use glam::{uvec2, vec2, UVec2, Vec2};
use hashbrown::HashMap;
use nohash_hasher::BuildNoHashHasher;
use rect_packer::DensePacker;
use crate::rect::Corners;

const RGBA_CHANNEL_COUNT: u32 = 4;
//TODO make this work
const ALLOW_ROTATION: bool = false;

/// Texture format of the source texture data
#[derive(Default, Clone, Copy, PartialEq, Eq)]
pub enum TextureFormat {
  /// The data is stored in RGBA format, with 1 byte (8 bits) per channel
  #[default]
  Rgba,

  /// The data is copied into the Alpha channel, with 1 byte (8 bits) per channel\
  /// Remaining channels are set to 255 (which can be easily shaded to any color)
  ///
  /// This format is useful for storing grayscale textures such as icons\
  /// (Please note that the internal representation is still RGBA, this is just a convenience feature)
  Grayscale,
}

/// Contains a reference to the texture data, and metadata associated with it
pub struct TextureAtlasMeta<'a> {
  /// Texture data\
  /// The data is stored in RGBA format, with 1 byte (8 bits) per channel
  pub data: &'a [u8],
  /// Current size of the texture atlas\
  /// Please note that this value might change
  pub size: UVec2,
  /// True if the atlas has been modified since the beginning of the current frame\
  /// If this function returns true, the texture atlas should be re-uploaded to the GPU before rendering\
  pub modified: bool,
}

/// Texture handle, stores the internal index of a texture within the texture atlas and can be cheaply copied.
///
/// Please note that dropping a handle does not deallocate the texture from the atlas, you must do it manually.
///
/// Only valid for the `UiInstance` that created it.\
/// Using it with other instances may result in panics or unexpected behavior.
///
/// Handle values are not guaranteed to be valid.\
/// Creating or transmuting an invalid handle is allowed and is *not* UB.
///
/// Internal value is an implementation detail and should not be relied upon.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
pub struct ImageHandle {
  pub(crate) index: u32,
}

#[derive(Clone, Copy, Debug)]
pub(crate) struct TextureAllocation {
  /// Position in the texture atlas\
  /// (This is an implementation detail and should not be exposed to the user)
  pub(crate) position: UVec2,

  /// Requested texture size
  pub size: UVec2,

  /// True if the texture was rotated by 90 degrees\
  /// (This is an implementation detail and should not be exposed to the user)
  pub(crate) rotated: bool,
}

/// Manages a texture atlas and the allocation of space within it\
/// The atlas is alllowed to grow and resize dynamically, as needed
pub(crate) struct TextureAtlasManager {
  packer: DensePacker,
  count: u32,
  size: UVec2,
  data: Vec<u8>,
  allocations: HashMap<u32, TextureAllocation, BuildNoHashHasher<u32>>,
  /// Items that have been removed from the allocation list, but still affect
  remove_queue: Vec<TextureAllocation>,
  /// True if the atlas has been modified in a way which requires a texture reupload
  /// since the beginning of the current frame
  modified: bool,

  /// If true, attempting to modify the atlas in a way which invalidates UVs will cause a panic\
  /// Used internally to ensure that the UVs do not become invalidated mid-render
  pub(crate) lock_atlas: bool,
}

impl TextureAtlasManager {
  /// Create a new texture atlas with the specified size\
  /// 512x512 is a good default size for most applications, and the texture atlas can grow dynamically as needed
  pub fn new(size: UVec2) -> Self {
    Self {
      packer: DensePacker::new(size.x as i32, size.y as i32),
      count: 0,
      size,
      data: vec![0; (size.x * size.y * RGBA_CHANNEL_COUNT) as usize],
      allocations: HashMap::default(),
      remove_queue: Vec::new(),
      modified: true,
      lock_atlas: false,
    }
  }

  /// Resize the texture atlas to the new size in-place, preserving the existing data
  pub fn resize(&mut self, new_size: UVec2) {
    if self.lock_atlas {
      panic!("Attempted to resize the texture atlas while the atlas is locked");
    }
    log::trace!("resizing texture atlas to {:?}", new_size);
    if self.size == new_size {
      log::warn!("Texture atlas is already the requested size");
      return
    }
    if new_size.x > self.size.x && new_size.y > self.size.y {
      self.packer.resize(new_size.x as i32, new_size.y as i32);
      //Resize the data array in-place
      self.data.resize((new_size.x * new_size.y * RGBA_CHANNEL_COUNT) as usize, 0);
      for y in (0..self.size.y).rev() {
        for x in (1..self.size.x).rev() {
          let idx = ((y * self.size.x + x) * RGBA_CHANNEL_COUNT) as usize;
          let new_idx = ((y * new_size.x + x) * RGBA_CHANNEL_COUNT) as usize;
          for c in 0..(RGBA_CHANNEL_COUNT as usize) {
            self.data[new_idx + c] = self.data[idx + c];
          }
        }
      }
    } else {
      //If scaling down, just recreate the atlas from scratch (since we need to re-pack everything anyway)
      todo!("Atlas downscaling is not implemented yet");
    }
    self.size = new_size;
    self.modified = true;
  }

  /// Ensure that a texture with specified size would fit without resizing on the next allocation attempt\
  pub fn ensure_fits(&mut self, size: UVec2) {
    // Plan A: try if any of the existing items in the remove queue would fit the texture
    // Plan B: purge the remove queue, recreate the packer and try again (might be expensive...!)
    // TODO: implement these
    // Plan C: resize the atlas
    let mut new_size = self.size;
    while !self.packer.can_pack(size.x as i32, size.y as i32, ALLOW_ROTATION) {
      new_size *= 2;
      self.packer.resize(new_size.x as i32, new_size.y as i32);
    }
    if new_size != self.size {
      self.resize(new_size);
    }
  }

  /// Allocate a new texture region in the atlas and return a handle to it\
  /// Returns None if the texture could not be allocated due to lack of space\
  /// Use `allocate` to allocate a texture and resize the atlas if necessary\
  /// Does not modify the texture data
  fn try_allocate(&mut self, size: UVec2) -> Option<ImageHandle> {
    log::trace!("Allocating texture of size {:?}", size);
    let result = self.packer.pack(size.x as i32, size.y as i32, ALLOW_ROTATION)?;
    let index = self.count;
    self.count += 1;
    let allocation = TextureAllocation {
      position: UVec2::new(result.x as u32, result.y as u32),
      size,
      //If the size does not match the requested size, the texture was rotated
      rotated: ALLOW_ROTATION && (result.width != size.x as i32),
    };
    self.allocations.insert_unique_unchecked(index, allocation);
    Some(ImageHandle { index })
  }

  /// Allocate a new texture region in the atlas and resize the atlas if necessary\
  /// This function should never fail under normal circumstances.\
  /// May modify the texture data if the atlas is resized
  pub fn allocate(&mut self, size: UVec2) -> ImageHandle {
    self.ensure_fits(size);
    self.try_allocate(size).unwrap()
  }

  /// Allocate a new texture region in the atlas and copy the data into it\
  /// This function may resize the atlas as needed, and should never fail under normal circumstances.
  pub(crate) fn add_rgba(&mut self, width: usize, data: &[u8]) -> ImageHandle {
    let size = uvec2(width as u32, (data.len() / (width * RGBA_CHANNEL_COUNT as usize)) as u32);
    let handle: ImageHandle = self.allocate(size);
    let allocation = self.allocations.get(&handle.index).unwrap();
    assert!(!allocation.rotated, "Rotated textures are not implemented yet");
    for y in 0..size.y {
      for x in 0..size.x {
        let src_idx = (y * size.x + x) * RGBA_CHANNEL_COUNT;
        let dst_idx = ((allocation.position.y + y) * self.size.x + allocation.position.x + x) * RGBA_CHANNEL_COUNT;
        for c in 0..RGBA_CHANNEL_COUNT as usize {
          self.data[dst_idx as usize + c] = data[src_idx as usize + c];
        }
      }
    }
    self.modified = true;
    handle
  }

  /// Works the same way as [`TextureAtlasManager::add`], but the input data is assumed to be grayscale (1 channel per pixel)\
  /// The data is copied into the alpha channel of the texture, while all the other channels are set to 255\
  /// May resize the atlas as needed, and should never fail under normal circumstances.
  pub(crate) fn add_grayscale(&mut self, width: usize, data: &[u8]) -> ImageHandle {
    let size = uvec2(width as u32, (data.len() / width) as u32);
    let handle = self.allocate(size);
    let allocation = self.allocations.get(&handle.index).unwrap();
    assert!(!allocation.rotated, "Rotated textures are not implemented yet");
    for y in 0..size.y {
      for x in 0..size.x {
        let src_idx = (y * size.x + x) as usize;
        let dst_idx = (((allocation.position.y + y) * self.size.x + allocation.position.x + x) * RGBA_CHANNEL_COUNT) as usize;
        self.data[dst_idx..(dst_idx + RGBA_CHANNEL_COUNT as usize)].copy_from_slice(&[255, 255, 255, data[src_idx]]);
      }
    }
    self.modified = true;
    handle
  }

  pub fn add(&mut self, width: usize, data: &[u8], format: TextureFormat) -> ImageHandle {
    match format {
      TextureFormat::Rgba => self.add_rgba(width, data),
      TextureFormat::Grayscale => self.add_grayscale(width, data),
    }
  }

  pub(crate) fn add_dummy(&mut self) {
    let handle = self.allocate((1, 1).into());
    assert!(handle.index == 0, "Dummy texture handle is not 0");
    assert!(self.get(handle).unwrap().position == (0, 0).into(), "Dummy texture position is not (0, 0)");
    self.data[0..4].copy_from_slice(&[255, 255, 255, 255]);
    self.modified = true;
  }

  pub fn modify(&mut self, handle: ImageHandle) {
    todo!()
  }

  pub fn remove(&mut self, handle: ImageHandle) {
    todo!()
  }

  pub fn get(&self, handle: ImageHandle) -> Option<&TextureAllocation> {
    self.allocations.get(&handle.index)
  }

  pub(crate) fn get_uv(&self, handle: ImageHandle) -> Option<Corners<Vec2>> {
    let info = self.get(handle)?;
    let atlas_size = self.meta().size.as_vec2();
    let p0x = info.position.x as f32 / atlas_size.x;
    let p1x = (info.position.x as f32 + info.size.x as f32) / atlas_size.x;
    let p0y = info.position.y as f32 / atlas_size.y;
    let p1y = (info.position.y as f32 + info.size.y as f32) / atlas_size.y;
    Some(Corners {
      top_left: vec2(p0x, p0y),
      top_right: vec2(p1x, p0y),
      bottom_left: vec2(p0x, p1y),
      bottom_right: vec2(p1x, p1y),
    })
  }

  /// Reset the `is_modified` flag
  pub(crate) fn reset_modified(&mut self) {
    self.modified = false;
  }

  pub fn meta(&self) -> TextureAtlasMeta {
    TextureAtlasMeta {
      data: &self.data,
      size: self.size,
      modified: self.modified,
    }
  }

  pub fn context(&self) -> ImageCtx {
    ImageCtx { atlas: self }
  }
}

impl Default for TextureAtlasManager {
  /// Create a new texture atlas with a default size of 512x512
  fn default() -> Self {
    Self::new(UVec2::new(512, 512))
  }
}

/// Context that allows read-only accss to image metadata
#[derive(Clone, Copy)]
pub struct ImageCtx<'a> {
  pub(crate) atlas: &'a TextureAtlasManager,
}

impl ImageCtx<'_> {
  /// Get size of the image with the specified handle
  ///
  /// Returns None if the handle is invalid for the current context
  pub fn get_size(&self, handle: ImageHandle) -> Option<UVec2> {
    self.atlas.get(handle).map(|a| a.size)
  }
}