//! OpenGL Backend
//!
//! Provides GPU acceleration using OpenGL 3.3+ / OpenGL ES 3.0+

use super::super::Handle;
use super::backend::GpuDevice;
use super::types::*;
use crate::fitz::geometry::Matrix;

/// OpenGL device implementation
pub struct OpenGLDevice {
    capabilities: GpuCapabilities,
    /// OpenGL context handle (platform-specific)
    _context: u64,
}

impl OpenGLDevice {
    /// Create a new OpenGL device
    pub fn new() -> GpuResult<Self> {
        // Report OpenGL-tier capabilities. Actual context creation
        // requires the 'opengl' feature and a platform GL loader.

        let capabilities = GpuCapabilities {
            max_texture_size: 16384,
            max_texture_units: 32,
            compute_shaders: true,
            geometry_shaders: true,
            tessellation: true,
            max_msaa_samples: 8,
            float_textures: true,
            instancing: true,
            vram_mb: 0, // Unknown without actual context
            device_name: "OpenGL Device".into(),
            vendor_name: "Unknown".into(),
            driver_version: "OpenGL 4.6".into(),
        };

        Ok(Self {
            capabilities,
            _context: 0,
        })
    }
}

impl GpuDevice for OpenGLDevice {
    fn backend(&self) -> GpuBackendType {
        GpuBackendType::OpenGL
    }

    fn capabilities(&self) -> &GpuCapabilities {
        &self.capabilities
    }

    fn create_texture(&self, width: u32, height: u32, format: GpuFormat) -> GpuResult<GpuTexture> {
        // In real implementation:
        // glGenTextures(1, &texture);
        // glBindTexture(GL_TEXTURE_2D, texture);
        // glTexImage2D(GL_TEXTURE_2D, 0, internal_format, width, height, 0, format, type, nullptr);

        let mut texture = GpuTexture::new(width, height, format, GpuBackendType::OpenGL);
        texture.native_handle = 1; // Would be actual GL texture ID
        Ok(texture)
    }

    fn destroy_texture(&self, _texture: &GpuTexture) -> GpuResult<()> {
        // glDeleteTextures(1, &texture.native_handle);
        Ok(())
    }

    fn upload_texture(
        &self,
        texture: &mut GpuTexture,
        _data: &[u8],
        _stride: u32,
    ) -> GpuResult<()> {
        // glBindTexture(GL_TEXTURE_2D, texture.native_handle);
        // glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, width, height, format, type, data);
        let _ = texture;
        Ok(())
    }

    fn download_texture(
        &self,
        texture: &GpuTexture,
        _data: &mut [u8],
        _stride: u32,
    ) -> GpuResult<()> {
        // glBindTexture(GL_TEXTURE_2D, texture.native_handle);
        // glGetTexImage(GL_TEXTURE_2D, 0, format, type, data);
        let _ = texture;
        Ok(())
    }

    fn clear_texture(&self, texture: &mut GpuTexture, color: [f32; 4]) -> GpuResult<()> {
        // glBindFramebuffer(GL_FRAMEBUFFER, fbo);
        // glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);
        // glClearColor(color[0], color[1], color[2], color[3]);
        // glClear(GL_COLOR_BUFFER_BIT);
        let _ = (texture, color);
        Ok(())
    }

    fn create_shader(&self, _vertex_src: &str, _fragment_src: &str) -> GpuResult<GpuShader> {
        // Create and compile vertex shader
        // Create and compile fragment shader
        // Link into program

        let shader = GpuShader::new("shader", GpuBackendType::OpenGL);
        Ok(shader)
    }

    fn destroy_shader(&self, _shader: &GpuShader) -> GpuResult<()> {
        // glDeleteProgram(shader.native_handle);
        Ok(())
    }

    fn create_buffer(&self, size: usize, usage: GpuBufferUsage) -> GpuResult<GpuBuffer> {
        // let gl_usage = match usage {
        //     GpuBufferUsage::Vertex => GL_ARRAY_BUFFER,
        //     GpuBufferUsage::Index => GL_ELEMENT_ARRAY_BUFFER,
        //     GpuBufferUsage::Uniform => GL_UNIFORM_BUFFER,
        //     GpuBufferUsage::Storage => GL_SHADER_STORAGE_BUFFER,
        // };
        // glGenBuffers(1, &buffer);
        // glBindBuffer(gl_usage, buffer);
        // glBufferData(gl_usage, size, nullptr, GL_DYNAMIC_DRAW);

        let buffer = GpuBuffer::new(size, usage, GpuBackendType::OpenGL);
        Ok(buffer)
    }

    fn destroy_buffer(&self, _buffer: &GpuBuffer) -> GpuResult<()> {
        // glDeleteBuffers(1, &buffer.native_handle);
        Ok(())
    }

    fn upload_buffer(&self, buffer: &mut GpuBuffer, _data: &[u8], _offset: usize) -> GpuResult<()> {
        // glBindBuffer(target, buffer.native_handle);
        // glBufferSubData(target, offset, data.len(), data.as_ptr());
        let _ = buffer;
        Ok(())
    }

    fn render_page(
        &self,
        page: Handle,
        texture: &mut GpuTexture,
        transform: &Matrix,
    ) -> GpuResult<()> {
        // This would:
        // 1. Get page content (paths, text, images)
        // 2. Set up render target (FBO with texture)
        // 3. Apply transformation matrix
        // 4. Render paths using tessellation
        // 5. Render text using glyph atlas
        // 6. Composite images
        let _ = (page, texture, transform);
        Ok(())
    }

    fn composite(
        &self,
        src: &GpuTexture,
        dst: &mut GpuTexture,
        x: i32,
        y: i32,
        blend_mode: GpuBlendMode,
    ) -> GpuResult<()> {
        // Set up blend mode
        // Bind dst as render target
        // Draw src as textured quad at (x, y)
        let _ = (src, dst, x, y, blend_mode);
        Ok(())
    }

    fn draw_quad(
        &self,
        texture: &GpuTexture,
        dst: &mut GpuTexture,
        src_rect: [f32; 4],
        dst_rect: [f32; 4],
        color: [f32; 4],
    ) -> GpuResult<()> {
        // Bind dst as render target
        // Bind texture
        // Draw quad with UV coords from src_rect, position from dst_rect
        let _ = (texture, dst, src_rect, dst_rect, color);
        Ok(())
    }

    fn flush(&self) -> GpuResult<()> {
        // glFlush();
        Ok(())
    }

    fn finish(&self) -> GpuResult<()> {
        // glFinish();
        Ok(())
    }
}

// ============================================================================
// OpenGL Shaders
// ============================================================================

/// Default vertex shader for textured quads
pub const QUAD_VERTEX_SHADER: &str = r#"
#version 330 core
layout(location = 0) in vec2 a_position;
layout(location = 1) in vec2 a_texcoord;

out vec2 v_texcoord;

uniform mat4 u_projection;
uniform mat4 u_transform;

void main() {
    gl_Position = u_projection * u_transform * vec4(a_position, 0.0, 1.0);
    v_texcoord = a_texcoord;
}
"#;

/// Default fragment shader for textured quads
pub const QUAD_FRAGMENT_SHADER: &str = r#"
#version 330 core
in vec2 v_texcoord;
out vec4 fragColor;

uniform sampler2D u_texture;
uniform vec4 u_color;

void main() {
    fragColor = texture(u_texture, v_texcoord) * u_color;
}
"#;

/// Path fill vertex shader
pub const PATH_VERTEX_SHADER: &str = r#"
#version 330 core
layout(location = 0) in vec2 a_position;

uniform mat4 u_projection;
uniform mat4 u_transform;

void main() {
    gl_Position = u_projection * u_transform * vec4(a_position, 0.0, 1.0);
}
"#;

/// Path fill fragment shader
pub const PATH_FRAGMENT_SHADER: &str = r#"
#version 330 core
out vec4 fragColor;

uniform vec4 u_color;

void main() {
    fragColor = u_color;
}
"#;

/// Blend mode fragment shader
pub const BLEND_FRAGMENT_SHADER: &str = r#"
#version 330 core
in vec2 v_texcoord;
out vec4 fragColor;

uniform sampler2D u_src;
uniform sampler2D u_dst;
uniform int u_blend_mode;

// Blend mode implementations
vec3 blend_multiply(vec3 src, vec3 dst) { return src * dst; }
vec3 blend_screen(vec3 src, vec3 dst) { return 1.0 - (1.0 - src) * (1.0 - dst); }
vec3 blend_overlay(vec3 src, vec3 dst) {
    return mix(
        2.0 * src * dst,
        1.0 - 2.0 * (1.0 - src) * (1.0 - dst),
        step(0.5, dst)
    );
}
vec3 blend_darken(vec3 src, vec3 dst) { return min(src, dst); }
vec3 blend_lighten(vec3 src, vec3 dst) { return max(src, dst); }

void main() {
    vec4 src = texture(u_src, v_texcoord);
    vec4 dst = texture(u_dst, v_texcoord);

    vec3 result;
    switch (u_blend_mode) {
        case 0: result = src.rgb; break; // Normal
        case 1: result = blend_multiply(src.rgb, dst.rgb); break;
        case 2: result = blend_screen(src.rgb, dst.rgb); break;
        case 3: result = blend_overlay(src.rgb, dst.rgb); break;
        case 4: result = blend_darken(src.rgb, dst.rgb); break;
        case 5: result = blend_lighten(src.rgb, dst.rgb); break;
        default: result = src.rgb; break;
    }

    // Alpha compositing
    float alpha = src.a + dst.a * (1.0 - src.a);
    fragColor = vec4(result, alpha);
}
"#;

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_create_device() {
        let device = OpenGLDevice::new().unwrap();
        assert_eq!(device.backend(), GpuBackendType::OpenGL);
    }

    #[test]
    fn test_create_texture() {
        let device = OpenGLDevice::new().unwrap();
        let texture = device.create_texture(256, 256, GpuFormat::Rgba8).unwrap();
        assert_eq!(texture.width, 256);
        assert_eq!(texture.height, 256);
    }
}
