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    use crate::rust::{
    	cmp::min,
    	iter::{FromIterator, IntoIterator},
    	mem,
    	slice,
    	vec::{self, Vec},
    };

    use crate::io;

    use super::{Deserialize, Error, Serialize, VarUint32};

    /// A map from non-contiguous `u32` keys to values of type `T`, which is
    /// serialized and deserialized ascending order of the keys. Normally used for
    /// relative dense maps with occasional "holes", and stored as an array.
    ///
    /// **SECURITY WARNING:** This code is currently subject to a denial of service
    /// attack if you create a map containing the key `u32::MAX`, which should never
    /// happen in normal data. It would be pretty easy to provide a safe
    /// deserializing mechanism which addressed this problem.
    #[derive(Debug, Default)]
    pub struct IndexMap<T> {
    	/// The number of non-`None` entries in this map.
    	len: usize,

    	/// A vector of entries. Missing entries are represented as `None`.
    	entries: Vec<Option<T>>,
    }

    impl<T> IndexMap<T> {
    	/// Create an empty `IndexMap`, preallocating enough space to store
    	/// `capacity` entries without needing to reallocate the underlying memory.
    	pub fn with_capacity(capacity: usize) -> IndexMap<T> {
    		IndexMap {
    			len: 0,
    			entries: Vec::with_capacity(capacity),
    		}
    	}

    	/// Clear the map.
    	pub fn clear(&mut self) {
    		self.entries.clear();
    		self.len = 0;
    	}

    	/// Return the name for the specified index, if it exists.
    	pub fn get(&self, idx: u32) -> Option<&T> {
    		match self.entries.get(idx as usize) {
    			Some(&Some(ref value)) => Some(value),
    			Some(&None) | None => None,
    		}
    	}

    	/// Does the map contain an entry for the specified index?
    	pub fn contains_key(&self, idx: u32) -> bool {
    		match self.entries.get(idx as usize) {
    			Some(&Some(_)) => true,
    			Some(&None) | None => false,
    		}
    	}

    	/// Insert a name into our map, returning the existing value if present.
    	///
    	/// Note: This API is designed for reasonably dense indices based on valid
    	/// data. Inserting a huge `idx` will use up a lot of RAM, and this function
    	/// will not try to protect you against that.
    	pub fn insert(&mut self, idx: u32, value: T) -> Option<T> {
    		let idx = idx as usize;
    		let result = if idx >= self.entries.len() {
    			// We need to grow the array, and add the new element at the end.
    			for _ in 0..(idx - self.entries.len()) {
    				// We can't use `extend(repeat(None)).take(n)`, because that
    				// would require `T` to implement `Clone`.
    				self.entries.push(None);
    			}
    			self.entries.push(Some(value));
    			debug_assert_eq!(idx + 1, self.entries.len());
    			self.len += 1;
    			None
    		} else {
    			// We're either replacing an existing element, or filling in a
    			// missing one.
    			let existing = self.entries[idx].take();
    			if existing.is_none() {
    				self.len += 1;
    			}
    			self.entries[idx] = Some(value);
    			existing
    		};
    		if mem::size_of::<usize>() > 4 {
    			debug_assert!(self.entries.len() <= (u32::max_value() as usize) + 1);
    		}
    		#[cfg(slow_assertions)]
    		debug_assert_eq!(self.len, self.slow_len());
    		result
    	}

    	/// Remove an item if present and return it.
    	pub fn remove(&mut self, idx: u32) -> Option<T> {
    		let result = match self.entries.get_mut(idx as usize) {
    			Some(value @ &mut Some(_)) => {
    				self.len -= 1;
    				value.take()
    			}
    			Some(&mut None) | None => None,
    		};
    		#[cfg(slow_assertions)]
    		debug_assert_eq!(self.len, self.slow_len());
    		result
    	}

    	/// The number of items in this map.
    	pub fn len(&self) -> usize {
    		#[cfg(slow_assertions)]
    		debug_assert_eq!(self.len, self.slow_len());
    		self.len
    	}

    	/// Is this map empty?
    	pub fn is_empty(&self) -> bool {
    		self.len == 0
    	}

    	/// This function is only compiled when `--cfg slow_assertions` is enabled.
    	/// It computes the `len` value using a slow algorithm.
    	///
    	/// WARNING: This turns a bunch of O(n) operations into O(n^2) operations.
    	/// We may want to remove it once the code is tested, or to put it behind
    	/// a feature flag named `slow_debug_checks`, or something like that.
    	#[cfg(slow_assertions)]
    	fn slow_len(&self) -> usize {
    		self.entries.iter().filter(|entry| entry.is_some()).count()
    	}

    	/// Create a non-consuming iterator over this `IndexMap`'s keys and values.
    	pub fn iter(&self) -> Iter<T> {
    		// Note that this does the right thing because we use `&self`.
    		self.into_iter()
    	}

    	/// Custom deserialization routine.
    	///
    	/// We will allocate an underlying array no larger than `max_entry_space` to
    	/// hold the data, so the maximum index must be less than `max_entry_space`.
    	/// This prevents mallicious *.wasm files from having a single entry with
    	/// the index `u32::MAX`, which would consume far too much memory.
    	///
    	/// The `deserialize_value` function will be passed the index of the value
    	/// being deserialized, and must deserialize the value.
    	pub fn deserialize_with<R, F>(
    		max_entry_space: usize,
    		deserialize_value: &F,
    		rdr: &mut R,
    	) -> Result<IndexMap<T>, Error>
    	where
    		R: io::Read,
    		F: Fn(u32, &mut R) -> Result<T, Error>,
    	{
    		let len: u32 = VarUint32::deserialize(rdr)?.into();
    		let mut map = IndexMap::with_capacity(len as usize);
    		let mut prev_idx = None;
    		for _ in 0..len {
    			let idx: u32 = VarUint32::deserialize(rdr)?.into();
    			if idx as usize >= max_entry_space {
    				return Err(Error::Other("index is larger than expected"));
    			}
    			match prev_idx {
    				Some(prev) if prev >= idx => {
    					// Supposedly these names must be "sorted by index", so
    					// let's try enforcing that and seeing what happens.
    					return Err(Error::Other("indices are out of order"));
    				}
    				_ => {
    					prev_idx = Some(idx);
    				}
    			}
    			let val = deserialize_value(idx, rdr)?;
    			map.insert(idx, val);
    		}
    		Ok(map)
    	}

    }

    impl<T: Clone> Clone for IndexMap<T> {
    	fn clone(&self) -> IndexMap<T> {
    		IndexMap {
    			len: self.len,
    			entries: self.entries.clone(),
    		}
    	}
    }

    impl<T: PartialEq> PartialEq<IndexMap<T>> for IndexMap<T> {
    	fn eq(&self, other: &IndexMap<T>) -> bool {
    		if self.len() != other.len() {
    			// If the number of non-`None` entries is different, we can't match.
    			false
    		} else {
    			// This is tricky, because one `Vec` might have a bunch of empty
    			// entries at the end which we want to ignore.
    			let smallest_len = min(self.entries.len(), other.entries.len());
    			self.entries[0..smallest_len].eq(&other.entries[0..smallest_len])
    		}
    	}
    }

    impl<T: Eq> Eq for IndexMap<T> {}

    impl<T> FromIterator<(u32, T)> for IndexMap<T> {
    	/// Create an `IndexMap` from an iterator.
    	///
    	/// Note: This API is designed for reasonably dense indices based on valid
    	/// data. Inserting a huge `idx` will use up a lot of RAM, and this function
    	/// will not try to protect you against that.
    	fn from_iter<I>(iter: I) -> Self
    	where
    		I: IntoIterator<Item = (u32, T)>,
    	{
    		let iter = iter.into_iter();
    		let (lower, upper_opt) = iter.size_hint();
    		let mut map = IndexMap::with_capacity(upper_opt.unwrap_or(lower));
    		for (idx, value) in iter {
    			map.insert(idx, value);
    		}
    		map
    	}
    }

    /// An iterator over an `IndexMap` which takes ownership of it.
    pub struct IntoIter<T> {
    	next_idx: u32,
    	remaining_len: usize,
    	iter: vec::IntoIter<Option<T>>,
    }

    impl<T> Iterator for IntoIter<T> {
    	type Item = (u32, T);

    	fn size_hint(&self) -> (usize, Option<usize>) {
    		(self.remaining_len, Some(self.remaining_len))
    	}

    	fn next(&mut self) -> Option<Self::Item> {
    		// Bail early if we know there are no more items. This also keeps us
    		// from repeatedly calling `self.iter.next()` once it has been
    		// exhausted, which is not guaranteed to keep returning `None`.
    		if self.remaining_len == 0 {
    			return None;
    		}
    		while let Some(value_opt) = self.iter.next() {
    			let idx = self.next_idx;
    			self.next_idx += 1;
    			if let Some(value) = value_opt {
    				self.remaining_len -= 1;
    				return Some((idx, value));
    			}
    		}
    		debug_assert_eq!(self.remaining_len, 0);
    		None
    	}
    }

    impl<T> IntoIterator for IndexMap<T> {
    	type Item = (u32, T);
    	type IntoIter = IntoIter<T>;

    	fn into_iter(self) -> IntoIter<T> {
    		IntoIter {
    			next_idx: 0,
    			remaining_len: self.len,
    			iter: self.entries.into_iter(),
    		}
    	}
    }

    /// An iterator over a borrowed `IndexMap`.
    pub struct Iter<'a, T: 'static> {
    	next_idx: u32,
    	remaining_len: usize,
    	iter: slice::Iter<'a, Option<T>>,
    }

    impl<'a, T: 'static> Iterator for Iter<'a, T> {
    	type Item = (u32, &'a T);

    	fn size_hint(&self) -> (usize, Option<usize>) {
    		(self.remaining_len, Some(self.remaining_len))
    	}

    	fn next(&mut self) -> Option<Self::Item> {
    		// Bail early if we know there are no more items. This also keeps us
    		// from repeatedly calling `self.iter.next()` once it has been
    		// exhausted, which is not guaranteed to keep returning `None`.
    		if self.remaining_len == 0 {
    			return None;
    		}
    		while let Some(value_opt) = self.iter.next() {
    			let idx = self.next_idx;
    			self.next_idx += 1;
    			if let &Some(ref value) = value_opt {
    				self.remaining_len -= 1;
    				return Some((idx, value));
    			}
    		}
    		debug_assert_eq!(self.remaining_len, 0);
    		None
    	}
    }

    impl<'a, T: 'static> IntoIterator for &'a IndexMap<T> {
    	type Item = (u32, &'a T);
    	type IntoIter = Iter<'a, T>;

    	fn into_iter(self) -> Iter<'a, T> {
    		Iter {
    			next_idx: 0,
    			remaining_len: self.len,
    			iter: self.entries.iter(),
    		}
    	}
    }

    impl<T> Serialize for IndexMap<T>
    where
    	T: Serialize,
    	Error: From<<T as Serialize>::Error>,
    {
    	type Error = Error;

    	fn serialize<W: io::Write>(self, wtr: &mut W) -> Result<(), Self::Error> {
    		VarUint32::from(self.len()).serialize(wtr)?;
    		for (idx, value) in self {
    			VarUint32::from(idx).serialize(wtr)?;
    			value.serialize(wtr)?;
    		}
    		Ok(())
    	}
    }

    impl<T: Deserialize> IndexMap<T>
    where
    	T: Deserialize,
    	Error: From<<T as Deserialize>::Error>,
    {
    	/// Deserialize a map containing simple values that support `Deserialize`.
    	/// We will allocate an underlying array no larger than `max_entry_space` to
    	/// hold the data, so the maximum index must be less than `max_entry_space`.
    	pub fn deserialize<R: io::Read>(
    		max_entry_space: usize,
    		rdr: &mut R,
    	) -> Result<Self, Error> {
    		let deserialize_value: fn(u32, &mut R) -> Result<T, Error> = |_idx, rdr| {
    			T::deserialize(rdr).map_err(Error::from)
    		};
    		Self::deserialize_with(max_entry_space, &deserialize_value, rdr)
    	}
    }

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

    	#[test]
    	fn default_is_empty_no_matter_how_we_look_at_it() {
    		let map = IndexMap::<String>::default();
    		assert_eq!(map.len(), 0);
    		assert!(map.is_empty());
    		assert_eq!(map.iter().collect::<Vec<_>>().len(), 0);
    		assert_eq!(map.into_iter().collect::<Vec<_>>().len(), 0);
    	}

    	#[test]
    	fn with_capacity_creates_empty_map() {
    		let map = IndexMap::<String>::with_capacity(10);
    		assert!(map.is_empty());
    	}

    	#[test]
    	fn clear_removes_all_values() {
    		let mut map = IndexMap::<String>::default();
    		map.insert(0, "sample value".to_string());
    		assert_eq!(map.len(), 1);
    		map.clear();
    		assert_eq!(map.len(), 0);
    	}

    	#[test]
    	fn get_returns_elements_that_are_there_but_nothing_else() {
    		let mut map = IndexMap::<String>::default();
    		map.insert(1, "sample value".to_string());
    		assert_eq!(map.len(), 1);
    		assert_eq!(map.get(0), None);
    		assert_eq!(map.get(1), Some(&"sample value".to_string()));
    		assert_eq!(map.get(2), None);
    	}

    	#[test]
    	fn contains_key_returns_true_when_a_key_is_present() {
    		let mut map = IndexMap::<String>::default();
    		map.insert(1, "sample value".to_string());
    		assert!(!map.contains_key(0));
    		assert!(map.contains_key(1));
    		assert!(!map.contains_key(2));
    	}

    	#[test]
    	fn insert_behaves_like_other_maps() {
    		let mut map = IndexMap::<String>::default();

    		// Insert a key which requires extending our storage.
    		assert_eq!(map.insert(1, "val 1".to_string()), None);
    		assert_eq!(map.len(), 1);
    		assert!(map.contains_key(1));

    		// Insert a key which requires filling in a hole.
    		assert_eq!(map.insert(0, "val 0".to_string()), None);
    		assert_eq!(map.len(), 2);
    		assert!(map.contains_key(0));

    		// Insert a key which replaces an existing key.
    		assert_eq!(
    			map.insert(1, "val 1.1".to_string()),
    			Some("val 1".to_string())
    		);
    		assert_eq!(map.len(), 2);
    		assert!(map.contains_key(1));
    		assert_eq!(map.get(1), Some(&"val 1.1".to_string()));
    	}

    	#[test]
    	fn remove_behaves_like_other_maps() {
    		let mut map = IndexMap::<String>::default();
    		assert_eq!(map.insert(1, "val 1".to_string()), None);

    		// Remove an out-of-bounds element.
    		assert_eq!(map.remove(2), None);
    		assert_eq!(map.len(), 1);

    		// Remove an in-bounds but missing element.
    		assert_eq!(map.remove(0), None);
    		assert_eq!(map.len(), 1);

    		// Remove an existing element.
    		assert_eq!(map.remove(1), Some("val 1".to_string()));
    		assert_eq!(map.len(), 0);
    	}

    	#[test]
    	fn partial_eq_works_as_expected_in_simple_cases() {
    		let mut map1 = IndexMap::<String>::default();
    		let mut map2 = IndexMap::<String>::default();
    		assert_eq!(map1, map2);

    		map1.insert(1, "a".to_string());
    		map2.insert(1, "a".to_string());
    		assert_eq!(map1, map2);

    		map1.insert(0, "b".to_string());
    		assert_ne!(map1, map2);
    		map1.remove(0);
    		assert_eq!(map1, map2);

    		map1.insert(1, "not a".to_string());
    		assert_ne!(map1, map2);
    	}

    	#[test]
    	fn partial_eq_is_smart_about_none_values_at_the_end() {
    		let mut map1 = IndexMap::<String>::default();
    		let mut map2 = IndexMap::<String>::default();

    		map1.insert(1, "a".to_string());
    		map2.insert(1, "a".to_string());

    		// Both maps have the same (idx, value) pairs, but map2 has extra space.
    		map2.insert(10, "b".to_string());
    		map2.remove(10);
    		assert_eq!(map1, map2);

    		// Both maps have the same (idx, value) pairs, but map1 has extra space.
    		map1.insert(100, "b".to_string());
    		map1.remove(100);
    		assert_eq!(map1, map2);

    		// Let's be paranoid.
    		map2.insert(1, "b".to_string());
    		assert_ne!(map1, map2);
    	}

    	#[test]
    	fn from_iterator_builds_a_map() {
    		let data = &[
    			// We support out-of-order values here!
    			(3, "val 3"),
    			(2, "val 2"),
    			(5, "val 5"),
    		];
    		let iter = data.iter().map(|&(idx, val)| (idx, val.to_string()));
    		let map = IndexMap::from_iter(iter);
    		assert_eq!(map.len(), 3);
    		assert_eq!(map.get(2), Some(&"val 2".to_string()));
    		assert_eq!(map.get(3), Some(&"val 3".to_string()));
    		assert_eq!(map.get(5), Some(&"val 5".to_string()));
    	}

    	#[test]
    	fn iterators_are_well_behaved() {
    		// Create a map with reasonably complex internal structure, making
    		// sure that we have both internal missing elements, and a bunch of
    		// missing elements at the end.
    		let data = &[(3, "val 3"), (2, "val 2"), (5, "val 5")];
    		let src_iter = data.iter().map(|&(idx, val)| (idx, val.to_string()));
    		let mut map = IndexMap::from_iter(src_iter);
    		map.remove(5);

    		// Make sure `size_hint` and `next` behave as we expect at each step.
    		{
    			let mut iter1 = map.iter();
    			assert_eq!(iter1.size_hint(), (2, Some(2)));
    			assert_eq!(iter1.next(), Some((2, &"val 2".to_string())));
    			assert_eq!(iter1.size_hint(), (1, Some(1)));
    			assert_eq!(iter1.next(), Some((3, &"val 3".to_string())));
    			assert_eq!(iter1.size_hint(), (0, Some(0)));
    			assert_eq!(iter1.next(), None);
    			assert_eq!(iter1.size_hint(), (0, Some(0)));
    			assert_eq!(iter1.next(), None);
    			assert_eq!(iter1.size_hint(), (0, Some(0)));
    		}

    		// Now do the same for a consuming iterator.
    		let mut iter2 = map.into_iter();
    		assert_eq!(iter2.size_hint(), (2, Some(2)));
    		assert_eq!(iter2.next(), Some((2, "val 2".to_string())));
    		assert_eq!(iter2.size_hint(), (1, Some(1)));
    		assert_eq!(iter2.next(), Some((3, "val 3".to_string())));
    		assert_eq!(iter2.size_hint(), (0, Some(0)));
    		assert_eq!(iter2.next(), None);
    		assert_eq!(iter2.size_hint(), (0, Some(0)));
    		assert_eq!(iter2.next(), None);
    		assert_eq!(iter2.size_hint(), (0, Some(0)));
    	}

    	#[test]
    	fn serialize_and_deserialize() {
    		let mut map = IndexMap::<String>::default();
    		map.insert(1, "val 1".to_string());

    		let mut output = vec![];
    		map.clone()
    			.serialize(&mut output)
    			.expect("serialize failed");

    		let mut input = io::Cursor::new(&output);
    		let deserialized = IndexMap::deserialize(2, &mut input).expect("deserialize failed");

    		assert_eq!(deserialized, map);
    	}

    	#[test]
    	fn deserialize_requires_elements_to_be_in_order() {
    		// Build a in-order example by hand.
    		let mut valid = vec![];
    		VarUint32::from(2u32).serialize(&mut valid).unwrap();
    		VarUint32::from(0u32).serialize(&mut valid).unwrap();
    		"val 0".to_string().serialize(&mut valid).unwrap();
    		VarUint32::from(1u32).serialize(&mut valid).unwrap();
    		"val 1".to_string().serialize(&mut valid).unwrap();
    		let map = IndexMap::<String>::deserialize(2, &mut io::Cursor::new(valid))
    			.expect("unexpected error deserializing");
    		assert_eq!(map.len(), 2);

    		// Build an out-of-order example by hand.
    		let mut invalid = vec![];
    		VarUint32::from(2u32).serialize(&mut invalid).unwrap();
    		VarUint32::from(1u32).serialize(&mut invalid).unwrap();
    		"val 1".to_string().serialize(&mut invalid).unwrap();
    		VarUint32::from(0u32).serialize(&mut invalid).unwrap();
    		"val 0".to_string().serialize(&mut invalid).unwrap();
    		let res = IndexMap::<String>::deserialize(2, &mut io::Cursor::new(invalid));
    		assert!(res.is_err());
    	}

    	#[test]
    	fn deserialize_enforces_max_idx() {
    		// Build an example with an out-of-bounds index by hand.
    		let mut invalid = vec![];
    		VarUint32::from(1u32).serialize(&mut invalid).unwrap();
    		VarUint32::from(5u32).serialize(&mut invalid).unwrap();
    		"val 5".to_string().serialize(&mut invalid).unwrap();
    		let res = IndexMap::<String>::deserialize(1, &mut io::Cursor::new(invalid));
    		assert!(res.is_err());
    	}
    }

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