text: //========================================================================
  // GLFW 3.0 - www.glfw.org
  //------------------------------------------------------------------------
  // Copyright (c) 2002-2006 Marcus Geelnard
  // Copyright (c) 2006-2010 Camilla Berglund <elmindreda@elmindreda.org>
  //
  // This software is provided 'as-is', without any express or implied
  // warranty. In no event will the authors be held liable for any damages
  // arising from the use of this software.
  //
  // Permission is granted to anyone to use this software for any purpose,
  // including commercial applications, and to alter it and redistribute it
  // freely, subject to the following restrictions:
  //
  // 1. The origin of this software must not be misrepresented; you must not
  //    claim that you wrote the original software. If you use this software
  //    in a product, an acknowledgment in the product documentation would
  //    be appreciated but is not required.
  //
  // 2. Altered source versions must be plainly marked as such, and must not
  //    be misrepresented as being the original software.
  //
  // 3. This notice may not be removed or altered from any source
  //    distribution.
  //
  //========================================================================
  
  #include "internal.h"
  
  #include <stdio.h>
  #include <string.h>
  #include <limits.h>
  #include <stdio.h>
  
  
  // Parses the client API version string and extracts the version number
  //
  static GLboolean parseGLVersion(int* api, int* major, int* minor, int* rev)
  {
      int i, _api = GLFW_OPENGL_API, _major, _minor = 0, _rev = 0;
      const char* version;
      const char* prefixes[] =
      {
          "OpenGL ES-CM ",
          "OpenGL ES-CL ",
          "OpenGL ES ",
          NULL
      };
  
      version = (const char*) glGetString(GL_VERSION);
      if (!version)
      {
          _glfwInputError(GLFW_PLATFORM_ERROR,
                          "Failed to retrieve context version string");
          return GL_FALSE;
      }
  
      for (i = 0;  prefixes[i];  i++)
      {
          const size_t length = strlen(prefixes[i]);
  
          if (strncmp(version, prefixes[i], length) == 0)
          {
              version += length;
              _api = GLFW_OPENGL_ES_API;
              break;
          }
      }
  
      if (!sscanf(version, "%d.%d.%d", &_major, &_minor, &_rev))
      {
          _glfwInputError(GLFW_PLATFORM_ERROR,
                          "No version found in context version string");
          return GL_FALSE;
      }
  
      *api = _api;
      *major = _major;
      *minor = _minor;
      *rev = _rev;
  
      return GL_TRUE;
  }
  
  
  //////////////////////////////////////////////////////////////////////////
  //////                       GLFW internal API                      //////
  //////////////////////////////////////////////////////////////////////////
  
  GLboolean _glfwIsValidContextConfig(_GLFWwndconfig* wndconfig)
  {
      if (wndconfig->clientAPI != GLFW_OPENGL_API &&
          wndconfig->clientAPI != GLFW_OPENGL_ES_API)
      {
          _glfwInputError(GLFW_INVALID_ENUM, "Invalid client API requested");
          return GL_FALSE;
      }
  
      if (wndconfig->clientAPI == GLFW_OPENGL_API)
      {
          if (wndconfig->glMajor < 1 || wndconfig->glMinor < 0 ||
              (wndconfig->glMajor == 1 && wndconfig->glMinor > 5) ||
              (wndconfig->glMajor == 2 && wndconfig->glMinor > 1) ||
              (wndconfig->glMajor == 3 && wndconfig->glMinor > 3))
          {
              // OpenGL 1.0 is the smallest valid version
              // OpenGL 1.x series ended with version 1.5
              // OpenGL 2.x series ended with version 2.1
              // OpenGL 3.x series ended with version 3.3
  
              _glfwInputError(GLFW_INVALID_VALUE,
                              "Invalid OpenGL version %i.%i requested",
                              wndconfig->glMajor, wndconfig->glMinor);
              return GL_FALSE;
          }
          else
          {
              // For now, let everything else through
          }
  
          if (wndconfig->glProfile)
          {
              if (wndconfig->glProfile != GLFW_OPENGL_CORE_PROFILE &&
                  wndconfig->glProfile != GLFW_OPENGL_COMPAT_PROFILE)
              {
                  _glfwInputError(GLFW_INVALID_ENUM,
                                  "Invalid OpenGL profile requested");
                  return GL_FALSE;
              }
  
              if (wndconfig->glMajor < 3 ||
                  (wndconfig->glMajor == 3 && wndconfig->glMinor < 2))
              {
                  // Desktop OpenGL context profiles are only defined for version 3.2
                  // and above
  
                  _glfwInputError(GLFW_INVALID_VALUE,
                                  "Context profiles only exist for "
                                  "OpenGL version 3.2 and above");
                  return GL_FALSE;
              }
          }
  
          if (wndconfig->glForward && wndconfig->glMajor < 3)
          {
              // Forward-compatible contexts are only defined for OpenGL version 3.0 and above
              _glfwInputError(GLFW_INVALID_VALUE,
                              "Forward compatibility only exist for OpenGL "
                              "version 3.0 and above");
              return GL_FALSE;
          }
      }
      else if (wndconfig->clientAPI == GLFW_OPENGL_ES_API)
      {
          if (wndconfig->glMajor < 1 || wndconfig->glMinor < 0 ||
              (wndconfig->glMajor == 1 && wndconfig->glMinor > 1) ||
              (wndconfig->glMajor == 2 && wndconfig->glMinor > 0))
          {
              // OpenGL ES 1.0 is the smallest valid version
              // OpenGL ES 1.x series ended with version 1.1
              // OpenGL ES 2.x series ended with version 2.0
  
              _glfwInputError(GLFW_INVALID_VALUE,
                              "Invalid OpenGL ES version %i.%i requested",
                              wndconfig->glMajor, wndconfig->glMinor);
              return GL_FALSE;
          }
          else
          {
              // For now, let everything else through
          }
  
          if (wndconfig->glProfile)
          {
              // OpenGL ES does not support profiles
              _glfwInputError(GLFW_INVALID_VALUE,
                              "Context profiles are not supported by OpenGL ES");
              return GL_FALSE;
          }
  
          if (wndconfig->glForward)
          {
              // OpenGL ES does not support forward-compatibility
              _glfwInputError(GLFW_INVALID_VALUE,
                              "Forward compatibility is not supported by OpenGL ES");
              return GL_FALSE;
          }
      }
  
      if (wndconfig->glRobustness)
      {
          if (wndconfig->glRobustness != GLFW_NO_RESET_NOTIFICATION &&
              wndconfig->glRobustness != GLFW_LOSE_CONTEXT_ON_RESET)
          {
              _glfwInputError(GLFW_INVALID_VALUE,
                              "Invalid context robustness mode requested");
              return GL_FALSE;
          }
      }
  
      return GL_TRUE;
  }
  
  const _GLFWfbconfig* _glfwChooseFBConfig(const _GLFWfbconfig* desired,
                                           const _GLFWfbconfig* alternatives,
                                           unsigned int count)
  {
      unsigned int i;
      unsigned int missing, leastMissing = UINT_MAX;
      unsigned int colorDiff, leastColorDiff = UINT_MAX;
      unsigned int extraDiff, leastExtraDiff = UINT_MAX;
      const _GLFWfbconfig* current;
      const _GLFWfbconfig* closest = NULL;
  
      for (i = 0;  i < count;  i++)
      {
          current = alternatives + i;
  
          if (desired->stereo > 0 && current->stereo == 0)
          {
              // Stereo is a hard constraint
              continue;
          }
  
          // Count number of missing buffers
          {
              missing = 0;
  
              if (desired->alphaBits > 0 && current->alphaBits == 0)
                  missing++;
  
              if (desired->depthBits > 0 && current->depthBits == 0)
                  missing++;
  
              if (desired->stencilBits > 0 && current->stencilBits == 0)
                  missing++;
  
              if (desired->auxBuffers > 0 && current->auxBuffers < desired->auxBuffers)
                  missing += desired->auxBuffers - current->auxBuffers;
  
              if (desired->samples > 0 && current->samples == 0)
              {
                  // Technically, several multisampling buffers could be
                  // involved, but that's a lower level implementation detail and
                  // not important to us here, so we count them as one
                  missing++;
              }
          }
  
          // These polynomials make many small channel size differences matter
          // less than one large channel size difference
  
          // Calculate color channel size difference value
          {
              colorDiff = 0;
  
              if (desired->redBits > 0)
              {
                  colorDiff += (desired->redBits - current->redBits) *
                               (desired->redBits - current->redBits);
              }
  
              if (desired->greenBits > 0)
              {
                  colorDiff += (desired->greenBits - current->greenBits) *
                               (desired->greenBits - current->greenBits);
              }
  
              if (desired->blueBits > 0)
              {
                  colorDiff += (desired->blueBits - current->blueBits) *
                               (desired->blueBits - current->blueBits);
              }
          }
  
          // Calculate non-color channel size difference value
          {
              extraDiff = 0;
  
              if (desired->alphaBits > 0)
              {
                  extraDiff += (desired->alphaBits - current->alphaBits) *
                               (desired->alphaBits - current->alphaBits);
              }
  
              if (desired->depthBits > 0)
              {
                  extraDiff += (desired->depthBits - current->depthBits) *
                               (desired->depthBits - current->depthBits);
              }
  
              if (desired->stencilBits > 0)
              {
                  extraDiff += (desired->stencilBits - current->stencilBits) *
                               (desired->stencilBits - current->stencilBits);
              }
  
              if (desired->accumRedBits > 0)
              {
                  extraDiff += (desired->accumRedBits - current->accumRedBits) *
                               (desired->accumRedBits - current->accumRedBits);
              }
  
              if (desired->accumGreenBits > 0)
              {
                  extraDiff += (desired->accumGreenBits - current->accumGreenBits) *
                               (desired->accumGreenBits - current->accumGreenBits);
              }
  
              if (desired->accumBlueBits > 0)
              {
                  extraDiff += (desired->accumBlueBits - current->accumBlueBits) *
                               (desired->accumBlueBits - current->accumBlueBits);
              }
  
              if (desired->accumAlphaBits > 0)
              {
                  extraDiff += (desired->accumAlphaBits - current->accumAlphaBits) *
                               (desired->accumAlphaBits - current->accumAlphaBits);
              }
  
              if (desired->samples > 0)
              {
                  extraDiff += (desired->samples - current->samples) *
                               (desired->samples - current->samples);
              }
  
              if (desired->sRGB)
              {
                  if (!current->sRGB)
                      extraDiff++;
              }
          }
  
          // Figure out if the current one is better than the best one found so far
          // Least number of missing buffers is the most important heuristic,
          // then color buffer size match and lastly size match for other buffers
  
          if (missing < leastMissing)
              closest = current;
          else if (missing == leastMissing)
          {
              if ((colorDiff < leastColorDiff) ||
                  (colorDiff == leastColorDiff && extraDiff < leastExtraDiff))
              {
                  closest = current;
              }
          }
  
          if (current == closest)
          {
              leastMissing = missing;
              leastColorDiff = colorDiff;
              leastExtraDiff = extraDiff;
          }
      }
  
      return closest;
  }
  
  GLboolean _glfwRefreshContextAttribs(void)
  {
      _GLFWwindow* window = _glfwPlatformGetCurrentContext();
  
      if (!parseGLVersion(&window->clientAPI,
                          &window->glMajor,
                          &window->glMinor,
                          &window->glRevision))
      {
          return GL_FALSE;
      }
  
  #if defined(_GLFW_USE_OPENGL)
      if (window->glMajor > 2)
      {
          // OpenGL 3.0+ uses a different function for extension string retrieval
          // We cache it here instead of in glfwExtensionSupported mostly to alert
          // users as early as possible that their build may be broken
  
          window->GetStringi = (PFNGLGETSTRINGIPROC) glfwGetProcAddress("glGetStringi");
          if (!window->GetStringi)
          {
              _glfwInputError(GLFW_PLATFORM_ERROR,
                              "Entry point retrieval is broken");
              return GL_FALSE;
          }
      }
  
      if (window->clientAPI == GLFW_OPENGL_API)
      {
          // Read back context flags (OpenGL 3.0 and above)
          if (window->glMajor >= 3)
          {
              GLint flags;
              glGetIntegerv(GL_CONTEXT_FLAGS, &flags);
  
              if (flags & GL_CONTEXT_FLAG_FORWARD_COMPATIBLE_BIT)
                  window->glForward = GL_TRUE;
  
              if (flags & GL_CONTEXT_FLAG_DEBUG_BIT)
                  window->glDebug = GL_TRUE;
              else if (glfwExtensionSupported("GL_ARB_debug_output"))
              {
                  // HACK: This is a workaround for older drivers (pre KHR_debug)
                  // not setting the debug bit in the context flags for debug
                  // contexts
                  window->glDebug = GL_TRUE;
              }
          }
  
          // Read back OpenGL context profile (OpenGL 3.2 and above)
          if (window->glMajor > 3 ||
              (window->glMajor == 3 && window->glMinor >= 2))
          {
              GLint mask;
              glGetIntegerv(GL_CONTEXT_PROFILE_MASK, &mask);
  
              if (mask & GL_CONTEXT_COMPATIBILITY_PROFILE_BIT)
                  window->glProfile = GLFW_OPENGL_COMPAT_PROFILE;
              else if (mask & GL_CONTEXT_CORE_PROFILE_BIT)
                  window->glProfile = GLFW_OPENGL_CORE_PROFILE;
          }
  
          // Read back robustness strategy
          if (glfwExtensionSupported("GL_ARB_robustness"))
          {
              // NOTE: We avoid using the context flags for detection, as they are
              // only present from 3.0 while the extension applies from 1.1
  
              GLint strategy;
              glGetIntegerv(GL_RESET_NOTIFICATION_STRATEGY_ARB, &strategy);
  
              if (strategy == GL_LOSE_CONTEXT_ON_RESET_ARB)
                  window->glRobustness = GLFW_LOSE_CONTEXT_ON_RESET;
              else if (strategy == GL_NO_RESET_NOTIFICATION_ARB)
                  window->glRobustness = GLFW_NO_RESET_NOTIFICATION;
          }
      }
      else
      {
          // Read back robustness strategy
          if (glfwExtensionSupported("GL_EXT_robustness"))
          {
              // NOTE: The values of these constants match those of the OpenGL ARB
              // one, so we can reuse them here
  
              GLint strategy;
              glGetIntegerv(GL_RESET_NOTIFICATION_STRATEGY_ARB, &strategy);
  
              if (strategy == GL_LOSE_CONTEXT_ON_RESET_ARB)
                  window->glRobustness = GLFW_LOSE_CONTEXT_ON_RESET;
              else if (strategy == GL_NO_RESET_NOTIFICATION_ARB)
                  window->glRobustness = GLFW_NO_RESET_NOTIFICATION;
          }
      }
  #endif // _GLFW_USE_OPENGL
  
      return GL_TRUE;
  }
  
  GLboolean _glfwIsValidContext(_GLFWwndconfig* wndconfig)
  {
      _GLFWwindow* window = _glfwPlatformGetCurrentContext();
  
      if (window->glMajor < wndconfig->glMajor ||
          (window->glMajor == wndconfig->glMajor &&
           window->glMinor < wndconfig->glMinor))
      {
          // The desired OpenGL version is greater than the actual version
          // This only happens if the machine lacks {GLX|WGL}_ARB_create_context
          // /and/ the user has requested an OpenGL version greater than 1.0
  
          // For API consistency, we emulate the behavior of the
          // {GLX|WGL}_ARB_create_context extension and fail here
  
          _glfwInputError(GLFW_VERSION_UNAVAILABLE, NULL);
          return GL_FALSE;
      }
  
      return GL_TRUE;
  }
  
  int _glfwStringInExtensionString(const char* string, const GLubyte* extensions)
  {
      const GLubyte* start;
      GLubyte* where;
      GLubyte* terminator;
  
      // It takes a bit of care to be fool-proof about parsing the
      // OpenGL extensions string. Don't be fooled by sub-strings,
      // etc.
      start = extensions;
      for (;;)
      {
          where = (GLubyte*) strstr((const char*) start, string);
          if (!where)
              return GL_FALSE;
  
          terminator = where + strlen(string);
          if (where == start || *(where - 1) == ' ')
          {
              if (*terminator == ' ' || *terminator == '\0')
                  break;
          }
  
          start = terminator;
      }
  
      return GL_TRUE;
  }
  
  
  //////////////////////////////////////////////////////////////////////////
  //////                        GLFW public API                       //////
  //////////////////////////////////////////////////////////////////////////
  
  GLFWAPI void glfwMakeContextCurrent(GLFWwindow* handle)
  {
      _GLFWwindow* window = (_GLFWwindow*) handle;
  
      _GLFW_REQUIRE_INIT();
  
      if (_glfwPlatformGetCurrentContext() == window)
          return;
  
      _glfwPlatformMakeContextCurrent(window);
  }
  
  GLFWAPI GLFWwindow* glfwGetCurrentContext(void)
  {
      _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
      return (GLFWwindow*) _glfwPlatformGetCurrentContext();
  }
  
  GLFWAPI void glfwSwapBuffers(GLFWwindow* handle)
  {
      _GLFWwindow* window = (_GLFWwindow*) handle;
      _GLFW_REQUIRE_INIT();
      _glfwPlatformSwapBuffers(window);
  }
  
  GLFWAPI void glfwSwapInterval(int interval)
  {
      _GLFW_REQUIRE_INIT();
  
      if (!_glfwPlatformGetCurrentContext())
      {
          _glfwInputError(GLFW_NO_CURRENT_CONTEXT, NULL);
          return;
      }
  
      _glfwPlatformSwapInterval(interval);
  }
  
  GLFWAPI int glfwExtensionSupported(const char* extension)
  {
      const GLubyte* extensions;
      _GLFWwindow* window;
  
      _GLFW_REQUIRE_INIT_OR_RETURN(GL_FALSE);
  
      window = _glfwPlatformGetCurrentContext();
      if (!window)
      {
          _glfwInputError(GLFW_NO_CURRENT_CONTEXT, NULL);
          return GL_FALSE;
      }
  
      if (!extension || *extension == '\0')
      {
          _glfwInputError(GLFW_INVALID_VALUE, NULL);
          return GL_FALSE;
      }
  
      if (window->glMajor < 3)
      {
          // Check if extension is in the old style OpenGL extensions string
  
          extensions = glGetString(GL_EXTENSIONS);
          if (!extensions)
          {
              _glfwInputError(GLFW_PLATFORM_ERROR,
                              "Failed to retrieve extension string");
              return GL_FALSE;
          }
  
          if (_glfwStringInExtensionString(extension, extensions))
              return GL_TRUE;
      }
  #if defined(_GLFW_USE_OPENGL)
      else
      {
          int i;
          GLint count;
  
          // Check if extension is in the modern OpenGL extensions string list
  
          glGetIntegerv(GL_NUM_EXTENSIONS, &count);
  
          for (i = 0;  i < count;  i++)
          {
              const char* en = (const char*) window->GetStringi(GL_EXTENSIONS, i);
              if (!en)
              {
                  _glfwInputError(GLFW_PLATFORM_ERROR,
                                  "Failed to retrieve extension string %i", i);
                  return GL_FALSE;
              }
  
              if (strcmp(en, extension) == 0)
                  return GL_TRUE;
          }
      }
  #endif // _GLFW_USE_OPENGL
  
      // Check if extension is in the platform-specific string
      return _glfwPlatformExtensionSupported(extension);
  }
  
  GLFWAPI GLFWglproc glfwGetProcAddress(const char* procname)
  {
      _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
  
      if (!_glfwPlatformGetCurrentContext())
      {
          _glfwInputError(GLFW_NO_CURRENT_CONTEXT, NULL);
          return NULL;
      }
  
      return _glfwPlatformGetProcAddress(procname);
  }
  
  