5 files changed, 126 insertions, 29 deletions
diff --git a/build.zig b/build.zig
index dbaf428..aea990e 100644
--- a/build.zig
+++ b/build.zig
@@ -1,9 +1,35 @@
 const std = @import("std");
 
-// Although this function looks imperative, note that its job is to
-// declaratively construct a build graph that will be executed by an external
-// runner.
-pub fn build(b: *std.Build) void {
+fn define_subproj(name: []const u8, b: *std.Build, target: std.Build.ResolvedTarget, optimize: std.builtin.OptimizeMode) !void {
+    const exe_path = try std.fmt.allocPrint(b.allocator, "src/{s}/main.zig", .{name});
+    const test_path = try std.fmt.allocPrint(b.allocator, "src/{s}/test.zig", .{name});
+    const run_task_name = try std.fmt.allocPrint(b.allocator, "run_{s}", .{name});
+    const run_task_desc = try std.fmt.allocPrint(b.allocator, "Run {s}", .{name});
+    const test_task_name = try std.fmt.allocPrint(b.allocator, "test_{s}", .{name});
+    const test_task_desc = try std.fmt.allocPrint(b.allocator, "Run tests for {s}", .{name});
+
+    const exe = b.addExecutable(.{
+        .name = name,
+        .root_source_file = b.path(exe_path),
+        .target = target,
+        .optimize = optimize,
+    });
+    b.installArtifact(exe);
+
+    const run_cmd = b.addRunArtifact(exe);
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    const run_step = b.step(run_task_name, run_task_desc);
+    run_step.dependOn(&run_cmd.step);
+
+    const test_exe = b.addTest(.{ .root_source_file = b.path(test_path), .target = target, .optimize = optimize });
+
+    const test_cmd = b.addRunArtifact(test_exe);
+    const test_step = b.step(test_task_name, test_task_desc);
+    test_step.dependOn(&test_cmd.step);
+}
+
+pub fn build(b: *std.Build) !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
@@ -54,24 +80,6 @@ pub fn build(b: *std.Build) void {
         test_step.dependOn(&run_exe_unit_tests.step);
     }
 
-    // monkey brains
-    const monkey_brain = b.addExecutable(.{
-        .name = "monkey_brain",
-        .root_source_file = b.path("src/monkey_brain/main.zig"),
-        .target = target,
-        .optimize = optimize,
-    });
-    b.installArtifact(monkey_brain);
-
-    const monkey_brain_run = b.addRunArtifact(monkey_brain);
-    monkey_brain_run.step.dependOn(b.getInstallStep());
-
-    const monkey_run_step = b.step("monkey_run", "Run the monkey brain");
-    monkey_run_step.dependOn(&monkey_brain_run.step);
-
-    const monkey_test_exec = b.addTest(.{ .root_source_file = b.path("src/monkey_brain/test.zig"), .target = target, .optimize = optimize });
-
-    const monkey_test_run = b.addRunArtifact(monkey_test_exec);
-    const monkey_test_step = b.step("monkey_test", "Run monkey brain cells test");
-    monkey_test_step.dependOn(&monkey_test_run.step);
+    try define_subproj("monkey_brain", b, target, optimize);
+    try define_subproj("monkey_learns", b, target, optimize);
 }
diff --git a/src/monkey_brain/test.zig b/src/monkey_brain/test.zig
index 4d4a04b..b3363e9 100644
--- a/src/monkey_brain/test.zig
+++ b/src/monkey_brain/test.zig
@@ -1,5 +1,3 @@
-pub const perceptron = @import("perceptron.zig");
-
-test {
-    @import("std").testing.refAllDecls(@This());
+comptime {
+    _ = @import("perceptron.zig");
 }
diff --git a/src/monkey_learns/linear_regression.zig b/src/monkey_learns/linear_regression.zig
index 70b786d..4cc5a9f 100644
--- a/src/monkey_learns/linear_regression.zig
+++ b/src/monkey_learns/linear_regression.zig
@@ -1 +1,88 @@
-// TODO
+const std = @import("std");
+const testing = std.testing;
+const math = std.math;
+
+const LinearRegression = struct {
+    const Self = @This();
+
+    weight: f64,
+    bias: f64,
+
+    fn init() LinearRegression {
+        return Self{ .weight = 0.0, .bias = 0.0 };
+    }
+
+    fn predict(self: Self, x: f64) f64 {
+        return self.weight * x + self.bias;
+    }
+
+    fn train(self: *LinearRegression, x: []const f64, y: []const f64, learning_rate: f64, epochs: usize) void {
+        const n: f64 = @floatFromInt(x.len);
+
+        for (0..epochs) |epoch| {
+            var total_error: f64 = 0;
+
+            for (x, y) |xi, yi| {
+                const prediction = self.predict(xi);
+                const err = prediction - yi;
+
+                self.weight -= learning_rate * err * xi;
+                self.bias -= learning_rate * err;
+
+                total_error += err * err;
+            }
+
+            const current_mse = total_error / n;
+
+            if (epoch % 1000 == 0 or epoch == epochs - 1) {
+                std.debug.print("Epoch {d}: MSE = {d:.6}\n", .{ epoch, current_mse });
+            }
+        }
+    }
+
+    // https://en.wikipedia.org/wiki/Mean_squared_error
+    fn loss_function(self: Self, x: []const f64, y: []const f64) f64 {
+        var squared_sum: f64 = 0.0;
+        for (x, y) |xi, yi| {
+            const predicted = self.predict(xi);
+            const err = predicted - yi;
+            squared_sum += err * err;
+        }
+        const n: f64 = @floatFromInt(x.len);
+        return squared_sum / n;
+    }
+};
+
+test "Linear Regression" {
+    // Initialize the model
+    var model = LinearRegression.init();
+
+    const x = [_]f64{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
+    const y = [_]f64{ 3.1, 4.9, 7.2, 9.1, 11.0, 12.8, 14.9, 17.2, 18.8, 21.1 };
+
+    const learning_rate: f64 = 0.01;
+    const epochs: usize = 1000;
+    model.train(&x, &y, learning_rate, epochs);
+
+    try testing.expect(@abs(model.weight - 2.0) < 0.1);
+    try testing.expect(@abs(model.bias - 1.0) < 0.1);
+
+    const test_x = [_]f64{ 0, 5, 10 };
+    const expected_y = [_]f64{ 1, 11, 21 };
+    for (test_x, expected_y) |xi, yi| {
+        const prediction = model.predict(xi);
+        try testing.expect(@abs(prediction - yi) < 0.5);
+    }
+
+    const mse = model.loss_function(&x, &y);
+    try testing.expect(mse < 0.1);
+
+    const new_x = [_]f64{ 11, 12, 13 };
+    const new_y = [_]f64{ 23.1, 24.9, 27.2 };
+    const new_mse = model.loss_function(&new_x, &new_y);
+    try testing.expect(new_mse < 0.2);
+
+    std.debug.print("\nTrained model: y = {d:.4}x + {d:.4}\n", .{ model.weight, model.bias });
+    std.debug.print("Mean Squared Error on training data: {d:.4}\n", .{mse});
+    std.debug.print("Mean Squared Error on new data: {d:.4}\n", .{new_mse});
+}
diff --git a/src/monkey_learns/main.zig b/src/monkey_learns/main.zig
new file mode 100644
index 0000000..902b554
--- /dev/null
+++ b/src/monkey_learns/main.zig
@@ -0,0 +1 @@
+pub fn main() void {}
diff --git a/src/monkey_learns/test.zig b/src/monkey_learns/test.zig
new file mode 100644
index 0000000..7c3472e
--- /dev/null
+++ b/src/monkey_learns/test.zig
@@ -0,0 +1,3 @@
+comptime {
+    _ = @import("linear_regression.zig");
+}