From 26b2c3e7e2b05f8bdca5e33240acd27a052d4328 Mon Sep 17 00:00:00 2001
From: John Bytheway <jbytheway@gmail.com>
Date: Thu, 9 Apr 2020 20:17:05 -0400
Subject: [PATCH] Change random test shuffling technique (refactored)

Previously a random test ordering was obtained by applying std::shuffle
to the tests in declaration order.  This has two problems:

- It depends on the declaration order, so the order in which the tests
  will be run will be platform-specific.
- When trying to debug accidental inter-test dependencies, it is helpful
  to be able to find a minimal subset of tests which exhibits the issue.
  However, any change to the set of tests being run will completely
  change the test ordering, making it difficult or impossible to reduce
  the set of tests being run in any reasonably efficient manner.

Therefore, change the randomization approach to resolve both these
issues.

Generate a random value based on the user-provided RNG seed.  Convert
every test case to an integer by hashing a combination of that value
with the test name.  Sort the test cases by this integer.

The test names and RNG are platform-independent, so this should be
consistent across platforms.  Also, removing one test does not change
the integer value associated with the remaining tests, so they remain in
the same order.

To hash, use the FNV-1a hash, except with the basis being our randomly
selected value rather than the fixed basis set in the algorithm.  Cannot
use std::hash, because it is important that the result be
platform-independent.
---
 .../catch_test_case_registry_impl.cpp         | 75 +++++++++++++++----
 1 file changed, 59 insertions(+), 16 deletions(-)

diff --git a/src/catch2/internal/catch_test_case_registry_impl.cpp b/src/catch2/internal/catch_test_case_registry_impl.cpp
index ab324a36..7d16fb50 100644
--- a/src/catch2/internal/catch_test_case_registry_impl.cpp
+++ b/src/catch2/internal/catch_test_case_registry_impl.cpp
@@ -16,27 +16,70 @@
 #include <catch2/catch_test_case_info.hpp>
 #include <catch2/catch_test_spec.hpp>
 
+#include <algorithm>
+#include <iterator>
+#include <random>
 #include <sstream>
 
 namespace Catch {
 
-    std::vector<TestCaseHandle> sortTests( IConfig const& config, std::vector<TestCaseHandle> const& unsortedTestCases ) {
-
-        std::vector<TestCaseHandle> sorted = unsortedTestCases;
-
-        switch( config.runOrder() ) {
-            case RunTests::InLexicographicalOrder:
-                std::sort( sorted.begin(), sorted.end() );
-                break;
-            case RunTests::InRandomOrder:
-                seedRng( config );
-                std::shuffle( sorted.begin(), sorted.end(), rng() );
-                break;
-            case RunTests::InDeclarationOrder:
-                // already in declaration order
-                break;
+namespace {
+    struct HashTest {
+        explicit HashTest(SimplePcg32& rng) {
+            basis = rng();
+            basis <<= 32;
+            basis |= rng();
         }
-        return sorted;
+
+        uint64_t basis;
+
+        uint64_t operator()(TestCaseInfo const& t) const {
+            // Modified FNV-1a hash
+            static constexpr uint64_t prime = 1099511628211;
+            uint64_t hash = basis;
+            for (const char c : t.name) {
+                hash ^= c;
+                hash *= prime;
+            }
+            return hash;
+        }
+    };
+} // end anonymous namespace
+
+    std::vector<TestCaseHandle> sortTests( IConfig const& config, std::vector<TestCaseHandle> const& unsortedTestCases ) {
+        switch (config.runOrder()) {
+        case RunTests::InDeclarationOrder:
+            return unsortedTestCases;
+
+        case RunTests::InLexicographicalOrder: {
+            std::vector<TestCaseHandle> sorted = unsortedTestCases;
+            std::sort(sorted.begin(), sorted.end());
+            return sorted;
+        }
+        case RunTests::InRandomOrder: {
+            seedRng(config);
+            HashTest h(rng());
+            std::vector<std::pair<uint64_t, TestCaseHandle>> indexed_tests;
+            indexed_tests.reserve(unsortedTestCases.size());
+
+            for (auto const& handle : unsortedTestCases) {
+                indexed_tests.emplace_back(h(handle.getTestCaseInfo()), handle);
+            }
+
+            std::sort(indexed_tests.begin(), indexed_tests.end());
+
+            std::vector<TestCaseHandle> randomized;
+            randomized.reserve(indexed_tests.size());
+
+            for (auto const& indexed : indexed_tests) {
+                randomized.push_back(indexed.second);
+            }
+
+            return randomized;
+        }
+        }
+
+        CATCH_INTERNAL_ERROR("Unknown test order value!");
     }
 
     bool isThrowSafe( TestCaseHandle const& testCase, IConfig const& config ) {