Add bignum.h and port exactfloat to use it.

This removes the dependency on OpenSSL for its bignum library. The main benefit
of this is that targetting things like WASM are much simpler since we don't need
to compile OpenSSL to WASM.

I didn't try as hard for performance optimization as OpenSSL (which also has a
hard requirement around constant-time operation for security purposes), but
benchmarks indicate addition and subtraction is generally the same and
multiplication is within a factor of two. For the size of bignums we expect in
S2 there shouldn't be a noticeable difference in performance.

Author: smcallis

CMakeLists.txt

@@ -60,7 +60,6 @@ add_definitions(-DABSL_MIN_LOG_LEVEL=1)
 if (NOT TARGET absl::base)
     find_package(absl REQUIRED)
 endif()
-find_package(OpenSSL REQUIRED)
 # pthreads isn't used directly, but this is still required for std::thread.
 find_package(Threads REQUIRED)
 
@@ -99,10 +98,6 @@ else()
     add_compile_options(-fsized-deallocation)
 endif()
 
-# If OpenSSL is installed in a non-standard location, configure with
-# something like:
-# OPENSSL_ROOT_DIR=/usr/local/opt/openssl cmake ..
-include_directories(${OPENSSL_INCLUDE_DIR})
 
 if (WITH_PYTHON)
     include_directories(${Python3_INCLUDE_DIRS})
@@ -230,7 +225,6 @@ endif()
 
 target_link_libraries(
     s2
-    ${OPENSSL_LIBRARIES}
     absl::absl_vlog_is_on
     absl::base
     absl::btree
@@ -617,6 +611,7 @@ if (BUILD_TESTS)
       src/s2/s2shapeutil_shape_edge_id_test.cc
       src/s2/s2shapeutil_visit_crossing_edge_pairs_test.cc
       src/s2/s2text_format_test.cc
+      src/s2/util/math/exactfloat/bignum_test.cc
       src/s2/s2validation_query_test.cc
       src/s2/s2wedge_relations_test.cc
       src/s2/s2winding_operation_test.cc

README.md

@@ -59,7 +59,6 @@ This issue may require revision of boringssl or exactfloat.
 *   [Abseil](https://github.com/abseil/abseil-cpp) >= LTS
     [`20250512`](https://github.com/abseil/abseil-cpp/releases/tag/20250512.1)
     (standard library extensions)
-*   [OpenSSL](https://github.com/openssl/openssl) (for its bignum library)
 *   [googletest testing framework >= 1.10](https://github.com/google/googletest)
     (to build tests and example programs, optional)
 
@@ -139,11 +138,6 @@ Disable building of shared libraries with `-DBUILD_SHARED_LIBS=OFF`.
 
 Enable the python interface with `-DWITH_PYTHON=ON`.
 
-If OpenSSL is installed in a non-standard location set `OPENSSL_ROOT_DIR`
-before running configure, for example on macOS:
-```
-OPENSSL_ROOT_DIR=/opt/homebrew/Cellar/openssl@3/3.1.0 cmake -DCMAKE_PREFIX_PATH=/opt/homebrew -DCMAKE_CXX_STANDARD=17
-```
 
 ## Installing
 
@@ -218,8 +212,6 @@ python -m build
 
 The resulting wheel will be in the `dist` directory.
 
-> If OpenSSL is in a non-standard location make sure to set `OPENSSL_ROOT_DIR`; 
-> see above for more information.
 
 ## Other S2 implementations
 

src/s2/util/math/exactfloat/BUILD

@@ -3,13 +3,17 @@ package(default_visibility = ["//visibility:public"])
 cc_library(
     name = "exactfloat",
     srcs = ["exactfloat.cc"],
-    hdrs = ["exactfloat.h"],
+    hdrs = ["exactfloat.h", "bignum.h"],
     deps = [
         "//s2/base:types",
         "//s2/base:port",
         "//s2/base:logging",
         "@abseil-cpp//absl/log:log",
         "@abseil-cpp//absl/log:absl_check",
-        "@boringssl//:crypto",
+        "@abseil-cpp//absl/strings:str_format",
+        "@abseil-cpp//absl/container:inlined_vector",
+        "@abseil-cpp//absl/numeric:bits",
+        "@abseil-cpp//absl/numeric:int128",
+        "@abseil-cpp//absl/strings:ascii",
     ],
 )