diff --git a/day11/build.zig b/day11/build.zig
new file mode 100644
index 0000000..a5872cc
--- /dev/null
+++ b/day11/build.zig
@@ -0,0 +1,27 @@
+const std = @import("std");
+
+pub fn build(b: *std.build.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("day11", "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/day11/input b/day11/input
new file mode 100644
index 0000000..fe97bfe
--- /dev/null
+++ b/day11/input
@@ -0,0 +1,10 @@
+1564524226
+1384554685
+7582264835
+8812672272
+1161463137
+7831762344
+2855527748
+6141737874
+8611458313
+8215372443
diff --git a/day11/src/main.zig b/day11/src/main.zig
new file mode 100644
index 0000000..24f452e
--- /dev/null
+++ b/day11/src/main.zig
@@ -0,0 +1,133 @@
+const std = @import("std");
+
+pub fn main() anyerror!void {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+    const alloc = &arena.allocator;
+
+    // Read our input
+    var f = try std.fs.cwd().openFile("input", .{});
+    defer f.close();
+    var contents = try f.readToEndAlloc(alloc, std.math.maxInt(u32));
+    defer alloc.free(contents);
+
+    var map: [100]u8 = undefined;
+    var lit = std.mem.tokenize(contents, "\n");
+    var line_number: u16 = 0;
+    while (lit.next()) |line| {
+        for (line) |c, i| {
+            map[i + @as(usize, line_number * 10)] = try std.fmt.parseInt(u8, line[i..i+1], 10);
+        }
+        line_number += 1;
+    }
+
+    var step: u16 = 0;
+    var flashed = std.AutoHashMap(usize, void).init(alloc);
+    defer flashed.deinit();
+    var flashes: u32 = 0;
+    while (step < 1000) {
+        // Increase energy level of octopuses by 1
+        for (map) |v, i| {
+            map[i] += 1;
+        }
+        // Check for high energy octopi
+        for (map) |v, i| {
+            if (v > 9) {
+                // Flash!
+                try flash(i, map[0..], &flashed);
+            }
+        }
+        //std.log.debug("Step {} had {} octopi flash", .{step, flashed.count()});
+        flashes += flashed.count();
+        if (flashed.count() == 100) {
+            std.log.info("[Part 2] On step {}, all octopi flashed", .{step+1});
+            break;
+        }
+        // Return all flashed octopi to 0 energy
+        var fit = flashed.keyIterator();
+        while (fit.next()) |fi| {
+            map[fi.*] = 0;
+        }
+        flashed.clearRetainingCapacity();
+        step += 1;
+        if (step == 100) {
+            std.log.info("[Part 1] After {} steps, {} octopi have flashed",
+                         .{step, flashes});
+        }
+        // std.log.debug("After step:\n", .{});
+        // var i: usize = 0;
+        // while (i < 10) : (i += 1) {
+        //     std.log.debug("{any}", .{map[i..i+10]});
+        // }
+        // std.log.debug("\n", .{});
+    }
+}
+
+fn flash(i: usize, map: []u8, flashed: *std.AutoHashMap(usize, void)) anyerror!void {
+    if (flashed.contains(i)) {
+        return;
+    }
+    try flashed.putNoClobber(i, undefined);
+    var adjacents = get_adjacent_points(i, 10, 10);
+    for (adjacents) |adj| {
+        if (adj) |a| {
+            if (!flashed.contains(a)) {
+                map[a] += 1;
+                if (map[a] > 9) {
+                    try flash(a, map, flashed);
+                }
+            }
+        }
+    }
+}
+
+fn get_adjacent_points(i: usize, width: usize, height: usize) [8]?usize {
+    var adjacents = [8] ?usize {
+        null,
+        null,
+        null,
+        null,
+        null,
+        null,
+        null,
+        null,
+    };
+    var ai: usize = 0;
+    var bottom = i < ((height - 1) * width);
+    var top = i >= width;
+    var left = (i % width) != 0;
+    var right = (i % width) != (width - 1);
+    if (top) {
+        if (left) {
+            adjacents[ai] = i - width - 1;
+            ai += 1;
+        }
+        adjacents[ai] = i - width;
+        ai += 1;
+        if (right) {
+            adjacents[ai] = i - width + 1;
+            ai += 1;
+        }
+    }
+    if (left) {
+        adjacents[ai] = i - 1;
+        ai += 1;
+    }
+    if (right) {
+        adjacents[ai] = i + 1;
+        ai += 1;
+    }
+    if (bottom) {
+        if (left) {
+            adjacents[ai] = i + width - 1;
+            ai += 1;
+        }
+        adjacents[ai] = i + width;
+        ai += 1;
+        if (right) {
+            adjacents[ai] = i + width + 1;
+            ai += 1;
+        }
+    }
+    return adjacents;
+}
diff --git a/day11/test b/day11/test
new file mode 100644
index 0000000..03743f6
--- /dev/null
+++ b/day11/test
@@ -0,0 +1,10 @@
+5483143223
+2745854711
+5264556173
+6141336146
+6357385478
+4167524645
+2176841721
+6882881134
+4846848554
+5283751526