feat: detect steps and play audio

app/src/main/cpp/jni_stepdetector.cpp

@@ -0,0 +1,52 @@
+//
+// Created by david on 03.03.2026.
+//
+
+#include <jni.h>
+#include "StepDetector.h"
+#include <new>
+#include <vector>
+
+extern "C" {
+
+jint throwIllegalArgumentException(JNIEnv *env, const char *message)
+{
+    jclass exClass;
+    const char *className = "java/lang/IllegalArgumentException";
+    exClass = env->FindClass(className);
+    if (exClass == nullptr) {
+        return -1;
+    }
+    return env->ThrowNew(exClass, message);
+}
+
+JNIEXPORT jlong JNICALL
+Java_at_lockstep_filter_StepDetector_native_1create(
+        JNIEnv *env,
+        jclass /*unused*/, jlong engineHandle) {
+    auto *listener = reinterpret_cast<StepListener *>(engineHandle);
+    // We use std::nothrow so `new` returns a nullptr if the engine creation fails
+    auto *detector = new(std::nothrow) StepDetector(listener);
+    return reinterpret_cast<jlong>(detector);
+}
+
+JNIEXPORT void JNICALL
+Java_at_lockstep_filter_StepDetector_native_1delete(
+        JNIEnv *env,
+        jclass /*unused*/, jlong handle) {
+    delete reinterpret_cast<StepDetector *>(handle);
+}
+
+JNIEXPORT void JNICALL
+Java_at_lockstep_filter_StepDetector_native_1filter(
+        JNIEnv *env,
+        jclass /*unused*/, jlong handle, jlong timestamp, jfloatArray values) {
+    if(values == nullptr) throwIllegalArgumentException(env, "values == null");
+    float* nativeValues = (float *)env->GetFloatArrayElements(values, 0);
+    jsize length = env->GetArrayLength(values);
+    std::vector<float> vecValues(nativeValues, nativeValues + length);
+    auto *detector = reinterpret_cast<StepDetector *>(handle);
+    detector->filter(vecValues);
+}
+
+}