Add support for includes in shader files

This can be used for keeping code for common functionality in one file
(e.g. lighting functions). The code which detects include statements is
pretty simple so it may break if it is used in a more complicated.
Circular includes are detected and treated as an error.

Author: asarium

code/graphics/opengl/gropenglshader.cpp

@@ -362,11 +362,10 @@ static SCP_string opengl_shader_get_header(shader_type type_id, int flags, shade
  * @param flags		integer variable holding a combination of SDR_* flags
  * @return			C-string holding the complete shader source code
  */
-static SCP_string opengl_load_shader(const char *filename)
-{
+static SCP_string opengl_load_shader(const char* filename) {
 	SCP_string content;
 	if (Enable_external_shaders) {
-		CFILE *cf_shader = cfopen(filename, "rt", CFILE_NORMAL, CF_TYPE_EFFECTS);
+		CFILE* cf_shader = cfopen(filename, "rt", CFILE_NORMAL, CF_TYPE_EFFECTS);
 
 		if (cf_shader != NULL) {
 			int len = cfilelength(cf_shader);
@@ -387,11 +386,86 @@ static SCP_string opengl_load_shader(const char *filename)
 	return content;
 }
 
+static void handle_includes_impl(SCP_vector<SCP_string>& include_stack,
+								 SCP_stringstream& output,
+								 int& include_counter,
+								 const SCP_string& filename,
+								 const SCP_string& original) {
+	include_stack.emplace_back(filename);
+	auto current_source_number = include_counter + 1;
+
+	const char* INCLUDE_STRING = "#include";
+	SCP_stringstream input(original);
+
+	int line_num = 1;
+	for (SCP_string line; std::getline(input, line);) {
+		auto include_start = line.find(INCLUDE_STRING);
+		if (include_start != SCP_string::npos) {
+			auto first_quote = line.find('"', include_start + strlen(INCLUDE_STRING));
+			auto second_quote = line.find('"', first_quote + 1);
+
+			if (first_quote == SCP_string::npos || second_quote == SCP_string::npos) {
+				Error(LOCATION,
+					  "Shader %s:%d: Malformed include line. Could not find both quote charaters.",
+					  filename.c_str(),
+					  line_num);
+			}
+
+			auto file_name = line.substr(first_quote + 1, second_quote - first_quote - 1);
+			auto existing_name = std::find_if(include_stack.begin(), include_stack.end(), [&file_name](const SCP_string& str) {
+				return str == file_name;
+			});
+			if (existing_name != include_stack.end()) {
+				SCP_stringstream stack_string;
+				for (auto& name : include_stack) {
+					stack_string << "\t" << name << "\n";
+				}
+
+				Error(LOCATION,
+					  "Shader %s:%d: Detected cyclic include! Previous includes (top level file first):\n%s",
+					  filename.c_str(),
+					  line_num,
+					  stack_string.str().c_str());
+			}
+
+			++include_counter;
+			// The second parameter defines which source string we are currently working with. We keep track of how many
+			// excludes have been in the file so far to specify this
+			output << "#line 1 " << include_counter + 1 << "\n";
+
+			handle_includes_impl(include_stack,
+								 output,
+								 include_counter,
+								 file_name,
+								 opengl_load_shader(file_name.c_str()));
+
+			// We are done with the include file so now we can return to the original file
+			output << "#line " << line_num + 1 << " " << current_source_number << "\n";
+		} else {
+			output << line << "\n";
+		}
+
+		++line_num;
+	}
+
+	include_stack.pop_back();
+}
+
+static SCP_string handle_includes(const char* filename, const SCP_string& original) {
+	SCP_stringstream output;
+	SCP_vector<SCP_string> include_stack;
+	auto include_counter = 0;
+
+	handle_includes_impl(include_stack, output, include_counter, filename, original);
+
+	return output.str();
+}
+
 static SCP_vector<SCP_string> opengl_get_shader_content(shader_type type_id, const char* filename, int flags, shader_stage stage) {
 	SCP_vector<SCP_string> parts;
 	parts.push_back(opengl_shader_get_header(type_id, flags, stage));
 
-	parts.push_back(opengl_load_shader(filename));
+	parts.push_back(handle_includes(filename, opengl_load_shader(filename)));
 
 	return parts;
 }