diff --git a/README.md b/README.md
new file mode 100644
index 0000000..83efa99
--- /dev/null
+++ b/README.md
@@ -0,0 +1,21 @@
+# zigeru
+
+zigeru is a IRC bot which implements the following commands:
+
+- `s/OLD/NEW/` -- posts the previous message by the user, where every
+  occurrence of OLD is replaced by NEW.
+
+## Getting started
+
+To enter into a development shell with all the tools needed for
+interacting with this project, run the following command:
+
+```
+$ nix develop
+```
+
+To run the tests: `zig build test`.
+
+To run the binary: `zig build run`.
+
+To build the binary: `zig build`.
diff --git a/flake.lock b/flake.lock
index ee94b44..b914351 100644
--- a/flake.lock
+++ b/flake.lock
@@ -1,5 +1,60 @@
 {
   "nodes": {
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1696426674,
+        "narHash": "sha256-kvjfFW7WAETZlt09AgDn1MrtKzP7t90Vf7vypd3OL1U=",
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "rev": "0f9255e01c2351cc7d116c072cb317785dd33b33",
+        "type": "github"
+      },
+      "original": {
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1705309234,
+        "narHash": "sha256-uNRRNRKmJyCRC/8y1RqBkqWBLM034y4qN7EprSdmgyA=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "1ef2e671c3b0c19053962c07dbda38332dcebf26",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "gitignore": {
+      "inputs": {
+        "nixpkgs": [
+          "zls",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1709087332,
+        "narHash": "sha256-HG2cCnktfHsKV0s4XW83gU3F57gaTljL9KNSuG6bnQs=",
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "rev": "637db329424fd7e46cf4185293b9cc8c88c95394",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "gitignore.nix",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1761373498,
@@ -16,9 +71,85 @@
         "type": "github"
       }
     },
+    "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1755704039,
+        "narHash": "sha256-gKlP0LbyJ3qX0KObfIWcp5nbuHSb5EHwIvU6UcNBg2A=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9cb344e96d5b6918e94e1bca2d9f3ea1e9615545",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-25.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "zls": "zls"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
+    "zig-overlay": {
+      "inputs": {
+        "flake-compat": "flake-compat",
+        "flake-utils": "flake-utils",
+        "nixpkgs": [
+          "zls",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1755864794,
+        "narHash": "sha256-hgnov6RLA+DD4Uocs/vCbiH3/3sKvqiJOKHpdhGyVAI=",
+        "owner": "mitchellh",
+        "repo": "zig-overlay",
+        "rev": "5cd601f8760d2383210b7b8c8a45fc79388f3ddf",
+        "type": "github"
+      },
+      "original": {
+        "owner": "mitchellh",
+        "repo": "zig-overlay",
+        "type": "github"
+      }
+    },
+    "zls": {
+      "inputs": {
+        "gitignore": "gitignore",
+        "nixpkgs": "nixpkgs_2",
+        "zig-overlay": "zig-overlay"
+      },
+      "locked": {
+        "lastModified": 1756048867,
+        "narHash": "sha256-GFzSHUljcxy7sM1PaabbkQUdUnLwpherekPWJFxXtnk=",
+        "owner": "zigtools",
+        "repo": "zls",
+        "rev": "ce6c8f02c78e622421cfc2405c67c5222819ec03",
+        "type": "github"
+      },
+      "original": {
+        "owner": "zigtools",
+        "ref": "0.15.0",
+        "repo": "zls",
+        "type": "github"
       }
     }
   },
diff --git a/flake.nix b/flake.nix
index dc1c419..17e2e91 100644
--- a/flake.nix
+++ b/flake.nix
@@ -3,15 +3,17 @@
 
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+    zls.url = "github:zigtools/zls?ref=0.15.0"; # TODO(jsb): Update this when bumping zig version
   };
 
-  outputs = { self, nixpkgs }:
+  outputs = { self, nixpkgs, zls }:
     let
       pkgs = nixpkgs.legacyPackages.x86_64-linux;
     in {
       devShells.x86_64-linux.default = pkgs.mkShell {
         buildInputs = [
           pkgs.zig
+          zls.packages.x86_64-linux.zls
         ];
       };
   };
diff --git a/src/bot.zig b/src/bot.zig
new file mode 100644
index 0000000..cf86d8e
--- /dev/null
+++ b/src/bot.zig
@@ -0,0 +1,183 @@
+const std = @import("std");
+
+pub const Command = union(enum) {
+    substitute: struct { author: []const u8, needle: []const u8, replacement: []const u8 },
+};
+
+pub const Result = union(enum) {
+    post_message: struct { content: []const u8 },
+};
+
+pub const Error = error{
+    OutOfMemory,
+    NoMessage,
+};
+
+pub const Message = struct {
+    timestamp: u32,
+    author: []const u8,
+    content: []const u8,
+};
+
+pub const Bot = struct {
+    backlog: [1024]?Message,
+    top: usize,
+    bottom: usize,
+    allocator: std.mem.Allocator,
+
+    pub fn new(allocator: std.mem.Allocator) Bot {
+        return Bot{
+            .backlog = .{null} ** 1024,
+            .top = 0,
+            .bottom = 0,
+            .allocator = allocator,
+        };
+    }
+
+    pub fn execute(self: *Bot, cmd: *const Command) Error!Result {
+        switch (cmd.*) {
+            .substitute => |command| {
+                const prev_msg = self.previous_message_by_author(command.author) orelse return Error.NoMessage;
+                const size = std.mem.replacementSize(u8, prev_msg.content, command.needle, command.replacement);
+                const output = try self.allocator.alloc(u8, size);
+                _ = std.mem.replace(u8, prev_msg.content, command.needle, command.replacement, output);
+                return Result{ .post_message = .{ .content = output } };
+            },
+            else => unreachable,
+        }
+    }
+
+    pub fn hear(self: *Bot, msg: Message) void {
+        self.backlog[self.top] = msg;
+        self.top = (self.top + 1) % 1024;
+        if (self.top == self.bottom) self.bottom = (self.bottom + 1) % self.backlog.len;
+    }
+
+    pub fn no_messages(self: *Bot) usize {
+        if (self.top < self.bottom) {
+            // we've bounded around, the backlog is full.
+            return self.backlog.len;
+        }
+        return self.top - self.bottom;
+    }
+
+    fn previous_idx(self: *Bot, idx: usize) usize {
+        if (idx == 0) {
+            return self.backlog.len - 1;
+        }
+        return idx - 1;
+    }
+
+    fn previous_message(self: *Bot, comptime pred: *const fn (Message) bool) ?Message {
+        var idx = self.previous_idx(self.top);
+        while (true) : (idx = self.previous_idx(idx)) {
+            if (self.backlog[idx] == null) {
+                return null;
+            }
+            const message = self.backlog[idx] orelse unreachable;
+            if (pred(message)) {
+                return message;
+            }
+            if (idx == self.bottom) {
+                // reached the start of the list
+                break;
+            }
+        }
+        return null;
+    }
+
+    fn previous_message_by_author(self: *Bot, author: []const u8) ?Message {
+        var idx = self.previous_idx(self.top);
+        while (true) : (idx = self.previous_idx(idx)) {
+            if (self.backlog[idx] == null) {
+                return null;
+            }
+            const message = self.backlog[idx] orelse unreachable;
+            if (std.mem.eql(u8, message.author, author)) {
+                return message;
+            }
+            if (idx == self.bottom) {
+                // reached the start of the list
+                break;
+            }
+        }
+        return null;
+    }
+};
+
+test "hear_wraps" {
+    var bot = Bot.new(std.testing.allocator);
+    const testMessage = Message{
+        .author = "Jassob",
+        .timestamp = 12345,
+        .content = "All your codebase are belong to us.\n",
+    };
+
+    for (0..1025) |_| {
+        bot.hear(testMessage);
+    }
+
+    try std.testing.expect(bot.top == 1);
+    try std.testing.expect(bot.bottom == 2);
+    try std.testing.expect(bot.no_messages() == 1024);
+}
+
+test "previous_message" {
+    var bot = Bot.new(std.testing.allocator);
+
+    const callback = struct {
+        fn callback(_: Message) bool {
+            return true;
+        }
+    }.callback;
+    const prev = bot.previous_message(callback);
+
+    try std.testing.expect(prev == null);
+}
+
+test "previous_message1" {
+    var bot = Bot.new(std.testing.allocator);
+    var contents: [10][]const u8 = .{undefined} ** 10;
+
+    for (0..10) |i| {
+        contents[i] = try std.fmt.allocPrint(std.testing.allocator, "{d}", .{i});
+        const msg = Message{
+            .author = "Jassob",
+            .timestamp = @as(u32, @intCast(i)),
+            .content = contents[i],
+        };
+        bot.hear(msg);
+    }
+
+    const callback = struct {
+        fn callback(msg: Message) bool {
+            return std.mem.eql(u8, msg.content, "8");
+        }
+    }.callback;
+    const prev = bot.previous_message(callback);
+
+    for (contents) |str| {
+        std.testing.allocator.free(str);
+    }
+
+    try std.testing.expect(prev != null);
+}
+
+test "execute_substitution_no_previous_message" {
+    var bot = Bot.new(std.testing.allocator);
+    const cmd = Command{ .substitute = .{ .author = "jassob", .needle = "What", .replacement = "what" } };
+    try std.testing.expectError(Error.NoMessage, bot.execute(&cmd));
+}
+
+test "execute_substitution" {
+    var bot = Bot.new(std.testing.allocator);
+    bot.hear(Message{ .timestamp = 1234, .author = "jassob", .content = "What" });
+    const cmd = Command{ .substitute = .{ .author = "jassob", .needle = "What", .replacement = "what" } };
+    const result = try bot.execute(&cmd);
+    switch (result) {
+        .post_message => |message| {
+            try std.testing.expectEqualDeep(message.content, "what");
+            std.testing.allocator.free(message.content);
+        },
+    }
+}
diff --git a/src/main.zig b/src/main.zig
index cebfe10..931ed37 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -3,25 +3,26 @@ const zigeru = @import("zigeru");
 
 pub fn main() !void {
     // Prints to stderr, ignoring potential errors.
-    std.debug.print("All your {s} are belong to us.\n", .{"codebase"});
-    try zigeru.bufferedPrint();
-}
-
-test "simple test" {
-    const gpa = std.testing.allocator;
-    var list: std.ArrayList(i32) = .empty;
-    defer list.deinit(gpa); // Try commenting this out and see if zig detects the memory leak!
-    try list.append(gpa, 42);
-    try std.testing.expectEqual(@as(i32, 42), list.pop());
-}
+    const message = zigeru.bot.Message{
+        .author = "Jassob",
+        .timestamp = 12345,
+        .content = "All your base are belong to us.\n",
+    };
 
-test "fuzz example" {
-    const Context = struct {
-        fn testOne(context: @This(), input: []const u8) anyerror!void {
-            _ = context;
-            // Try passing `--fuzz` to `zig build test` and see if it manages to fail this test case!
-            try std.testing.expect(!std.mem.eql(u8, "canyoufindme", input));
-        }
+    var gpa = std.heap.DebugAllocator(.{}){};
+    var bot = zigeru.bot.Bot.new(gpa.allocator());
+    bot.hear(message);
+    const command = zigeru.bot.Command{
+        .substitute = .{
+            .author = "Jassob",
+            .needle = "base",
+            .replacement = "codebase",
+        },
     };
-    try std.testing.fuzz(Context{}, Context.testOne, .{});
+    const result = try bot.execute(&command);
+    switch (result) {
+        .post_message => |msg| {
+            std.debug.print("{s}", .{msg.content});
+        },
+    }
 }
diff --git a/src/root.zig b/src/root.zig
index 94c7cd0..2f461df 100644
--- a/src/root.zig
+++ b/src/root.zig
@@ -1,23 +1,3 @@
 //! By convention, root.zig is the root source file when making a library.
 const std = @import("std");
-
-pub fn bufferedPrint() !void {
-    // Stdout is for the actual output of your application, for example if you
-    // are implementing gzip, then only the compressed bytes should be sent to
-    // stdout, not any debugging messages.
-    var stdout_buffer: [1024]u8 = undefined;
-    var stdout_writer = std.fs.File.stdout().writer(&stdout_buffer);
-    const stdout = &stdout_writer.interface;
-
-    try stdout.print("Run `zig build test` to run the tests.\n", .{});
-
-    try stdout.flush(); // Don't forget to flush!
-}
-
-pub fn add(a: i32, b: i32) i32 {
-    return a + b;
-}
-
-test "basic add functionality" {
-    try std.testing.expect(add(3, 7) == 10);
-}
+pub const bot = @import("bot.zig");