//! Bitmasks are a set of bits that can be indexed and queried by the values of
//! various enum types.

use std::fmt;
use std::ops::{Deref, DerefMut};
use enum_iterator::{IterableEnum, EnumIterator};

/// A generic trait that can be used to index by a given type into a set of bits.
pub trait BitmaskTrait {
	/// The underlying array storage data type.
	type Array: Sized;
	/// The type that the bitmask can be indexed by.
	type Index: Copy + IterableEnum;
	/// Empty default data.
	const ZERO: Self::Array;

	/// The default empty state of the bitmask.
	fn array_default() -> Self::Array;
	/// A representation of the bitmask as a slice of bytes.
	fn array_slice(array: &Self::Array) -> &[u8];
	/// A mutable representation of the bitmask as a slice of bytes.
	fn array_slice_mut(array: &mut Self::Array) -> &mut [u8];
	/// Computes a normalized index value.
	fn index(index: Self::Index) -> usize;
	/// Validates whether a given index is valid and in range for the bitmask.
	///
	/// Dynamically allocated bitmasks may not be large enough to contain all indices.
	fn index_valid(_array: &Self::Array, _index: Self::Index) -> bool { true }
}

impl BitmaskTrait for Vec<u8> {
	type Array = Self;
	type Index = u16;
	const ZERO: Self = Vec::new();

	fn array_default() -> Self::Array { vec![0u8; 0x10] }
	fn array_slice(array: &Self::Array) -> &[u8] { array }
	fn array_slice_mut(array: &mut Self::Array) -> &mut [u8] { array }
	fn index(index: Self::Index) -> usize { index as _ }
	fn index_valid(array: &Self::Array, index: Self::Index) -> bool { array.len() > index as usize }
}

/// A dynamically allocated bitmask.
pub type BitmaskVec = Bitmask<Vec<u8>>;

impl Bitmask<Vec<u8>> {
	/// Reallocate the bitmask to fit all valid code values for the given event
	/// type.
	pub fn resize(&mut self, kind: ::EventKind) {
		self.data_mut().resize((kind.count_bits().unwrap_or(0x80) + 7) / 8, 0);
	}
}

#[derive(Copy, Clone)]
/// A set of bits that can be indexed by specific enum values.
pub struct Bitmask<T: BitmaskTrait> {
	mask: T::Array,
}

impl<T: BitmaskTrait> Default for Bitmask<T> {
	fn default() -> Self {
		Bitmask {
			mask: T::array_default(),
		}
	}
}

impl<T: BitmaskTrait> Bitmask<T> {
	/// A new empty bitmask.
	pub const EMPTY: Self = Self {
		mask: T::ZERO,
	};

	/// Extracts the underlying bitmask data.
	pub fn into_inner(self) -> T::Array {
		self.mask
	}

	/// Iterates over all set bits.
	pub fn iter(&self) -> BitmaskIterator<T> {
		self.into_iter()
	}

	/// Borrows the underlying bitmask data.
	pub fn data(&self) -> &T::Array {
		&self.mask
	}

	/// Mutably borrows the underlying bitmask data.
	pub fn data_mut(&mut self) -> &mut T::Array {
		&mut self.mask
	}

	fn slice(&self) -> &[u8] {
		T::array_slice(&self.mask)
	}

	fn slice_mut(&mut self) -> &mut [u8] {
		T::array_slice_mut(&mut self.mask)
	}

	fn index(index: T::Index) -> (usize, usize) {
		let index = T::index(index);
		(index / 8, index % 8)
	}

	fn index_valid(&self, index: T::Index) -> bool {
		T::index_valid(&self.mask, index)
	}

	/// Unset all bits in the bitmask.
	pub fn clear(&mut self) {
		for b in self.slice_mut() {
			*b = 0;
		}
	}

	/// Gets the status of an index in the bitmask.
	pub fn get(&self, index: T::Index) -> bool {
		let (offset, shift) = Self::index(index);
		(self.slice()[offset] & (1u8 << shift)) != 0
	}

	/// Sets the status of an index in the bitmask.
	pub fn insert(&mut self, index: T::Index) {
		let (offset, shift) = Self::index(index);
		let v = &mut self.slice_mut()[offset];
		*v |= 1u8 << shift;
	}

	/// Clears the status of an index in the bitmask.
	pub fn remove(&mut self, index: T::Index) {
		let (offset, shift) = Self::index(index);
		let v = &mut self.slice_mut()[offset];
		*v &= !(1u8 << shift);
	}

	/// Inverts the status of an index in the bitmask.
	pub fn toggle(&mut self, index: T::Index) {
		let (offset, shift) = Self::index(index);
		self.slice_mut()[offset] ^= 1u8 << shift;
	}

	/// Merges the provided indices into the bitmask.
	pub fn or<I: IntoIterator<Item=T::Index>>(&mut self, indices: I) {
		for index in indices {
			self.insert(index);
		}
	}
}

impl<'a, T: BitmaskTrait> IntoIterator for &'a Bitmask<T> {
	type Item = T::Index;
	type IntoIter = BitmaskIterator<'a, T>;

	fn into_iter(self) -> Self::IntoIter {
		BitmaskIterator {
			mask: self,
			iter: T::Index::iter(),
		}
	}
}

/// An iterator over the set bits of a bitmask.
pub struct BitmaskIterator<'a, T: BitmaskTrait + 'a> {
	mask: &'a Bitmask<T>,
	iter: EnumIterator<T::Index>,
}

impl<'a, T: BitmaskTrait> Iterator for BitmaskIterator<'a, T> {
	type Item = T::Index;

	fn next(&mut self) -> Option<Self::Item> {
		let mask = &self.mask;
		self.iter.by_ref().take_while(|i| mask.index_valid(*i)).filter(|i| mask.get(*i)).next()
	}
}

impl<T: BitmaskTrait> fmt::Debug for Bitmask<T> where T::Index: fmt::Debug {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
		f.debug_list().entries(self.into_iter()).finish()
	}
}

impl<T: BitmaskTrait> Deref for Bitmask<T> {
	type Target = [u8];

	fn deref(&self) -> &Self::Target {
		self.slice()
	}
}

impl<T: BitmaskTrait> DerefMut for Bitmask<T> {
	fn deref_mut(&mut self) -> &mut Self::Target {
		self.slice_mut()
	}
}

impl<T: BitmaskTrait> AsRef<[u8]> for Bitmask<T> {
	fn as_ref(&self) -> &[u8] {
		self.slice()
	}
}

impl<T: BitmaskTrait> AsMut<[u8]> for Bitmask<T> {
	fn as_mut(&mut self) -> &mut [u8] {
		self.slice_mut()
	}
}