Merge pull request #7 from Progmasoft/feature/native-property-engine

Add a native property-testing engine with choice-sequence shrinking

6 files changed, 764 insertions, 0 deletions

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 79660687..3d4db3b3 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -160,8 +160,11 @@ target_link_libraries(SelfTest PRIVATE Catch2WithMain)
 add_executable(ProgmasoftSelfTest
   Progmasoft/Generators.tests.cpp
   Progmasoft/Property.tests.cpp
+  Progmasoft/ForAll.tests.cpp
   Progmasoft/Results.tests.cpp
   Progmasoft/Snapshot.tests.cpp
+  Progmasoft/Stateful.tests.cpp
+  Progmasoft/Strategy.tests.cpp
 )
 target_link_libraries(ProgmasoftSelfTest PRIVATE Progmasoft::Catch3 Catch2WithMain)
 target_compile_features(ProgmasoftSelfTest PRIVATE cxx_std_20)
diff --git a/tests/Progmasoft/BUILD.bazel b/tests/Progmasoft/BUILD.bazel
index d2eeca92..853d99d4 100644
--- a/tests/Progmasoft/BUILD.bazel
+++ b/tests/Progmasoft/BUILD.bazel
@@ -7,10 +7,13 @@ package(default_visibility = ["//visibility:public"])
 cc_test(
     name = "property_tests",
     srcs = [
+        "ForAll.tests.cpp",
         "Generators.tests.cpp",
         "Property.tests.cpp",
         "Results.tests.cpp",
         "Snapshot.tests.cpp",
+        "Stateful.tests.cpp",
+        "Strategy.tests.cpp",
     ],
     copts = select({
         "@platforms//os:windows": ["/std:c++20"],
diff --git a/tests/Progmasoft/ForAll.tests.cpp b/tests/Progmasoft/ForAll.tests.cpp
new file mode 100644
index 00000000..b88477a6
--- /dev/null
+++ b/tests/Progmasoft/ForAll.tests.cpp
@@ -0,0 +1,314 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#include <Progmasoft/Catch3.hpp>
+
+#include <algorithm>
+#include <cstdint>
+#include <filesystem>
+#include <numeric>
+#include <set>
+#include <stdexcept>
+#include <string>
+#include <vector>
+
+namespace {
+
+    using Progmasoft::Catch3::ForAll;
+    using Progmasoft::Catch3::PropertyOptions;
+    using Progmasoft::Catch3::PropertyStatus;
+    using Progmasoft::Catch3::SizeRange;
+    namespace Gen = Progmasoft::Catch3::Gen;
+
+    PropertyOptions Quick( std::size_t trials = 200 ) {
+        PropertyOptions options;
+        options.Trials = trials;
+        options.Seed = 0xCA7C3;
+        options.MaxShrinkSteps = 5000;
+        return options;
+    }
+
+} // namespace
+
+TEST_CASE( "ForAll passes and counts its trials",
+           "[progmasoft][forall]" ) {
+    const auto result = ForAll( Quick( 150 ),
+                                Gen::Integers<int>( -100, 100 ),
+                                Gen::Integers<int>( -100, 100 ),
+                                []( int left, int right ) {
+                                    return left + right == right + left;
+                                } );
+    CHECK( result.Status == PropertyStatus::kPassed );
+    CHECK( result.TrialsRun == 150 );
+    CHECK( result.Describe() == "property passed: 150 trials" );
+}
+
+TEST_CASE( "ForAll shrinks integers to the boundary of the failure",
+           "[progmasoft][forall]" ) {
+    const auto result = ForAll(
+        Quick(), Gen::Integers<int>(), []( int value ) { return value <= 1000; } );
+    REQUIRE( result.Status == PropertyStatus::kFailed );
+    REQUIRE( result.Failure.has_value() );
+    CHECK( result.Failure->Counterexample == "1001 (0x3e9)" );
+    CHECK( result.Failure->Reason == "property returned false" );
+    CHECK( result.Failure->Replay.rfind( "c3.", 0 ) == 0 );
+}
+
+TEST_CASE( "ForAll shrinks collections to their essential element",
+           "[progmasoft][forall]" ) {
+    const auto result = ForAll(
+        Quick(),
+        Gen::Vectors( Gen::Integers<int>( 0, 100 ) ),
+        []( const std::vector<int>& values ) {
+            return std::none_of( values.begin(), values.end(),
+                                 []( int value ) { return value >= 10; } );
+        } );
+    REQUIRE( result.Status == PropertyStatus::kFailed );
+    CHECK( result.Failure->Counterexample == "{ 10 }" );
+}
+
+TEST_CASE( "ForAll shrinks through dependent and filtered strategies",
+           "[progmasoft][forall]" ) {
+    const auto dependent =
+        Gen::Integers<int>( 0, 20 ).FlatMap( []( int size ) {
+            return Gen::Vectors(
+                Gen::Integers<int>( 0, 100 ),
+                SizeRange{ static_cast<std::size_t>( size ),
+                           static_cast<std::size_t>( size ) } );
+        } );
+    const auto sized = ForAll( Quick(), dependent,
+                               []( const std::vector<int>& values ) {
+                                   return values.size() < 3;
+                               } );
+    REQUIRE( sized.Status == PropertyStatus::kFailed );
+    CHECK( sized.Failure->Counterexample == "{ 0, 0, 0 }" );
+
+    const auto even = Gen::Integers<int>( 0, 100000 ).Filter(
+        []( int value ) { return value % 2 == 0; } );
+    const auto filtered =
+        ForAll( Quick(), even, []( int value ) { return value <= 100; } );
+    INFO( filtered.Describe() );
+    REQUIRE( filtered.Status == PropertyStatus::kFailed );
+    CHECK( filtered.Failure->Counterexample == "102" );
+
+    const auto mapped = ForAll(
+        Quick(),
+        Gen::Integers<int>( 0, 1000 ).Map( []( int value ) {
+            return std::to_string( value );
+        } ),
+        []( const std::string& text ) { return text.size() < 3; } );
+    REQUIRE( mapped.Status == PropertyStatus::kFailed );
+    CHECK( mapped.Failure->Counterexample == "\"100\"" );
+}
+
+TEST_CASE( "ForAll shrinks several arguments together",
+           "[progmasoft][forall]" ) {
+    const auto result = ForAll( Quick(),
+                                Gen::Integers<int>( 0, 1000 ),
+                                Gen::Integers<int>( 0, 1000 ),
+                                []( int left, int right ) {
+                                    return left + right < 100;
+                                } );
+    REQUIRE( result.Status == PropertyStatus::kFailed );
+    CHECK( result.Failure->Counterexample == "{ 0, 100 }" );
+}
+
+TEST_CASE( "A replay token reproduces exactly the reported input",
+           "[progmasoft][forall]" ) {
+    const auto body = []( const std::vector<int>& values ) {
+        return std::accumulate( values.begin(), values.end(), 0 ) < 50;
+    };
+    const auto strategy = Gen::Vectors( Gen::Integers<int>( 0, 30 ) );
+    const auto first = ForAll( Quick(), strategy, body );
+    REQUIRE( first.Status == PropertyStatus::kFailed );
+
+    auto options = Quick();
+    options.Replay = first.Failure->Replay;
+    const auto replayed = ForAll( options, strategy, body );
+    REQUIRE( replayed.Status == PropertyStatus::kFailed );
+    CHECK( replayed.TrialsRun == 1 );
+    CHECK( replayed.Failure->Counterexample == first.Failure->Counterexample );
+    CHECK( replayed.Failure->Replay == first.Failure->Replay );
+    CHECK( first.Describe().find( "replay: set PropertyOptions::Replay to \"" +
+                                  first.Failure->Replay + "\"" ) !=
+           std::string::npos );
+
+    // The same options give the same report.
+    CHECK( ForAll( Quick(), strategy, body ).Describe() == first.Describe() );
+
+    options.Replay = "not a token";
+    CHECK( ForAll( options, strategy, body ).Status ==
+           PropertyStatus::kInvalidConfiguration );
+}
+
+TEST_CASE( "Property bodies assert, assume and note without exceptions",
+           "[progmasoft][forall]" ) {
+    const auto failing = ForAll(
+        Quick(), Gen::Integers<int>( 0, 1000 ), []( int value ) {
+            Progmasoft::Catch3::Note( "half", value / 2 );
+            CATCH3_PROPERTY_ASSERT( value < 500 );
+        } );
+    REQUIRE( failing.Status == PropertyStatus::kFailed );
+    CHECK( failing.Failure->Counterexample == "500 (0x1f4)" );
+    CHECK( failing.Failure->Reason.find(
+               "CATCH3_PROPERTY_ASSERT( value < 500 )" ) == 0 );
+    CHECK( failing.Failure->Notes == std::vector<std::string>{ "half = 250" } );
+
+    const auto assumed = ForAll(
+        Quick(), Gen::Integers<int>( 0, 9 ), []( int value ) {
+            CATCH3_PROPERTY_ASSUME( value % 2 == 0 );
+            CATCH3_PROPERTY_ASSERT( value % 2 == 0 );
+        } );
+    CHECK( assumed.Status == PropertyStatus::kPassed );
+    CHECK( assumed.Discarded > 0 );
+
+    auto strict = Quick( 50 );
+    strict.MaxDiscardRatio = 1;
+    const auto gaveUp = ForAll( strict, Gen::Integers<int>( 0, 9 ),
+                                []( int ) { CATCH3_PROPERTY_ASSUME( false ); } );
+    CHECK( gaveUp.Status == PropertyStatus::kGaveUp );
+    CHECK( gaveUp.Describe().find( "property gave up" ) == 0 );
+}
+
+TEST_CASE( "Exceptions from bodies fail and from strategies error",
+           "[progmasoft][forall]" ) {
+    const auto throwing = ForAll(
+        Quick(), Gen::Integers<int>( 0, 100 ), []( int value ) -> bool {
+            if ( value > 10 ) { throw std::runtime_error( "too big" ); }
+            return true;
+        } );
+    REQUIRE( throwing.Status == PropertyStatus::kFailed );
+    CHECK( throwing.Failure->Counterexample == "11" );
+    CHECK( throwing.Failure->Reason ==
+           "property threw std::exception: too big" );
+
+    const auto broken = ForAll(
+        Quick(),
+        Gen::Integers<int>( 0, 10 ).Map( []( int ) -> int {
+            throw std::runtime_error( "cannot build" );
+        } ),
+        []( int ) { return true; } );
+    CHECK( broken.Status == PropertyStatus::kErrored );
+    CHECK( broken.Error == "strategy threw std::exception: cannot build" );
+}
+
+TEST_CASE( "Classify and Cover report and enforce statistics",
+           "[progmasoft][forall]" ) {
+    auto options = Quick( 400 );
+    options.ShowStatistics = true;
+    const auto covered = ForAll(
+        options, Gen::Integers<int>( -100, 100 ), []( int value ) {
+            Progmasoft::Catch3::Classify( value == 0, "zero" );
+            Progmasoft::Catch3::Cover( 20.0, value < 0, "negative" );
+            return true;
+        } );
+    CHECK( covered.Status == PropertyStatus::kPassed );
+    CHECK( covered.Describe().find( "negative:" ) != std::string::npos );
+
+    const auto uncovered = ForAll(
+        Quick( 400 ), Gen::Integers<int>( 0, 1000 ), []( int value ) {
+            Progmasoft::Catch3::Cover( 50.0, value == 7, "seven" );
+            return true;
+        } );
+    CHECK( uncovered.Status == PropertyStatus::kInsufficientCoverage );
+    CHECK( uncovered.Describe().find( "required 50%" ) != std::string::npos );
+}
+
+TEST_CASE( "Targeted search runs after the regular trials",
+           "[progmasoft][forall]" ) {
+    auto options = Quick( 100 );
+    options.TargetSteps = 300;
+    const auto result = ForAll(
+        options, Gen::Integers<std::int64_t>( 0, 1LL << 40 ), []( std::int64_t value ) {
+            Progmasoft::Catch3::Target( static_cast<double>( value ) );
+            return value < ( 1LL << 39 ) + ( 1LL << 38 );
+        } );
+    // Climbing towards the largest values must reach the failing region.
+    REQUIRE( result.Status == PropertyStatus::kFailed );
+    CHECK( result.Failure->Counterexample == "824633720832 (0xc000000000)" );
+}
+
+TEST_CASE( "The example database replays a stored failure first",
+           "[progmasoft][forall]" ) {
+    const auto directory = std::filesystem::temp_directory_path() /
+                           "catch3-property-database-test";
+    std::filesystem::remove_all( directory );
+    auto options = Quick();
+    options.DatabasePath = directory.string();
+    options.Name = "database test";
+    int threshold = 200;
+    const auto body = [&]( int value ) { return value < threshold; };
+
+    const auto first = ForAll( options, Gen::Integers<int>( 0, 1000 ), body );
+    REQUIRE( first.Status == PropertyStatus::kFailed );
+    CHECK( first.Failure->Counterexample == "200" );
+    CHECK( !std::filesystem::is_empty( directory ) );
+
+    // A new seed would need its own trials; the stored example fails at
+    // once.
+    options.Seed = 0xD1FFE7;
+    const auto second = ForAll( options, Gen::Integers<int>( 0, 1000 ), body );
+    REQUIRE( second.Status == PropertyStatus::kFailed );
+    CHECK( second.TrialsRun == 1 );
+    CHECK( second.Failure->Counterexample == "200" );
+
+    // Once fixed, the example passes and is dropped.
+    threshold = 2000;
+    const auto fixed = ForAll( options, Gen::Integers<int>( 0, 1000 ), body );
+    CHECK( fixed.Status == PropertyStatus::kPassed );
+    CHECK( std::filesystem::is_empty( directory ) );
+    std::filesystem::remove_all( directory );
+}
+
+TEST_CASE( "Relations check differential, metamorphic and round-trip oracles",
+           "[progmasoft][forall][relations]" ) {
+    using namespace Progmasoft::Catch3;
+    const auto sorted = []( std::vector<int> values ) {
+        std::sort( values.begin(), values.end() );
+        return values;
+    };
+    // A "fast" sort that loses duplicates.
+    const auto lossy = []( const std::vector<int>& values ) {
+        const std::set<int> unique( values.begin(), values.end() );
+        return std::vector<int>( unique.begin(), unique.end() );
+    };
+    const auto differential = ForAll(
+        Quick(), Gen::Vectors( Gen::Integers<int>( 0, 50 ) ),
+        Equivalent( sorted, lossy ) );
+    REQUIRE( differential.Status == PropertyStatus::kFailed );
+    CHECK( differential.Failure->Counterexample == "{ 0, 0 }" );
+    CHECK( differential.Failure->Notes ==
+           std::vector<std::string>{ "reference: { 0, 0 }",
+                                     "candidate: { 0 }" } );
+
+    const auto sum = []( const std::vector<int>& values ) {
+        return std::accumulate( values.begin(), values.end(), 0LL );
+    };
+    const auto reversed = []( std::vector<int> values ) {
+        std::reverse( values.begin(), values.end() );
+        return values;
+    };
+    CHECK( ForAll( Quick(), Gen::Vectors( Gen::Integers<int>( -9, 9 ) ),
+                   Metamorphic( sum, reversed, std::equal_to<>() ) )
+               .Succeeded() );
+
+    const auto encode = []( int value ) { return std::to_string( value ); };
+    const auto decode = []( const std::string& text ) {
+        return std::stoi( text );
+    };
+    CHECK( ForAll( Quick(), Gen::Integers<int>(), RoundTrip( encode, decode ) )
+               .Succeeded() );
+    CHECK( ForAll( Quick(), Gen::Vectors( Gen::Integers<int>( 0, 9 ) ),
+                   Idempotent( sorted ) )
+               .Succeeded() );
+}
+
+TEST_CASE( "CATCH3_CHECK_FOR_ALL reports through Catch2",
+           "[progmasoft][forall]" ) {
+    CATCH3_CHECK_FOR_ALL( Gen::Integers<int>( 0, 100 ),
+                          []( int value ) { return value >= 0; } );
+    CATCH3_REQUIRE_FOR_ALL( Quick( 20 ),
+                            Gen::Strings( Gen::PrintableCharacters() ),
+                            []( const std::string& text ) {
+                                return text.size() == text.length();
+                            } );
+}
diff --git a/tests/Progmasoft/Stateful.tests.cpp b/tests/Progmasoft/Stateful.tests.cpp
new file mode 100644
index 00000000..48e2c918
--- /dev/null
+++ b/tests/Progmasoft/Stateful.tests.cpp
@@ -0,0 +1,129 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#include <Progmasoft/Catch3.hpp>
+
+#include <cstddef>
+#include <deque>
+#include <string>
+#include <vector>
+
+namespace {
+
+    using Progmasoft::Catch3::Action;
+    using Progmasoft::Catch3::Command;
+    using Progmasoft::Catch3::PropertyOptions;
+    using Progmasoft::Catch3::PropertyStatus;
+    using Progmasoft::Catch3::SizeRange;
+    namespace Gen = Progmasoft::Catch3::Gen;
+
+    // A queue with a bug: past three elements it silently drops the
+    // oldest one.
+    class SmallQueue {
+    public:
+        void Push( int value ) {
+            if ( values_.size() == 3 ) { values_.pop_front(); }
+            values_.push_back( value );
+        }
+        int Pop() {
+            const int value = values_.front();
+            values_.pop_front();
+            return value;
+        }
+        [[nodiscard]] std::size_t Size() const { return values_.size(); }
+
+    private:
+        std::deque<int> values_;
+    };
+
+    using Model = std::deque<int>;
+
+    std::vector<Command<Model, SmallQueue>> QueueCommands() {
+        Command<Model, SmallQueue> push{
+            "push",
+            []( const Model& ) {
+                return Gen::Integers<int>( 0, 100 ).Map( []( int value ) {
+                    return Action<Model, SmallQueue>{
+                        "push(" + std::to_string( value ) + ")",
+                        [value]( Model& model, SmallQueue& queue ) {
+                            model.push_back( value );
+                            queue.Push( value );
+                            CATCH3_PROPERTY_ASSERT( queue.Size() ==
+                                                    model.size() );
+                        } };
+                } );
+            } };
+        Command<Model, SmallQueue> pop{
+            "pop",
+            []( const Model& ) {
+                return Gen::Just( Action<Model, SmallQueue>{
+                    "pop", []( Model& model, SmallQueue& queue ) {
+                        const int expected = model.front();
+                        model.pop_front();
+                        CATCH3_PROPERTY_ASSERT( queue.Pop() == expected );
+                    } } );
+            },
+            []( const Model& model ) { return !model.empty(); } };
+        return { push, pop };
+    }
+
+} // namespace
+
+TEST_CASE( "Stateful tests find and shrink a failing command sequence",
+           "[progmasoft][stateful]" ) {
+    PropertyOptions options;
+    options.Trials = 300;
+    options.Seed = 0x57A7E;
+    options.MaxShrinkSteps = 5000;
+    const auto result = Progmasoft::Catch3::CheckStateful<Model, SmallQueue>(
+        options,
+        [] { return Model{}; },
+        []( const Model& ) { return SmallQueue{}; },
+        QueueCommands() );
+    REQUIRE( result.Status == PropertyStatus::kFailed );
+    // The shortest failure: the fourth push drops an element.
+    CHECK( result.Failure->Counterexample ==
+           "{ push(0), push(0), push(0), push(0) }" );
+    CHECK( result.Failure->Reason.find(
+               "CATCH3_PROPERTY_ASSERT( queue.Size() == model.size() )" ) ==
+           0 );
+}
+
+TEST_CASE( "Stateful tests pass for a correct system and respect preconditions",
+           "[progmasoft][stateful]" ) {
+    struct Correct {
+        std::deque<int> Values;
+    };
+    Command<Model, Correct> push{
+        "push", []( const Model& ) {
+            return Gen::Integers<int>( 0, 9 ).Map( []( int value ) {
+                return Action<Model, Correct>{
+                    "push", [value]( Model& model, Correct& system ) {
+                        model.push_back( value );
+                        system.Values.push_back( value );
+                    } };
+            } );
+        } };
+    Command<Model, Correct> pop{
+        "pop",
+        []( const Model& ) {
+            return Gen::Just( Action<Model, Correct>{
+                "pop", []( Model& model, Correct& system ) {
+                    // The precondition guarantees an element.
+                    CATCH3_PROPERTY_ASSERT( !model.empty() );
+                    CATCH3_PROPERTY_ASSERT( system.Values.front() ==
+                                            model.front() );
+                    model.pop_front();
+                    system.Values.pop_front();
+                } } );
+        },
+        []( const Model& model ) { return !model.empty(); } };
+    PropertyOptions options;
+    options.Trials = 200;
+    const auto result = Progmasoft::Catch3::CheckStateful<Model, Correct>(
+        options,
+        [] { return Model{}; },
+        []( const Model& ) { return Correct{}; },
+        { push, pop },
+        SizeRange{ 0, 50 } );
+    CHECK( result.Status == PropertyStatus::kPassed );
+}
diff --git a/tests/Progmasoft/Strategy.tests.cpp b/tests/Progmasoft/Strategy.tests.cpp
new file mode 100644
index 00000000..5a1d5a2a
--- /dev/null
+++ b/tests/Progmasoft/Strategy.tests.cpp
@@ -0,0 +1,312 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-1.1
+#include <Progmasoft/Catch3.hpp>
+
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+#include <limits>
+#include <map>
+#include <optional>
+#include <set>
+#include <stdexcept>
+#include <string>
+#include <vector>
+
+namespace {
+
+    using Progmasoft::Catch3::ChoiceSource;
+    using Progmasoft::Catch3::SizeRange;
+    using Progmasoft::Catch3::Strategy;
+    namespace Gen = Progmasoft::Catch3::Gen;
+
+    template <typename ValueType>
+    std::vector<ValueType> Sample( const Strategy<ValueType>& strategy,
+                                   std::size_t count,
+                                   std::uint64_t seed ) {
+        std::vector<ValueType> values;
+        for ( std::size_t index = 0; index < count; ++index ) {
+            ChoiceSource source( seed + index, index % 101 );
+            auto value = strategy.Draw( source );
+            if ( value.has_value() ) { values.push_back( *value ); }
+        }
+        return values;
+    }
+
+    // Draws once, then replays the recorded choices: the same value must
+    // come back, which is what shrinking and replay tokens rely on.
+    template <typename ValueType>
+    bool ReplaysIdentically( const Strategy<ValueType>& strategy,
+                             std::uint64_t seed ) {
+        ChoiceSource original( seed, 60 );
+        const auto value = strategy.Draw( original );
+        const auto choices = original.Choices();
+        ChoiceSource replay( choices, 60 );
+        const auto again = strategy.Draw( replay );
+        return value == again && replay.Choices() == choices &&
+               !replay.Overran();
+    }
+
+} // namespace
+
+TEST_CASE( "Choice sources replay, clamp and run out to zero",
+           "[progmasoft][strategy]" ) {
+    const std::vector<std::uint64_t> recorded{ 3, 99, 1 };
+    ChoiceSource replay( recorded, 10 );
+    CHECK( replay.Draw( 10 ) == 3 );
+    CHECK( replay.Draw( 7 ) == 7 ); // clamped to the new maximum
+    CHECK( replay.DrawBoolean( 0.0 ) );
+    CHECK_FALSE( replay.Overran() );
+    CHECK( replay.Draw( 5 ) == 0 ); // past the end: the simplest choice
+    CHECK( replay.Overran() );
+    CHECK( replay.Choices() == std::vector<std::uint64_t>{ 3, 7, 1, 0 } );
+
+    ChoiceSource generated( 42, 50 );
+    for ( int index = 0; index < 1000; ++index ) {
+        CHECK( generated.Draw( 9 ) <= 9 );
+        CHECK( generated.DrawBiased( 1000 ) <= 1000 );
+    }
+    CHECK( generated.Choices().size() == 2000 );
+}
+
+TEST_CASE( "Integer strategies respect every kind of range",
+           "[progmasoft][strategy]" ) {
+    const auto crossing = Gen::Integers<int>( -50, 70 );
+    for ( int value : Sample( crossing, 2000, 1 ) ) {
+        CHECK( value >= -50 );
+        CHECK( value <= 70 );
+    }
+    const auto negative = Gen::Integers<std::int64_t>( -900, -100 );
+    for ( auto value : Sample( negative, 2000, 2 ) ) {
+        CHECK( value >= -900 );
+        CHECK( value <= -100 );
+    }
+    const auto positive = Gen::Integers<std::uint8_t>( 10, 20 );
+    for ( auto value : Sample( positive, 2000, 3 ) ) {
+        CHECK( value >= 10 );
+        CHECK( value <= 20 );
+    }
+    const auto full = Gen::Integers<std::int64_t>();
+    const auto values = Sample( full, 4000, 4 );
+    CHECK( std::any_of( values.begin(), values.end(), []( auto value ) {
+        return value == std::numeric_limits<std::int64_t>::max();
+    } ) );
+    CHECK( std::any_of( values.begin(), values.end(), []( auto value ) {
+        return value < 0;
+    } ) );
+    const auto upToZero = Gen::Integers<std::int64_t>(
+        std::numeric_limits<std::int64_t>::lowest(), 0 );
+    for ( auto value : Sample( upToZero, 2000, 5 ) ) { CHECK( value <= 0 ); }
+
+    // The simplest choices give the value closest to zero.
+    const std::vector<std::uint64_t> zeros;
+    ChoiceSource simplest( zeros, 0 );
+    CHECK( crossing.Draw( simplest ) == 0 );
+    ChoiceSource simplestNegative( zeros, 0 );
+    CHECK( negative.Draw( simplestNegative ) == -100 );
+    ChoiceSource simplestPositive( zeros, 0 );
+    CHECK( positive.Draw( simplestPositive ) == 10 );
+
+    CHECK_THROWS_AS( Gen::Integers<int>( 2, 1 ), std::invalid_argument );
+    for ( std::uint64_t seed = 0; seed < 50; ++seed ) {
+        CHECK( ReplaysIdentically( full, seed ) );
+    }
+}
+
+TEST_CASE( "Float strategies stay finite and in range unless allowed",
+           "[progmasoft][strategy]" ) {
+    using Progmasoft::Catch3::FloatOptions;
+    const auto bounded = Gen::Floats<double>(
+        FloatOptions{ .Minimum = -2.5, .Maximum = 1000.0 } );
+    for ( double value : Sample( bounded, 3000, 6 ) ) {
+        CHECK( std::isfinite( value ) );
+        CHECK( value >= -2.5 );
+        CHECK( value <= 1000.0 );
+    }
+    const auto everything = Gen::Floats<double>(
+        FloatOptions{ .AllowNaN = true, .AllowInfinity = true } );
+    const auto values = Sample( everything, 3000, 7 );
+    CHECK( std::any_of( values.begin(), values.end(), []( double value ) {
+        return std::isnan( value );
+    } ) );
+    CHECK( std::any_of( values.begin(), values.end(), []( double value ) {
+        return std::isinf( value );
+    } ) );
+    const auto positive = Gen::Floats<float>(
+        FloatOptions{ .Minimum = 1.0, .Maximum = 2.0 } );
+    const std::vector<std::uint64_t> zeros;
+    ChoiceSource simplest( zeros, 0 );
+    CHECK( positive.Draw( simplest ) == 1.0F );
+    CHECK_THROWS_AS( Gen::Floats<double>( FloatOptions{ .Minimum = 2.0,
+                                                        .Maximum = 1.0 } ),
+                     std::invalid_argument );
+}
+
+TEST_CASE( "Collection strategies respect their size ranges",
+           "[progmasoft][strategy]" ) {
+    const auto vectors = Gen::Vectors( Gen::Integers<int>( 0, 9 ),
+                                       SizeRange{ 2, 5 } );
+    for ( const auto& value : Sample( vectors, 1000, 8 ) ) {
+        CHECK( value.size() >= 2 );
+        CHECK( value.size() <= 5 );
+    }
+    const auto unique = Gen::UniqueVectors( Gen::Integers<int>( 0, 30 ),
+                                            SizeRange{ 0, 20 } );
+    for ( const auto& value : Sample( unique, 1000, 9 ) ) {
+        CHECK( std::set<int>( value.begin(), value.end() ).size() ==
+               value.size() );
+    }
+    const auto sets = Gen::Sets( Gen::Integers<int>( 0, 3 ),
+                                 SizeRange{ 4, 4 } );
+    for ( const auto& value : Sample( sets, 200, 10 ) ) {
+        CHECK( value == std::set<int>{ 0, 1, 2, 3 } );
+    }
+    const auto maps = Gen::Maps( Gen::Integers<int>( 0, 100 ),
+                                 Gen::Booleans(),
+                                 SizeRange{ 1, 10 } );
+    for ( const auto& value : Sample( maps, 500, 11 ) ) {
+        CHECK( !value.empty() );
+        CHECK( value.size() <= 10 );
+    }
+    const auto permutations = Gen::Permutations( std::vector<int>{ 1, 2, 3, 4, 5 } );
+    for ( auto value : Sample( permutations, 300, 12 ) ) {
+        std::sort( value.begin(), value.end() );
+        CHECK( value == std::vector<int>{ 1, 2, 3, 4, 5 } );
+    }
+    const std::vector<std::uint64_t> zeros;
+    ChoiceSource simplest( zeros, 0 );
+    CHECK( permutations.Draw( simplest ) ==
+           std::vector<int>{ 1, 2, 3, 4, 5 } );
+    CHECK_THROWS_AS( Gen::Vectors( Gen::Booleans(), SizeRange{ 3, 2 } ),
+                     std::invalid_argument );
+    for ( std::uint64_t seed = 0; seed < 50; ++seed ) {
+        CHECK( ReplaysIdentically( vectors, seed ) );
+        CHECK( ReplaysIdentically( maps, seed ) );
+    }
+}
+
+TEST_CASE( "Text strategies produce valid UTF-8 without surrogates",
+           "[progmasoft][strategy]" ) {
+    const auto text = Gen::Utf8Strings( SizeRange{ 0, 30 }, 0, 0x10FFFF );
+    for ( const auto& value : Sample( text, 1000, 13 ) ) {
+        for ( std::size_t index = 0; index + 1 < value.size(); ++index ) {
+            const auto lead = static_cast<unsigned char>( value[index] );
+            const auto next = static_cast<unsigned char>( value[index + 1] );
+            // ED A0..BF would encode a surrogate.
+            CHECK_FALSE( ( lead == 0xED && next >= 0xA0 ) );
+        }
+    }
+    const auto printable = Gen::Strings( Gen::PrintableCharacters(),
+                                         SizeRange{ 1, 8 } );
+    for ( const auto& value : Sample( printable, 500, 14 ) ) {
+        CHECK( !value.empty() );
+        CHECK( value.size() <= 8 );
+        CHECK( std::all_of( value.begin(), value.end(), []( char c ) {
+            return c >= 0x20 && c < 0x7F;
+        } ) );
+    }
+}
+
+TEST_CASE( "Combinators compose and keep replays stable",
+           "[progmasoft][strategy]" ) {
+    const auto even = Gen::Integers<int>( 0, 1000 ).Filter(
+        []( int value ) { return value % 2 == 0; } );
+    for ( int value : Sample( even, 500, 15 ) ) { CHECK( value % 2 == 0 ); }
+
+    const auto sized = Gen::Integers<int>( 0, 6 ).FlatMap( []( int size ) {
+        return Gen::Vectors( Gen::Just( size ),
+                             SizeRange{ static_cast<std::size_t>( size ),
+                                        static_cast<std::size_t>( size ) } );
+    } );
+    for ( const auto& value : Sample( sized, 500, 16 ) ) {
+        CHECK( std::all_of( value.begin(), value.end(), [&]( int element ) {
+            return element == static_cast<int>( value.size() );
+        } ) );
+    }
+
+    const auto mixed = Gen::OneOf( Gen::Just( std::string( "a" ) ),
+                                   Gen::Just( std::string( "b" ) ) );
+    const auto mixedValues = Sample( mixed, 200, 17 );
+    CHECK( std::count( mixedValues.begin(), mixedValues.end(), "a" ) > 0 );
+    CHECK( std::count( mixedValues.begin(), mixedValues.end(), "b" ) > 0 );
+
+    const auto weighted = Gen::Frequency<int>( { { 1, Gen::Just( 0 ) },
+                                                 { 9, Gen::Just( 1 ) } } );
+    const auto weightedValues = Sample( weighted, 2000, 18 );
+    CHECK( std::count( weightedValues.begin(), weightedValues.end(), 1 ) >
+           std::count( weightedValues.begin(), weightedValues.end(), 0 ) );
+
+    const auto trees = Gen::Recursive(
+        Gen::Just( 1 ),
+        []( const Strategy<int>& children ) {
+            return Gen::Tuples( children, children )
+                .Map( []( std::tuple<int, int> pair ) {
+                    return std::get<0>( pair ) + std::get<1>( pair );
+                } );
+        },
+        4 );
+    for ( int leaves : Sample( trees, 500, 19 ) ) {
+        CHECK( leaves >= 1 );
+        CHECK( leaves <= 16 ); // depth 4 at most: 2^4 leaves
+    }
+
+    const auto never = Gen::Integers<int>( 0, 10 ).Filter(
+        []( int ) { return false; }, 5 );
+    ChoiceSource source( 1, 50 );
+    CHECK_FALSE( never.Draw( source ).has_value() );
+}
+
+TEST_CASE( "Replay tokens round-trip and reject malformed text",
+           "[progmasoft][strategy]" ) {
+    using Progmasoft::Catch3::Detail::DecodeChoices;
+    using Progmasoft::Catch3::Detail::EncodeChoices;
+    const std::vector<std::uint64_t> choices{
+        0, 1, 35, 36, std::numeric_limits<std::uint64_t>::max() };
+    const auto token = EncodeChoices( 73, choices );
+    CHECK( token == "c3.21.0-1-z-10-3w5e11264sgsf" );
+    const auto decoded = DecodeChoices( token );
+    REQUIRE( decoded.has_value() );
+    CHECK( decoded->Size == 73 );
+    CHECK( decoded->Choices == choices );
+
+    const auto empty = DecodeChoices( EncodeChoices( 0, {} ) );
+    REQUIRE( empty.has_value() );
+    CHECK( empty->Choices.empty() );
+
+    for ( const char* malformed :
+          { "", "c3.", "c3.1", "x3.1.2", "c3.1.2-", "c3.1.-2", "c3.2t.1",
+            "c3.1.2--3", "c3.1.zzzzzzzzzzzzzzz" } ) {
+        CHECK_FALSE( DecodeChoices( malformed ).has_value() );
+    }
+}
+
+TEST_CASE( "The choice shrinker finds simple failing sequences",
+           "[progmasoft][strategy]" ) {
+    using Progmasoft::Catch3::Detail::IsSimpler;
+    using Progmasoft::Catch3::Detail::ShrinkChoices;
+    CHECK( IsSimpler( std::vector<std::uint64_t>{ 9 },
+                      std::vector<std::uint64_t>{ 0, 0 } ) );
+    CHECK( IsSimpler( std::vector<std::uint64_t>{ 1, 9 },
+                      std::vector<std::uint64_t>{ 2, 0 } ) );
+    CHECK_FALSE( IsSimpler( std::vector<std::uint64_t>{ 2, 0 },
+                            std::vector<std::uint64_t>{ 2, 0 } ) );
+
+    // Fails when the first two choices sum to at least 10.
+    const auto sumAtLeastTen = []( const std::vector<std::uint64_t>& choices )
+        -> std::optional<std::vector<std::uint64_t>> {
+        std::vector<std::uint64_t> consumed = choices;
+        consumed.resize( 2, 0 );
+        if ( consumed[0] + consumed[1] >= 10 ) { return consumed; }
+        return std::nullopt;
+    };
+    std::size_t calls = 0;
+    const auto minimized = ShrinkChoices(
+        { 7, 9, 4, 4, 1 }, 10000, sumAtLeastTen, calls );
+    // Shortlex: two choices, the first as small as the sum allows.
+    CHECK( minimized == std::vector<std::uint64_t>{ 0, 10 } );
+    CHECK( calls > 0 );
+
+    std::size_t limitedCalls = 0;
+    (void)ShrinkChoices( { 7, 9, 4, 4, 1 }, 3,