Issue/1687 (#1697)

Co-authored-by: Bart Louwers <bart@emeel.net>
Co-authored-by: mwilsnd <mwilsnd@gmail.com>

test/util/hash.test.cpp

@@ -0,0 +1,54 @@
+#include <mbgl/test/util.hpp>
+
+#include <mbgl/util/hash.hpp>
+
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <iomanip>
+#include <random>
+#include <vector>
+
+using namespace mbgl;
+
+TEST(OrderIndependentHash, Permutations) {
+    // Try collections of up to this many elements
+    constexpr int maxSize = 6;
+    // Try this many different sets of values for each size
+    constexpr int iterations = 10;
+    // Use values up to this large
+    constexpr size_t maxVal = 10;
+
+    std::seed_seq seed{0xf00dLL};
+    std::default_random_engine engine(seed);
+    std::uniform_int_distribution<size_t> distribution(0, maxVal);
+
+    for (int curSize = 1; curSize <= maxSize; ++curSize) {
+        for (int ii = 0; ii < iterations; ++ii) {
+            // Generate `curSize` random values
+            std::vector<size_t> values;
+            for (int jj = 0; jj < curSize; ++jj) {
+                values.push_back(distribution(engine));
+            }
+
+            // Generate the hash for each permutation, comparing subsequent values to the first one.
+            bool isFirst = true;
+            size_t initialValue = 0;
+            do {
+                const auto curValue = util::order_independent_hash(values.begin(), values.end());
+                if (isFirst || curValue != initialValue) {
+                    for (auto v : values) {
+                        std::cout << v << ",";
+                    }
+                    std::cout << " - " << std::hex << curValue << std::endl;
+                }
+                if (isFirst) {
+                    initialValue = curValue;
+                    isFirst = false;
+                } else {
+                    EXPECT_EQ(initialValue, curValue);
+                }
+            } while (std::next_permutation(values.begin(), values.end()));
+        }
+    }
+}