Implement conditional compilation of PNG renderer

A feature test is done for this by default, but it can optionally
be explicitly enabled or disabled (and the build will fail if it
is set to enabled but libpng cannot be found).
Run cmake with -DSAXBOSPIRAL_PNG_SUPPORT=<ON|OFF> to specify your
build preference.

Fixes #131

Author: saxbophone

CMakeLists.txt

@@ -89,12 +89,43 @@ if(CMAKE_BUILD_TYPE STREQUAL "Debug" OR CMAKE_BUILD_TYPE STREQUAL "")
     enable_c_compiler_flag_if_supported("-Werror")
 endif()
 
-# dependencies
+# begin dependencies
 # add custom dependencies directory
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
 # libpng
-find_package(PNG 1 EXACT REQUIRED)
-include_directories(${PNG_INCLUDE_DIR})
+# work out whether we have or have not requested PNG support, or don't care (default)
+if(NOT DEFINED SAXBOSPIRAL_PNG_SUPPORT)
+    # try and find libpng v1.x, but don't fail if we can't
+    message(STATUS "PNG output support will be enabled if possible")
+    find_package(PNG 1 EXACT)
+    # set SAXBOSPIRAL_PNG_SUPPORT based on value of PNG_FOUND
+    if(PNG_FOUND)
+        set(SAXBOSPIRAL_PNG_SUPPORT ON)
+    else()
+        set(SAXBOSPIRAL_PNG_SUPPORT OFF)
+    endif()
+elseif(SAXBOSPIRAL_PNG_SUPPORT)
+    # find libpng v1.x and fail the build if we can't
+    message(STATUS "PNG output support explicitly enabled")
+    find_package(PNG 1 EXACT REQUIRED)
+else()
+    # we've explicitly disabled PNG support, so don't include libpng
+    # issue a message saying so
+    message(STATUS "PNG output support explicitly disabled")
+endif()
+
+# include libpng directories and add feature test macro if support is enabled
+if(SAXBOSPIRAL_PNG_SUPPORT)
+    include_directories(${PNG_INCLUDE_DIR})
+    # feature test macro
+    add_definitions(-DSAXBOSPIRAL_PNG_SUPPORT)
+    # issue message
+    message(STATUS "PNG output support enabled")
+else()
+    # issue message
+    message(STATUS "PNG output support disabled")
+endif()
+# end dependencies
 
 # C source files
 file(
@@ -114,12 +145,14 @@ set_target_properties(
     saxbospiral PROPERTIES VERSION ${SAXBOSPIRAL_VERSION_STRING}
     SOVERSION ${PROJECT_VERSION_MAJOR}
 )
-# Link libsaxbospiral with libpng so we get libpng symbols
-target_link_libraries(saxbospiral ${PNG_LIBRARY})
+# Link libsaxbospiral with libpng so we get libpng symbols (if support enabled)
+if(SAXBOSPIRAL_PNG_SUPPORT)
+    target_link_libraries(saxbospiral ${PNG_LIBRARY})
+endif()
 
 add_executable(sxp_test tests.c)
 
-target_link_libraries(sxp_test saxbospiral ${PNG_LIBRARY})
+target_link_libraries(sxp_test saxbospiral)
 
 install(
     TARGETS saxbospiral

saxbospiral/render_backends/backend_png.c

@@ -5,6 +5,11 @@
  * This compilation unit provides functionality to render a bitmap struct to a
  * PNG image (stored in a buffer).
  *
+ * NOTE: PNG output support may have not been enabled in the compiled version
+ * of libsaxbospiral that you have. If support is not enabled, the library
+ * boolean constant SXBP_PNG_SUPPORT will be set to false and the one public
+ * function defined in this library will return SXBP_NOT_IMPLEMENTED.
+ *
  *
  *
  * Copyright (C) 2016, Joshua Saxby [email protected]
@@ -22,10 +27,16 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 #include <assert.h>
+// only include these extra dependencies if support for PNG output was enabled
+#ifdef SAXBOSPIRAL_PNG_SUPPORT
 #include <stdlib.h>
 #include <string.h>
+#endif
 
+// only include libpng if support for it was enabled
+#ifdef SAXBOSPIRAL_PNG_SUPPORT
 #include <png.h>
+#endif
 
 #include "../saxbospiral.h"
 #include "../render.h"
@@ -36,6 +47,8 @@
 extern "C"{
 #endif
 
+// only define the following private functions if libpng support was enabled
+#ifdef SAXBOSPIRAL_PNG_SUPPORT
 // private custom libPNG buffer write function
 static void buffer_write_data(
     png_structp png_ptr, png_bytep data, png_size_t length
@@ -80,11 +93,13 @@ static void cleanup_png_lib(
         free(row);
     }
 }
+#endif // SAXBOSPIRAL_PNG_SUPPORT
 
 /*
  * given a bitmap_t struct and a pointer to a blank buffer_t, write the bitmap
  * data as a PNG image to the buffer, using libpng.
- * returns a status struct containing error information, if any
+ * returns a status struct containing error information
+ * returns SXBP_NOT_IMPLEMENTED if PNG support was not enabled
  *
  * Asserts:
  * - That bitmap.pixels is not NULL
@@ -96,6 +111,11 @@ sxbp_status_t sxbp_render_backend_png(
     // preconditional assertsions
     assert(bitmap.pixels != NULL);
     assert(buffer->bytes == NULL);
+    // only do PNG operations if support is enabled
+    #ifndef SAXBOSPIRAL_PNG_SUPPORT
+    // return SXBP_NOT_IMPLEMENTED
+    return SXBP_NOT_IMPLEMENTED;
+    #else
     // result status
     sxbp_status_t result;
     // init buffer
@@ -186,6 +206,7 @@ sxbp_status_t sxbp_render_backend_png(
     // status ok
     result = SXBP_OPERATION_OK;
     return result;
+    #endif // SAXBOSPIRAL_PNG_SUPPORT
 }
 
 #ifdef __cplusplus

saxbospiral/render_backends/backend_png.h

@@ -5,6 +5,11 @@
  * This compilation unit provides functionality to render a bitmap struct to a
  * PNG image (stored in a buffer).
  *
+ * NOTE: PNG output support may have not been enabled in the compiled version
+ * of libsaxbospiral that you have. If support is not enabled, the library
+ * boolean constant SXBP_PNG_SUPPORT will be set to false and the one public
+ * function defined in this library will return SXBP_NOT_IMPLEMENTED.
+ *
  *
  *
  * Copyright (C) 2016, Joshua Saxby [email protected]
@@ -35,7 +40,8 @@ extern "C"{
 /*
  * given a bitmap_t struct and a pointer to a blank buffer_t, write the bitmap
  * data as a PNG image to the buffer, using libpng.
- * returns a status struct containing error information, if any
+ * returns a status struct containing error information
+ * returns SXBP_NOT_IMPLEMENTED if PNG support was not enabled
  *
  * Asserts:
  * - That bitmap.pixels is not NULL

saxbospiral/saxbospiral.c

@@ -21,6 +21,8 @@
  * You should have received a copy of the GNU Affero General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
+#include <stdbool.h>
+
 #include "saxbospiral.h"
 
 
@@ -35,6 +37,12 @@ const sxbp_version_t LIB_SXBP_VERSION = {
     .patch = SAXBOSPIRAL_VERSION_PATCH,
     .string = SAXBOSPIRAL_VERSION_STRING,
 };
+// flag for whether PNG output support has been compiled in based, on macro
+#ifdef SAXBOSPIRAL_PNG_SUPPORT
+const bool SXBP_PNG_SUPPORT = true;
+#else
+const bool SXBP_PNG_SUPPORT = false;
+#endif
 
 /*
  * computes a version_hash_t for a given version_t,

saxbospiral/saxbospiral.h

@@ -41,6 +41,8 @@ typedef struct sxbp_version_t {
 } sxbp_version_t;
 
 extern const sxbp_version_t LIB_SXBP_VERSION;
+// flag for whether PNG output support has been compiled in based, on macro
+extern const bool SXBP_PNG_SUPPORT;
 
 // used for indexing and comparing different versions in order
 typedef uint32_t sxbp_version_hash_t;
@@ -54,10 +56,11 @@ sxbp_version_hash_t sxbp_version_hash(sxbp_version_t version);
 // enum for function error information
 typedef enum sxbp_status_t {
     SXBP_STATE_UNKNOWN = 0, // unknown, the default state
+    SXBP_OPERATION_OK, // no problem
     SXBP_OPERATION_FAIL, // generic failure state
     SXBP_MALLOC_REFUSED, // memory allocation or re-allocation was refused
     SXBP_IMPOSSIBLE_CONDITION, // condition thought to be impossible detected
-    SXBP_OPERATION_OK, // no problem
+    SXBP_NOT_IMPLEMENTED, // function is not implemented / enabled
 } sxbp_status_t;
 
 // type for representing a cartesian direction