/*
    Copyright © 2020, Luna Nielsen
    Distributed under the 2-Clause BSD License, see LICENSE file.
    
    Authors: Luna Nielsen
*/
module engine.render.texture.font;
import bindbc.freetype;
import std.exception;
import std.conv : text;
import std.string;
import std.format;
import engine;

private {
    FT_Library lib;
    Shader fontShader;
    Camera2D fontCamera;

    // VAO and VP can be reused
    GLuint vao;
    GLint vp;

    /// How many entries in a Font batch
    enum EntryCount = 10_000;

    // Various variables that make it easier to reference sizes
    enum VecSize = 2;
    enum UVSize = 2;
    enum ColorSize = 4;
    enum VertsCount = 6;
    enum DataLength = VecSize+UVSize+ColorSize;
    enum DataSize = DataLength*VertsCount;

    vec2 transformVerts(vec2 position, mat4 matrix) {
        return vec2(matrix*vec4(position.x, position.y, 0, 1));
    }

    vec2 transformVertsR(vec2 position, mat4 matrix) {
        return vec2(vec4(position.x, position.y, 0, 1)*matrix);
    }
}

/**
    Initialize the font subsystem
*/
void initFontSystem() {
    int err = FT_Init_FreeType(&lib);
    if (err != FT_Err_Ok) {
        AppLog.fatal("Font System", "FreeType returned error %s", err);
    }

    // Set up batching
    glGenVertexArrays(1, &vao);

    fontCamera = new Camera2D();
    fontShader = new Shader(import("shaders/font.vert"), import("shaders/font.frag"));
    vp = fontShader.getUniformLocation("vp");
}

/**
    A font
*/
class Font {
private:
    //
    //    Glyph managment
    //
    struct Glyph {
        vec4i area;
        vec2 advance;
        vec2 bearing;
    }

    struct GlyphIndex {
        dchar c;
        int size;
    }

    vec2 metrics;

    FT_Face fontFace;
    TexturePacker fontPacker;
    Texture fontTexture;
    Glyph[GlyphIndex] glyphs;
    int size;

    // Generates glyph for the specified character
    bool genGlyphFor(dchar c) {

        // Find the character's index in the font
        uint index = FT_Get_Char_Index(fontFace, c);

        // Load it if possible, otherwise tell the outside code
        FT_Load_Glyph(fontFace, index, FT_LOAD_RENDER);
        if (fontFace.glyph is null) return false;

        // Get width/height of character
        immutable(uint) width = fontFace.glyph.bitmap.width;
        immutable(uint) height = fontFace.glyph.bitmap.rows;

        // Get length of pixel data, then get it in to a slice
        size_t dataLength = fontFace.glyph.bitmap.pitch*fontFace.glyph.bitmap.rows;
        ubyte[] pixData = fontFace.glyph.bitmap.buffer[0..dataLength];

        // Find space in the texture and pack the glyph in there
        vec4i area = fontPacker.packTexture(vec2i(width, height));
        fontTexture.setDataRegion(pixData, area.x, area.y, width, height);

        // Add glyph to listing
        glyphs[GlyphIndex(c, size)] = Glyph(
            // Area (in texture)
            area, 

            // Advance
            vec2(fontFace.glyph.advance.x >> 6, fontFace.glyph.advance.y >> 6), 

            // Bearing
            vec2(fontFace.glyph.bitmap_left, fontFace.glyph.bitmap_top)
        );
        return true;
    }

    //
    //    Font Batching
    //
    float[DataSize*EntryCount] data;
    size_t dataOffset;
    size_t tris;

    GLuint buffer;

    void addVertexData(vec2 position, vec2 uvs, vec4 color) {
        data[dataOffset..dataOffset+DataLength] = [position.x, position.y, uvs.x, uvs.y, color.x, color.y, color.z, color.w];
        dataOffset += DataLength;
    }

public:

    /**
        Destroys the font
    */
    ~this() {
        destroy(fontTexture);
        FT_Done_Face(fontFace);
        glDeleteBuffers(1, &buffer);
    }

    /**
        Constructs a new font
    */
    this(string file, int size) {
        int err = FT_New_Face(lib, file.toStringz, 0, &fontFace);

        enforce(err != FT_Err_Unknown_File_Format, "Unknown file format for %s".format(file));
        enforce(err == FT_Err_Ok, "Error %s while loading font file".format(err));

        // Change size of text
        changeSize(size);

        // Select unicode so we can render i18n text
        FT_Select_Charmap(fontFace, FT_ENCODING_UNICODE);

        // Create the texture
        fontTexture = new Texture(8192, 8192, GL_RED, 1);
        fontPacker = new TexturePacker(vec2i(8192, 8192));

        glBindVertexArray(vao);
        glGenBuffers(1, &buffer);
        glBindBuffer(GL_ARRAY_BUFFER, buffer);
        glBufferData(GL_ARRAY_BUFFER, float.sizeof*data.length, data.ptr, GL_DYNAMIC_DRAW);
    }


    //
    //    Glyph managment
    //

    /**
        Changes size of font
    */
    final void changeSize(int size) {

        // Don't try to change size when it's the same
        if (size == this.size) return;

        // Set the size of the font
        FT_Set_Pixel_Sizes(fontFace, 0, size);
        metrics = vec2(size, fontFace.size.metrics.height >> 6);
        this.size = size;
    }

    /**
        Gets the advance of a glyph
    */
    vec2 advance(dchar glyph) {
        
        // Newline is special
        if (glyph == '\n') return vec2(0);

        auto idx = GlyphIndex(glyph, size);

        // Make sure glyph is present
        if (idx !in glyphs) enforce(genGlyphFor(glyph), "Could not find glyph for character %s".format(glyph));
        
        // Return the advance of the glyphs
        return glyphs[idx].advance;
    }

    /**
        Measure size (width, height) of rectangle needed to contain the specified text
    */
    vec2 measure(dstring text) {

        int lines = 1;
        float curLineLen = 0;
        vec2 size = vec2(0);
        
        foreach(dchar c; text) {
            if (c == '\n') {
                lines++;
                curLineLen = 0;
                continue;
            }

            auto idx = GlyphIndex(c, this.size);

            // Try to generate glyph if not present
            if (idx !in glyphs) {
                genGlyphFor(c);

                // At this point if the glyph does not exist, skip it
                if (idx !in glyphs) continue;
            }

            // Bitshift the X advance to make it be in pixels.
            curLineLen += glyphs[idx].advance.x;

            // Update the bounding box if the length extends
            if (curLineLen > size.x) size.x = curLineLen;
        }
        size.y = metrics.y*lines;
        return size;
    }


    //
    //    Font Batching
    //

    /**
        Basic string draw function
    */
    void draw(dstring text, vec2 position, vec4 color=vec4(1)) {
        vec2 next = position;
        size_t line;

        foreach(dchar c; text) {  

            // Skip newline
            if (c == '\n') {
                line++;
                next.x = position.x;
                next.y += metrics.y;
                continue;
            }

            auto idx = GlyphIndex(c, size);
                  
            // Load character if neccesary
            if (idx !in glyphs) {
                genGlyphFor(c);

                // At this point if the glyph does not exist, skip it
                if (idx !in glyphs) continue;
            }

            draw(c, next, vec2(0), 0, color);
            next.x += glyphs[idx].advance.x;
        }
    }

    /**
        draws a character
    */
    void draw(dchar c, vec2 position, vec2 origin=vec2(0), float rotation=0, vec4 color=vec4(1)) {
        
        auto idx = GlyphIndex(c, size);

        // Load character if neccesary
        if (idx !in glyphs) {
            genGlyphFor(c);

            // At this point if the glyph does not exist, skip it
            if (idx !in glyphs) return;
        }

        // Flush if neccesary
        if (dataOffset == DataSize*EntryCount) flush();
        vec4 area = glyphs[idx].area;
        vec2 bearing = glyphs[idx].bearing;

        vec2 pos = vec2(
            position.x + bearing.x,
            (position.y - bearing.y)+metrics.y
        );

        mat4 transform =
            mat4.translation(-origin.x, -origin.y, 0) *
            mat4.translation(pos.x, pos.y, 0) *
            mat4.translation(origin.x, origin.y, 0) *
            mat4.zrotation(rotation) * 
            mat4.translation(-origin.x, -origin.y, 0) *
            mat4.scaling(area.z, area.w, 0);


        vec4 uvArea = vec4(
            (area.x)+0.25,
            (area.y)+0.25,
            (area.x+area.z)-0.25,
            (area.y+area.w)-0.25,
        );

        // Triangle 1
        addVertexData(vec2(0, 1).transformVerts(transform), vec2(uvArea.x, uvArea.w), color);
        addVertexData(vec2(1, 0).transformVerts(transform), vec2(uvArea.z, uvArea.y), color);
        addVertexData(vec2(0, 0).transformVerts(transform), vec2(uvArea.x, uvArea.y), color);
        
        // Triangle 2
        addVertexData(vec2(0, 1).transformVerts(transform), vec2(uvArea.x, uvArea.w), color);
        addVertexData(vec2(1, 1).transformVerts(transform), vec2(uvArea.z, uvArea.w), color);
        addVertexData(vec2(1, 0).transformVerts(transform), vec2(uvArea.z, uvArea.y), color);

        tris += 2;
    }

    /**
        Flush font rendering
    */
    void flush() {

        // Disable depth testing for the batcher
        glDisable(GL_DEPTH_TEST);
        // Bind VAO
        glBindVertexArray(vao);

        // Bind just in case some shennanigans happen
        glBindBuffer(GL_ARRAY_BUFFER, buffer);

        // Update with this draw round's data
        glBufferSubData(GL_ARRAY_BUFFER, 0, dataOffset*float.sizeof, data.ptr);

        // Bind the texture
        fontTexture.bind();

        // Use our sprite batcher shader
        fontShader.use();
        fontShader.setUniform(vp, fontCamera.matrix);

        // Vertex Buffer
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(
            0,
            VecSize,
            GL_FLOAT,
            GL_FALSE,
            DataLength*GLfloat.sizeof,
            null,
        );

        // UV Buffer
        glEnableVertexAttribArray(1);
        glVertexAttribPointer(
            1,
            UVSize,
            GL_FLOAT,
            GL_FALSE,
            DataLength*GLfloat.sizeof,
            cast(GLvoid*)(UVSize*GLfloat.sizeof),
        );

        // UV Buffer
        glEnableVertexAttribArray(2);
        glVertexAttribPointer(
            2,
            ColorSize,
            GL_FLOAT,
            GL_FALSE,
            DataLength*GLfloat.sizeof,
            cast(GLvoid*)((VecSize+UVSize)*GLfloat.sizeof),
        );

        // Draw the triangles
        glDrawArrays(GL_TRIANGLES, 0, cast(int)(tris*3));

        // Reset the batcher's state
        glDisableVertexAttribArray(0);
        glDisableVertexAttribArray(1);
        glDisableVertexAttribArray(2);
        dataOffset = 0;
        tris = 0;

        // Re-enable depth test Clear depth buffer
        glEnable(GL_DEPTH_TEST);
    }

}