diff --git a/Chapter18/accelerometer_interface.cpp b/Chapter18/accelerometer_interface.cpp
index af37787..3525837 100644
--- a/Chapter18/accelerometer_interface.cpp
+++ b/Chapter18/accelerometer_interface.cpp
@@ -3,7 +3,7 @@
 
 class accelerometer {
 public:
-    struct data{
+    struct data {
         float x;
         float y;
         float z;
@@ -13,25 +13,49 @@ public:
         c_50_hz,
         c_100_hz,
     };
-    virtual void set_sampling_rate(sampling_rate) = 0;
+    enum error {
+        ok,
+        not_supported
+    };
+    virtual error set_sampling_rate(sampling_rate) = 0;
     virtual data get_data() = 0;
 };
 
+class i2c {
+public:
+    virtual void write() = 0;
+};
+
+class i2c_stm32 : public i2c {
+public:
+    void write() override {
+        printf("i2c::write...\r\n");
+    }
+};
+
 class adxl_345 : public accelerometer {
 public:
-    void set_sampling_rate(sampling_rate) override {
+    adxl_345(i2c &i2c_obj) : i2c_(i2c_obj) {}
+    error set_sampling_rate(sampling_rate) override {
         printf("adxl_345: setting sampling rate\r\n");
+        i2c_.write();
+        return error::ok;
     }
     data get_data() override {
         return data{0.02f, 0.981f, 0.03f};
     }
+private:
+    i2c &i2c_;
 };
 
 class tap_detection_algo {
 public:
     tap_detection_algo(accelerometer &accel) : 
         accel_(accel) {
-            accel_.set_sampling_rate(accelerometer::sampling_rate::c_100_hz);
+            auto err = accel_.set_sampling_rate(accelerometer::sampling_rate::c_100_hz);
+            if(err == accelerometer::error::not_supported) {
+                // try another sampling rate and adapt
+            }
     }
     bool run () {
         auto accel_data = accel_.get_data();
@@ -45,7 +69,8 @@ private:
 };
 
 int main() {
-    adxl_345 accel;
+    i2c_stm32 i2c1;
+    adxl_345 accel(i2c1);
     tap_detection_algo algo(accel);
     algo.run();
 
diff --git a/Chapter18/tap_detection_algo.cpp b/Chapter18/tap_detection_algo.cpp
new file mode 100644
index 0000000..7aac6be
--- /dev/null
+++ b/Chapter18/tap_detection_algo.cpp
@@ -0,0 +1,131 @@
+#include <cmath>
+#include <vector>
+#include <gtest/gtest.h>
+
+class accelerometer {
+public:
+    struct data {
+        float x;
+        float y;
+        float z;
+    };
+
+    enum class sampling_rate {
+        c_20_hz,
+        c_50_hz,
+        c_100_hz,
+    };
+
+    enum error {
+        ok,
+        not_supported
+    };
+
+    virtual error set_sampling_rate(sampling_rate) = 0;
+    virtual data get_data() = 0;
+};
+
+class tap_detection_algo {
+public:
+    tap_detection_algo(accelerometer &accel)
+        : accel_(accel), first_sample_(true) {}
+
+    bool run() {
+        auto current = accel_.get_data();
+
+        if (first_sample_) {
+            prev_ = current;
+            first_sample_ = false;
+            return false;
+        }
+
+        bool tap = (std::fabs(current.x - prev_.x) > c_threshold) ||
+                   (std::fabs(current.y - prev_.y) > c_threshold) ||
+                   (std::fabs(current.z - prev_.z) > c_threshold);
+        prev_ = current;
+        return tap;
+    }
+
+private:
+    static constexpr float c_threshold = 0.5f; 
+    accelerometer &accel_;
+    accelerometer::data prev_;
+    bool first_sample_ = true;
+};
+
+class fake_accel : public accelerometer {
+public:
+    fake_accel(const std::vector<data>& samples)
+        : samples_(samples), index_(0) {}
+
+    error set_sampling_rate(sampling_rate) override {
+        return error::ok;
+    }
+
+    data get_data() override {
+        if (index_ < samples_.size()) {
+            return samples_[index_++];
+        }
+        return samples_.back();
+    }
+
+private:
+    std::vector<data> samples_;
+    size_t index_;
+};
+
+// Test 1: No tap is detected if there is no significant change between samples.
+TEST(TapDetectionAlgoTest, NoTapWhenNoSignificantChange) {
+    std::vector<accelerometer::data> samples = {
+        {0.0f, 1.0f, 0.0f},
+        {0.0f, 1.0f, 0.0f},
+        {0.0f, 1.0f, 0.0f}
+    };
+    fake_accel fakeAccel(samples);
+    tap_detection_algo algo(fakeAccel);
+
+    // The first call initializes the previous reading.
+    EXPECT_FALSE(algo.run());
+    // The subsequent calls show no significant change.
+    EXPECT_FALSE(algo.run());
+    EXPECT_FALSE(algo.run());
+}
+
+// Test 2: A tap is detected when there is a sudden change in the readings.
+TEST(TapDetectionAlgoTest, DetectTapOnSuddenChange) {
+    std::vector<accelerometer::data> samples = {
+        {0.0f, 1.0f, 0.0f},  // initial reading
+        {0.0f, 1.0f, 0.0f},  // no change -> false
+        {0.0f, 2.0f, 0.0f}   // significant change on y axis (delta = 1.0 > 0.5) -> true
+    };
+    fake_accel fakeAccel(samples);
+    tap_detection_algo algo(fakeAccel);
+
+    EXPECT_FALSE(algo.run()); 
+    EXPECT_FALSE(algo.run());
+    EXPECT_TRUE(algo.run());  
+}
+
+// Test 3: Detect multiple tap events in a sequence.
+TEST(TapDetectionAlgoTest, MultipleTapsDetection) {
+    std::vector<accelerometer::data> samples = {
+        {0.0f, 1.0f, 0.0f},  // initial reading
+        {0.0f, 1.0f, 0.0f},  // no change -> false
+        {0.0f, 2.0f, 0.0f},  // tap detected (change: 1.0 > threshold)
+        {0.0f, 2.1f, 0.0f},  // small change -> false
+        {0.0f, 3.0f, 0.0f}   // tap detected (change: 0.9 > threshold)
+    };
+    fake_accel fakeAccel(samples);
+    tap_detection_algo algo(fakeAccel);
+
+    EXPECT_FALSE(algo.run());
+    EXPECT_FALSE(algo.run());
+    EXPECT_TRUE(algo.run()); 
+    EXPECT_FALSE(algo.run());
+    EXPECT_TRUE(algo.run());
+}
+
+int main(int argc, char **argv) {
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}
\ No newline at end of file