fuzz: fuzz diagram creation and comparison

Co-authored-by: Suhas Daftuar <sdaftuar@chaincode.com>
Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>

src/Makefile.test.include

@@ -315,6 +315,7 @@ test_fuzz_fuzz_SOURCES = \
  test/fuzz/deserialize.cpp \
  test/fuzz/eval_script.cpp \
  test/fuzz/fee_rate.cpp \
+ test/fuzz/feeratediagram.cpp \
  test/fuzz/fees.cpp \
  test/fuzz/flatfile.cpp \
  test/fuzz/float.cpp \

src/test/fuzz/feeratediagram.cpp

@@ -0,0 +1,119 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <stdint.h>
+
+#include <vector>
+
+#include <util/feefrac.h>
+#include <policy/rbf.h>
+
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+
+#include <assert.h>
+
+namespace {
+
+/** Evaluate a diagram at a specific size, returning the fee as a fraction.
+ *
+ * Fees in diagram cannot exceed 2^32, as the returned evaluation could overflow
+ * the FeeFrac::fee field in the result. */
+FeeFrac EvaluateDiagram(int32_t size, Span<const FeeFrac> diagram)
+{
+    assert(diagram.size() > 0);
+    unsigned not_above = 0;
+    unsigned not_below = diagram.size() - 1;
+    // If outside the range of diagram, extend begin/end.
+    if (size < diagram[not_above].size) return {diagram[not_above].fee, 1};
+    if (size > diagram[not_below].size) return {diagram[not_below].fee, 1};
+    // Perform bisection search to locate the diagram segment that size is in.
+    while (not_below > not_above + 1) {
+        unsigned mid = (not_below + not_above) / 2;
+        if (diagram[mid].size <= size) not_above = mid;
+        if (diagram[mid].size >= size) not_below = mid;
+    }
+    // If the size matches a transition point between segments, return its fee.
+    if (not_below == not_above) return {diagram[not_below].fee, 1};
+    // Otherwise, interpolate.
+    auto dir_coef = diagram[not_below] - diagram[not_above];
+    assert(dir_coef.size > 0);
+    // Let A = diagram[not_above] and B = diagram[not_below]
+    const auto& point_a = diagram[not_above];
+    // We want to return:
+    //     A.fee + (B.fee - A.fee) / (B.size - A.size) * (size - A.size)
+    //   = A.fee + dir_coef.fee / dir_coef.size * (size - A.size)
+    //   = (A.fee * dir_coef.size + dir_coef.fee * (size - A.size)) / dir_coef.size
+    assert(size >= point_a.size);
+    return {point_a.fee * dir_coef.size + dir_coef.fee * (size - point_a.size), dir_coef.size};
+}
+
+std::weak_ordering CompareFeeFracWithDiagram(const FeeFrac& ff, Span<const FeeFrac> diagram)
+{
+    return FeeRateCompare(FeeFrac{ff.fee, 1}, EvaluateDiagram(ff.size, diagram));
+}
+
+std::partial_ordering CompareDiagrams(Span<const FeeFrac> dia1, Span<const FeeFrac> dia2)
+{
+    bool all_ge = true;
+    bool all_le = true;
+    for (const auto p1 : dia1) {
+        auto cmp = CompareFeeFracWithDiagram(p1, dia2);
+        if (std::is_lt(cmp)) all_ge = false;
+        if (std::is_gt(cmp)) all_le = false;
+    }
+    for (const auto p2 : dia2) {
+        auto cmp = CompareFeeFracWithDiagram(p2, dia1);
+        if (std::is_lt(cmp)) all_le = false;
+        if (std::is_gt(cmp)) all_ge = false;
+    }
+    if (all_ge && all_le) return std::partial_ordering::equivalent;
+    if (all_ge && !all_le) return std::partial_ordering::greater;
+    if (!all_ge && all_le) return std::partial_ordering::less;
+    return std::partial_ordering::unordered;
+}
+
+void PopulateChunks(FuzzedDataProvider& fuzzed_data_provider, std::vector<FeeFrac>& chunks)
+{
+    chunks.clear();
+
+    LIMITED_WHILE(fuzzed_data_provider.ConsumeBool(), 50)
+    {
+        chunks.emplace_back(fuzzed_data_provider.ConsumeIntegralInRange<int64_t>(INT32_MIN>>1, INT32_MAX>>1), fuzzed_data_provider.ConsumeIntegralInRange<int32_t>(1, 1000000));
+    }
+    return;
+}
+
+} // namespace
+
+FUZZ_TARGET(build_and_compare_feerate_diagram)
+{
+    // Generate a random set of chunks
+    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
+    std::vector<FeeFrac> chunks1, chunks2;
+    FeeFrac empty{0, 0};
+
+    PopulateChunks(fuzzed_data_provider, chunks1);
+    PopulateChunks(fuzzed_data_provider, chunks2);
+
+    std::vector<FeeFrac> diagram1{BuildDiagramFromChunks(chunks1)};
+    std::vector<FeeFrac> diagram2{BuildDiagramFromChunks(chunks2)};
+
+    assert(diagram1.front() == empty);
+    assert(diagram2.front() == empty);
+
+    auto real = CompareFeerateDiagram(diagram1, diagram2);
+    auto sim = CompareDiagrams(diagram1, diagram2);
+    assert(real == sim);
+
+    // Do explicit evaluation at up to 1000 points, and verify consistency with the result.
+    LIMITED_WHILE(fuzzed_data_provider.remaining_bytes(), 1000) {
+        int32_t size = fuzzed_data_provider.ConsumeIntegralInRange<int32_t>(0, diagram2.back().size);
+        auto eval1 = EvaluateDiagram(size, diagram1);
+        auto eval2 = EvaluateDiagram(size, diagram2);
+        auto cmp = FeeRateCompare(eval1, eval2);
+        if (std::is_lt(cmp)) assert(!std::is_gt(real));
+        if (std::is_gt(cmp)) assert(!std::is_lt(real));
+    }
+}