Host-authoritative multiplayer and cross-platform determinism (0.4.0)

.github/workflows/ci.yml

@@ -158,9 +158,11 @@ jobs:
     container: archlinux:latest
     steps:
       - name: Install build dependencies
+        # Wine runs the determinism tests (the toolchain file makes it the
+        # tests' emulator when it is installed at configure time).
         run: |
           pacman -Syu --noconfirm --needed \
-            base-devel git cmake ninja mingw-w64-gcc shaderc glslang
+            base-devel git cmake ninja mingw-w64-gcc shaderc glslang wine
 
       - uses: actions/checkout@v7
 
@@ -178,6 +180,20 @@ jobs:
       - name: Build
         run: cmake --build --preset mingw-cross-release
 
+      # The simulation must give the same bits as the Linux and MSVC builds
+      # (docs/physics.md, "The same results on every platform"). Only these
+      # tests: the native jobs run the rest.
+      - name: Test determinism under Wine
+        env:
+          # A prefix of the container's own user (Wine refuses one it does
+          # not own), without the Mono and Gecko installers.
+          WINEPREFIX: /tmp/wineprefix
+          WINEDEBUG: -all
+          WINEDLLOVERRIDES: mscoree,mshtml=
+        run: |
+          wineboot --init
+          ctest --preset mingw-cross-release -R '^Determinism[.]'
+
       - name: Package
         run: cpack --preset mingw-portable
 

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.25...4.0)
 
 project(OpenMM2
-    VERSION 0.3.1
+    VERSION 0.4.0
     DESCRIPTION "Open source reimplementation of Midtown Madness 2"
     HOMEPAGE_URL "https://github.com/lowlevelmetal/OpenMM2"
     LANGUAGES C CXX)

cmake/CompilerOptions.cmake

@@ -6,6 +6,29 @@ add_library(OpenMM2::options ALIAS openmm2_options)
 
 target_compile_features(openmm2_options INTERFACE cxx_std_23)
 
+# Floating point: the same results on every compiler and platform, so that a
+# host and its clients (and replays, tests and the opponent sweep) simulate a
+# sample alike whichever toolchain built them (docs/physics.md, "The same
+# results on every platform"). Every float and double operation is rounded
+# as IEEE 754 says, in the order the source writes it:
+#   * no contraction of a * b + c into a fused multiply-add, which rounds
+#     once instead of twice: GCC defaults to -ffp-contract=fast for C++ and
+#     Clang to on, which fuse wherever the target has FMA (-march=native,
+#     x86-64-v3 distribution builds, every AArch64 build). MSVC 2022 and
+#     later fuse only with /fp:contract or /fp:fast, and x64 has no FMA
+#     below /arch:AVX2, which the project never sets;
+#   * no fast-math reassociation or reciprocal approximations;
+#   * SSE2 (x86-64) or AArch64 arithmetic, where float and double
+#     expressions are evaluated in their own type (FLT_EVAL_METHOD 0); a
+#     32-bit x87 build would round differently and is not supported.
+# The C runtime's sin, cos, atan2, exp, log and pow still differ between
+# platforms; the simulation uses OpenMM2's own (src/core/Libm.h).
+if(MSVC)
+    target_compile_options(openmm2_options INTERFACE /fp:precise)
+else()
+    target_compile_options(openmm2_options INTERFACE -ffp-contract=off -fno-fast-math)
+endif()
+
 if(MSVC)
     target_compile_options(openmm2_options INTERFACE
         /W4 /permissive- /utf-8 /Zc:__cplusplus /Zc:preprocessor /EHsc

docs/bangers.md

@@ -183,3 +183,37 @@ room of the player's car (type 4), an opponent's or a network player's
 its room, so a prop still moving vanishes when the cars leave it behind. A
 prop still standing (`dgUnhitBangerInstance`) keeps lvlInstance's empty
 Detach.
+
+## Network games
+
+MM2 sends nothing about props: every machine knocks its own with its own
+simulation of every car (`mmNetObject`, `mmGameMulti`). OpenMM2's host is the
+authority instead (docs/multiplayer.md, "Props"); BangerSet carries what that
+needs, all of it inert in a single-player race:
+
+* The placed props' indices name them on every machine (they are placed in
+  the same order everywhere); a hit instance remembers the placed prop it came
+  from (`Instance::source`, with `part` for a BREAKnn piece) or the tag its
+  thrower gave a car part (`Instance::tag`). The ring's slots count how often
+  they were handed out (`generation`). In a network race the ring grows by
+  40 slots, up to 40 a player, when it is about to wrap onto a prop that
+  still moves or was knocked less than 10 s ago (`setRingGrowth`; MM2's
+  ring of 40 makes such props disappear in a pile-up).
+* `takeKnocks` (with `recordKnocks(true)`): the placed props that broke loose
+  (`dgUnhitBangerInstance::Impact`) and what hit them.
+* A client (`setReplica`): only its own car and the cars it simulates with
+  it (the other players' near ones, the shared police and knocked traffic
+  cars) may touch its props, and the props it simulates one another but
+  no placed prop (`phys::Instance::acceptsContact`); `breakPlaced` takes a
+  prop out of its room as Impact does but without a body; `restoreStanding`
+  undoes a knock its car predicted that the host did not make (Reset for one
+  prop); `showMirror` / `hideMirror` show the host's ring slots in instances
+  of their own, drawn where the host has them. An active attached to a
+  mirror starts from the host's motion (`dgBangerActive::Attach` starts a
+  placed or resting prop at rest).
+* A client replaying its car (`phys::World::replaySample`) meets a prop with
+  the body an active would give it (`heldInertia`: `dgBangerActive::Attach`'s
+  mass and inertia, a mirror's host motion), which the world moves through
+  the replay only; a prop its car broke loose in the last second stands
+  again for the samples up to the one that broke it loose (`replayGhost`),
+  its pieces left out; the later samples meet the pieces.