Emscripten SDK support

Adds support for the Emscripten SDK via CMake.

3rdParty/libmpq/CMakeLists.txt

@@ -1,4 +1,6 @@
-find_package(ZLIB REQUIRED)
+if(NOT TARGET ZLIB::ZLIB)
+  find_package(ZLIB REQUIRED)
+endif()
 
 if(NOT TARGET BZip2::BZip2)
   find_package(BZip2 REQUIRED)

CMake/Dependencies.cmake

@@ -1,8 +1,13 @@
 # Options that control whether to use system dependencies or build them from source,
 # and whether to link them statically.
 include(functions/dependency_options)
+include(functions/emscripten_system_library)
 
-if(USE_SDL1)
+if(EMSCRIPTEN)
+  # We use `USE_PTHREADS=1` here to get a version of SDL2 that supports threads.
+  emscripten_system_library("SDL2" SDL2::SDL2 USE_SDL=2 USE_PTHREADS=1)
+  list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/3rdParty/SDL2/CMake")
+elseif(USE_SDL1)
   find_package(SDL REQUIRED)
   include_directories(${SDL_INCLUDE_DIR})
 else()
@@ -52,21 +57,25 @@ macro(_find_SDL_image QUIET_OR_REQUIRED)
   endif()
 endmacro()
 
-if(NOT DEFINED DEVILUTIONX_SYSTEM_SDL_IMAGE)
-  _find_SDL_image(QUIET)
-  if(SDL_image_FOUND)
-    message("-- Found SDL_image")
+if(EMSCRIPTEN)
+  emscripten_system_library("SDL_image" SDL2::SDL2_image USE_SDL_IMAGE=2 "SDL2_IMAGE_FORMATS='[\"png\"]'")
+else()
+  if(NOT DEFINED DEVILUTIONX_SYSTEM_SDL_IMAGE)
+    _find_SDL_image(QUIET)
+    if(SDL_image_FOUND)
+      message("-- Found SDL_image")
+    else()
+      message("-- Suitable system SDL_image package not found, will use SDL_image from source")
+      set(DEVILUTIONX_SYSTEM_SDL_IMAGE OFF)
+    endif()
+  endif()
+  dependency_options("SDL_image" DEVILUTIONX_SYSTEM_SDL_IMAGE ON DEVILUTIONX_STATIC_SDL_IMAGE)
+  if(DEVILUTIONX_SYSTEM_SDL_IMAGE)
+    _find_SDL_image(REQUIRED)
   else()
-    message("-- Suitable system SDL_image package not found, will use SDL_image from source")
-    set(DEVILUTIONX_SYSTEM_SDL_IMAGE OFF)
+    add_subdirectory(3rdParty/SDL_image)
   endif()
 endif()
-dependency_options("SDL_image" DEVILUTIONX_SYSTEM_SDL_IMAGE ON DEVILUTIONX_STATIC_SDL_IMAGE)
-if(DEVILUTIONX_SYSTEM_SDL_IMAGE)
-  _find_SDL_image(REQUIRED)
-else()
-  add_subdirectory(3rdParty/SDL_image)
-endif()
 
 if(NOT NOSOUND)
   dependency_options("SDL_audiolib" DEVILUTIONX_SYSTEM_SDL_AUDIOLIB OFF DEVILUTIONX_STATIC_SDL_AUDIOLIB)
@@ -110,6 +119,10 @@ else()
   add_subdirectory(3rdParty/bzip2)
 endif()
 
+if(EMSCRIPTEN)
+  emscripten_system_library("zlib" ZLIB::ZLIB USE_ZLIB=1)
+endif()
+
 add_subdirectory(3rdParty/libsmackerdec)
 
 if(WIN32)

CMake/Platforms.cmake

@@ -58,3 +58,7 @@ endif()
 if(IOS)
   include(platforms/ios)
 endif()
+
+if(EMSCRIPTEN)
+  include(platforms/emscripten)
+endif()

CMake/functions/emscripten_system_library.cmake

@@ -0,0 +1,17 @@
+# This function defines a target that points to an Emscripten system library.
+#
+# Arguments:
+#   LIB_NAME: a human-readable library name.
+#   TARGET_NAME: the library target name
+#   ...ARGN: Emscripten flags.
+#
+# Example:
+#   emscripten_system_library("SDL2_image" SDL2::SDL2_image USE_SDL_IMAGE=2 "SDL2_IMAGE_FORMATS='[\"png\"]'")
+function(emscripten_system_library LIB_NAME TARGET_NAME)
+  add_library(${TARGET_NAME} INTERFACE IMPORTED GLOBAL)
+  foreach(arg ${ARGN})
+    target_compile_options(${TARGET_NAME} INTERFACE "SHELL:-s ${arg}")
+    target_link_options(${TARGET_NAME} INTERFACE "SHELL:-s ${arg}")
+  endforeach()
+  message("-- 📚 ${LIB_NAME}: Emscripten system library via ${ARGN}")
+endfunction()

CMake/platforms/emscripten.cmake

@@ -0,0 +1,10 @@
+set(BUILD_TESTING OFF)
+set(DISABLE_ZERO_TIER ON)
+set(DEVILUTIONX_SYSTEM_SDL_AUDIOLIB OFF)
+set(DEVILUTIONX_SYSTEM_LIBSODIUM OFF)
+set(DEVILUTIONX_SYSTEM_LIBFMT OFF)
+
+# Emscripten ports do have a bzip2 but it fails to link with this error:
+#   warning: _BZ2_bzDecompress may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
+#   error: undefined symbol: BZ2_bzDecompressEnd (referenced by top-level compiled C/C++ code)
+set(DEVILUTIONX_SYSTEM_BZIP2 OFF)

docs/building.md

@@ -353,6 +353,18 @@ You can do this by selecting the DevilutionX icon, then hold right mouse button
 select Icons -> Information in the top menu.
 </details>
 
+<details><summary>Emscripten</summary>
+
+Emscripten port is a work in progress. It builds but does not do more than that currently.
+
+To build, install the [Emscripten SDK](https://emscripten.org/docs/getting_started/downloads.html), then run:
+
+~~~ bash
+emcmake cmake -S. -Bbuild-em -DCMAKE_BUILD_TYPE=Release
+cmake --build build-em -j $(getconf _NPROCESSORS_ONLN)
+~~~
+</details>
+
 <details><summary><b>CMake build options</b></summary>
 
 ### General