/*
    This file is part of Magnum.

    Copyright © 2010, 2011, 2012, 2013, 2014, 2015, 2016
              Vladimír Vondruš <mosra@centrum.cz>

    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
    to deal in the Software without restriction, including without limitation
    the rights to use, copy, modify, merge, publish, distribute, sublicense,
    and/or sell copies of the Software, and to permit persons to whom the
    Software is furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
    DEALINGS IN THE SOFTWARE.
*/

#ifndef NEW_GLSL
#define in varying
#define value gl_FragColor.x
#define texture texture2D
#endif

#ifndef RUNTIME_CONST
#define const
#endif

#if (defined(GL_ES) && __VERSION__ >= 300) || (!defined(GL_ES) && __VERSION__ >= 150)
#define TEXELFETCH_USABLE
#endif

#ifdef EXPLICIT_UNIFORM_LOCATION
layout(location = 0) uniform int radius;
layout(location = 1) uniform vec2 scaling;
#else
uniform lowp int radius;
uniform mediump vec2 scaling;
#endif

#ifdef EXPLICIT_TEXTURE_LAYER
layout(binding = 8) uniform sampler2D textureData;
#else
uniform lowp sampler2D textureData;
#endif

#ifdef TEXELFETCH_USABLE
#ifndef GL_ES
layout(pixel_center_integer) in mediump vec4 gl_FragCoord;
#endif
#else
#ifdef EXPLICIT_UNIFORM_LOCATION
layout(location = 2) uniform vec2 imageSizeInverted;
#else
uniform mediump vec2 imageSizeInverted;
#endif
#endif

#ifdef NEW_GLSL
out lowp float value;
#endif

#ifdef TEXELFETCH_USABLE
mediump ivec2 rotate(const mediump ivec2 vec) {
    return ivec2(-vec.y, vec.x);
}

bool hasValue(const mediump ivec2 position, const mediump ivec2 offset) {
    return texelFetch(textureData, position+offset, 0).r > 0.5;
}
#else
mediump vec2 rotate(const mediump vec2 vec) {
    return vec2(-vec.y, vec.x);
}

bool hasValue(const mediump vec2 position, const mediump vec2 offset) {
    return texture(textureData, position+offset).r > 0.5;
}
#endif

void main() {
    #ifdef TEXELFETCH_USABLE
    #ifndef GL_ES
    const mediump ivec2 position = ivec2(gl_FragCoord.xy*scaling);
    #else
    const mediump ivec2 position = ivec2((gl_FragCoord.xy - vec2(0.5))*scaling);
    #endif
    #else
    const mediump vec2 position = (gl_FragCoord.xy - vec2(0.5))*scaling*imageSizeInverted;
    #endif

    /* If pixel at the position is inside (1), we are looking for nearest pixel
       outside and the value will be positive (> 0.5). If it is outside (0), we
       are looking for nearest pixel inside and the value will be negative
       (< 0.5). */
    #ifdef TEXELFETCH_USABLE
    const bool isInside = hasValue(position, ivec2(0, 0));
    #else
    const bool isInside = hasValue(position, vec2(0.0, 0.0));
    #endif
    const highp float sign = isInside ? 1.0 : -1.0;

    /* Minimal found distance is just out of the radius (i.e. infinity) */
    highp float minDistanceSquared = float((radius+1)*(radius+1));

    /* Go in circles around the point and find nearest value */
    int radiusLimit = radius;
    for(int i = 1; i <= radiusLimit; ++i) {
        for(int j = 0, jmax = i*2; j < jmax; ++j) {
            #ifdef TEXELFETCH_USABLE
            const lowp ivec2 offset = ivec2(-i+j, i);
            #else
            const lowp vec2 pixelOffset = vec2(float(-i+j), float(i)) + vec2(0.5);
            const lowp vec2 offset = pixelOffset*imageSizeInverted;
            #endif

            /* If any of the four values is opposite of what is on the pixel,
               we found nearest value */
            if(hasValue(position, offset) == !isInside ||
               hasValue(position, rotate(offset)) == !isInside ||
               hasValue(position, rotate(rotate(offset))) == !isInside ||
               hasValue(position, rotate(rotate(rotate(offset)))) == !isInside) {
                #ifdef TEXELFETCH_USABLE
                const mediump float distanceSquared = dot(vec2(offset), vec2(offset));
                #else
                const mediump float distanceSquared = dot(pixelOffset, pixelOffset);
                #endif

                /* Set smaller distance, if found, or continue with lookup for
                   smaller */
                if(minDistanceSquared < distanceSquared) continue;
                else minDistanceSquared = distanceSquared;

                /* Set radius limit to max radius which can contain smaller
                   value, e.g. for distance 3.5 we can find smaller value even
                   in radius 3 */
                #ifdef NEW_GLSL
                radiusLimit = min(radius, int(floor(length(vec2(offset)))));
                #else
                radiusLimit = int(min(float(radius), floor(length(vec2(offset)))));
                #endif
            }
        }
    }

    /* Final signed distance, normalized from [-radius-1, radius+1] to [0, 1] */
    value = sign*sqrt(minDistanceSquared)/float(radius*2+2)+0.5;
}