// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

use crate::bit_iterator::{BitIndexIterator, BitIterator, BitSliceIterator};
use crate::buffer::BooleanBuffer;
use crate::{Buffer, MutableBuffer};

/// A [`BooleanBuffer`] used to encode validity (null values) for Arrow arrays
///
/// In the [Arrow specification], array validity is encoded in a packed bitmask with a
/// `true` value indicating the corresponding slot is not null, and `false` indicating
/// that it is null.
///
/// # See also
/// * [`NullBufferBuilder`] for creating `NullBuffer`s
///
/// [Arrow specification]: https://arrow.apache.org/docs/format/Columnar.html#validity-bitmaps
/// [`NullBufferBuilder`]: crate::NullBufferBuilder
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct NullBuffer {
    buffer: BooleanBuffer,
    null_count: usize,
}

impl NullBuffer {
    /// Create a new [`NullBuffer`] computing the null count
    pub fn new(buffer: BooleanBuffer) -> Self {
        let null_count = buffer.len() - buffer.count_set_bits();
        Self { buffer, null_count }
    }

    /// Create a new [`NullBuffer`] of length `len` where all values are null
    pub fn new_null(len: usize) -> Self {
        Self {
            buffer: BooleanBuffer::new_unset(len),
            null_count: len,
        }
    }

    /// Create a new [`NullBuffer`] of length `len` where all values are valid
    ///
    /// Note: it is more efficient to not set the null buffer if it is known to
    /// be all valid (aka all values are not null)
    pub fn new_valid(len: usize) -> Self {
        Self {
            buffer: BooleanBuffer::new_set(len),
            null_count: 0,
        }
    }

    /// Create a new [`NullBuffer`] with the provided `buffer` and `null_count`
    ///
    /// # Safety
    ///
    /// `buffer` must contain `null_count` `0` bits
    pub unsafe fn new_unchecked(buffer: BooleanBuffer, null_count: usize) -> Self {
        Self { buffer, null_count }
    }

    /// Computes the union of the nulls in two optional [`NullBuffer`]
    ///
    /// This is commonly used by binary operations where the result is NULL if either
    /// of the input values is NULL. Handling the null mask separately in this way
    /// can yield significant performance improvements over an iterator approach
    pub fn union(lhs: Option<&NullBuffer>, rhs: Option<&NullBuffer>) -> Option<NullBuffer> {
        match (lhs, rhs) {
            (Some(lhs), Some(rhs)) => Some(Self::new(lhs.inner() & rhs.inner())),
            (Some(n), None) | (None, Some(n)) => Some(n.clone()),
            (None, None) => None,
        }
    }

    /// Computes the union of the nulls in multiple optional [`NullBuffer`]s
    ///
    /// See [`union`](Self::union)
    pub fn union_many<'a>(
        nulls: impl IntoIterator<Item = Option<&'a NullBuffer>>,
    ) -> Option<NullBuffer> {
        // Unwrap to BooleanBuffer because BitAndAssign is not implemented for NullBuffer
        let mut buffers = nulls.into_iter().filter_map(|nb| nb.map(NullBuffer::inner));
        let first = buffers.next()?;
        let mut result = first.clone();
        for buf in buffers {
            result &= buf;
        }
        Some(Self::new(result))
    }

    /// Returns true if all nulls in `other` also exist in self
    pub fn contains(&self, other: &NullBuffer) -> bool {
        if other.null_count == 0 {
            return true;
        }
        let lhs = self.inner().bit_chunks().iter_padded();
        let rhs = other.inner().bit_chunks().iter_padded();
        lhs.zip(rhs).all(|(l, r)| (l & !r) == 0)
    }

    /// Returns a new [`NullBuffer`] where each bit in the current null buffer
    /// is repeated `count` times. This is useful for masking the nulls of
    /// the child of a FixedSizeListArray based on its parent
    pub fn expand(&self, count: usize) -> Self {
        let capacity = self.buffer.len().checked_mul(count).unwrap();
        let mut buffer = MutableBuffer::new_null(capacity);

        // Expand each bit within `null_mask` into `element_len`
        // bits, constructing the implicit mask of the child elements
        for i in 0..self.buffer.len() {
            if self.is_null(i) {
                continue;
            }
            for j in 0..count {
                crate::bit_util::set_bit(buffer.as_mut(), i * count + j)
            }
        }
        Self {
            buffer: BooleanBuffer::new(buffer.into(), 0, capacity),
            null_count: self.null_count * count,
        }
    }

    /// Returns the length of this [`NullBuffer`] in bits
    #[inline]
    pub fn len(&self) -> usize {
        self.buffer.len()
    }

    /// Returns the offset of this [`NullBuffer`] in bits
    #[inline]
    pub fn offset(&self) -> usize {
        self.buffer.offset()
    }

    /// Returns true if this [`NullBuffer`] is empty
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.buffer.is_empty()
    }

    /// Free up unused memory.
    pub fn shrink_to_fit(&mut self) {
        self.buffer.shrink_to_fit();
    }

    /// Returns the null count for this [`NullBuffer`]
    #[inline]
    pub fn null_count(&self) -> usize {
        self.null_count
    }

    /// Returns `true` if the value at `idx` is not null
    #[inline]
    pub fn is_valid(&self, idx: usize) -> bool {
        self.buffer.value(idx)
    }

    /// Returns `true` if the value at `idx` is null
    #[inline]
    pub fn is_null(&self, idx: usize) -> bool {
        !self.is_valid(idx)
    }

    /// Returns the packed validity of this [`NullBuffer`] not including any offset
    #[inline]
    pub fn validity(&self) -> &[u8] {
        self.buffer.values()
    }

    /// Slices this [`NullBuffer`] by the provided `offset` and `length`
    pub fn slice(&self, offset: usize, len: usize) -> Self {
        Self::new(self.buffer.slice(offset, len))
    }

    /// Returns an iterator over the bits in this [`NullBuffer`]
    ///
    /// * `true` indicates that the corresponding value is not NULL
    /// * `false` indicates that the corresponding value is NULL
    ///
    /// Note: [`Self::valid_indices`] will be significantly faster for most use-cases
    pub fn iter(&self) -> BitIterator<'_> {
        self.buffer.iter()
    }

    /// Returns a [`BitIndexIterator`] over the valid indices in this [`NullBuffer`]
    ///
    /// Valid indices indicate the corresponding value is not NULL
    pub fn valid_indices(&self) -> BitIndexIterator<'_> {
        self.buffer.set_indices()
    }

    /// Returns a [`BitSliceIterator`] yielding contiguous ranges of valid indices
    ///
    /// Valid indices indicate the corresponding value is not NULL
    pub fn valid_slices(&self) -> BitSliceIterator<'_> {
        self.buffer.set_slices()
    }

    /// Calls the provided closure for each index in this null mask that is set
    #[inline]
    pub fn try_for_each_valid_idx<E, F: FnMut(usize) -> Result<(), E>>(
        &self,
        f: F,
    ) -> Result<(), E> {
        if self.null_count == self.len() {
            return Ok(());
        }
        self.valid_indices().try_for_each(f)
    }

    /// Returns the inner [`BooleanBuffer`]
    #[inline]
    pub fn inner(&self) -> &BooleanBuffer {
        &self.buffer
    }

    /// Returns the inner [`BooleanBuffer`]
    #[inline]
    pub fn into_inner(self) -> BooleanBuffer {
        self.buffer
    }

    /// Returns the underlying [`Buffer`]
    #[inline]
    pub fn buffer(&self) -> &Buffer {
        self.buffer.inner()
    }

    /// Create a [`NullBuffer`] from an *unsliced* validity bitmap (`offset = 0` **bits**) of length `len`.
    ///
    /// Returns `None` if there are no nulls (all values valid).
    pub fn from_unsliced_buffer(buffer: impl Into<Buffer>, len: usize) -> Option<Self> {
        let bb = BooleanBuffer::new(buffer.into(), 0, len);
        let nb = NullBuffer::new(bb);
        (nb.null_count() > 0).then_some(nb)
    }

    /// Claim memory used by this null buffer in the provided memory pool.
    #[cfg(feature = "pool")]
    pub fn claim(&self, pool: &dyn crate::MemoryPool) {
        // NullBuffer wraps a BooleanBuffer which wraps a Buffer
        self.buffer.inner().claim(pool);
    }
}

impl<'a> IntoIterator for &'a NullBuffer {
    type Item = bool;
    type IntoIter = BitIterator<'a>;

    fn into_iter(self) -> Self::IntoIter {
        self.buffer.iter()
    }
}

impl From<BooleanBuffer> for NullBuffer {
    fn from(value: BooleanBuffer) -> Self {
        Self::new(value)
    }
}

impl From<&[bool]> for NullBuffer {
    fn from(value: &[bool]) -> Self {
        BooleanBuffer::from(value).into()
    }
}

impl<const N: usize> From<&[bool; N]> for NullBuffer {
    fn from(value: &[bool; N]) -> Self {
        value[..].into()
    }
}

impl From<Vec<bool>> for NullBuffer {
    fn from(value: Vec<bool>) -> Self {
        BooleanBuffer::from(value).into()
    }
}

impl FromIterator<bool> for NullBuffer {
    fn from_iter<T: IntoIterator<Item = bool>>(iter: T) -> Self {
        BooleanBuffer::from_iter(iter).into()
    }
}

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

    #[test]
    fn test_size() {
        // This tests that the niche optimisation eliminates the overhead of an option
        assert_eq!(
            std::mem::size_of::<NullBuffer>(),
            std::mem::size_of::<Option<NullBuffer>>()
        );
    }

    #[test]
    fn test_from_unsliced_buffer_with_nulls() {
        // 0b10110010 → null(0), valid(1), null(2), null(3), valid(4), valid(5), null(6), valid(7)
        let buf = Buffer::from([0b10110010u8]);
        let result = NullBuffer::from_unsliced_buffer(buf, 8);
        assert!(result.is_some());
        let nb = result.unwrap();
        assert_eq!(nb.len(), 8);
        assert_eq!(nb.null_count(), 4);
        assert!(nb.is_null(0));
        assert!(nb.is_valid(1));
        assert!(nb.is_null(2));
        assert!(nb.is_null(3));
        assert!(nb.is_valid(4));
        assert!(nb.is_valid(5));
        assert!(nb.is_null(6));
        assert!(nb.is_valid(7));
    }

    #[test]
    fn test_from_unsliced_buffer_all_valid() {
        // All bits set = all valid, no nulls
        let buf = Buffer::from([0b11111111u8]);
        let result = NullBuffer::from_unsliced_buffer(buf, 8);
        assert!(result.is_none());
    }

    #[test]
    fn test_from_unsliced_buffer_all_null() {
        // No bits set = all null
        let buf = Buffer::from([0b00000000u8]);
        let result = NullBuffer::from_unsliced_buffer(buf, 8);
        assert!(result.is_some());
        let nb = result.unwrap();
        assert_eq!(nb.len(), 8);
        assert_eq!(nb.null_count(), 8);
    }

    #[test]
    fn test_from_unsliced_buffer_empty() {
        let buf = Buffer::from([]);
        let result = NullBuffer::from_unsliced_buffer(buf, 0);
        assert!(result.is_none());
    }

    #[test]
    fn test_union_many_all_none() {
        let result = NullBuffer::union_many([None, None, None]);
        assert!(result.is_none());
    }

    #[test]
    fn test_union_many_single_some() {
        let a = NullBuffer::from(&[true, false, true, true]);
        let result = NullBuffer::union_many([Some(&a)]);
        assert_eq!(result, Some(a));
    }

    #[test]
    fn test_union_many_two_inputs() {
        let a = NullBuffer::from(&[true, false, true, true]);
        let b = NullBuffer::from(&[true, true, false, true]);
        let result = NullBuffer::union_many([Some(&a), Some(&b)]);
        let expected = NullBuffer::union(Some(&a), Some(&b));
        assert_eq!(result, expected);
    }

    #[test]
    fn test_union_many_three_inputs() {
        let a = NullBuffer::from(&[true, false, true, true]);
        let b = NullBuffer::from(&[true, true, false, true]);
        let c = NullBuffer::from(&[false, true, true, true]);
        let result = NullBuffer::union_many([Some(&a), Some(&b), Some(&c)]);
        let expected = NullBuffer::from(&[false, false, false, true]);
        assert_eq!(result, Some(expected));
    }

    #[test]
    fn test_union_many_mixed_none() {
        let a = NullBuffer::from(&[true, false, true, true]);
        let b = NullBuffer::from(&[false, true, true, true]);
        let result = NullBuffer::union_many([Some(&a), None, Some(&b)]);
        let expected = NullBuffer::union(Some(&a), Some(&b));
        assert_eq!(result, expected);
    }

    #[test]
    fn test_union_many_empty_slice() {
        let result = NullBuffer::union_many([] as [Option<&NullBuffer>; 0]);
        assert!(result.is_none());
    }
}
