@@ -138,6 +138,22 @@ case.

 [use]: https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glUseProgram.xhtml

+### Preprocessing

+

+A simple preprocessor is run before shader sources are passed to OpenGL.

+

+At present the preprocessor does not interpret line continuations or multi-line

+comments correctly on lines it acts upon. Other lines are left alone so they

+can be used there without issue.

+

+#### `#version`

+

+Each supplied shader file is checked for the existence of one, and only one,

+[`#version`][]. An error is thrown if this is not the case. (The OpenGL default

+is to use `#version 110` if none is provided.)

+

+[`#version`]: https://www.khronos.org/opengl/wiki/Core_Language_(GLSL)#Version

+

 ## Dependencies

 Public (interface):

new file mode 100644

@@ -0,0 +1,7 @@

+#version 1.10

+

+

+void main()

+{

+    gl_Position = gl_Vertex;

+}

new file mode 100644

@@ -0,0 +1,4 @@

+void main()

+{

+    gl_Position = gl_Vertex;

+}

new file mode 100644

@@ -0,0 +1,8 @@

+#version 110

+#version 110

+

+

+void main()

+{

+    gl_Position = gl_Vertex;

+}

@@ -6,8 +6,11 @@

 #include <cstring>

 #include <fstream>

 #include <memory>

+#include <regex>

+#include <sstream>

 #include <stdexcept>

 #include <string>

+#include <tuple>

 #include <utility>

 #include <vector>

@@ -16,6 +19,9 @@

 #include <str.hpp>

+using Here = std::tuple<std::string, int, std::string>;

+

+

 // NOLINTNEXTLINE

 #define GLSHADER_INIT_(NAME, INIT) decltype(NAME) NAME INIT;

 GLSHADER_INIT_(Shader::root_, {})

@@ -70,6 +76,106 @@ static void info_log_action_(

 }

+static std::string source_(

+    std::string const & error,

+    std::string const & path,

+    std::string const & root

+)

+{

+    // Set here error.

+    auto const here_error = [](Here const & here)

+    {

+        return STR(

+            std::get<0>(here) << ":" <<

+            std::get<1>(here) << ": " <<

+            std::get<2>(here)

+        );

+    };

+

+    // Set full path.

+    auto path_full = path;

+    if (!root.empty())

+        path_full = STR(root << "/" << path_full);

+

+    // Define and open input stream.

+    auto istream = std::ifstream{path_full};

+    if (!istream)

+        throw std::runtime_error{STR(

+            error << "; " <<

+            "could not open file '" << path_full << "':\n" <<

+            std::strerror(errno)

+        )};

+

+    // Define output stream.

+    auto ostream = std::ostringstream{};

+

+    // Define parse regexes.

+    static auto const re_ignored = std::regex{R"(\s*//.*$)"};

+    static auto const re_words   = std::regex{R"((\w+(?:\s+\w+)*))"};

+    static auto const re_version = std::regex{R"(\s*#\s*version\s*(.*))"};

+

+    // Parse.

+    auto version_number = 0;

+    auto line_number    = 0;

+    auto line           = std::string{};

+    auto match          = std::smatch{};

+    auto here           = [&]()

+    {

+        return Here{path_full, line_number, line};

+    };

+    while (++line_number, std::getline(istream, line))

+    {

+        // Remove ignored.

+        auto const content = std::regex_replace(line, re_ignored, "");

+

+        // Process version.

+        if (std::regex_match(content, match, re_version))

+        {

+            // Parse.

+            auto const words = match.str(1);

+            if (!std::regex_match(words, match, re_words))

+                throw std::runtime_error{STR(

+                    error << "; " <<

+                    "malformed #version:\n" <<

+                    here_error(here())

+                )};

+            auto const version = match.str(1);

+

+            // Check for errors.

+            if (version_number)

+                throw std::runtime_error{STR(

+                    error << "; " <<

+                    "found repeated #version:\n" <<

+                    here_error(here())

+                )};

+

+            // Process.

+            version_number = std::stoi(version);

+

+            // Output.

+            ostream << line << "\n";

+        }

+

+        // Non-processed line.

+        else

+        {

+            // Output.

+            ostream << line << "\n";

+        }

+    }

+

+    // Check for version.

+    if (!version_number)

+        throw std::runtime_error{STR(

+            error << "; " <<

+            "found no #version."

+        )};

+

+    // Return.

+    return ostream.str();

+}

+

+

 Shader::Shader(Paths const & paths)

 :

     program_{0},

@@ -157,17 +263,7 @@ Shader::Shader(Paths const & paths)

             // Set shader source.

             auto const source_error = STR("Failed to source " << shader_name);

-            auto path_full = path;

-            if (!root_.empty())

-                path_full = STR(root_ << "/" << path_full);

-            auto source_istream = std::ifstream{path_full};

-            if (!source_istream)

-                throw std::runtime_error{STR(

-                    source_error << "; " <<

-                    "could not open file '" << path_full << "':\n" <<

-                    std::strerror(errno)

-                )};

-            auto const source = STR(source_istream.rdbuf());

+            auto const source = source_(source_error, path, root_);

             auto const sources = std::array<char const *, 1>{{

                 source.c_str()

             }};

@@ -53,6 +53,25 @@ GLTEST(2, 0, 640, 480, glshader)

         "shader program not current."

     )

+    // Version.

+    GLTEST_EXPECT_EXCEPTION(false,

+        Shader({"tests/version_none.vert"}),

+        "Failed to source shader 'tests/version_none.vert' of shader program 'tests/version_none.vert'; "

+        "found no #version."

+    )

+    GLTEST_EXPECT_EXCEPTION(false,

+        Shader({"tests/version_malformed.vert"}),

+        "Failed to source shader 'tests/version_malformed.vert' of shader program 'tests/version_malformed.vert'; "

+        "malformed #version:\n"

+        "assets/shaders/tests/version_malformed.vert:1: #version 1.10"

+    )

+    GLTEST_EXPECT_EXCEPTION(false,

+        Shader({"tests/version_repeated.vert"}),

+        "Failed to source shader 'tests/version_repeated.vert' of shader program 'tests/version_repeated.vert'; "

+        "found repeated #version:\n"

+        "assets/shaders/tests/version_repeated.vert:2: #version 110"

+    )

+

     auto all = Shader({

         "tests/all.vert",

         "tests/all.frag",