FCMP++: output_to_tuple {output pubkey, commitment} -> {O,I,C}

diff --git a/src/fcmp_pp/CMakeLists.txt b/src/fcmp_pp/CMakeLists.txt
index 32ca1b4..d5eebdc 100644
--- a/src/fcmp_pp/CMakeLists.txt
+++ b/src/fcmp_pp/CMakeLists.txt
@@ -27,10 +27,14 @@
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 set(fcmp_pp_sources
-  fcmp_pp_crypto.cpp)
+  curve_trees.cpp
+  fcmp_pp_crypto.cpp
+  fcmp_pp_types.cpp)
 
 monero_find_all_headers(fcmp_pp_headers "${CMAKE_CURRENT_SOURCE_DIR}")
 
+add_subdirectory(fcmp_pp_rust)
+
 monero_add_library(fcmp_pp
   ${fcmp_pp_sources}
   ${fcmp_pp_headers})
diff --git a/src/fcmp_pp/curve_trees.cpp b/src/fcmp_pp/curve_trees.cpp
new file mode 100644
index 0000000..a533390
--- /dev/null
+++ b/src/fcmp_pp/curve_trees.cpp
@@ -0,0 +1,109 @@
+// Copyright (c) 2024, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "curve_trees.h"
+
+#include "crypto/crypto.h"
+#include "fcmp_pp_crypto.h"
+#include "fcmp_pp_types.h"
+#include "profile_tools.h"
+
+
+namespace fcmp_pp
+{
+namespace curve_trees
+{
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+OutputTuple output_to_tuple(const OutputPair &output_pair)
+{
+    const crypto::public_key &output_pubkey = output_pubkey_cref(output_pair);
+    const crypto::ec_point &commitment      = commitment_cref(output_pair);
+
+    TIME_MEASURE_NS_START(derive_key_image_generator_ns);
+
+    // Derive key image generator using the original output pubkey.
+    // CRITICAL: it's CRITICALLY important that we use the original output pubkey here, and not `O` calculated later.
+    // If we were to use `O`, then an output with torsion could be double spent before and after the FCMP++ fork, since
+    // it would have 1 key image before the fork (tied to the output pubkey) and a distinct key image (tied to torsion
+    // cleared `O`).
+    crypto::ec_point I;
+    crypto::derive_key_image_generator(output_pubkey, use_biased_hash_to_point(output_pair), I);
+
+    TIME_MEASURE_NS_FINISH(derive_key_image_generator_ns);
+
+    LOG_PRINT_L3("derive_key_image_generator_ns: " << derive_key_image_generator_ns);
+
+    // Get O and C from output pubkey and commitment
+    crypto::ec_point O = output_pubkey;
+    crypto::ec_point C = commitment;
+
+    // If the output has already been checked for torsion, then we don't need to clear torsion here
+    if (!output_checked_for_torsion(output_pair))
+    {
+        TIME_MEASURE_NS_START(clear_torsion_ns);
+
+        if (!fcmp_pp::get_valid_torsion_cleared_point_vartime(output_pubkey, O))
+            throw std::runtime_error("O is invalid for insertion to tree");
+        if (!fcmp_pp::get_valid_torsion_cleared_point_vartime(commitment, C))
+            throw std::runtime_error("C is invalid for insertion to tree");
+
+        if (O != output_pubkey)
+            LOG_PRINT_L2("Output pubkey has torsion: " << output_pubkey);
+        if (C != commitment)
+            LOG_PRINT_L2("Commitment has torsion: " << commitment);
+
+        TIME_MEASURE_NS_FINISH(clear_torsion_ns);
+
+        LOG_PRINT_L3("clear_torsion_ns: " << clear_torsion_ns);
+    }
+
+#if !defined(NDEBUG)
+    {
+        // Debug build safety checks
+        crypto::ec_point O_debug;
+        crypto::ec_point C_debug;
+        assert(fcmp_pp::get_valid_torsion_cleared_point_vartime(output_pubkey, O_debug));
+        assert(fcmp_pp::get_valid_torsion_cleared_point_vartime(commitment, C_debug));
+        assert(O == O_debug);
+        assert(C == C_debug);
+    }
+#endif
+
+    // Redundant check for safety
+    if (O == fcmp_pp::EC_I)
+        throw std::runtime_error("O cannot equal identity");
+    if (C == fcmp_pp::EC_I)
+        throw std::runtime_error("C cannot equal identity");
+
+    return output_tuple_from_bytes(O, I, C);
+}
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+} //namespace curve_trees
+} //namespace fcmp_pp
diff --git a/src/fcmp_pp/curve_trees.h b/src/fcmp_pp/curve_trees.h
new file mode 100644
index 0000000..ceb62f3
--- /dev/null
+++ b/src/fcmp_pp/curve_trees.h
@@ -0,0 +1,46 @@
+// Copyright (c) 2024, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include "crypto/crypto.h"
+#include "fcmp_pp_types.h"
+#include "misc_log_ex.h"
+
+
+namespace fcmp_pp
+{
+namespace curve_trees
+{
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+OutputTuple output_to_tuple(const OutputPair &output_pair);
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+} //namespace curve_trees
+} //namespace fcmp_pp
diff --git a/src/fcmp_pp/fcmp_pp_rust/CMakeLists.txt b/src/fcmp_pp/fcmp_pp_rust/CMakeLists.txt
new file mode 100644
index 0000000..bdad6f5
--- /dev/null
+++ b/src/fcmp_pp/fcmp_pp_rust/CMakeLists.txt
@@ -0,0 +1,27 @@
+# Copyright (c) 2024, The Monero Project
+#
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are
+# permitted provided that the following conditions are met:
+#
+# 1. Redistributions of source code must retain the above copyright notice, this list of
+#    conditions and the following disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright notice, this list
+#    of conditions and the following disclaimer in the documentation and/or other
+#    materials provided with the distribution.
+#
+# 3. Neither the name of the copyright holder nor the names of its contributors may be
+#    used to endorse or promote products derived from this software without specific
+#    prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+# EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+# THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+# PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+# STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+# THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/src/fcmp_pp/fcmp_pp_rust/fcmp++.h b/src/fcmp_pp/fcmp_pp_rust/fcmp++.h
new file mode 100644
index 0000000..a4f2e93
--- /dev/null
+++ b/src/fcmp_pp/fcmp_pp_rust/fcmp++.h
@@ -0,0 +1,39 @@
+// Copyright (c) 2025, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <stdint.h>
+
+
+struct OutputTuple
+{
+  uint8_t O[32];
+  uint8_t I[32];
+  uint8_t C[32];
+};
diff --git a/src/fcmp_pp/fcmp_pp_types.cpp b/src/fcmp_pp/fcmp_pp_types.cpp
new file mode 100644
index 0000000..517f69f
--- /dev/null
+++ b/src/fcmp_pp/fcmp_pp_types.cpp
@@ -0,0 +1,108 @@
+// Copyright (c) 2024, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "fcmp_pp_types.h"
+
+#include "misc_log_ex.h"
+
+namespace fcmp_pp
+{
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+// Helpers
+//----------------------------------------------------------------------------------------------------------------------
+OutputTuple output_tuple_from_bytes(const crypto::ec_point &O, const crypto::ec_point &I, const crypto::ec_point &C)
+{
+    OutputTuple output_tuple;
+
+    static_assert(sizeof(output_tuple.O) == sizeof(O), "unexpected sizeof O");
+    static_assert(sizeof(output_tuple.I) == sizeof(I), "unexpected sizeof I");
+    static_assert(sizeof(output_tuple.C) == sizeof(C), "unexpected sizeof C");
+
+    memcpy(output_tuple.O, &O, sizeof(O));
+    memcpy(output_tuple.I, &I, sizeof(I));
+    memcpy(output_tuple.C, &C, sizeof(C));
+
+    return output_tuple;
+}
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+const crypto::public_key &output_pubkey_cref(const OutputPair &output_pair)
+{
+    struct output_pair_visitor
+    {
+        const crypto::public_key &operator()(const LegacyOutputPair &o) const
+        { return o.output_pubkey; }
+        const crypto::public_key &operator()(const CarrotOutputPairV1 &o) const
+        { return o.output_pubkey; }
+    };
+
+    return std::visit(output_pair_visitor{}, output_pair);
+}
+//----------------------------------------------------------------------------------------------------------------------
+const crypto::ec_point &commitment_cref(const OutputPair &output_pair)
+{
+    struct output_pair_visitor
+    {
+        const crypto::ec_point &operator()(const LegacyOutputPair &o) const
+        { return o.commitment; }
+        const crypto::ec_point &operator()(const CarrotOutputPairV1 &o) const
+        { return o.commitment; }
+    };
+
+    return std::visit(output_pair_visitor{}, output_pair);
+}
+//----------------------------------------------------------------------------------------------------------------------
+bool output_checked_for_torsion(const OutputPair &output_pair)
+{
+    struct output_pair_visitor
+    {
+        bool operator()(const LegacyOutputPair&) const
+        { return false; }
+        bool operator()(const CarrotOutputPairV1&) const
+        { return true; }
+    };
+
+    return std::visit(output_pair_visitor{}, output_pair);
+}
+//----------------------------------------------------------------------------------------------------------------------
+bool use_biased_hash_to_point(const OutputPair &output_pair)
+{
+    struct output_pair_visitor
+    {
+        bool operator()(const LegacyOutputPair&) const
+        { return true; }
+        bool operator()(const CarrotOutputPairV1&) const
+        { return false; }
+    };
+
+    return std::visit(output_pair_visitor{}, output_pair);
+}
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+}//namespace fcmp_pp
diff --git a/src/fcmp_pp/fcmp_pp_types.h b/src/fcmp_pp/fcmp_pp_types.h
new file mode 100644
index 0000000..80a7839
--- /dev/null
+++ b/src/fcmp_pp/fcmp_pp_types.h
@@ -0,0 +1,115 @@
+// Copyright (c) 2024, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <cstring>
+#include <type_traits>
+#include <variant>
+
+#include "crypto/crypto.h"
+#include "fcmp_pp_rust/fcmp++.h"
+
+
+namespace fcmp_pp
+{
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+// Rust types
+//----------------------------------------------------------------------------------------------------------------------
+using OutputTuple = ::OutputTuple;
+//----------------------------------------------------------------------------------------------------------------------
+OutputTuple output_tuple_from_bytes(const crypto::ec_point &O, const crypto::ec_point &I, const crypto::ec_point &C);
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+// C++ types
+//----------------------------------------------------------------------------------------------------------------------
+//   Curve trees types
+//----------------------------------------------------------------------------------------------------------------------
+// Output pubkey and commitment, ready to be converted to a leaf tuple
+// - From {output_pubkey,commitment} -> {O,C} -> {O,I,C} -> {O.x,O.y,I.x,I.y,C.x,C.y}
+// - Output pairs do NOT necessarily have torsion cleared. We need the output pubkey as it exists in the chain in order
+//   to derive the correct I (when deriving {O.x,O.y,I.x,I.y,C.x,C.y}). Torsion clearing O before deriving I from O
+//   would enable spending a torsioned output once before FCMP++ fork and again with a different key image via FCMP++.
+template<typename T>
+struct OutputPairTemplate
+{
+    crypto::public_key output_pubkey;
+    // Uses the ec_point type to avoid a circular dep to ringct/rctTypes.h, and to differentiate from output_pubkey
+    crypto::ec_point commitment;
+
+    OutputPairTemplate(const crypto::public_key &_output_pubkey, const crypto::ec_point &_commitment):
+        output_pubkey(_output_pubkey),
+        commitment(_commitment)
+    {};
+
+    OutputPairTemplate():
+        output_pubkey{},
+        commitment{}
+    {};
+
+    // WARNING: not constant time
+    bool operator==(const OutputPairTemplate &other) const
+    {
+        return output_pubkey == other.output_pubkey
+            && commitment == other.commitment;
+    }
+};
+
+// May have torsion, use biased key image generator for I
+struct LegacyOutputPair : public OutputPairTemplate<LegacyOutputPair>{};
+// No torsion, use unbiased key image generator for I
+struct CarrotOutputPairV1 : public OutputPairTemplate<CarrotOutputPairV1>{};
+
+static_assert(sizeof(LegacyOutputPair)   == (32+32), "sizeof LegacyOutputPair unexpected");
+static_assert(sizeof(CarrotOutputPairV1) == (32+32), "sizeof CarrotOutputPairV1 unexpected");
+
+static_assert(std::has_unique_object_representations_v<LegacyOutputPair>);
+static_assert(std::has_unique_object_representations_v<CarrotOutputPairV1>);
+
+using OutputPair = std::variant<LegacyOutputPair, CarrotOutputPairV1>;
+
+const crypto::public_key &output_pubkey_cref(const OutputPair &output_pair);
+const crypto::ec_point &commitment_cref(const OutputPair &output_pair);
+
+bool output_checked_for_torsion(const OutputPair &output_pair);
+bool use_biased_hash_to_point(const OutputPair &output_pair);
+//----------------------------------------------------------------------------------------------------------------------
+//----------------------------------------------------------------------------------------------------------------------
+}//namespace fcmp_pp
+
+// WARNING: not constant time
+inline bool operator==(const fcmp_pp::OutputTuple &a, const fcmp_pp::OutputTuple &b)
+{
+    static_assert(sizeof(fcmp_pp::OutputTuple) == (sizeof(a.O) + sizeof(a.I) + sizeof(a.C)),
+        "unexpected sizeof OutputTuple for == implementation");
+    return
+        (memcmp(a.O, b.O, sizeof(a.O)) == 0) &&
+        (memcmp(a.I, b.I, sizeof(a.I)) == 0) &&
+        (memcmp(a.C, b.C, sizeof(a.C)) == 0);
+}
diff --git a/src/fcmp_pp/ffi_api_c_compat.c b/src/fcmp_pp/ffi_api_c_compat.c
new file mode 100644
index 0000000..c92510e
--- /dev/null
+++ b/src/fcmp_pp/ffi_api_c_compat.c
@@ -0,0 +1,3 @@
+#include "fcmp++.h"
+
+int main() { return 0; }