Add thin wrapper to expose witness generation via FFI.

.github/actions/compile-witness-generator/action.yml

@@ -69,6 +69,16 @@ runs:
         CIRCUIT_FILENAME: ${{ steps.parse-circuit-path.outputs.CIRCUIT_FILENAME }}
       run: circom --c --r1cs --no_asm --O2 "${CIRCUIT_FILENAME}"
 
+    - name: Copy ${{ inputs.circuit-name-display }} FFI Sources
+      shell: bash
+      env:
+        SOURCES_ROOT: ${{ github.workspace }}/src
+        CIRCUIT_CPP_PATH: ${{ steps.parse-circuit-path.outputs.CIRCUIT_CPP_PATH }}
+      run: |
+        cp "${SOURCES_ROOT}/ffi.cpp" "${CIRCUIT_CPP_PATH}/ffi.cpp"
+        cp "${SOURCES_ROOT}/ffi.hpp" "${CIRCUIT_CPP_PATH}/ffi.hpp"
+        cp "${SOURCES_ROOT}/types.hpp" "${CIRCUIT_CPP_PATH}/types.hpp"
+
     # TODO: Instead of replace, make a fork that generates the appropriate Makefile
     - name: Replace ${{ inputs.circuit-name-display }}'s Makefile
       shell: bash
@@ -114,6 +124,8 @@ runs:
         mv "${CIRCUIT_CPP_PATH}/calcwit.hpp" "${CIRCUIT_CPP_PATH}/include/"
         mv "${CIRCUIT_CPP_PATH}/circom.hpp" "${CIRCUIT_CPP_PATH}/include/"
         mv "${CIRCUIT_CPP_PATH}/fr.hpp" "${CIRCUIT_CPP_PATH}/include/"
+        mv "${CIRCUIT_CPP_PATH}/ffi.hpp" "${CIRCUIT_CPP_PATH}/include/"
+        mv "${CIRCUIT_CPP_PATH}/types.hpp" "${CIRCUIT_CPP_PATH}/include/"
 
     - name: Upload ${{ inputs.circuit-name-display }}
       uses: actions/upload-artifact@de65e23aa2b7e23d713bb51fbfcb6d502f8667d8

.github/resources/witness-generator/Makefile

@@ -20,11 +20,13 @@ else
     # On non-macOS targets we assume objcopy is available on PATH.
     OBJCOPY := objcopy
 endif
-SRCS            := main.cpp calcwit.cpp fr.cpp $(PROJECT).cpp
-OBJS            := $(SRCS:.cpp=.o)
-LIB_SRCS        := calcwit.cpp fr.cpp $(PROJECT).cpp
+COMMON_SRCS     := main.cpp calcwit.cpp fr.cpp $(PROJECT).cpp
+COMMON_OBJS     := $(COMMON_SRCS:.cpp=.o)
+LIB_ONLY_SRCS   := ffi.cpp
+LIB_ONLY_OBJS   := $(LIB_ONLY_SRCS:.cpp=.o)
+LIB_SRCS        := $(COMMON_SRCS) $(LIB_ONLY_SRCS)
 LIB_OBJS        := $(LIB_SRCS:.cpp=.o)
-DEPS_HPP        := circom.hpp calcwit.hpp fr.hpp
+DEPS_HPP        := circom.hpp calcwit.hpp fr.hpp types.hpp ffi.hpp
 BIN             := $(PROJECT)
 ifeq ($(OS),windows)
 LIB_EXT         := .lib
@@ -61,7 +63,7 @@ windows-lib: CXXFLAGS=$(CXXFLAGS_COMMON) -fPIC -I/include -Duint="unsigned int"
 windows-lib: $(LIB)
 
 # ---- Rules ----
-$(BIN): $(OBJS)
+$(BIN): $(COMMON_OBJS)
 	$(CXX) $(LDFLAGS) $^ $(LDLIBS) -o $@
 
 $(LIB): $(LIB_OBJS)
@@ -72,4 +74,4 @@ $(LIB): $(LIB_OBJS)
 	$(CXX) $(CXXFLAGS) -c $< -o $@
 
 clean:
-	rm -f $(OBJS) $(LIB_OBJS) $(BIN) $(LIB)
+	rm -f $(COMMON_OBJS) $(LIB_ONLY_OBJS) $(BIN) $(LIB)

src/ffi.cpp

@@ -0,0 +1,39 @@
+#include "ffi.hpp"
+
+#include <string>
+#include <vector>
+
+#include <nlohmann/json.hpp>
+
+#include "calcwit.hpp"
+#include "circom.hpp"
+#include "fr.hpp"
+
+using json = nlohmann::json;
+
+// ---- Forward declarations from circom-generated main.cpp ----
+
+bool check_valid_number(std::string& s, uint base);
+void json2FrElements(json val, std::vector<FrElement>& vval);
+json::value_t check_type(std::string prefix, json in);
+void qualify_input(std::string prefix, json& in, json& in1);
+void qualify_input_list(std::string prefix, json& in, json& in1);
+
+// ---- File-based entry point (wraps circom-generated main) ----
+
+extern "C" int generate_witness_from_files(const char* dat, const char* inputs, const char* output) {
+    char* argv[] = {
+        (char*)dat,
+        (char*)inputs,
+        (char*)output,
+        nullptr
+    };
+    return main(3, argv);
+}
+
+// ---- Memory-based entry point ----
+
+extern "C" int generate_witness(const WitnessInput input, Bytes* output) {
+    // TODO
+    return 0;
+}

src/ffi.hpp

@@ -0,0 +1,40 @@
+#ifndef FFI_HPP
+#define FFI_HPP
+
+#include "types.hpp"
+
+/// Inputs for witness generation.
+struct WitnessInput {
+    /// Contents of the circuit's .dat file.
+    const ConstBytes dat;
+    /// Null-terminated JSON string of circuit inputs.
+    const char* inputs_json;
+};
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/// Generates a witness by delegating to the circom-generated CLI entry point.
+///
+/// # Parameters
+///
+/// - `dat`: Path to the .dat file. Must be extensionless.
+/// - `inputs`: Path to the inputs file for the circuit. Must be a JSON file.
+/// - `output`: Path where the output witness file will be written.
+int generate_witness_from_files(const char* dat, const char* inputs, const char* output);
+
+/// Generates a witness from in-memory buffers.
+///
+/// # Parameters
+///
+/// - `input`: The `WitnessInput` struct containing the circuit information.
+/// - `output`: On success, this will be populated with the generated witness bytes.
+///  The caller is responsible for freeing this buffer.
+int generate_witness(const WitnessInput input, Bytes* output);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // FFI_HPP

src/types.hpp

@@ -0,0 +1,26 @@
+#ifndef TYPES_HPP
+#define TYPES_HPP
+
+#include <stddef.h>
+#include <stdint.h>
+
+/// A pointer to a contiguous sequence of elements with a known length.
+///
+/// # Parameters
+///
+/// - `T`: The element type. Use `const T` for immutable data.
+template<typename T>
+struct Slice {
+    /// Pointer to the first element.
+    T* data;
+    /// Number of elements.
+    size_t size;
+};
+
+/// Mutable byte buffer.
+using Bytes = Slice<uint8_t>;
+
+/// Immutable byte buffer.
+using ConstBytes = Slice<const uint8_t>;
+
+#endif // TYPES_HPP