#include <string.h> /* memcpy, memset */

#include <cairo.h>
#include <gdk-pixbuf/gdk-pixbuf.h> /* includes glib.h */
#include <glib/gprintf.h>
#include <libxml/parser.h>
#include <libxml/tree.h>
#include <svgtiny.h>

/* This "header" includes extra code that we've copy/pasted from GDK */
#include "gdk_pixbuf_get_from_surface.h"

/* Declare the two functions that we export. Their names aren't
 * mentioned in gdk-pixbuf-io.h, but they are required by the
 * implementation, as you can confirm from the API docs or in
 * gdk-pixbuf-io.c. The G_MODULE_EXPORT macro is defined in
 * glib's gmodule.h */
G_MODULE_EXPORT void fill_vtable(GdkPixbufModule* module);
G_MODULE_EXPORT void fill_info(GdkPixbufFormat *info);


/* The width and height of the viewport that we'll render the SVG
 * into. The final "picture" may not actually be this size; based on
 * the height, width, viewBox, and preserveAspectRatio attributes in
 * the SVG itself, libsvgtiny may scale, stretch, offset, etc. the
 * paths to make them fit nicely into the viewport. */
#define VIEWPORT_WIDTH 512
#define VIEWPORT_HEIGHT 512

/* Convenient typedefs for libsvgtiny */
typedef struct svgtiny_diagram diagram_t;
typedef struct svgtiny_shape shape_t;


/* Our main data structure. One of these is created when we start
 * loading an SVG, and it persists while the SVG is being read. It
 * contains a few boilerplate members, and the svg_data/svg_data_size
 * fields that we use to keep track of the buffer where the SVG file
 * lives. */
typedef struct {
  GdkPixbufModuleUpdatedFunc  updated_func;
  GdkPixbufModulePreparedFunc prepared_func;
  gpointer                    user_data;

  /* The SVG "file" that we're building in memory. */
  gchar*                      svg_data;

  /* How far into svg_data are we? This should always point to the
     next empty byte. If (for example) svg_data_size is 2, then
     svg_data[0] and svg_data[1] are used, but svg_data[2] is free. */
  size_t svg_data_size;

} SvgTinyContext;


/**
 * @brief Render an svgtiny path using cairo.
 *
 * This was lovingly borrowed from @c examples/svgtiny_display_x11.c
 * in libsvgtiny itself, and modified to use a transparent background.
 *
 * @param cr
 *   A pointer to a valid cairo context.
 *
 * @param path
 *   A pointer to an svgtiny shape that will be rendered on the
 *   cairo context's target surface.
 */
static void render_path(cairo_t* cr, shape_t* path) {
  unsigned int j;

  cairo_new_path(cr);
  for (j = 0; j != path->path_length; ) {
    switch ((int) path->path[j]) {
    case svgtiny_PATH_MOVE:
      cairo_move_to(cr,
                    path->path[j + 1],
                    path->path[j + 2]);
      j += 3;
      break;
    case svgtiny_PATH_CLOSE:
      cairo_close_path(cr);
      j += 1;
      break;
    case svgtiny_PATH_LINE:
      cairo_line_to(cr,
                    path->path[j + 1],
                    path->path[j + 2]);
      j += 3;
      break;
    case svgtiny_PATH_BEZIER:
      cairo_curve_to(cr,
                     path->path[j + 1],
                     path->path[j + 2],
                     path->path[j + 3],
                     path->path[j + 4],
                     path->path[j + 5],
                     path->path[j + 6]);
      j += 7;
    }
  }
  if (path->fill != svgtiny_TRANSPARENT) {
    cairo_set_source_rgba(cr,
                          svgtiny_RED(path->fill) / 255.0,
                          svgtiny_GREEN(path->fill) / 255.0,
                          svgtiny_BLUE(path->fill) / 255.0,
                          1);
    cairo_fill_preserve(cr);
  }
  if (path->stroke != svgtiny_TRANSPARENT) {
    cairo_set_source_rgba(cr,
                          svgtiny_RED(path->stroke) / 255.0,
                          svgtiny_GREEN(path->stroke) / 255.0,
                          svgtiny_BLUE(path->stroke) / 255.0,
                          1);
    cairo_set_line_width(cr, path->stroke_width);
    cairo_stroke_preserve(cr);
  }
}

/**
 * @brief Parse a buffer of SVG data into a diagram_t structure.
 *
 * @param buffer
 *   The buffer containing the SVG document.
 *
 * @param bytecount
 *   The number of bytes in @c buffer.
 *
 * @return If successful, a pointer to a @c diagram_t structure is
 *   returned; if not, @c NULL is returned. You are expected to @c
 *   svgtiny_free the result if it is valid.
 */
static diagram_t* svgtiny_diagram_from_buffer(const gchar* buffer,
                                              gsize bytecount,
                                              guint width,
                                              guint height,
                                              GError** error) {
  diagram_t* diagram;
  svgtiny_code code;

  diagram = svgtiny_create();
  if (!diagram) {
    g_set_error_literal(error,
                        G_FILE_ERROR,
                        G_FILE_ERROR_NOMEM,
                        "out of memory in svgtiny_create()");
    return NULL;
  }

  g_assert((int)width >= 0);
  g_assert((int)height >= 0);

  /* There's a thread-safety issue in libwapcaplet that can cause
   * svgtiny_parse() to crash if you load lots of SVGs at once:
   *
   *   https://bugs.netsurf-browser.org/mantis/view.php?id=2857
   *
   * Putting a lock around svgtiny_parse() is a pretty simple solution
   * and looks like it does the trick.
  */
  static GMutex mutex;
  g_mutex_lock(&mutex);
  code = svgtiny_parse(diagram,
                       buffer,
                       bytecount, "",
                       (int)width,
                       (int)height);
  g_mutex_unlock (&mutex);

  switch(code) {
    case svgtiny_OK:
      /* The one success case. */
      return diagram;
    case svgtiny_OUT_OF_MEMORY:
      g_set_error_literal(error,
                          G_FILE_ERROR,
                          G_FILE_ERROR_NOMEM,
                          "out of memory in svgtiny_parse()");
      break;
    case svgtiny_LIBDOM_ERROR:
      g_set_error_literal(error,
                          GDK_PIXBUF_ERROR,
                          GDK_PIXBUF_ERROR_CORRUPT_IMAGE,
                          "invalid XML DOM in svgtiny_parse()");
      break;
    case svgtiny_NOT_SVG:
      g_set_error_literal(error,
                          GDK_PIXBUF_ERROR,
                          GDK_PIXBUF_ERROR_CORRUPT_IMAGE,
                          "missing <svg> element in svgtiny_parse()");
      break;
    case svgtiny_SVG_ERROR:
      g_set_error(error,
                  GDK_PIXBUF_ERROR,
                  GDK_PIXBUF_ERROR_CORRUPT_IMAGE,
                  "SVG format error in svgtiny_parse() on line %i: %s",
                  diagram->error_line,
                  diagram->error_message);
      break;
  }

  /* All other cases above are failure */
  return NULL;
}

/**
 * @brief Create a cairo context from a libsvgtiny diagram.
 *
 * @param diagram
 *   A pointer to a valid libsvgtiny diagram.
 *
 * @return If successful, a pointer to a @c cairo_t context structure
 *   is returned; if not, @c NULL is returned. You are expected to @c
 *   cairo_destroy the result if it is valid.
 */
static cairo_t* cairo_context_from_diagram(const diagram_t* diagram) {
  cairo_t* cr;
  cairo_surface_t* surface;
  cairo_status_t crs;
  unsigned int i;

  surface = cairo_image_surface_create(CAIRO_FORMAT_ARGB32,
                                       diagram->width,
                                       diagram->height);

  crs = cairo_surface_status(surface);
  if (crs != CAIRO_STATUS_SUCCESS) {
    g_fprintf(stderr,
              "cairo_image_surface_create failed: %s\n",
              cairo_status_to_string(crs));
    cairo_surface_destroy(surface);
    return NULL;
  }

  cr = cairo_create(surface);
  crs = cairo_status(cr);

  /* Immediately destroy the surface which is now accessible as
     cr->target */
  cairo_surface_destroy(surface);

  if (crs != CAIRO_STATUS_SUCCESS) {
    g_fprintf(stderr,
              "cairo_create failed: %s\n",
              cairo_status_to_string(crs));
    cairo_destroy(cr);
    return NULL;
  }

  cairo_set_source_rgba(cr, 0, 0, 0, 0);
  cairo_paint(cr);

  /* Loop through the shapes in the diagram... */
  for (i = 0; i != diagram->shape_count; i++) {

    /* If this shape is a path, just render it. */
    if (diagram->shape[i].path) {
      render_path(cr, &diagram->shape[i]);
    }

    /* If this shape is text... */
    if (diagram->shape[i].text) {
      /* Figure out what color to use from the R/G/B components of the
         shape's stroke. */
      cairo_set_source_rgba(cr,
                            svgtiny_RED(diagram->shape[i].stroke) / 255.0,
                            svgtiny_GREEN(diagram->shape[i].stroke) / 255.0,
                            svgtiny_BLUE(diagram->shape[i].stroke) / 255.0,
                            1);
      /* Then move to the actual position of the text within the
         shape... */
      cairo_move_to(cr,
                    diagram->shape[i].text_x,
                    diagram->shape[i].text_y);

      /* and draw it. */
      cairo_show_text(cr, diagram->shape[i].text);
    }
  }


  /* Check the status again just for good measure? */
  crs = cairo_status(cr);
  if (crs != CAIRO_STATUS_SUCCESS) {
    g_fprintf(stderr,
              "cairo error: %s\n",
              cairo_status_to_string(crs));
    cairo_destroy(cr);
    return NULL;
  }

  return cr;
}

/**
 * @brief Create a GdkPixbuf from a buffer of SVG data.
 *
 * @param buffer
 *   The buffer containing the SVG document.
 *
 * @param bytecount
 *   The number of bytes in @c buffer.
 *
 * @param error
 *   The address of a @c GError pointer that we use to return errors.
 *
 * @return If successful, a valid pointer to a @c GdkPixbuf is
 *   returned; if not, @c NULL is returned and @c error is populated.
 */
static GdkPixbuf* gdk_pixbuf_from_svg_buffer(const gchar* buffer,
                                             gsize bytecount,
                                             GError** error) {
  diagram_t* diagram;
  cairo_t* cr = 0;
  GdkPixbuf* pb;
  GError* sub_error = NULL;

  diagram = svgtiny_diagram_from_buffer(buffer,
                                        bytecount,
                                        VIEWPORT_WIDTH,
                                        VIEWPORT_HEIGHT,
                                        &sub_error);
  if (!diagram) {
    g_propagate_error(error, sub_error);
    return NULL;
  }

  cr = cairo_context_from_diagram(diagram);
  if (!cr) {
    svgtiny_free(diagram);
    g_set_error_literal(error,
                        GDK_PIXBUF_ERROR,
                        GDK_PIXBUF_ERROR_CORRUPT_IMAGE,
                        "could not create Cairo surface from SVG diagram");
    return NULL;
  }


  /* I've gone back and forth on this about five times: we use the
   * diagram width and height, and not the viewport width and height.
   * This can ultimately render an image that's larger than the
   * viewport size, but I think GDK will resize the final pixbuf
   * anyway. More importantly, rendering small icons at a larger
   * (viewport) size seems to make the whole thing go ape-shit.
   * So for now I'm back in the diagram camp.
   */
  pb = gdk_pixbuf_get_from_surface(cairo_get_target(cr),
                                   0,
                                   0,
                                   diagram->width,
                                   diagram->height);


  if (!pb) {
    g_set_error_literal(error,
                        GDK_PIXBUF_ERROR,
                        GDK_PIXBUF_ERROR_FAILED,
                        "failed to obtain a GdkPixbuf from Cairo surface");
  }

  return pb;
}


/**
 * @brief Our implementation of GdkPixbufModuleBeginLoadFunc, as
 *   defined in gdk-pixbuf-io.h.
 */
static gpointer gdk_pixbuf_begin_load(GdkPixbufModuleSizeFunc size_func,
                                      GdkPixbufModulePreparedFunc prep_func,
                                      GdkPixbufModuleUpdatedFunc updated_func,
                                      gpointer user_data,
                                      GError **error) {

  SvgTinyContext* context = g_new(SvgTinyContext, 1);

  context->prepared_func  = prep_func;
  context->updated_func   = updated_func;
  context->user_data      = user_data;

  context->svg_data       = NULL;
  context->svg_data_size  = 0;

  return context;
}


/**
 * @brief Our implementation of GdkPixbufModuleIncrementLoadFunc, as
 *   defined in gdk-pixbuf-io.h.
 */
static gboolean gdk_pixbuf_load_increment(gpointer data,
                                          const guchar* buf,
                                          guint buf_size,
                                          GError** error) {
  SvgTinyContext* context = (SvgTinyContext*)data;

  if (buf_size == 0) {
    return TRUE;
  }

  context->svg_data = g_realloc(context->svg_data,
                                context->svg_data_size + buf_size);
  memcpy(context->svg_data + context->svg_data_size, buf, buf_size);
  context->svg_data_size += buf_size;

  return TRUE;
}

/**
 * @brief Convenience function to execute the "updated" callback
 *   stored in our @c context.
 */
static void emit_updated(SvgTinyContext* context, GdkPixbuf* pixbuf) {
  if (context->updated_func != NULL) {
    (*context->updated_func)(pixbuf,
                             0,
                             0,
                             gdk_pixbuf_get_width(pixbuf),
                             gdk_pixbuf_get_height(pixbuf),
                             context->user_data);
  }
}

/**
 * @brief Convenience function to execute the "prepared" callback
 *   stored in our @c context.
 */
static void emit_prepared(SvgTinyContext* context, GdkPixbuf* pixbuf) {
  if (context->prepared_func != NULL) {
    (*context->prepared_func)(pixbuf, NULL, context->user_data);
  }
}



/**
 * @brief Replace one GTK <xi:include> element by its data.
 *
 * @param node
 *   A pointer to an <xi:include> element node.
 *
 * @return TRUE if we replaced the node, and FALSE otherwise.
 *
 */
static gboolean process_one_xinclude(xmlNode* node) {
  xmlChar* href;

  href = xmlGetProp(node, BAD_CAST "href");
  if (href == NULL) {
    /* We only process XIncludes with base64 data hrefs */
    return FALSE;
  }

  if (xmlStrncmp(href, BAD_CAST "data:text/xml;base64,", 21)) {
    /* We only process XIncludes with base64 data hrefs */
    return FALSE;
  }

  xmlChar* hrefdata = href+21;

  /* Verify that hrefdata is base64-encoded (and that it's safe to
     cast to a signed gchar pointer). I'm assuming that everyone is
     using the RFC 4648 encoding? */
  for (unsigned int i=0; i < xmlStrlen(hrefdata); i++) {
    if (hrefdata[i] > 'z') {
      return FALSE;
    }
    if (hrefdata[i] < '0' && hrefdata[i] != '+' && hrefdata[i] != '/') {
      return FALSE;
    }
  }

  /* WARNING: the xmlChar and guchar types here are compatible, but
     the decoded data is not necessarily NULL-terminated, while all of
     the libxml2 functions that operate on a xmlChar pointer assume
     that they are. */
  gsize decoded_size;
  xmlChar* decoded = g_base64_decode((const gchar*)hrefdata, &decoded_size);

  /* This cast is safe because signed and unsigned chars are the same size,
     and xmlReadMemory is going to treat the data as binary anyway. */
  xmlDoc* xinc_doc = xmlReadMemory((const char*)decoded,
                                   decoded_size,
                                   "xinclude.xml",
                                   NULL,
                                   0);
  g_free(decoded);

  if (xinc_doc == NULL) {
    return FALSE;
  }

  xmlNode* xinc_root = xmlDocGetRootElement(xinc_doc);
  if (xinc_root == NULL || xmlStrcmp(xinc_root->name, BAD_CAST "svg")) {
    return FALSE;
  }

  /* Replace the original xinclude "node" with the children of this
     "svg" node. Do the order of the nodes in an SVG matter? I don't
     know, but we go to a little bit of extra trouble here to ensure
     that we put the replacement in the right place, i.e. after its
     previous sibling (if there is one). */

  xmlNode* p = xmlPreviousElementSibling(node);
  xmlNode* cur_node;

  /* If there is no previous sibling element, do one AddChild()
     first. Then we're back to the case of a previous sibling. */
  if (p) {
    cur_node = xmlFirstElementChild(xinc_root);
  }
  else {
    p = node->parent;
    cur_node = xmlFirstElementChild(xinc_root);
    if (cur_node) {
      /* Without the xmlCopyNode, I get segfaults, and I don't care to
	 investigate why. */
      p = xmlAddChild(p, xmlCopyNode(cur_node,1));
      xmlReconciliateNs(p->doc, p);

      cur_node = xmlNextElementSibling(cur_node);
    }
  }

  g_assert(p != NULL); /* xmlAddChild didn't fail */

  xmlUnlinkNode(node);
  xmlFreeNode(node);

  while (cur_node) {
    p = xmlAddNextSibling(p, xmlCopyNode(cur_node,1));
    xmlReconciliateNs(p->doc, p);
    cur_node = xmlNextElementSibling(cur_node);
  }

  xmlFreeDoc(xinc_doc);

  return TRUE;
}

/**
 * @brief Replace all GTK <xi:include> elements in a tree by their data.
 *
 * @param node
 *   A node pointer, to the root of the tree.
 *
 * @return TRUE if we replaced any <xi:include> element nodes, and
 *   FALSE otherwise.
 *
 */
static gboolean process_child_xincludes(xmlNode* a_node) {
  gboolean result = FALSE;
  xmlNode* cur_node = a_node;
  xmlNode* next_node;

  g_assert(cur_node == NULL || cur_node->type == XML_ELEMENT_NODE);

  while (cur_node) {
    if (!xmlStrcmp(cur_node->name, BAD_CAST "include")) {
      /* process_one_xinclude() clobbers this node, so we need
         to get its successor before calling that function. */
      next_node = xmlNextElementSibling(cur_node);
      if (process_one_xinclude(cur_node)) {
        result = TRUE;
      }
      cur_node = next_node;
      continue;
    }

    if (process_child_xincludes(xmlFirstElementChild(cur_node))) {
      result = TRUE;
    }
    cur_node = xmlNextElementSibling(cur_node);
  }

  return result;
}


/**
 * @brief Process GTK <xi:include> elements in an SVG buffer.
 *
 * GTK is very cute. Its gtk-encode-symbolic-svg tool wraps your SVG
 * in its own boilerplate, but then rather than including your SVG
 * data verbatim, it includes it via a sketchy XInclude that looks
 * like,
 *
 *   <xi:include href="data:text/xml;base64,PD94bWwgd..."/>
 *
 * Librsvg knows how to parse that, but libxml2 doesn't (the latter
 * can handle an XInclude, just not a base64-encoded data reference).
 * Fortunately, you can read the source to gtk-encode-symbolic-svg,
 * and just see what the format of that line will be. Here we're going
 * to parse out the base64 data, decode it, strip out its opening
 * <xml> and <svg> tags, and then replace the original <xi:include>
 * element with the result.
 *
 * @param buffer
 *   A buffer containing SVG file data.
 *
 * @param buf_size
 *   The size of @c buffer (which may not be NULL-terminated).
 *
 * @param new_size
 *   A pointer to the size of the new buffer, valid only if the
 *   return value is non-NULL.
 *
 * @return A pointer to a buffer where the <xi:include> has been
 * processed. If no replacements were made, the result will be @c
 * NULL; otherwise, you are expected to @c free it when you are done.
 */
static gchar* process_gtk_symbolic_svg_xinclude(const gchar* buffer,
                                                gsize buf_size,
                                                gsize* new_size) {

  xmlDoc* doc = xmlReadMemory(buffer,buf_size,"symbolic.xml",NULL,0);
  if (doc == NULL) {
    return NULL;
  }

  xmlNode* root_element = xmlDocGetRootElement(doc);
  if (root_element == NULL) {
    return NULL;
  }

  gchar* result = NULL;
  if (process_child_xincludes(root_element)) {
    /* If we actually replaced something, we need to return the new
       document in a buffer. */
    xmlChar *xmlbuf;
    int xmlbuf_size;
    xmlDocDumpFormatMemory(doc, &xmlbuf, &xmlbuf_size, 1);
    /* We're going to free() this later on with g_free() instead of
       xmlFree(), so the two "byte" types had better be the same
       size. */
    g_assert(sizeof(xmlChar) == sizeof(gchar));
    *new_size = (gsize)xmlbuf_size;
    result = (gchar*)xmlbuf;
  }

  xmlFreeDoc(doc);
  xmlCleanupParser();
  return result;
}


/**
 * @brief Our implementation of GdkPixbufModuleStopLoadFunc, as
 *   defined in gdk-pixbuf-io.h.
 */
static gboolean gdk_pixbuf_stop_load(gpointer data, GError **error) {
  SvgTinyContext* context = (SvgTinyContext*)data;
  GdkPixbuf* pixbuf = NULL;
  GError* sub_error = NULL;

  g_assert(context != NULL);
  if (context->svg_data == NULL || context->svg_data_size == 0) {
    /* Is it possible to begin/stop with no increments in between?
     * I sure don't know. Let's play it safe. */
    return FALSE;
  }

  /* If we're inside of gtk-encode-symbolic-svg right now, we need to
     process the insane librsvg-specific XInclude directive it hands
     us before proceeding. */
  gsize newsize;
  gchar* newdata = process_gtk_symbolic_svg_xinclude(context->svg_data,
                                                     context->svg_data_size,
                                                     &newsize);
  if (newdata != NULL) {
    g_free(context->svg_data);
    context->svg_data = newdata;
    context->svg_data_size = newsize;
  }

  /* OK, we've got an SVG with no XIncludes now. */
  pixbuf = gdk_pixbuf_from_svg_buffer(context->svg_data,
                                      context->svg_data_size,
                                      &sub_error);

  if (pixbuf != NULL) {
    emit_prepared(context, pixbuf);
    emit_updated(context, pixbuf);
    g_object_unref(pixbuf);
  }
  else {
    g_propagate_error(error, sub_error);
    return FALSE;
  }
  g_free(context->svg_data);
  g_free(context);

  return TRUE;
}


/**
 * @brief Our implementation of GdkPixbufModuleFillVtableFunc, as
 *   defined in gdk-pixbuf-io.h.
 */
void fill_vtable(GdkPixbufModule* module) {
  module->begin_load = gdk_pixbuf_begin_load;
  module->load_increment = gdk_pixbuf_load_increment;
  module->stop_load = gdk_pixbuf_stop_load;
}

/**
 * @brief Our implementation of GdkPixbufModuleFillInfoFunc, as
 *   defined in gdk-pixbuf-io.h.
 */
void fill_info(GdkPixbufFormat* info) {
  /* Borrowed from librsvg-2.40.21 */
  static const GdkPixbufModulePattern signature[] = {
    {  " <svg",  "*    ", 100 },
    {  " <!DOCTYPE svg",  "*             ", 100 },
    { NULL, NULL, 0 }
  };

  /* I'm not sure if we should support gzipped svg here? */
  static const gchar *mime_types[] = {
    "image/svg+xml",
    "image/svg",
    "image/svg-xml",
    "image/vnd.adobe.svg+xml",
    "text/xml-svg",
    "image/svg+xml-compressed",
    NULL
  };

  static const gchar *extensions[] = {
    "svg",
    "svgz",
    "svg.gz",
    NULL
  };

  info->name        = "svg";
  info->signature   = (GdkPixbufModulePattern*)signature;
  info->description = "Scalable Vector Graphics";
  info->mime_types  = (gchar**)mime_types;
  info->extensions  = (gchar**)extensions;
  info->flags       = GDK_PIXBUF_FORMAT_SCALABLE;
  info->license     = "AGPL3";
}


/**
 * @brief Entry point of the svg2png test program.
 */
int main(int argc, char** argv) {
  char* svgpath;
  char* pngpath;
  GError* err = NULL;
  GdkPixbuf* pb;

  /* Parse arguments, and maybe print usage */
  if (argc < 3) {
    g_printf("Usage: %s INPUT OUTPUT\n", argv[0]);
    g_printf("Convert an SVG file (INPUT) to a PNG file (OUTPUT)\n");
    return 2;
  }

  svgpath = argv[1];
  pngpath = argv[2];

  pb = gdk_pixbuf_new_from_file(svgpath, &err);
  if (!pb) {
    g_fprintf(stderr,
              "Error %d in gdk_pixbuf_new_from_file: %s\n",
              err->code,
              err->message);
    g_error_free(err);
    return 1;
  }

  gdk_pixbuf_save(pb, pngpath, "png", NULL, NULL);
  g_object_unref(pb);
  return 0;
}