# Eigen in android

### First setup NDK and javah for android studio
1. after open Android Studio, choosing 'Configure' -> 'Preferences' -> 'Tools' -> 'External Tools'
2. click the add button
* javah:
* Name, Descriptions type anything you want
* Tool Settings:
Program: choose the path of your javah
Parameters: -v -jni -d ```$ModuleFileDir$\src\main\jni $FileClass$```
Working directory: ```$SourcepathEntry$```
* NDK
* Name, Descriptions type anything you want
* Tool Settings:
Program: choose the path of your ndk
Parameters: ```NDK_PROJECT_PATH=$ModuleFileDir$/build/intermediates/ndk ```

Workint directory: ```$SourcepathEntry$```
* Done!
### create project
1. create jni folder under app/src/main
2. download eigen http://eigen.tuxfamily.org/index.php?title=Main_Page#Download
3. then copy Eigen Folder to jni folder
4. create a java class(here I call Eigenlib)

package com.example.eugenechou.matrixcal;

public class Eigenlib {
    static {
        System.loadLibrary("eigenlib"); // the .so name we will name later
    }
    // the function to be implement
    public static native double[] mul(double[] matrix, int row, int col);
}

5. use javah(click Tools->External Tools->External Tools->javah) to generate .h file (packageName_Eigenlib.h)

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_eugenechou_matrixcal_Eigenlib */

#ifndef _Included_com_example_eugenechou_matrixcal_Eigenlib
#define _Included_com_example_eugenechou_matrixcal_Eigenlib
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_example_eugenechou_matrixcal_Eigenlib
 * Method:    mul
 * Signature: ([DII)[D
 */
JNIEXPORT jdoubleArray JNICALL Java_com_example_eugenechou_matrixcal_Eigenlib_mul
  (JNIEnv *, jclass, jdoubleArray, jint, jint);

#ifdef __cplusplus
}
#endif
#endif

6. create .cpp file under jni(here I name test.cpp)

#include <iostream>
#include <math.h>
#include <jni.h>
#include <exception>
#include <Eigen/Dense>

using namespace Eigen;

extern "C"{
//declare constant here
double a[9] = {1, 1, 1, 1, 1, 1, 1, 1, 1};
double b[9] = {1, 1, 1, 1, 1, 1, 1, 1, 1};

jdoubleArray
Java_com_example_eugenechou_matrixcal_Eigenlib_mul
(JNIEnv *env, jclass obj, jdoubleArray matrix, jint row, jint col){
    jsize length = env->GetArrayLength(matrix);

    if(row != col) return NULL;
    if(length != row * col) return NULL;

    MatrixXd Mat(row, col), const1(row, col), const2(row, col);
    jdouble *array = env->GetDoubleArrayElements(matrix, 0);

    // convert the array to matrix
    int k=0;
    for(int i=0 ; i<row ; i++)
        for(int j=0 ; j<col ; j++){
            Mat(i, j) = array[k];
            const1(i, j) = a[k];
            const2(i, j) = b[k];
            k++;
        }
    // multiplication
    MatrixXd result = const1 * const2 * Mar;

    //convert matrix back to array
    k=0;
    for(int i=0 ; i<row ; i++)
        for(int j=0 ; j<col ; j++)
            array[k++] = result(i, j);

    int len = (int) row * col;
    jdoubleArray ret = env->NewDoubleArray(len);
    env->SetDoubleArrayRegion(ret, 0, len, array);
    env->ReleaseDoubleArrayElements(matrix, array, 0);
    return ret;
}
}

7. create Android.mk under jni

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_C_INCLUDES += $(LOCAL_PATH)
LOCAL_LDLIBS += -llog -ldl

LOCAL_MODULE := eigenlib
LOCAL_SRC_FILES := eigenlib.cpp

include $(BUILD_SHARED_LIBRARY)

8. create Application.mk under jni

APP_ABI := all
APP_STL := c++_shared
APP_CPPFLAGS += -fexceptions

9. use ndk(click Tools->External Tools->External Tools->NDK) to build .so file
10. calling in java

package com.example.eugenechou.matrixcal;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TextView textView = (TextView)findViewById(R.id.TextView1);
        StringBuffer sb = new StringBuffer();

        Eigenlib eigenlib = new Eigenlib();
        double[] matrix = {2, 2, 2, 2, 2, 2, 2, 2, 2};
        int row = 3, col = 3;
        sb.append("matrix: \n");
        int len = matrix.length;
        for(int i=0 ; i<len ; i++){
            sb.append(matrix[i]);
            if(i != len - 1) sb.append(" ");
            if((i+1) % row == 0) sb.append("\n");
        }
        sb.append("\n");
        sb.append("\n");
        sb.append("Result: \n");
        double[] f = eigenlib.mul(matrix, row, col);
        if(f == null) sb.append("Result error!");
        else{
            len = f.length;
            for(int i=0 ; i<len ; i++){
                sb.append(f[i]);
                if(i != len-1) sb.append(" ");
                if((i+1) % row == 0) sb.append("\n");
            }
        }
        textView.setText(new String(sb));
    }
}

###### tags: `note`