DDM 2021-11-16

package com.joythis.android.distanceindays;

import android.app.Activity;

import java.util.Calendar;

public class AmCalendar {
    Activity mActivity; //to bridge to Activity tools
    /*
    Calendar objects represent
    month as 0-based data
    days of week as 1-based data
     */
    private Calendar mCalendar;
    private int mYear; //2021
    private int mMonth; //11
    private int mDay; //10

    private int mAbsDayInYear;
    private int mDayOfWeek;

    /*
    constructs an AmCalendar object
    corresponding to the current date
    default constructor - the constructor without
    arguments
     */
    public AmCalendar(Activity pA){
        this.mActivity = pA;
        this.mCalendar = Calendar.getInstance();
        this.mYear = this.mCalendar.get(Calendar.YEAR);
        this.mMonth =
            1+this.mCalendar.get(Calendar.MONTH);
        this.mDay = this.mCalendar.get(Calendar.DATE);
        this.mAbsDayInYear =
            this.mCalendar.get(Calendar.DAY_OF_YEAR);
        this.mDayOfWeek =
            this.mCalendar.get(Calendar.DAY_OF_WEEK);
    }//AmCalendar

    public AmCalendar(
        Activity pA,
        int pYear,
        int pMonth, //natural
        int pDay
    ){
        this.mActivity = pA;

        this.mCalendar = Calendar.getInstance();
        this.mCalendar.clear();
        this.mCalendar.set(
            Calendar.YEAR,
            pYear
        );
        //because Calendar's month are 0-based
        //after having received a "natural" month
        //we must discount it 1
        this.mCalendar.set(
            Calendar.MONTH,
            pMonth-1
        );
        this.mCalendar.set(
            Calendar.DATE,
            pDay
        );
        this.mYear = pYear;
        this.mMonth = pMonth;
        this.mDay = pDay;
        this.mAbsDayInYear = this.mCalendar.get(
            Calendar.DAY_OF_YEAR
        );
        //1 - Sunday, 2 - Monday ... 7 - Saturday
        this.mDayOfWeek = this.mCalendar.get(
            Calendar.DAY_OF_WEEK
        );

        this.mDayOfWeek = this.mDayOfWeek;
    }//AmCalendar

    /*
    computes the distance, in days,
    relatively to the device's current date
     */
    public long distanceInDays(){
        /*
        One needs to compute the difference
        between 2 moments
        moments represented as Calendar objects
        one of the 2 moments is the user's date
        the other moment is the system's date
        BOTH can be represented as Calendar objects
        All Calendar objects provide a method named
        getTimeInMillis
        which computes the time in milliseconds
        since the "Epoch" time (1970-1-1)
        So, the subtraction of these ints (long)
        gives the wanted number (in ms)
         */
        Calendar cNow = Calendar.getInstance();
        long lNow = cNow.getTimeInMillis();

        long lUser = this.mCalendar.getTimeInMillis();

        long lDiffMs = Math.abs(lNow-lUser);
        long lDiffDays = lDiffMs/1000/60/60/24;
        return lDiffDays;
    }//distanceInDays

    /*
    computes the distance, in days, relatively
    to a given date (as a Calendar object)
     */
    public int distanceInDays(Calendar pC){
        return 0;
    }//distanceInDays


    public int distanceInDays(int y, int m, int d){
        return 0;
    }

    public String toString(){
        String strRet = "";

        String strDayOfWeek = "";
        /* 0=>"sunday", ... 6=>"saturday"*/
        String[] dow = this.mActivity.getResources().
                getStringArray(R.array.aDaysOfWeek);
        /* 1=>"sunday" .. 7=>"saturday"*/
        strDayOfWeek = dow[this.mDayOfWeek-1];

        strRet = String.format(
            "%d-%d-%d #%d (%d = %s) => %d day(s) distant",
            this.mYear,
            this.mMonth,
            this.mDay,
            this.mAbsDayInYear,
            this.mDayOfWeek,
            strDayOfWeek,
            distanceInDays()
        );

        return strRet;
    }//toString

    public static boolean validYear(int pYear){
        return pYear>=1;
    }//validYear

    public static boolean validMonth(int pMonth){
        return pMonth>=1 && pMonth<=12;
    }//validMonth

    public static boolean validDay(
        int pY, int pM, int pD
    ){
        boolean bY = validYear(pY);
        boolean bM = validMonth(pM);
        if (bY && bM)
            return pD>=1 && pD<=daysInMonth(pY, pM);
        return false;
    }//validDay

    public final static int IMPOSSIBLE_MONTH = -1;
    public static int daysInMonth(int pY, int pM){
        switch(pM){
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                return 31;
            case 4:
            case 6:
            case 9:
            case 11:
                return 30;
            case 2:
                return leapYear(pY) ? 29:28;
            default:
                return IMPOSSIBLE_MONTH;
        }//switch
    }//daysInMonth

    public static boolean leapYear(int pY){
        boolean bM4 = pY%4==0;
        boolean bM100 = pY%100==0;
        boolean bM400 = pY%400==0;

        return bM400 || (bM4 && !bM100);
    }//leapYear
}//AmCalendar


************

package com.joythis.android.distanceindays;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import java.util.Calendar;

public class MainActivity extends AppCompatActivity {
    AmUtil mUtil;
    TextView mTvResults;
    AmCalendar mCalendar;
    EditText mEtYear, mEtMonth, mEtDay;
    Button mBtnComputeDistance;

    View.OnClickListener mClickHandler =
    new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            switch(v.getId()){
                case R.id.idBtnComputeDistance:
                    actionComputeDistance();
                    break;
            }//switch
        }//onClick
    };

    void actionComputeDistance(){
        int y,m,d;
        try {
            y = AmUtil.readIntFromEditText(mEtYear);
            m = AmUtil.readIntFromEditText(mEtMonth);
            d = AmUtil.readIntFromEditText(mEtDay);

            boolean bValidDate =
                AmCalendar.validDay(y,m,d);

            if (bValidDate){
                //instantiate and write a corresponding
                //AmCalendar object
                AmCalendar cUserDate =
                    new AmCalendar(this, y,m,d);
                mTvResults.setText(cUserDate.toString());
                long lDays = cUserDate.distanceInDays();
                mTvResults.setText(
                    cUserDate.toString());
            }
            else{
                //TODO
                mUtil.fb("Invalid Date!");
            }

        }
        catch (Exception e){
            //TODO
            mUtil.fb("Empty field.");
        }

    }//actionComputeDistance

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.rl_distance_in_days_v1);

        init();
    }//onCreate

    void init(){
        mUtil = new AmUtil (this);
        mEtYear = findViewById(R.id.idEtYear);
        mEtMonth = findViewById(R.id.idEtMonth);
        mEtDay = findViewById(R.id.idEtDay);
        mBtnComputeDistance = findViewById(R.id.idBtnComputeDistance);

        String[] aDaysOfWeek =
            getResources().getStringArray
                (R.array.aDaysOfWeek);

        mTvResults = findViewById(R.id.idTvResults);

        mBtnComputeDistance.setOnClickListener(
            mClickHandler
        );
        displayCurrentYMD();

    }//init

    void displayCurrentYMD(){
        Calendar c = Calendar.getInstance();
        int y = c.get(Calendar.YEAR);
        int m = c.get(Calendar.MONTH)+1;
        int d = c.get(Calendar.DATE);
        mEtYear.setText(String.valueOf(y));
        mEtMonth.setText(String.valueOf(m));
        mEtDay.setText(String.valueOf(d));
    }//displayCurrentYMD

    void auxTests(){
        //testing
        for (int day=1; day<=31; day+=1){
            AmCalendar calendarAtDay = new AmCalendar
                    (MainActivity.this, 2021, 1, day);
            String strDay = calendarAtDay.toString();
            mTvResults.setText(
                    strDay+"\n"+
                            mTvResults.getText().toString()
            );
        }//for
    }//auxTests

    /*
    EX1: on init make sure the EditTexts for
    year, month and day, are initialized to the
    device's current year, month and day

    EX2: create and print an AmCalendar object,
    corresponding to the user date
     */
}//MainActivity