image_modifications.cpp

/*
	Copyright (C) 2009 - 2024
	by Iris Morelle <shadowm2006@gmail.com>
	Part of the Battle for Wesnoth Project https://www.wesnoth.org/

	This program is free software; you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation; either version 2 of the License, or
	(at your option) any later version.
	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY.

	See the COPYING file for more details.
*/

#include "image_modifications.hpp"

#include "color.hpp"
#include "config.hpp"
#include "game_config.hpp"
#include "picture.hpp"
#include "lexical_cast.hpp"
#include "log.hpp"
#include "serialization/string_utils.hpp"
#include "team.hpp"
#include "utils/from_chars.hpp"

#include "formula/formula.hpp"
#include "formula/callable.hpp"

#define GETTEXT_DOMAIN "wesnoth-lib"

static lg::log_domain log_display("display");
#define ERR_DP LOG_STREAM(err, log_display)

namespace image {

/** Adds @a mod to the queue (unless mod is nullptr). */
void modification_queue::push(std::unique_ptr<modification> mod)
{
	priorities_[mod->priority()].push_back(std::move(mod));
}

/** Removes the top element from the queue */
void modification_queue::pop()
{
	map_type::iterator top_pair = priorities_.begin();
	auto& top_vector = top_pair->second;

	// Erase the top element.
	top_vector.erase(top_vector.begin());
	if(top_vector.empty()) {
		// We need to keep the map clean.
		priorities_.erase(top_pair);
	}
}

/** Returns the number of elements in the queue. */
std::size_t modification_queue::size() const
{
	std::size_t count = 0;
	for(const map_type::value_type& pair : priorities_) {
		count += pair.second.size();
	}

	return count;
}

/** Returns the top element in the queue . */
modification * modification_queue::top() const
{
	return priorities_.begin()->second.front().get();
}


namespace {

/** A function used to parse modification arguments */
using mod_parser = std::function<std::unique_ptr<modification>(std::string_view)>;

/** A map of all registered mod parsers
 *
 * The mapping is between the modification name and the parser function pointer
 * An example of an entry would be "TC" -> &parse_TC_mod
 */
std::map<std::string, mod_parser, std::less<>> mod_parsers;

/** Decodes a single modification using an appropriate mod_parser
 *
 * @param encoded_mod A string representing a single modification
 *
 * @return A pointer to the decoded modification object
 * @retval nullptr if the string is invalid or a parser isn't found
 */
std::unique_ptr<modification> decode_modification(const std::string& encoded_mod)
{
	const std::vector<std::string> split = utils::parenthetical_split(encoded_mod);

	if(split.size() != 2) {
		ERR_DP << "error parsing image modifications: " << encoded_mod;
		return nullptr;
	}

	const std::string& mod_type = split[0];
	const std::string& args = split[1];

	if(const auto parser = mod_parsers.find(mod_type); parser != mod_parsers.end()) {
		return std::invoke(parser->second, args);
	} else {
		ERR_DP << "unknown image function in path: " << mod_type;
		return nullptr;
	}
}

} // end anon namespace


modification::imod_exception::imod_exception(const std::stringstream& message_stream)
	: message(message_stream.str())
{
	this->store();
}

modification::imod_exception::imod_exception(const std::string& message)
	: message(message)
{
	this->store();
}

/** Decodes the modification string
 *
 * Important:
 * It creates new objects which need to be deleted after use
 *
 * @param encoded_mods A string representing any number of modifications
 *
 * @return A modification_queue filled with decoded modification pointers
 */
modification_queue modification::decode(const std::string& encoded_mods)
{
	modification_queue mods;

	for(const std::string& encoded_mod : utils::parenthetical_split(encoded_mods, '~')) {
		if(auto mod = decode_modification(encoded_mod)) {
			mods.push(std::move(mod));
		}
	}

	return mods;
}

surface rc_modification::operator()(const surface& src) const
{
	// unchecked
	return recolor_image(src, rc_map_);
}

surface fl_modification::operator()(const surface& src) const
{
	surface ret = src;

	if(horiz_  && vert_ ) {
		// Slightly faster than doing both a flip and a flop.
		ret = rotate_180_surface(ret);
	} else if(horiz_) {
		ret = flip_surface(ret);
	} else if(vert_) {
		ret = flop_surface(ret);
	}

	return ret;
}

surface rotate_modification::operator()(const surface& src) const
{
	// Convert the number of degrees to the interval [0,360].
	const int normalized = degrees_ >= 0 ?
		degrees_ - 360 * (degrees_ / 360) :
		degrees_ + 360 * (1 + (-degrees_) / 360); // In case compilers disagree as to what -90/360 is.

	switch ( normalized )
	{
		case 0:   return src;
		case 90:  return rotate_90_surface(src, true);
		case 180: return rotate_180_surface(src);
		case 270: return rotate_90_surface(src, false);
		case 360: return src;
	}

	return rotate_any_surface(src, normalized, zoom_, offset_);
}

surface gs_modification::operator()(const surface& src) const
{
	return greyscale_image(src);
}

surface crop_transparency_modification::operator()(const surface& src) const
{
	return get_non_transparent_portion(src);
}

surface bw_modification::operator()(const surface& src) const
{
	return monochrome_image(src, threshold_);
}

surface sepia_modification::operator()(const surface &src) const
{
	return sepia_image(src);
}

surface negative_modification::operator()(const surface &src) const
{
	return negative_image(src, red_, green_, blue_);
}

surface plot_alpha_modification::operator()(const surface& src) const
{
	return alpha_to_greyscale(src);
}

surface wipe_alpha_modification::operator()(const surface& src) const
{
	return wipe_alpha(src);
}

// TODO: Is this useful enough to move into formula/callable_objects?
class pixel_callable : public wfl::formula_callable
{
public:
	pixel_callable(SDL_Point p, color_t clr, uint32_t w, uint32_t h)
		: p(p), clr(clr), w(w), h(h)
	{}

	void get_inputs(wfl::formula_input_vector& inputs) const override
	{
		add_input(inputs, "x");
		add_input(inputs, "y");
		add_input(inputs, "u");
		add_input(inputs, "v");
		add_input(inputs, "red");
		add_input(inputs, "green");
		add_input(inputs, "blue");
		add_input(inputs, "alpha");
		add_input(inputs, "height");
		add_input(inputs, "width");
	}

	wfl::variant get_value(const std::string& key) const override
	{
		using wfl::variant;
		if(key == "x") {
			return variant(p.x);
		} else if(key == "y") {
			return variant(p.y);
		} else if(key == "red") {
			return variant(clr.r);
		} else if(key == "green") {
			return variant(clr.g);
		} else if(key == "blue") {
			return variant(clr.b);
		} else if(key == "alpha") {
			return variant(clr.a);
		} else if(key == "width") {
			return variant(w);
		} else if(key == "height") {
			return variant(h);
		} else if(key == "u") {
			return variant(p.x / static_cast<float>(w));
		} else if(key == "v") {
			return variant(p.y / static_cast<float>(h));
		}

		return variant();
	}

private:
	SDL_Point p;
	color_t clr;
	uint32_t w, h;
};

surface adjust_alpha_modification::operator()(const surface & src) const
{
	if(src == nullptr) {
		return nullptr;
	}

	wfl::formula new_alpha(formula_);

	surface nsurf = src.clone();

	if(nsurf == nullptr) {
		PLAIN_LOG << "could not make neutral surface...";
		return nullptr;
	}

	{
		surface_lock lock(nsurf);
		uint32_t* cur = lock.pixels();
		uint32_t* const end = cur + nsurf->w * src->h;
		uint32_t* const beg = cur;

		while(cur != end) {
			color_t pixel;
			pixel.a = (*cur) >> 24;
			pixel.r = (*cur) >> 16;
			pixel.g = (*cur) >> 8;
			pixel.b = (*cur);

			int i = cur - beg;
			SDL_Point p;
			p.y = i / nsurf->w;
			p.x = i % nsurf->w;

			pixel_callable px(p, pixel, nsurf->w, nsurf->h);
			pixel.a = std::min<unsigned>(new_alpha.evaluate(px).as_int(), 255);
			*cur = (pixel.a << 24) + (pixel.r << 16) + (pixel.g << 8) + pixel.b;

			++cur;
		}
	}

	return nsurf;
}

surface adjust_channels_modification::operator()(const surface & src) const
{
	if(src == nullptr) {
		return nullptr;
	}

	wfl::formula new_red(formulas_[0]);
	wfl::formula new_green(formulas_[1]);
	wfl::formula new_blue(formulas_[2]);
	wfl::formula new_alpha(formulas_[3]);

	surface nsurf = src.clone();

	if(nsurf == nullptr) {
		PLAIN_LOG << "could not make neutral surface...";
		return nullptr;
	}

	{
		surface_lock lock(nsurf);
		uint32_t* cur = lock.pixels();
		uint32_t* const end = cur + nsurf->w * src->h;
		uint32_t* const beg = cur;

		while(cur != end) {
			color_t pixel;
			pixel.a = (*cur) >> 24;
			pixel.r = (*cur) >> 16;
			pixel.g = (*cur) >> 8;
			pixel.b = (*cur);

			int i = cur - beg;
			SDL_Point p;
			p.y = i / nsurf->w;
			p.x = i % nsurf->w;

			pixel_callable px(p, pixel, nsurf->w, nsurf->h);
			pixel.r = std::min<unsigned>(new_red.evaluate(px).as_int(), 255);
			pixel.g = std::min<unsigned>(new_green.evaluate(px).as_int(), 255);
			pixel.b = std::min<unsigned>(new_blue.evaluate(px).as_int(), 255);
			pixel.a = std::min<unsigned>(new_alpha.evaluate(px).as_int(), 255);
			*cur = (pixel.a << 24) + (pixel.r << 16) + (pixel.g << 8) + pixel.b;

			++cur;
		}
	}

	return nsurf;
}

surface crop_modification::operator()(const surface& src) const
{
	SDL_Rect area = slice_;
	if(area.w == 0) {
		area.w = src->w;
	}

	if(area.h == 0) {
		area.h = src->h;
	}

	/*
	 * Unlike other image functions cut_surface does not convert the input
	 * surface to a neutral surface, nor does it convert its return surface
	 * to an optimised surface.
	 *
	 * Since it seems to work for most cases, rather change this caller instead
	 * of the function signature. (The issue was discovered in bug #20876).
	 */
	return cut_surface(src, area);
}

surface blit_modification::operator()(const surface& src) const
{
	if(x_ >= src->w) {
		std::stringstream sstr;
		sstr << "~BLIT(): x-coordinate '"
			<< x_ << "' larger than destination image's width '"
			<< src->w << "' no blitting performed.\n";

		throw imod_exception(sstr);
	}

	if(y_ >= src->h) {
		std::stringstream sstr;
		sstr << "~BLIT(): y-coordinate '"
			<< y_ << "' larger than destination image's height '"
			<< src->h << "' no blitting performed.\n";

		throw imod_exception(sstr);
	}

	if(surf_->w + x_ < 0) {
		std::stringstream sstr;
		sstr << "~BLIT(): offset and width '"
			<< x_ + surf_->w << "' less than zero no blitting performed.\n";

		throw imod_exception(sstr);
	}

	if(surf_->h + y_ < 0) {
		std::stringstream sstr;
		sstr << "~BLIT(): offset and height '"
			<< y_ + surf_->h << "' less than zero no blitting performed.\n";

		throw imod_exception(sstr);
	}

	surface nsrc = src.clone();
	SDL_Rect r {x_, y_, 0, 0};
	sdl_blit(surf_, nullptr, nsrc, &r);
	return nsrc;
}

surface mask_modification::operator()(const surface& src) const
{
	if(src->w == mask_->w &&  src->h == mask_->h && x_ == 0 && y_ == 0) {
		return mask_surface(src, mask_);
	}

	SDL_Rect r {x_, y_, 0, 0};
	surface new_mask(src->w, src->h);
	sdl_blit(mask_, nullptr, new_mask, &r);
	return mask_surface(src, new_mask);
}

surface light_modification::operator()(const surface& src) const {
	if(src == nullptr) { return nullptr; }

	// light_surface wants a neutral surface having same dimensions
	surface nsurf;
	if(surf_->w != src->w || surf_->h != src->h) {
		nsurf = scale_surface(surf_, src->w, src->h);
	} else {
		nsurf = surf_;
	}

	return light_surface(src, nsurf);
}

surface scale_modification::operator()(const surface& src) const
{
	point size = target_size_;

	if(size.x <= 0) {
		size.x = src->w;
	} else if(flags_ & X_BY_FACTOR) {
		size.x = src->w * (static_cast<double>(size.x) / 100);
	}

	if(size.y <= 0) {
		size.y = src->h;
	} else if(flags_ & Y_BY_FACTOR) {
		size.y = src->h * (static_cast<double>(size.y) / 100);
	}

	if(flags_ & PRESERVE_ASPECT_RATIO) {
		const auto ratio = std::min(
			static_cast<long double>(size.x) / src->w,
			static_cast<long double>(size.y) / src->h
		);

		size = {
			static_cast<int>(src->w * ratio),
			static_cast<int>(src->h * ratio)
		};
	}

	if(flags_ & SCALE_SHARP) {
		return scale_surface_sharp(src, size.x, size.y);
	} else {
		return scale_surface_legacy(src, size.x, size.y);
	}
}

surface xbrz_modification::operator()(const surface& src) const
{
	if(z_ == 1) {
		return src;
	}

	return scale_surface_xbrz(src, z_);
}

/*
 * The Opacity IPF doesn't seem to work with surface-wide alpha and instead needs per-pixel alpha.
 * If this is needed anywhere else it can be moved back to sdl/utils.*pp.
 */
surface o_modification::operator()(const surface& src) const
{
	surface nsurf = src.clone();

	if(nsurf == nullptr) {
		PLAIN_LOG << "could not make neutral surface...";
		return nullptr;
	}

	uint8_t alpha_mod = float_to_color(opacity_);

	{
		surface_lock lock(nsurf);
		uint32_t* beg = lock.pixels();
		uint32_t* end = beg + nsurf->w * src->h;

		while(beg != end) {
			uint8_t alpha = (*beg) >> 24;

			if(alpha) {
				uint8_t r, g, b;
				r = (*beg) >> 16;
				g = (*beg) >> 8;
				b = (*beg);

				alpha = color_multiply(alpha, alpha_mod);
				*beg = (alpha << 24) + (r << 16) + (g << 8) + b;
			}

			++beg;
		}
	}

	return nsurf;
}

surface cs_modification::operator()(const surface& src) const
{
	return((r_ != 0 || g_ != 0 || b_ != 0)
		? adjust_surface_color(src, r_, g_, b_)
		: src
	);
}

surface blend_modification::operator()(const surface& src) const
{
	return blend_surface(src, static_cast<double>(a_), color_t(r_, g_, b_));
}

surface bl_modification::operator()(const surface& src) const
{
	return blur_alpha_surface(src, depth_);
}

surface background_modification::operator()(const surface &src) const
{
	surface ret = src.clone();
	SDL_FillRect(ret, nullptr, SDL_MapRGBA(ret->format, color_.r, color_.g,
					    color_.b, color_.a));
	sdl_blit(src, nullptr, ret, nullptr);
	return ret;
}

surface swap_modification::operator()(const surface &src) const
{
	return swap_channels_image(src, red_, green_, blue_, alpha_);
}

namespace {

struct parse_mod_registration
{
	parse_mod_registration(const char* name, mod_parser parser)
	{
		mod_parsers[name] = parser;
	}
};

/** A macro for automatic modification parser registration
 *
 * It automatically registers the created parser in the mod_parsers map
 * It should be used just like a function header (look at the uses below)
 * It should only be used within an anonymous namespace
 *
 * @param type The modification type to be registered (unquoted)
 * @param args_var The name for the string argument provided
 */
#define REGISTER_MOD_PARSER(type, args_var)                                                           \
    static std::unique_ptr<modification> parse_##type##_mod(std::string_view);                        \
    static parse_mod_registration parse_##type##_mod_registration_aux(#type, &parse_##type##_mod);    \
    static std::unique_ptr<modification> parse_##type##_mod(std::string_view args_var)                \

// Color-range-based recoloring
REGISTER_MOD_PARSER(TC, args)
{
	const auto params = utils::split_view(args,',');

	if(params.size() < 2) {
		ERR_DP << "too few arguments passed to the ~TC() function";

		return nullptr;
	}

	const int side_n = utils::from_chars<int>(params[0]).value_or(-1);
	if(side_n < 1) {
		ERR_DP << "Invalid side (" << params[0] << ") passed to the ~TC() function";
		return nullptr;
	}

	//
	// Pass argseters for RC functor
	//
	if(!game_config::tc_info(params[1]).size()){
		ERR_DP << "could not load TC info for '" << params[1] << "' palette";
		ERR_DP << "bailing out from TC";

		return nullptr;
	}

	color_range_map rc_map;
	try {
		const color_range& new_color = team::get_side_color_range(side_n);
		const std::vector<color_t>& old_color = game_config::tc_info(params[1]);

		rc_map = recolor_range(new_color,old_color);
	} catch(const config::error& e) {
		ERR_DP << "caught config::error while processing TC: " << e.message;
		ERR_DP << "bailing out from TC";

		return nullptr;
	}

	return std::make_unique<rc_modification>(rc_map);
}

// Team-color-based color range selection and recoloring
REGISTER_MOD_PARSER(RC, args)
{
	const auto recolor_params = utils::split_view(args,'>');

	if(recolor_params.size() <= 1) {
		return nullptr;
	}

	//
	// recolor source palette to color range
	//
	color_range_map rc_map;
	try {
		const color_range& new_color = game_config::color_info(recolor_params[1]);
		const std::vector<color_t>& old_color = game_config::tc_info(recolor_params[0]);

		rc_map = recolor_range(new_color,old_color);
	} catch (const config::error& e) {
		ERR_DP
			<< "caught config::error while processing color-range RC: "
			<< e.message;
		ERR_DP << "bailing out from RC";
		rc_map.clear();
	}

	return std::make_unique<rc_modification>(rc_map);
}

// Palette switch
REGISTER_MOD_PARSER(PAL, args)
{
	const auto remap_params = utils::split_view(args,'>');

	if(remap_params.size() < 2) {
		ERR_DP << "not enough arguments passed to the ~PAL() function: " << args;

		return nullptr;
	}

	try {
		color_range_map rc_map;
		const std::vector<color_t>& old_palette = game_config::tc_info(remap_params[0]);
		const std::vector<color_t>& new_palette =game_config::tc_info(remap_params[1]);

		for(std::size_t i = 0; i < old_palette.size() && i < new_palette.size(); ++i) {
			rc_map[old_palette[i]] = new_palette[i];
		}

		return std::make_unique<rc_modification>(rc_map);
	} catch(const config::error& e) {
		ERR_DP
			<< "caught config::error while processing PAL function: "
			<< e.message;
		ERR_DP
			<< "bailing out from PAL";

		return nullptr;
	}
}

// Flip/flop
REGISTER_MOD_PARSER(FL, args)
{
	bool horiz = (args.empty() || args.find("horiz") != std::string::npos);
	bool vert = (args.find("vert") != std::string::npos);

	return std::make_unique<fl_modification>(horiz, vert);
}

// Rotations
REGISTER_MOD_PARSER(ROTATE, args)
{
	const auto slice_params = utils::split_view(args, ',', utils::STRIP_SPACES);

	switch(slice_params.size()) {
		case 0:
			return std::make_unique<rotate_modification>();
		case 1:
			return std::make_unique<rotate_modification>(
				utils::from_chars<int>(slice_params[0]).value_or(0));
		case 2:
			return std::make_unique<rotate_modification>(
				utils::from_chars<int>(slice_params[0]).value_or(0),
				utils::from_chars<int>(slice_params[1]).value_or(0));
		case 3:
			return std::make_unique<rotate_modification>(
				utils::from_chars<int>(slice_params[0]).value_or(0),
				utils::from_chars<int>(slice_params[1]).value_or(0),
				utils::from_chars<int>(slice_params[2]).value_or(0));
	}
	return nullptr;
}

// Grayscale
REGISTER_MOD_PARSER(GS, )
{
	return std::make_unique<gs_modification>();
}

// crop transparent padding
REGISTER_MOD_PARSER(CROP_TRANSPARENCY, )
{
	return std::make_unique<crop_transparency_modification>();
}

// TODO: should this be made a more general util function?
bool in_range(int val, int min, int max)
{
	return min <= val && val <= max;
}

// Black and white
REGISTER_MOD_PARSER(BW, args)
{
	const auto params = utils::split_view(args, ',');

	if(params.size() != 1) {
		ERR_DP << "~BW() requires  exactly one argument";
		return nullptr;
	}

	// TODO: maybe get this directly as uint8_t?
	const auto threshold = utils::from_chars<int>(params[0]);
	if(!threshold) {
		ERR_DP << "unsupported argument in ~BW() function";
		return nullptr;
	}

	if(!in_range(*threshold, 0, 255)) {
		ERR_DP << "~BW() argument out of range 0 - 255";
		return nullptr;
	}

	return std::make_unique<bw_modification>(*threshold);
}

// Sepia
REGISTER_MOD_PARSER(SEPIA, )
{
	return std::make_unique<sepia_modification>();
}

// Negative
REGISTER_MOD_PARSER(NEG, args)
{
	const auto params = utils::split_view(args, ',');

	switch(params.size()) {
	case 0:
		// apparently -1 may be a magic number but this is the threshold
		// value required to fully invert a channel
		return std::make_unique<negative_modification>(-1, -1, -1);

	case 1: {
		const auto threshold = utils::from_chars<int>(params[0]);

		if(threshold && in_range(*threshold, -1, 255)) {
			return std::make_unique<negative_modification>(*threshold, *threshold, *threshold);
		} else {
			ERR_DP << "unsupported argument value in ~NEG() function";
			return nullptr;
		}
	}

	case 3: {
		const auto thR = utils::from_chars<int>(params[0]);
		const auto thG = utils::from_chars<int>(params[1]);
		const auto thB = utils::from_chars<int>(params[2]);

		if(thR && thG && thB && in_range(*thR, -1, 255) && in_range(*thG, -1, 255) && in_range(*thB, -1, 255)) {
			return std::make_unique<negative_modification>(*thR, *thG, *thB);
		} else {
			ERR_DP << "unsupported argument value in ~NEG() function";
			return nullptr;
		}
	}

	default:
		ERR_DP << "~NEG() requires 0, 1 or 3 arguments";
		return nullptr;
	}
}

// Plot Alpha
REGISTER_MOD_PARSER(PLOT_ALPHA, )
{
	return std::make_unique<plot_alpha_modification>();
}

// Wipe Alpha
REGISTER_MOD_PARSER(WIPE_ALPHA, )
{
	return std::make_unique<wipe_alpha_modification>();
}

// Adjust Alpha
REGISTER_MOD_PARSER(ADJUST_ALPHA, args)
{
	// Formulas may contain commas, so use parenthetical split to ensure that they're properly considered a single argument.
	// (A comma in a formula is only valid in function parameters or list/map literals, so this should always work.)
	const std::vector<std::string>& params = utils::parenthetical_split(args, ',', "([", ")]");

	if(params.size() != 1) {
		ERR_DP << "~ADJUST_ALPHA() requires exactly 1 arguments";
		return nullptr;
	}

	return std::make_unique<adjust_alpha_modification>(params.at(0));
}

// Adjust Channels
REGISTER_MOD_PARSER(CHAN, args)
{
	// Formulas may contain commas, so use parenthetical split to ensure that they're properly considered a single argument.
	// (A comma in a formula is only valid in function parameters or list/map literals, so this should always work.)
	const std::vector<std::string>& params = utils::parenthetical_split(args, ',', "([", ")]");

	if(params.size() < 1 || params.size() > 4) {
		ERR_DP << "~CHAN() requires 1 to 4 arguments";
		return nullptr;
	}

	return std::make_unique<adjust_channels_modification>(params);
}

// Color-shift
REGISTER_MOD_PARSER(CS, args)
{
	const auto factors = utils::split_view(args, ',');
	const std::size_t s = factors.size();

	if(s == 0) {
		ERR_DP << "no arguments passed to the ~CS() function";
		return nullptr;
	}

	int r = 0, g = 0, b = 0;

	r = utils::from_chars<int>(factors[0]).value_or(0);

	if(s > 1 ) {
		g = utils::from_chars<int>(factors[0]).value_or(0);
	}
	if(s > 2 ) {
		b = utils::from_chars<int>(factors[0]).value_or(0);
	}

	return std::make_unique<cs_modification>(r, g , b);
}

// Color blending
REGISTER_MOD_PARSER(BLEND, args)
{
	const auto params = utils::split_view(args, ',');

	if(params.size() != 4) {
		ERR_DP << "~BLEND() requires exactly 4 arguments";
		return nullptr;
	}

	float opacity = 0.0f;
	const std::string_view& opacity_str = params[3];
	const std::string_view::size_type p100_pos = opacity_str.find('%');

	if(p100_pos == std::string::npos)
		opacity = lexical_cast_default<float>(opacity_str);
	else {
		// make multiplier
		const std::string_view parsed_field = opacity_str.substr(0, p100_pos);
		opacity = lexical_cast_default<float>(parsed_field);
		opacity /= 100.0f;
	}

	return std::make_unique<blend_modification>(
		utils::from_chars<int>(params[0]).value_or(0),
		utils::from_chars<int>(params[1]).value_or(0),
		utils::from_chars<int>(params[2]).value_or(0),
		opacity);
}

// Crop/slice
REGISTER_MOD_PARSER(CROP, args)
{
	const auto slice_params = utils::split_view(args, ',', utils::STRIP_SPACES);
	const std::size_t s = slice_params.size();

	if(s == 0 || (s == 1 && slice_params[0].empty())) {
		ERR_DP << "no arguments passed to the ~CROP() function";
		return nullptr;
	}

	SDL_Rect slice_rect { 0, 0, 0, 0 };

	slice_rect.x = utils::from_chars<int16_t>(slice_params[0]).value_or(0);

	if(s > 1) {
		slice_rect.y = utils::from_chars<int16_t>(slice_params[1]).value_or(0);
	}
	if(s > 2) {
		slice_rect.w = utils::from_chars<uint16_t>(slice_params[2]).value_or(0);
	}
	if(s > 3) {
		slice_rect.h = utils::from_chars<uint16_t>(slice_params[3]).value_or(0);
	}

	return std::make_unique<crop_modification>(slice_rect);
}

static bool check_image(const image::locator& img, std::stringstream & message)
{
	if(img.file_exists()) return true;
	message << " image not found: '" << img.get_filename() << "'\n";
	ERR_DP << message.str();
	return false;
}

// Blit
REGISTER_MOD_PARSER(BLIT, args)
{
	std::vector<std::string> param = utils::parenthetical_split(args, ',');
	const std::size_t s = param.size();

	if(s == 0 || (s == 1 && param[0].empty())){
		ERR_DP << "no arguments passed to the ~BLIT() function";
		return nullptr;
	}

	if(s > 3){
		ERR_DP << "too many arguments passed to the ~BLIT() function";
		return nullptr;
	}

	int x = 0, y = 0;

	if(s == 3) {
		x = utils::from_chars<int>(param[1]).value_or(0);
		y = utils::from_chars<int>(param[2]).value_or(0);
	}

	const image::locator img(param[0]);
	std::stringstream message;
	message << "~BLIT():";
	if(!check_image(img, message))
		return nullptr;
	surface surf = get_surface(img);

	return std::make_unique<blit_modification>(surf, x, y);
}

// Mask
REGISTER_MOD_PARSER(MASK, args)
{
	std::vector<std::string> param = utils::parenthetical_split(args, ',');
	const std::size_t s = param.size();

	if(s == 0 || (s == 1 && param[0].empty())){
		ERR_DP << "no arguments passed to the ~MASK() function";
		return nullptr;
	}

	int x = 0, y = 0;

	if(s == 3) {
		x = utils::from_chars<int>(param[1]).value_or(0);
		y = utils::from_chars<int>(param[2]).value_or(0);
	}

	if(x < 0 || y < 0) {
		ERR_DP << "negative position arguments in ~MASK() function";
		return nullptr;
	}

	const image::locator img(param[0]);
	std::stringstream message;
	message << "~MASK():";
	if(!check_image(img, message))
		return nullptr;
	surface surf = get_surface(img);

	return std::make_unique<mask_modification>(surf, x, y);
}

// Light
REGISTER_MOD_PARSER(L, args)
{
	if(args.empty()){
		ERR_DP << "no arguments passed to the ~L() function";
		return nullptr;
	}

	surface surf = get_surface(std::string{args}); // FIXME: string_view for image::locator::value
	return std::make_unique<light_modification>(surf);
}

namespace
{
std::pair<int, bool> parse_scale_value(std::string_view arg)
{
	if(const std::size_t pos = arg.rfind('%'); pos != std::string_view::npos) {
		return { utils::from_chars<int>(arg.substr(0, pos)).value_or(0), true };
	} else {
		return { utils::from_chars<int>(arg).value_or(0), false };
	}
}

/** Common helper function to parse scaling IPF inputs. */
std::optional<std::pair<point, uint8_t>> parse_scale_args(std::string_view args)
{
	const auto scale_params = utils::split_view(args, ',', utils::STRIP_SPACES);
	const std::size_t num_args = scale_params.size();

	if(num_args == 0 || (num_args == 1 && scale_params[0].empty())) {
		return std::nullopt;
	}

	uint8_t flags = 0;
	std::array<int, 2> parsed_sizes{0,0};

	for(unsigned i = 0; i < std::min<unsigned>(2, num_args); ++i) {
		const auto& [size, relative] = parse_scale_value(scale_params[i]);

		if(size < 0) {
			ERR_DP << "Negative size passed to scaling IPF. Original image dimension will be used instead";
			continue;
		}

		parsed_sizes[i] = size;

		if(relative) {
			flags |= (i == 0 ? scale_modification::X_BY_FACTOR : scale_modification::Y_BY_FACTOR);
		}
	}

	return std::pair{point{parsed_sizes[0], parsed_sizes[1]}, flags};
}

} // namespace

// Scale
REGISTER_MOD_PARSER(SCALE, args)
{
	if(auto params = parse_scale_args(args)) {
		constexpr uint8_t mode = scale_modification::SCALE_LINEAR | scale_modification::FIT_TO_SIZE;
		return std::make_unique<scale_modification>(params->first, mode | params->second);
	} else {
		ERR_DP << "no arguments passed to the ~SCALE() function";
		return nullptr;
	}
}

REGISTER_MOD_PARSER(SCALE_SHARP, args)
{
	if(auto params = parse_scale_args(args)) {
		constexpr uint8_t mode = scale_modification::SCALE_SHARP | scale_modification::FIT_TO_SIZE;
		return std::make_unique<scale_modification>(params->first, mode | params->second);
	} else {
		ERR_DP << "no arguments passed to the ~SCALE_SHARP() function";
		return nullptr;
	}
}

REGISTER_MOD_PARSER(SCALE_INTO, args)
{
	if(auto params = parse_scale_args(args)) {
		constexpr uint8_t mode = scale_modification::SCALE_LINEAR | scale_modification::PRESERVE_ASPECT_RATIO;
		return std::make_unique<scale_modification>(params->first, mode | params->second);
	} else {
		ERR_DP << "no arguments passed to the ~SCALE_INTO() function";
		return nullptr;
	}
}

REGISTER_MOD_PARSER(SCALE_INTO_SHARP, args)
{
	if(auto params = parse_scale_args(args)) {
		constexpr uint8_t mode = scale_modification::SCALE_SHARP | scale_modification::PRESERVE_ASPECT_RATIO;
		return std::make_unique<scale_modification>(params->first, mode | params->second);
	} else {
		ERR_DP << "no arguments passed to the ~SCALE_INTO_SHARP() function";
		return nullptr;
	}
}

// xBRZ
REGISTER_MOD_PARSER(XBRZ, args)
{
	int z = utils::from_chars<int>(args).value_or(0);
	if(z < 1 || z > 5) {
		z = 5; //only values 2 - 5 are permitted for xbrz scaling factors.
	}

	return std::make_unique<xbrz_modification>(z);
}

// Gaussian-like blur
REGISTER_MOD_PARSER(BL, args)
{
	const int depth = std::max<int>(0, utils::from_chars<int>(args).value_or(0));
	return std::make_unique<bl_modification>(depth);
}

// Opacity-shift
REGISTER_MOD_PARSER(O, args)
{
	const std::string::size_type p100_pos = args.find('%');
	float num = 0.0f;
	if(p100_pos == std::string::npos) {
		num = lexical_cast_default<float, std::string_view>(args);
	} else {
		// make multiplier
		const std::string_view parsed_field = args.substr(0, p100_pos);
		num = lexical_cast_default<float, std::string_view>(parsed_field);
		num /= 100.0f;
	}

	return std::make_unique<o_modification>(num);
}

//
// ~R(), ~G() and ~B() are the children of ~CS(). Merely syntactic sugar.
// Hence they are at the end of the evaluation.
//
// Red component color-shift
REGISTER_MOD_PARSER(R, args)
{
	const int r = utils::from_chars<int>(args).value_or(0);
	return std::make_unique<cs_modification>(r, 0, 0);
}

// Green component color-shift
REGISTER_MOD_PARSER(G, args)
{
	const int g = utils::from_chars<int>(args).value_or(0);
	return std::make_unique<cs_modification>(0, g, 0);
}

// Blue component color-shift
REGISTER_MOD_PARSER(B, args)
{
	const int b = utils::from_chars<int>(args).value_or(0);
	return std::make_unique<cs_modification>(0, 0, b);
}

REGISTER_MOD_PARSER(NOP, )
{
	return nullptr;
}

// Only used to tag terrain images which should not be color-shifted by ToD
REGISTER_MOD_PARSER(NO_TOD_SHIFT, )
{
	return nullptr;
}

// Fake image function used by GUI2 portraits until
// Mordante gets rid of it. *tsk* *tsk*
REGISTER_MOD_PARSER(RIGHT, )
{
	return nullptr;
}

// Add a background color.
REGISTER_MOD_PARSER(BG, args)
{
	int c[4] { 0, 0, 0, SDL_ALPHA_OPAQUE };
	const auto factors = utils::split_view(args, ',');

	for(int i = 0; i < std::min<int>(factors.size(), 4); ++i) {
		c[i] = utils::from_chars<int>(factors[i]).value_or(0);
	}

	return std::make_unique<background_modification>(color_t(c[0], c[1], c[2], c[3]));
}

// Channel swap
REGISTER_MOD_PARSER(SWAP, args)
{
	const auto params = utils::split_view(args, ',', utils::STRIP_SPACES);

	// accept 3 arguments (rgb) or 4 (rgba)
	if(params.size() != 3 && params.size() != 4) {
		ERR_DP << "incorrect number of arguments in ~SWAP() function, they must be 3 or 4";
		return nullptr;
	}

	channel redValue, greenValue, blueValue, alphaValue;
	// compare the parameter's value with the constants defined in the channels enum
	if(params[0] == "red") {
		redValue = RED;
	} else if(params[0] == "green") {
		redValue = GREEN;
	} else if(params[0] == "blue") {
		redValue = BLUE;
	} else if(params[0] == "alpha") {
		redValue = ALPHA;
	} else {
		ERR_DP << "unsupported argument value in ~SWAP() function: " << params[0];
		return nullptr;
	}

	// wash, rinse and repeat for the other three channels
	if(params[1] == "red") {
		greenValue = RED;
	} else if(params[1] == "green") {
		greenValue = GREEN;
	} else if(params[1] == "blue") {
		greenValue = BLUE;
	} else if(params[1] == "alpha") {
		greenValue = ALPHA;
	} else {
		ERR_DP << "unsupported argument value in ~SWAP() function: " << params[0];
		return nullptr;
	}

	if(params[2] == "red") {
		blueValue = RED;
	} else if(params[2] == "green") {
		blueValue = GREEN;
	} else if(params[2] == "blue") {
		blueValue = BLUE;
	} else if(params[2] == "alpha") {
		blueValue = ALPHA;
	} else {
		ERR_DP << "unsupported argument value in ~SWAP() function: " << params[0];
		return nullptr;
	}

	// additional check: the params vector may not have a fourth elementh
	// if so, default to the same channel
	if(params.size() == 3) {
		alphaValue = ALPHA;
	} else {
		if(params[3] == "red") {
			alphaValue = RED;
		} else if(params[3] == "green") {
			alphaValue = GREEN;
		} else if(params[3] == "blue") {
			alphaValue = BLUE;
		} else if(params[3] == "alpha") {
			alphaValue = ALPHA;
		} else {
			ERR_DP << "unsupported argument value in ~SWAP() function: " << params[3];
			return nullptr;
		}
	}

	return std::make_unique<swap_modification>(redValue, greenValue, blueValue, alphaValue);
}

} // end anon namespace

} /* end namespace image */