#version 120

/*
 *  SPDX-License-Identifier: GPL-2.0-or-later
 *
 *  Copyright (C) 2020-2024  The DOSBox Staging Team
 *  Copyright (C) 2004-2020  The DOSBox Team
 *
 *  Contributors:
 *   - 2004, Sjoerd van der Berg <harekiet@users.sourceforge.net>: authored
 *           https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@1817
 *
 *   - 2020, jmarsh <jmarsh@vogons.org>: converted to OpenGL fragment shader
 *           https://svn.code.sf.net/p/dosbox/code-0/dosbox/trunk@4319
 *
 *  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; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */

#pragma force_single_scan
#pragma force_no_pixel_doubling

varying vec2 v_texCoord;
uniform sampler2D rubyTexture;
uniform vec2 rubyInputSize;
uniform vec2 rubyOutputSize;
uniform vec2 rubyTextureSize;

#if defined(VERTEX)
attribute vec4 a_position;

void main()
{
	gl_Position = a_position;
	v_texCoord = vec2(a_position.x + 1.0, 1.0 - a_position.y) * rubyInputSize;
}

#elif defined(FRAGMENT)

vec3 getadvinterp2xtexel(vec2 coord)
{
	vec2 base = floor(coord / vec2(2.0)) + vec2(0.5);
	vec3 c4 = texture2D(rubyTexture, base / rubyTextureSize).xyz;
	vec3 c1 = texture2D(rubyTexture, (base - vec2(0.0, 1.0)) / rubyTextureSize).xyz;
	vec3 c7 = texture2D(rubyTexture, (base + vec2(0.0, 1.0)) / rubyTextureSize).xyz;
	vec3 c3 = texture2D(rubyTexture, (base - vec2(1.0, 0.0)) / rubyTextureSize).xyz;
	vec3 c5 = texture2D(rubyTexture, (base + vec2(1.0, 0.0)) / rubyTextureSize).xyz;

	bool outer = c1 != c7 && c3 != c5;
	bool c3c1 = outer && c3 == c1;
	bool c1c5 = outer && c1 == c5;
	bool c3c7 = outer && c3 == c7;
	bool c7c5 = outer && c7 == c5;

	vec3 l00 = mix(c3, c4, c3c1 ? 3.0 / 8.0 : 1.0);
	vec3 l01 = mix(c5, c4, c1c5 ? 3.0 / 8.0 : 1.0);
	vec3 l10 = mix(c3, c4, c3c7 ? 3.0 / 8.0 : 1.0);
	vec3 l11 = mix(c5, c4, c7c5 ? 3.0 / 8.0 : 1.0);

	coord = max(floor(mod(coord, 2.0)), 0.0);
	/* 2x2 output:
	 *    |x=0|x=1
	 * y=0|l00|l01
	 * y=1|l10|l11
	 */

	return mix(mix(l00, l01, coord.x), mix(l10, l11, coord.x), coord.y);
}

void main()
{
	vec2 coord = v_texCoord;
	coord -= 0.5;
	vec3 c0 = getadvinterp2xtexel(coord);
	vec3 c1 = getadvinterp2xtexel(coord + vec2(1.0, 0.0));
	vec3 c2 = getadvinterp2xtexel(coord + vec2(0.0, 1.0));
	vec3 c3 = getadvinterp2xtexel(coord + vec2(1.0));

	coord = fract(max(coord, 0.0));
	gl_FragColor = vec4(mix(mix(c0, c1, coord.x), mix(c2, c3, coord.x), coord.y), 1.0);
}
#endif
