Make: Test Android cross-compilation in CI

Author: ashvardanian

.github/workflows/prerelease.yml

@@ -457,15 +457,16 @@ jobs:
           cmake --build build_artifacts --config RelWithDebInfo
 
         # We can't run the produced builds, but we can make sure they exist
-      - name: Test artifacts presense
+      - name: Test artifacts presence
         run: |
           test -e build_artifacts/libusearch_c.so
           test -e build_artifacts/libusearch_sqlite.so
 
-  test_ubuntu_android_ndk:
-    name: Android NDK Build
+  test_android:
+    name: Android
     runs-on: ubuntu-22.04
     strategy:
+      fail-fast: false
       matrix:
         include:
           - processor: armv7a
@@ -478,7 +479,7 @@ jobs:
       - uses: actions/checkout@v4
       - run: git submodule update --init --recursive
 
-      - name: Install NDK ndk
+      - name: Install NDK
         run: |
           ${ANDROID_HOME}/cmdline-tools/latest/bin/sdkmanager --install "ndk;${{ env.ANDROID_NDK_VERSION }}"
 
@@ -498,9 +499,8 @@ jobs:
           cmake --build build_artifacts --config RelWithDebInfo
 
         # We can't run the produced builds, but we can make sure they exist
-      - name: Test artifacts presense
-        run: |
-          test -e build_artifacts/libusearch_c.so
+      - name: Test artifacts presence
+        run: test -e build_artifacts/libusearch_c.so
 
         # Rust
       - name: Set up Rust
@@ -521,5 +521,20 @@ jobs:
           echo "CARGO_${{ matrix.target }}=${TOOLCHAIN}/llvm-ar" >> ${GITHUB_ENV}
 
       - name: Build Rust
+        run: cargo build --target ${{ matrix.target }}
+      
+        # Java
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: "21"
+          distribution: "adopt"
+      
+        # We skip native compilation with Gradle and focus just on the Java part,
+        # assuming the we won't be able to tun the tests without emulation
+      - name: Test Java bindings with Android library
         run: |
-          cargo build --target ${{ matrix.target }}
+          mkdir -p build/libs/usearch/shared/
+          cp build_artifacts/libusearch_c.so build/libs/usearch/shared/
+          javac -cp . java/cloud/unum/usearch/*.java
+          echo "Java compilation successful with Android NDK library"

CONTRIBUTING.md

@@ -156,6 +156,38 @@ cmake -D CMAKE_BUILD_TYPE=Release \
 cmake --build build_artifacts --config Release
 ```
 
+For Android development, you can cross-compile for ARM architectures without requiring the full Android NDK setup.
+Here's an example targeting 32-bit ARM (`armeabi-v7a`):
+
+```sh
+sudo apt-get update
+sudo apt-get install -y clang lld crossbuild-essential-armhf
+
+# Cross-compile for 32-bit ARM (Android compatible)
+CMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
+cmake -B build_artifacts \
+  -D CMAKE_C_COMPILER=clang \
+  -D CMAKE_CXX_COMPILER=clang++ \
+  -D CMAKE_C_COMPILER_TARGET=armv7-linux-gnueabihf \
+  -D CMAKE_CXX_COMPILER_TARGET=armv7-linux-gnueabihf \
+  -D CMAKE_SYSTEM_NAME=Linux \
+  -D CMAKE_SYSTEM_PROCESSOR=armv7 \
+  -D CMAKE_C_FLAGS="--target=armv7-linux-gnueabihf -march=armv7-a" \
+  -D CMAKE_CXX_FLAGS="--target=armv7-linux-gnueabihf -march=armv7-a" \
+  -D CMAKE_BUILD_TYPE=RelWithDebInfo \
+  -D USEARCH_BUILD_LIB_C=1 \
+  -D USEARCH_BUILD_TEST_CPP=0 \
+  -D USEARCH_BUILD_BENCH_CPP=0 \
+  -D USEARCH_USE_SIMSIMD=0 \
+  -D USEARCH_USE_FP16LIB=1
+
+cmake --build build_artifacts --config RelWithDebInfo
+file build_artifacts/libusearch_c.so # Verify the output
+# Should show: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, ...
+```
+
+The resulting `libusearch_c.so` can be used in Android projects by placing it in `src/main/jniLibs/armeabi-v7a/` for 32-bit ARM or `arm64-v8a/` for 64-bit ARM.
+
 ## Python 3
 
 Python bindings are built using PyBind11 and are available on [PyPi](https://pypi.org/project/usearch/).