),
+        };
+    }
+
+    pub fn deinit(self: *Self) void {
+        self.str_map.deinit();
+    }
+
+    pub fn borrow(self: *Self) *std.StringHashMap(i32) {
+        self.mutex.lock();
+        return &self.str_map;
+    }
+
+    pub fn restore(self: *Self) void {
+        self.mutex.unlock();
+    }
+};
+
+fn returnMutexMap() MutexStringHashMap {
+    return MutexStringHashMap.init();
+}
+
+var LazyMutexMap = Lazy(MutexStringHashMap, returnMutexMap).init();
+
+test "test lazy mutex map" {
+    var obj = LazyMutexMap.get();
+    defer obj.*.deinit();
+
+    var map = obj.*.borrow();
+    try map.*.put("v", 0);
+    obj.*.restore();
+
+    var threads: [10]std.Thread = undefined;
+
+    for (&threads) |*handle| {
+        handle.* = try std.Thread.spawn(.{}, struct {
+            fn thread_fn(x: u8) !void {
+                _ = x;
+                var o = LazyMutexMap.get();
+                var m = o.*.borrow();
+
+                const v = m.get("v").?;
+                try m.put("v", v + 1);
+                o.*.restore();
+            }
+        }.thread_fn, .{0});
+    }
+
+    for (threads) |handle| handle.join();
+
+    const map2 = obj.*.borrow();
+    try testing.expectEqual(@as(i32, 10), map2.*.get("v").?);
+}
diff --git a/src/singlethread.zig b/src/singlethread.zig
new file mode 100644
index 0000000..2738d16
--- /dev/null
+++ b/src/singlethread.zig
@@ -0,0 +1,4 @@
+pub fn OnceCell(comptime T: type) type {
+    _ = T;
+    return struct {};
+}