/*
 * Released under the terms of the Apache 2.0 license with LLVM
 * exception. See `LICENSE` for details.
 */

//! Index sets: sets of integers that represent indices into a space.

use alloc::vec::Vec;
use core::cell::Cell;

use crate::FxHashMap;

const SMALL_ELEMS: usize = 12;

/// A hybrid large/small-mode sparse mapping from integer indices to
/// elements.
///
/// The trailing `(u32, u64)` elements in each variant is a one-item
/// cache to allow fast access when streaming through.
#[derive(Clone, Debug)]
enum AdaptiveMap {
    Small {
        len: u32,
        keys: [u32; SMALL_ELEMS],
        values: [u64; SMALL_ELEMS],
    },
    Large(FxHashMap<u32, u64>),
}

const INVALID: u32 = 0xffff_ffff;

impl AdaptiveMap {
    fn new() -> Self {
        Self::Small {
            len: 0,
            keys: [INVALID; SMALL_ELEMS],
            values: [0; SMALL_ELEMS],
        }
    }

    #[inline(always)]
    fn get_or_insert<'a>(&'a mut self, key: u32) -> &'a mut u64 {
        // Check whether the key is present and we are in small mode;
        // if no to both, we need to expand first.
        let small_mode_idx = match self {
            &mut Self::Small {
                len,
                ref mut keys,
                ref values,
            } => {
                // Perform this scan but do not return right away;
                // doing so runs into overlapping-borrow issues
                // because the current non-lexical lifetimes
                // implementation is not able to see that the `self`
                // mutable borrow on return is only on the
                // early-return path.
                if let Some(i) = keys[..len as usize].iter().position(|&k| k == key) {
                    Some(i)
                } else if len != SMALL_ELEMS as u32 {
                    debug_assert!(len < SMALL_ELEMS as u32);
                    None
                } else if let Some(i) = values.iter().position(|&v| v == 0) {
                    // If an existing value is zero, reuse that slot.
                    keys[i] = key;
                    Some(i)
                } else {
                    *self = Self::Large(keys.iter().copied().zip(values.iter().copied()).collect());
                    None
                }
            }
            _ => None,
        };

        match self {
            Self::Small { len, keys, values } => {
                // If we found the key already while checking whether
                // we need to expand above, use that index to return
                // early.
                if let Some(i) = small_mode_idx {
                    return &mut values[i];
                }
                // Otherwise, the key must not be present; add a new
                // entry.
                debug_assert!(*len < SMALL_ELEMS as u32);
                let idx = *len as usize;
                *len += 1;
                keys[idx] = key;
                values[idx] = 0;
                &mut values[idx]
            }
            Self::Large(map) => map.entry(key).or_insert(0),
        }
    }

    #[inline(always)]
    fn get_mut(&mut self, key: u32) -> Option<&mut u64> {
        match self {
            &mut Self::Small {
                len,
                ref keys,
                ref mut values,
            } => {
                for i in 0..len {
                    if keys[i as usize] == key {
                        return Some(&mut values[i as usize]);
                    }
                }
                None
            }
            &mut Self::Large(ref mut map) => map.get_mut(&key),
        }
    }
    #[inline(always)]
    fn get(&self, key: u32) -> Option<u64> {
        match self {
            &Self::Small {
                len,
                ref keys,
                ref values,
            } => {
                for i in 0..len {
                    if keys[i as usize] == key {
                        let value = values[i as usize];
                        return Some(value);
                    }
                }
                None
            }
            &Self::Large(ref map) => {
                let value = map.get(&key).cloned();
                value
            }
        }
    }
    fn iter<'a>(&'a self) -> AdaptiveMapIter<'a> {
        match self {
            &Self::Small {
                len,
                ref keys,
                ref values,
            } => AdaptiveMapIter::Small(&keys[0..len as usize], &values[0..len as usize]),
            &Self::Large(ref map) => AdaptiveMapIter::Large(map.iter()),
        }
    }

    fn is_empty(&self) -> bool {
        match self {
            AdaptiveMap::Small { values, .. } => values.iter().all(|&value| value == 0),
            AdaptiveMap::Large(m) => m.values().all(|&value| value == 0),
        }
    }
}

enum AdaptiveMapIter<'a> {
    Small(&'a [u32], &'a [u64]),
    Large(hashbrown::hash_map::Iter<'a, u32, u64>),
}

impl<'a> core::iter::Iterator for AdaptiveMapIter<'a> {
    type Item = (u32, u64);

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        match self {
            &mut Self::Small(ref mut keys, ref mut values) => {
                if keys.is_empty() {
                    None
                } else {
                    let (k, v) = ((*keys)[0], (*values)[0]);
                    *keys = &(*keys)[1..];
                    *values = &(*values)[1..];
                    Some((k, v))
                }
            }
            &mut Self::Large(ref mut it) => it.next().map(|(&k, &v)| (k, v)),
        }
    }
}

/// A conceptually infinite-length set of indices that allows union
/// and efficient iteration over elements.
#[derive(Clone)]
pub struct IndexSet {
    elems: AdaptiveMap,
    cache: Cell<(u32, u64)>,
}

const BITS_PER_WORD: usize = 64;

impl IndexSet {
    pub fn new() -> Self {
        Self {
            elems: AdaptiveMap::new(),
            cache: Cell::new((INVALID, 0)),
        }
    }

    #[inline(always)]
    fn elem(&mut self, bit_index: usize) -> &mut u64 {
        let word_index = (bit_index / BITS_PER_WORD) as u32;
        if self.cache.get().0 == word_index {
            self.cache.set((INVALID, 0));
        }
        self.elems.get_or_insert(word_index)
    }

    #[inline(always)]
    fn maybe_elem_mut(&mut self, bit_index: usize) -> Option<&mut u64> {
        let word_index = (bit_index / BITS_PER_WORD) as u32;
        if self.cache.get().0 == word_index {
            self.cache.set((INVALID, 0));
        }
        self.elems.get_mut(word_index)
    }

    #[inline(always)]
    fn maybe_elem(&self, bit_index: usize) -> Option<u64> {
        let word_index = (bit_index / BITS_PER_WORD) as u32;
        if self.cache.get().0 == word_index {
            Some(self.cache.get().1)
        } else {
            self.elems.get(word_index)
        }
    }

    #[inline(always)]
    pub fn set(&mut self, idx: usize, val: bool) {
        let bit = idx % BITS_PER_WORD;
        if val {
            *self.elem(idx) |= 1 << bit;
        } else if let Some(word) = self.maybe_elem_mut(idx) {
            *word &= !(1 << bit);
        }
    }

    pub fn assign(&mut self, other: &Self) {
        self.elems = other.elems.clone();
        self.cache = other.cache.clone();
    }

    #[inline(always)]
    pub fn get(&self, idx: usize) -> bool {
        let bit = idx % BITS_PER_WORD;
        if let Some(word) = self.maybe_elem(idx) {
            (word & (1 << bit)) != 0
        } else {
            false
        }
    }

    pub fn union_with(&mut self, other: &Self) -> bool {
        let mut changed = 0;
        for (word_idx, bits) in other.elems.iter() {
            if bits == 0 {
                continue;
            }
            let word_idx = word_idx as usize;
            let self_word = self.elem(word_idx * BITS_PER_WORD);
            changed |= bits & !*self_word;
            *self_word |= bits;
        }
        changed != 0
    }

    pub fn iter<'a>(&'a self) -> impl Iterator<Item = usize> + 'a {
        self.elems.iter().flat_map(|(word_idx, bits)| {
            let word_idx = word_idx as usize;
            SetBitsIter(bits).map(move |i| BITS_PER_WORD * word_idx + i)
        })
    }

    /// Is the adaptive data structure in "small" mode? This is meant
    /// for testing assertions only.
    pub(crate) fn is_small(&self) -> bool {
        match &self.elems {
            &AdaptiveMap::Small { .. } => true,
            _ => false,
        }
    }

    /// Is the set empty?
    pub(crate) fn is_empty(&self) -> bool {
        self.elems.is_empty()
    }
}

pub struct SetBitsIter(u64);

impl Iterator for SetBitsIter {
    type Item = usize;

    #[inline]
    fn next(&mut self) -> Option<usize> {
        // Build an `Option<NonZeroU64>` so that on the nonzero path,
        // the compiler can optimize the trailing-zeroes operator
        // using that knowledge.
        core::num::NonZeroU64::new(self.0).map(|nz| {
            let bitidx = nz.trailing_zeros();
            self.0 &= self.0 - 1; // clear highest set bit
            bitidx as usize
        })
    }
}

impl core::fmt::Debug for IndexSet {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        let vals = self.iter().collect::<Vec<_>>();
        write!(f, "{:?}", vals)
    }
}

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

    #[test]
    fn test_set_bits_iter() {
        let mut vec = IndexSet::new();
        let mut sum = 0;
        for i in 0..1024 {
            if i % 17 == 0 {
                vec.set(i, true);
                sum += i;
            }
        }

        let mut checksum = 0;
        for bit in vec.iter() {
            debug_assert!(bit % 17 == 0);
            checksum += bit;
        }

        debug_assert_eq!(sum, checksum);
    }

    #[test]
    fn test_expand_remove_zero_elems() {
        let mut vec = IndexSet::new();
        // Set 12 different words (this is the max small-mode size).
        for i in 0..12 {
            vec.set(64 * i, true);
        }
        // Now clear a bit, and set a bit in a different word. We
        // should still be in small mode.
        vec.set(64 * 5, false);
        vec.set(64 * 100, true);
        debug_assert!(vec.is_small());
    }
}