4 files changed, 94 insertions, 5 deletions
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");
+}