diff --git a/day15/build.zig b/day15/build.zig
new file mode 100644
index 0000000..6af6b1e
--- /dev/null
+++ b/day15/build.zig
@@ -0,0 +1,27 @@
+const Builder = @import("std").build.Builder;
+
+pub fn build(b: *Builder) void {
+    // Standard target options allows the person running `zig build` to choose
+    // what target to build for. Here we do not override the defaults, which
+    // means any target is allowed, and the default is native. Other options
+    // for restricting supported target set are available.
+    const target = b.standardTargetOptions(.{});
+
+    // Standard release options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall.
+    const mode = b.standardReleaseOptions();
+
+    const exe = b.addExecutable("day15", "src/main.zig");
+    exe.setTarget(target);
+    exe.setBuildMode(mode);
+    exe.install();
+
+    const run_cmd = exe.run();
+    run_cmd.step.dependOn(b.getInstallStep());
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+}
diff --git a/day15/src/main.zig b/day15/src/main.zig
new file mode 100644
index 0000000..5d3ad9a
--- /dev/null
+++ b/day15/src/main.zig
@@ -0,0 +1,203 @@
+const std = @import("std");
+
+pub fn main() anyerror!void {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+    var gpa = &arena.allocator;
+
+    //var f = try std.fs.cwd().openFile("input", .{});
+    //var contents = try f.readToEndAlloc(gpa, std.math.maxInt(u32));
+
+    // Starting numbers 0,8,15,2,12,1,4
+    var seed = [_]u64 {0, 8, 15, 2, 12, 1, 4};
+    var game = try MemoryGame.create(gpa);
+    defer game.deinit();
+
+    try game.seed_game(seed[0..]);
+    try game.run_until(2020);
+    std.log.info("Round {} spoke {}", .{game.round-1, game.last_number});
+
+    // Part 2
+    try game.run_until(30000000);
+    std.log.info("Round {} spoke {}", .{game.round-1, game.last_number});
+}
+
+const NumberEntry = struct {
+    last: u64,
+    second_last: ?u64 = null,
+};
+
+const MemoryGame = struct {
+    map: std.hash_map.AutoHashMap(u64, NumberEntry),
+    allocator: *std.mem.Allocator,
+    round: u64 = 1, // Starting at one to match the puzzle
+    last_number: u64 = 0,
+
+    pub fn create(a: *std.mem.Allocator) !*MemoryGame {
+        var self = try a.create(MemoryGame);
+        errdefer a.destroy(self);
+        self.* = .{
+            .map = std.hash_map.AutoHashMap(u64, NumberEntry).init(a),
+            .allocator = a,
+        };
+        return self;
+    }
+
+    pub fn deinit(self: *MemoryGame) void {
+        self.map.deinit();
+        self.allocator.destroy(self);
+    }
+
+    // Assumes there's not repeated number in the seed
+    pub fn seed_game(self: *MemoryGame, seed: []u64) !void {
+        for (seed) |s, k| {
+            try self.map.put(s, NumberEntry{.last = self.round});
+            self.last_number = s;
+            self.round += 1;
+        }
+    }
+
+    // Returns the number spoken, but also updates everything
+    pub fn do_round(self: *MemoryGame) !u64 {
+        // Consider last number "spoken"
+        // Has it been said before?
+        var retval : u64 = 0;
+        if (self.map.get(self.last_number)) |v| {
+            //std.log.debug("Found previously spoken number {} for round {}. Previous spoken on round {}, has a previous occurence: {}\n",
+            //               .{self.last_number, self.round, v.last, v.second_last != null});
+            if ((v.last == (self.round-1)) and (v.second_last == null)) {
+                // The previously spoken number was new
+                retval = 0;
+                if (self.map.get(0)) |v2| {
+                    self.map.putAssumeCapacity(0, NumberEntry {.last = self.round, .second_last = v2.last});
+                }
+                else {
+                    // 0 is our new number
+                    try self.map.put(0, NumberEntry {.last = self.round});
+                }
+            }
+            else {
+                // The previous number was already spoken, so we speak the diff
+                retval = v.last - v.second_last.?;
+                if (self.map.get(retval)) |v2| {
+                    self.map.putAssumeCapacity(retval, NumberEntry {.last = self.round, .second_last = v2.last});
+                }
+                else {
+                    try self.map.put(retval, NumberEntry {.last = self.round});
+                }
+            }
+        }
+        else {
+            unreachable;
+        }
+        self.last_number = retval;
+        self.round += 1;
+        return retval;
+    }
+
+    pub fn run_until(self: *MemoryGame, target_round: u64) !void {
+        var last : u64 = 0;
+        while (self.round <= target_round) {
+            //std.log.warn("Round {} before running do_round()", .{self.round});
+            last = try self.do_round();
+            //std.log.warn("Round {} spoke {}", .{self.round-1, self.last_number});
+        }
+    }
+};
+
+test "036_ten_turns" {
+    var seed = [_]u64 {0, 3, 6};
+    var game = try MemoryGame.create(std.testing.allocator);
+    defer game.deinit();
+    try game.seed_game(seed[0..]);
+    if (game.map.get(0)) |value| {
+        std.testing.expectEqual(value.last, 1);
+    }
+    if (game.map.get(3)) |value| {
+        std.testing.expectEqual(value.last, 2);
+    }
+    if (game.map.get(6)) |value| {
+        std.testing.expectEqual(value.last, 3);
+    }
+
+    var response = try game.do_round();
+    std.testing.expectEqual(response, 0); // Round 4
+
+    response = try game.do_round();
+    std.testing.expectEqual(response, 3); // Round 5
+
+    response = try game.do_round();
+    std.testing.expectEqual(response, 3); // ROund 6
+
+    response = try game.do_round();
+    std.testing.expectEqual(response, 1); // Round 7
+
+    response = try game.do_round();
+    std.testing.expectEqual(response, 0); // Round 8
+
+    response = try game.do_round();
+    std.testing.expectEqual(response, 4); // Roiund 9
+
+    response = try game.do_round();
+    std.testing.expectEqual(response, 0); // Round 10
+}
+
+test "132_to_2020" {
+    var seed = [_]u64 {1, 3, 2};
+    var game = try MemoryGame.create(std.testing.allocator);
+    defer game.deinit();
+    try game.seed_game(seed[0..]);
+    try game.run_until(2020);
+    //std.log.warn("Last number spoken was {}", .{game.last_number});
+    std.testing.expectEqual(game.last_number, 1);
+}
+
+test "213_to_2020" {
+    var seed = [_]u64 {2, 1, 3};
+    var game = try MemoryGame.create(std.testing.allocator);
+    defer game.deinit();
+    try game.seed_game(seed[0..]);
+    try game.run_until(2020);
+    //std.log.warn("Last number spoken was {}", .{game.last_number});
+    std.testing.expectEqual(game.last_number, 10);
+}
+
+test "123_to_2020" {
+    var seed = [_]u64 {1, 2, 3};
+    var game = try MemoryGame.create(std.testing.allocator);
+    defer game.deinit();
+    try game.seed_game(seed[0..]);
+    try game.run_until(2020);
+    //std.log.warn("Last number spoken was {}", .{game.last_number});
+    std.testing.expectEqual(game.last_number, 27);
+}
+
+test "231_to_2020" {
+    var seed = [_]u64 {2, 3, 1};
+    var game = try MemoryGame.create(std.testing.allocator);
+    defer game.deinit();
+    try game.seed_game(seed[0..]);
+    try game.run_until(2020);
+    //std.log.warn("Last number spoken was {}", .{game.last_number});
+    std.testing.expectEqual(game.last_number, 78);
+}
+
+test "321_to_2020" {
+    var seed = [_]u64 {3, 2, 1};
+    var game = try MemoryGame.create(std.testing.allocator);
+    defer game.deinit();
+    try game.seed_game(seed[0..]);
+    try game.run_until(2020);
+    //std.log.warn("Last number spoken was {}", .{game.last_number});
+    std.testing.expectEqual(game.last_number, 438);
+}
+
+test "312_to_2020" {
+    var seed = [_]u64 {3, 1, 2};
+    var game = try MemoryGame.create(std.testing.allocator);
+    defer game.deinit();
+    try game.seed_game(seed[0..]);
+    try game.run_until(2020);
+    //std.log.warn("Last number spoken was {}", .{game.last_number});
+    std.testing.expectEqual(game.last_number, 1836);
+}