hash non-numeric input when used as PRNG seed, fixes #800 (#801)

src/main/java/org/javarosa/core/model/ItemsetBinding.java

@@ -2,6 +2,7 @@
 
 import static org.javarosa.core.model.FormDef.getAbsRef;
 import static org.javarosa.xform.parse.RandomizeHelper.shuffle;
+import static org.javarosa.xpath.expr.XPathFuncExpr.toLongHash;
 import static org.javarosa.xpath.expr.XPathFuncExpr.toNumeric;
 
 import java.io.DataInputStream;
@@ -36,6 +37,7 @@
 import org.javarosa.model.xform.XPathReference;
 import org.javarosa.xpath.XPathConditional;
 import org.javarosa.xpath.XPathException;
+import org.javarosa.xpath.XPathNodeset;
 import org.javarosa.xpath.expr.XPathNumericLiteral;
 import org.javarosa.xpath.expr.XPathPathExpr;
 
@@ -309,10 +311,21 @@ private static MultipleItemsData getFilteredAndBoundSelections(MultipleItemsData
     }
 
     private Long resolveRandomSeed(DataInstance model, EvaluationContext ec) {
+        XPathNodeset seedNode = null;
         if (randomSeedNumericExpr != null)
             return ((Double) randomSeedNumericExpr.eval(model, ec)).longValue();
-        if (randomSeedPathExpr != null)
-            return toNumeric(randomSeedPathExpr.eval(model, ec)).longValue();
+        if (randomSeedPathExpr != null) {
+            seedNode = randomSeedPathExpr.eval(model, ec);
+            Double asDouble = toNumeric(seedNode);
+            if (asDouble == Double.NaN) {
+                // Reasonable attempts at reading the node's value as a number failed.
+                // Fall back to deriving the seed from it using hashing.
+                // See https://github.com/getodk/javarosa/issues/800
+                return toLongHash(seedNode);
+            } else {
+                return asDouble.longValue();
+            }
+        }
         return null;
     }
 

src/main/java/org/javarosa/xpath/expr/XPathFuncExpr.java

@@ -22,6 +22,8 @@
 import java.io.DataOutputStream;
 import java.io.IOException;
 import java.math.BigDecimal;
+import java.nio.ByteBuffer;
+import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.Date;
@@ -56,6 +58,7 @@
 import org.javarosa.xpath.XPathUnhandledException;
 import org.jetbrains.annotations.NotNull;
 import org.joda.time.DateTime;
+import org.bouncycastle.crypto.digests.SHA256Digest;
 
 /**
  * Representation of an xpath function expression.
@@ -699,6 +702,26 @@ public static Double toDouble(Object o) {
 
     }
 
+    /**
+     * convert a string value to an integer by:
+     * - encoding it as utf-8
+     * - hashing it with sha256 (available cross-platform, including via browser crypto API)
+     * - interpreting the first 8 bytes of the hash as a long
+     */
+    public static long toLongHash(String sourceString) {
+        byte[] hasheeBuf = sourceString.getBytes(Charset.forName("UTF-8"));
+        SHA256Digest hasher = new SHA256Digest();
+        hasher.update(hasheeBuf, 0, hasheeBuf.length);
+        byte[] digestBuf = new byte[32];
+        hasher.doFinal(digestBuf, 0);
+        return (ByteBuffer.wrap(digestBuf)).getLong(0);
+    }
+
+    public static long toLongHash(Object o) {
+        String sourceString = (String) unpack(o);
+        return toLongHash(sourceString);
+    }
+
     /**
      * convert a value to a number using xpath's type conversion rules (note that xpath itself makes
      * no distinction between integer and floating point numbers)

src/test/java/org/javarosa/xpath/expr/XPathFuncAsSomethingTest.java

@@ -0,0 +1,42 @@
+package org.javarosa.xpath.expr;
+
+import org.junit.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.not;
+import static org.javarosa.xpath.expr.XPathFuncExpr.toLongHash;
+import static org.javarosa.xpath.expr.XPathFuncExpr.toNumeric;
+
+import java.util.Date;
+
+public class XPathFuncAsSomethingTest {
+
+    @Test
+    public void toLongHashHashesWell() {
+        assertThat(toLongHash("Hello"), equalTo(1756278180214341157L));
+        assertThat(toLongHash(""), equalTo(-2039914840885289964L));
+    }
+
+    @Test
+    public void toNumericHandlesBooleans() {
+        assertThat(toNumeric(true), equalTo(1.0));
+        assertThat(toNumeric(false), equalTo(0.0));
+    }
+
+    @Test
+    public void toNumericHandlesStrings() {
+        assertThat(toNumeric("  123  "), equalTo(123.0));
+        assertThat(toNumeric("  123.0  "), equalTo(123.0));
+        assertThat(toNumeric("  123.4  "), equalTo(123.4));
+        assertThat(toNumeric("  123,4  "), equalTo(123.4));
+
+        assertThat(toNumeric("0x12"), not(18.0));
+        assertThat(toNumeric("0x12"), equalTo(Double.NaN));
+    }
+
+    @Test
+    public void toNumericHandlesDates() {
+        assertThat(toNumeric(new Date(86400 * 1000L)), equalTo(1.0));
+    }
+}