Note: There aren't any Java methods yet. When I initially set out I had plans of writing methods only for whatever application needs so did not bother writing exhaustive (or general) bridge functions. Apologies for the repo name that appears to indicate that the Java bit would be present.

• Android NDK on Linux (I've not gotten around to trying it on Mac or Windows).
• python2
• make
• bash shell

Makefiles for building fftw3 for android

1. Download the fftw3 source from http://fftw.org/download.html (The github repo is not 'ready-to-use' they advise against using it) and untar it somewhere convenient.

2. Assuming the fftw3 sources are under fftw-3.3.8, change to the directory and clone this repo under the name jni.

    cd fftw-3.3.8
    git clone git@github.com:aniline/fftw-android-jni.git jni
   

3. Set your Android ANDROID_NDK_HOME (or NDK_DIR) path to your NDK root directory. for e.g:

    export ANDROID_NDK_HOME=/home/myhome/android-ndk-r16e
   

4. Configure fftw3 so a correct config.h is available.

   The script jni/configure.sh could be used for this. It requires the environment variables:

   Variable	Purpose
   ANDROID_NDK_HOME or NDK_DIR	NDK root directory
   APP_PLATFORM	SDK level (android-14, android-21 etc.)
   NDK_ARCH	arch-arm (32bit) or arch-arm64

   So one could run configure for arm64 like, this :

    APP_PLATFORM=android-21 NDK_ARCH=arch-arm64 bash jni/configure.sh
   

   or arm (32 bit) like, say, this:

    APP_PLATFORM=android-21 NDK_ARCH=arch-arm bash jni/configure.sh
   

5. The configure script also launches the compile at the end. You could break it and run jni/build.sh to just build it later. The variables set by the configure.sh are saved to jni/.env. So just running bash jni/build.sh should build with the previously configured setting.

• Reasonably new NDK has clang set as the default toolchain. We override and set it to gcc-4.9 (using NDK_TOOLCHAIN_VERSION=4.9).
• There is a find command in the configure script which tries to find out the gcc compilers available in the NDK. Its ugly.
• Directly ndk-build set would build for all the architectures on that particular SDK level (say 21). But the configure step (using the configure.sh script) would configure fftw for only one platform. So the resulting builds for architectures other than the one configured for might be trouble. Some more work is needed to automagically configure for all architecures on a given level.