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POProGuard can cause incorrect calculations
 

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    <p>I have met a pretty strange bug. The following small piece of code uses a rather simple math.</p> <pre><code>protected double C_n_k(int n, int k) { if(k&lt;0 || k&gt;n) return 0; double s=1; for(int i=1;i&lt;=k;i++) s=s*(n+1-i)/i; return s; } </code></pre> <p><strong>Edit</strong> Using ProGuard can make it go wrong on some devices. I have it confirmed on HTC One S Android 4.1.1 build 3.16.401.8, but judging by e-mails I got, a lot of phones with Android 4+ are affected. For some of them (Galaxy S3), american operator-branded phones are affected, while international versions are not. Many phones are not affected. </p> <p>Below is the code of activity which calculates C(n,k) for 1&lt;=n&lt;25 and 0&lt;=k&lt;=n. On device mentioned above the first session gives correct results, but the subsequent launches show incorrect results, each time in different positions.</p> <p>I have 3 questions:</p> <ol> <li><p>How can it be? Even if ProGuard made something wrong, calculations should be consistent between devices and sessions.</p></li> <li><p>How can we avoid it? I know substituting <code>double</code> by <code>long</code> is fine in this case, but it is not a universal method. Dropping using <code>double</code> or releasing not-obfuscated versions is out of question.</p></li> <li><p>What Android versions are affected? I was quite quick with fixing it in the game, so I just know that many players have seen it, and at least the most had Android 4.0</p></li> </ol> <p>Overflow is out of question, because sometimes I see mistake in calculating <code>C(3,3)=3/1*2/2*1/3</code>. Usually incorrect numbers start somewhere in C(10,...), and look like a phone has "forgotten" to make some divisions.</p> <p>My SDK tools are 22.3 (the latest), and I have seen it in builds created by both Eclipse and IntelliJ IDEA.</p> <p>Activity code:</p> <pre><code>package com.karmangames.mathtest; import android.app.Activity; import android.os.Bundle; import android.text.method.ScrollingMovementMethod; import android.widget.TextView; public class MathTestActivity extends Activity { /** * Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); String s=""; for(int n=0;n&lt;=25;n++) for(int k=0;k&lt;=n;k++) { double v=C_n_k_double(n,k); s+="C("+n+","+k+")="+v+(v==C_n_k_long(n,k) ? "" : " Correct is "+C_n_k_long(n,k))+"\n"; if(k==n) s+="\n"; } System.out.println(s); ((TextView)findViewById(R.id.text)).setText(s); ((TextView)findViewById(R.id.text)).setMovementMethod(new ScrollingMovementMethod()); } protected double C_n_k_double(int n, int k) { if(k&lt;0 || k&gt;n) return 0; //C_n^k double s=1; for(int i=1;i&lt;=k;i++) s=s*(n+1-i)/i; return s; } protected double C_n_k_long(int n, int k) { if(k&lt;0 || k&gt;n) return 0; //C_n^k long s=1; for(int i=1;i&lt;=k;i++) s=s*(n+1-i)/i; return (double)s; } } </code></pre> <p>main.xml:</p> <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt; &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent" &gt; &lt;TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:id="@+id/text" android:text="Hello World!" /&gt; &lt;/LinearLayout&gt; </code></pre> <p>Example of wrong calculation results (remember, it's different every time I try it)</p> <pre><code>C(0,0)=1.0 C(1,0)=1.0 C(1,1)=1.0 C(2,0)=1.0 C(2,1)=2.0 C(2,2)=1.0 C(3,0)=1.0 C(3,1)=3.0 C(3,2)=3.0 C(3,3)=1.0 C(4,0)=1.0 C(4,1)=4.0 C(4,2)=6.0 C(4,3)=4.0 C(4,4)=1.0 C(5,0)=1.0 C(5,1)=5.0 C(5,2)=10.0 C(5,3)=10.0 C(5,4)=30.0 Correct is 5.0 C(5,5)=1.0 C(6,0)=1.0 C(6,1)=6.0 C(6,2)=15.0 C(6,3)=40.0 Correct is 20.0 C(6,4)=90.0 Correct is 15.0 C(6,5)=144.0 Correct is 6.0 C(6,6)=120.0 Correct is 1.0 C(7,0)=1.0 C(7,1)=7.0 C(7,2)=21.0 C(7,3)=35.0 C(7,4)=105.0 Correct is 35.0 C(7,5)=504.0 Correct is 21.0 C(7,6)=840.0 Correct is 7.0 C(7,7)=720.0 Correct is 1.0 C(8,0)=1.0 C(8,1)=8.0 C(8,2)=28.0 C(8,3)=112.0 Correct is 56.0 C(8,4)=70.0 C(8,5)=1344.0 Correct is 56.0 C(8,6)=3360.0 Correct is 28.0 C(8,7)=5760.0 Correct is 8.0 C(8,8)=5040.0 Correct is 1.0 C(9,0)=1.0 C(9,1)=9.0 C(9,2)=36.0 C(9,3)=168.0 Correct is 84.0 C(9,4)=756.0 Correct is 126.0 C(9,5)=3024.0 Correct is 126.0 C(9,6)=10080.0 Correct is 84.0 C(9,7)=25920.0 Correct is 36.0 C(9,8)=45360.0 Correct is 9.0 C(9,9)=40320.0 Correct is 1.0 C(10,0)=1.0 C(10,1)=10.0 C(10,2)=45.0 C(10,3)=120.0 C(10,4)=210.0 C(10,5)=252.0 C(10,6)=25200.0 Correct is 210.0 C(10,7)=120.0 C(10,8)=315.0 Correct is 45.0 C(10,9)=16800.0 Correct is 10.0 C(10,10)=1.0 </code></pre>
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    1. COSounds very unlikely. Have you tried logging each value of `s` in that loop to figure out how the calculation differs?
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    2. COWhich is "correct?" if either?
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    3. COFWIW, in math, C(12, 4) = 495.
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