choice randomization: better approximation of JR behaviour, fixes #49 (#241)

Choice randomization: better approximation of JR behaviour, fixes #49

Co-authored-by: eyelidlessness <eyelidlessness@users.noreply.github.com>

Author: brontolosone

.changeset/strange-brooms-rush.md

@@ -0,0 +1,7 @@
+---
+"@getodk/xpath": patch
+---
+
+Choice list order randomization seed handling: better correspondence with JavaRosa behaviour,
+including the addition of derivation of seeds from non-numeric inputs.
+Previously, entering a non-integer in a form field seed input would result in an exception being thrown.

packages/xpath/src/functions/xforms/node-set.ts

@@ -1,3 +1,5 @@
+import { SHA256 } from 'crypto-js';
+
 import type { XPathNode } from '../../adapter/interface/XPathNode.ts';
 import type { XPathDOMProvider } from '../../adapter/xpathDOMProvider.ts';
 import { LocationPathEvaluation } from '../../evaluations/LocationPathEvaluation.ts';
@@ -384,8 +386,44 @@ export const randomize = new NodeSetFunction(
 
 		const nodeResults = Array.from(results.values());
 		const nodes = nodeResults.map(({ value }) => value);
-		const seed = seedExpression?.evaluate(context).toNumber();
 
-		return seededRandomize(nodes, seed);
+		if (seedExpression === undefined) return seededRandomize(nodes);
+
+		const seed = seedExpression.evaluate(context);
+		const asNumber = seed.toNumber(); // TODO: There are some peculiarities to address: https://github.com/getodk/web-forms/issues/240
+		let finalSeed: number | bigint | undefined;
+		if (Number.isNaN(asNumber)) {
+			// Specific behaviors for when a seed value is not interpretable as numeric.
+			// We still want to derive a seed in those cases, see https://github.com/getodk/javarosa/issues/800
+			const seedString = seed.toString();
+			if (seedString === '') {
+				finalSeed = 0; // special case: JR behaviour
+			} else {
+				// any other string, we'll convert to a number via a digest function
+				finalSeed = toBigIntHash(seedString);
+			}
+		} else {
+			finalSeed = asNumber;
+		}
+		return seededRandomize(nodes, finalSeed);
 	}
 );
+
+const toBigIntHash = (text: string): bigint => {
+	/**
+	Hash text with sha256, and interpret the first 64 bits of output
+	(the first and second int32s ("words") of CryptoJS digest output)
+	as an int64 (in JS represented in a BigInt).
+	Thus the entropy of the hash is reduced to 64 bits, which
+	for some applications is sufficient.
+	The underlying representations are big-endian regardless of the endianness
+	of the machine this runs on, as is the equivalent JavaRosa implementation.
+	({@link https://github.com/getodk/javarosa/blob/ab0e8f4da6ad8180ac7ede5bc939f3f261c16edf/src/main/java/org/javarosa/xpath/expr/XPathFuncExpr.java#L718-L726 | see here}).
+	*/
+	const buffer = new ArrayBuffer(8);
+	const dataview = new DataView(buffer);
+	SHA256(text)
+		.words.slice(0, 2)
+		.forEach((val, ix) => dataview.setInt32(ix * Int32Array.BYTES_PER_ELEMENT, val));
+	return dataview.getBigInt64(0);
+};

packages/xpath/src/lib/collections/sort.ts

@@ -14,12 +14,18 @@ class UnseededPseudoRandomNumberGenerator implements PseudoRandomNumberGenerator
 	}
 }
 
-class SeededPseudoRandomNumberGenerator implements PseudoRandomNumberGenerator {
+class ParkMillerPRNG implements PseudoRandomNumberGenerator {
 	protected seed: number;
 
-	constructor(seed: Int) {
-		let initialSeed = seed % SEED_MODULO_OPERAND;
-
+	constructor(seed: Int | bigint) {
+		let initialSeed: number;
+		if (typeof seed === 'bigint') {
+			// the result of the modulo operation is always smaller than Number.MAX_SAFE_INTEGER,
+			// thus it's safe to convert to a Number.
+			initialSeed = Number(BigInt(seed) % BigInt(SEED_MODULO_OPERAND));
+		} else {
+			initialSeed = seed % SEED_MODULO_OPERAND;
+		}
 		if (initialSeed <= 0) {
 			initialSeed += MAX_INT_32 - 1;
 		}
@@ -38,17 +44,38 @@ class SeededPseudoRandomNumberGenerator implements PseudoRandomNumberGenerator {
 	}
 }
 
-const isInt = (value: number): value is Int => value % 1 === 0;
+class JavaRosaPRNG extends ParkMillerPRNG {
+	// Per issue #49 (https://github.com/getodk/web-forms/issues/49) this is intended to be "bug-or-feature-compatible"
+	// with JavaRosa's implementation; org.javarosa.core.model.ItemsetBinding.resolveRandomSeed takes the .longValue() of
+	// the double produced by randomSeedPathExpr.eval() — see https://github.com/getodk/javarosa/blob/6ce13527c/src/main/java/org/javarosa/core/model/ItemsetBinding.java#L311:L317 .
+	// That results in a 0L when the double is NaN, which happens (for instance) when there
+	// is a string that does not look like a number (which is a problem in itself, as any non-numeric
+	// looking string will then result in the same seed of 0 — see https://github.com/getodk/javarosa/issues/800).
+	// We'll emulate Java's Double -> Long conversion here (for NaN and some other double values)
+	// so that we produce the same randomization as JR.
+
+	constructor(seed: Int | bigint) {
+		let finalSeed: number | bigint;
+		// In Java, a NaN double's .longValue is 0
+		if (Number.isNaN(seed)) finalSeed = 0;
+		// In Java, an Infinity double's .longValue() is 2**63 -1, which is larger than Number.MAX_SAFE_INTEGER, thus we'll need a BigInt.
+		else if (seed === Infinity) finalSeed = 2n ** 63n - 1n;
+		// Analogous with the above conversion, but for -Infinity
+		else if (seed === -Infinity) finalSeed = -(2n ** 63n);
+		// A Java Double's .longValue drops the fractional part.
+		else if (typeof seed === 'number' && !Number.isInteger(seed)) finalSeed = Math.trunc(seed);
+		else finalSeed = seed;
+		super(finalSeed);
+	}
+}
 
-export const seededRandomize = <T>(values: readonly T[], seed?: number): T[] => {
+export const seededRandomize = <T>(values: readonly T[], seed?: number | bigint): T[] => {
 	let generator: PseudoRandomNumberGenerator;
 
 	if (seed == null) {
 		generator = new UnseededPseudoRandomNumberGenerator();
-	} else if (!isInt(seed)) {
-		throw 'todo not an int';
 	} else {
-		generator = new SeededPseudoRandomNumberGenerator(seed);
+		generator = new JavaRosaPRNG(seed);
 	}
 
 	const { length } = values;

packages/xpath/test/xforms/randomize.test.ts

@@ -18,6 +18,9 @@ describe('randomize()', () => {
 	});
 
 	const SELECTOR = '//xhtml:div[@id="FunctionRandomize"]/xhtml:div';
+	const MIRROR = 'mirror';
+	const MIRROR_HASH_VALUE = 5989458117437254;
+	const MIRROR_HASH_SORT_ORDER = 'ACBEDF';
 
 	describe('shuffles nodesets', () => {
 		beforeEach(() => {
@@ -44,7 +47,10 @@ describe('randomize()', () => {
               <p>3</p>
               <p>4</p>
             </div>
-          </body>
+            <div id="testFunctionNodeset3">
+              <p>${MIRROR}</p>
+            </div>
+					</body>
         </html>`,
 				{ namespaceResolver }
 			);
@@ -74,8 +80,15 @@ describe('randomize()', () => {
 			{ seed: 1, expected: 'BFEACD' },
 			{ seed: 11111111, expected: 'ACDBFE' },
 			{ seed: 'int(1)', expected: 'BFEACD' },
+			{ seed: 1.1, expected: 'BFEACD' },
+			{ seed: 0, expected: 'CBEAFD' },
+			{ seed: NaN, expected: 'CBEAFD' },
+			{ seed: Infinity, expected: 'CBEAFD' },
+			{ seed: -Infinity, expected: 'CFBEAD' },
 			{ seed: 'floor(1.1)', expected: 'BFEACD' },
 			{ seed: '//xhtml:div[@id="testFunctionNodeset2"]/xhtml:p', expected: 'BFEACD' },
+			{ seed: MIRROR_HASH_VALUE, expected: MIRROR_HASH_SORT_ORDER },
+			{ seed: '//xhtml:div[@id="testFunctionNodeset3"]/xhtml:p', expected: MIRROR_HASH_SORT_ORDER },
 		].forEach(({ seed, expected }) => {
 			it(`with a seed: ${seed}`, () => {
 				const expression = `randomize(${SELECTOR}, ${seed})`;
@@ -88,15 +101,13 @@ describe('randomize()', () => {
 		});
 	});
 
-	[
-		{ expression: 'randomize()' },
-		{ expression: `randomize(${SELECTOR}, 'a')` },
-		{ expression: `randomize(${SELECTOR}, 1, 2)` },
-	].forEach(({ expression }) => {
-		it.fails(`${expression} with invalid args, throws an error`, () => {
-			testContext.evaluate(expression);
-		});
-	});
+	[{ expression: 'randomize()' }, { expression: `randomize(${SELECTOR}, 1, 2)` }].forEach(
+		({ expression }) => {
+			it.fails(`${expression} with invalid argument count, throws an error`, () => {
+				testContext.evaluate(expression);
+			});
+		}
+	);
 
 	it('randomizes nodes', () => {
 		testContext = createXFormsTestContext(`