summa-core 2.0.0

//! Bitpacking utilities for compact integer encoding
//!
//! Implements SIMD-friendly bitpacking for posting list compression.
//! Uses PForDelta-style encoding with exceptions for outliers.
//!
//! Optimizations:
//! - SIMD-accelerated unpacking (when available)
//! - Hillis-Steele parallel prefix sum for delta decoding
//! - Binary search within decoded blocks
//! - Variable block sizes based on posting list length

use crate::structures::simd;
use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
use std::io::{self, Read, Write};

/// Block size for bitpacking (128 integers per block for SIMD alignment)
pub const HORIZONTAL_BP128_BLOCK_SIZE: usize = 128;

/// Small block size for short posting lists (better cache locality)
pub const SMALL_BLOCK_SIZE: usize = 32;

/// Threshold for using small blocks (posting lists shorter than this use small blocks)
pub const SMALL_BLOCK_THRESHOLD: usize = 256;

/// Pack a block of 128 u32 values using the specified bit width
pub fn pack_block(
    values: &[u32; HORIZONTAL_BP128_BLOCK_SIZE],
    bit_width: u8,
    output: &mut Vec<u8>,
) {
    pack_block_n(values, bit_width, output);
}

/// Pack `values` at `width` bits each (little-endian bit order) into `out`.
/// This is the one little-endian bit packer; every value must fit `width`.
pub(super) fn pack_block_n(values: &[u32], width: u8, out: &mut Vec<u8>) {
    debug_assert!(width <= 32, "bit width exceeds u32");
    debug_assert!(
        width == 32 || values.iter().all(|&v| v >> width == 0),
        "value exceeds the packed width"
    );
    if width == 0 || values.is_empty() {
        return;
    }
    if width == 32 {
        for &v in values {
            out.extend_from_slice(&v.to_le_bytes());
        }
        return;
    }
    let start = out.len();
    out.resize(start + (values.len() * width as usize).div_ceil(8), 0);
    let dst = &mut out[start..];
    let mut bit_pos = 0usize;
    for &v in values {
        let mut acc = (v as u64) << (bit_pos & 7);
        let mut byte = bit_pos >> 3;
        let mut remaining = (bit_pos & 7) + width as usize;
        while remaining > 0 {
            dst[byte] |= acc as u8;
            acc >>= 8;
            byte += 1;
            remaining = remaining.saturating_sub(8);
        }
        bit_pos += width as usize;
    }
}

/// Unpack a block of 128 u32 values without requiring trailing padding.
///
/// Panics if the width exceeds 32 or the encoded input is truncated.
pub fn unpack_block(input: &[u8], bit_width: u8, output: &mut [u32; HORIZONTAL_BP128_BLOCK_SIZE]) {
    unpack_block_n(input, bit_width, output, HORIZONTAL_BP128_BLOCK_SIZE);
}

/// Unpack `n` horizontally packed integers, without reading outside `input`.
/// Byte-aligned widths reuse SIMD widening; other widths use bounded word
/// loads with a scalar tail. Shared by packed postings and patched low bits.
///
/// Panics if the width exceeds 32 or the input/output extents are too short.
#[inline]
pub fn unpack_block_n(input: &[u8], bit_width: u8, output: &mut [u32], n: usize) {
    assert!(bit_width <= 32, "bit width exceeds u32");
    let output = &mut output[..n];
    let bytes = n
        .checked_mul(usize::from(bit_width))
        .expect("packed input length overflows usize")
        .div_ceil(8);
    let input = &input[..bytes];
    match bit_width {
        0 => output.fill(0),
        8 => simd::unpack_8bit(input, output, n),
        16 => simd::unpack_16bit(input, output, n),
        32 => simd::unpack_32bit(input, output, n),
        _ => {
            let mask = (1u64 << bit_width) - 1;
            let mut bit_pos = 0usize;
            for slot in output {
                let byte = bit_pos >> 3;
                let word = if byte + 8 <= input.len() {
                    u64::from_le_bytes(input[byte..byte + 8].try_into().unwrap())
                } else {
                    let mut word = 0u64;
                    for (i, &b) in input[byte..].iter().enumerate() {
                        word |= (b as u64) << (i * 8);
                    }
                    word
                };
                *slot = ((word >> (bit_pos & 7)) & mask) as u32;
                bit_pos += usize::from(bit_width);
            }
        }
    }
}

/// Binary search within a decoded block to find first element >= target
/// Returns the index within the block, or block.len() if not found
#[inline]
pub fn binary_search_block(block: &[u32], target: u32) -> usize {
    match block.binary_search(&target) {
        Ok(idx) => idx,
        Err(idx) => idx,
    }
}

/// Bitpacked block with skip info for block-max pruning
#[derive(Debug, Clone)]
pub struct HorizontalBP128Block {
    /// Delta-encoded doc_ids (bitpacked)
    pub doc_deltas: Vec<u8>,
    /// Bit width for doc deltas
    pub doc_bit_width: u8,
    /// Term frequencies (bitpacked)
    pub term_freqs: Vec<u8>,
    /// Bit width for term frequencies
    pub tf_bit_width: u8,
    /// First doc_id in this block (absolute)
    pub first_doc_id: u32,
    /// Last doc_id in this block (absolute)
    pub last_doc_id: u32,
    /// Number of docs in this block
    pub num_docs: u16,
    /// Maximum term frequency in this block (for BM25F upper bound calculation)
    pub max_tf: u32,
    /// Maximum impact score in this block (for MaxScore pruning)
    /// This is computed using BM25F with conservative length normalization
    pub max_block_score: f32,
}

impl HorizontalBP128Block {
    /// Serialize the block
    pub fn serialize<W: Write>(&self, writer: &mut W) -> io::Result<()> {
        writer.write_u32::<LittleEndian>(self.first_doc_id)?;
        writer.write_u32::<LittleEndian>(self.last_doc_id)?;
        writer.write_u16::<LittleEndian>(self.num_docs)?;
        writer.write_u8(self.doc_bit_width)?;
        writer.write_u8(self.tf_bit_width)?;
        writer.write_u32::<LittleEndian>(self.max_tf)?;
        writer.write_f32::<LittleEndian>(self.max_block_score)?;

        // Write doc deltas
        writer.write_u16::<LittleEndian>(self.doc_deltas.len() as u16)?;
        writer.write_all(&self.doc_deltas)?;

        // Write term freqs
        writer.write_u16::<LittleEndian>(self.term_freqs.len() as u16)?;
        writer.write_all(&self.term_freqs)?;

        Ok(())
    }

    /// Deserialize a block
    pub fn deserialize<R: Read>(reader: &mut R) -> io::Result<Self> {
        let first_doc_id = reader.read_u32::<LittleEndian>()?;
        let last_doc_id = reader.read_u32::<LittleEndian>()?;
        let num_docs = reader.read_u16::<LittleEndian>()?;
        let doc_bit_width = reader.read_u8()?;
        let tf_bit_width = reader.read_u8()?;
        let max_tf = reader.read_u32::<LittleEndian>()?;
        let max_block_score = reader.read_f32::<LittleEndian>()?;

        let doc_deltas_len = reader.read_u16::<LittleEndian>()? as usize;
        let mut doc_deltas = vec![0u8; doc_deltas_len];
        reader.read_exact(&mut doc_deltas)?;

        let term_freqs_len = reader.read_u16::<LittleEndian>()? as usize;
        let mut term_freqs = vec![0u8; term_freqs_len];
        reader.read_exact(&mut term_freqs)?;

        Ok(Self {
            doc_deltas,
            doc_bit_width,
            term_freqs,
            tf_bit_width,
            first_doc_id,
            last_doc_id,
            num_docs,
            max_tf,
            max_block_score,
        })
    }

    /// Decode doc_ids from this block
    pub fn decode_doc_ids(&self) -> Vec<u32> {
        let mut output = vec![0u32; self.num_docs as usize];
        self.decode_doc_ids_into(&mut output);
        output
    }

    /// Decode doc_ids into a pre-allocated buffer (avoids allocation)
    #[inline]
    pub fn decode_doc_ids_into(&self, output: &mut [u32]) -> usize {
        let count = self.num_docs as usize;
        if count == 0 {
            return 0;
        }

        // Fused unpack + delta decode - no intermediate buffer needed
        simd::unpack_delta_decode(
            &self.doc_deltas,
            self.doc_bit_width,
            output,
            self.first_doc_id,
            count,
        );

        count
    }

    /// Decode term frequencies from this block
    pub fn decode_term_freqs(&self) -> Vec<u32> {
        let mut output = vec![0u32; self.num_docs as usize];
        self.decode_term_freqs_into(&mut output);
        output
    }

    /// Decode term frequencies into a pre-allocated buffer (avoids allocation)
    #[inline]
    pub fn decode_term_freqs_into(&self, output: &mut [u32]) -> usize {
        let count = self.num_docs as usize;
        if count == 0 {
            return 0;
        }

        // Use slice-based unpack to avoid temp buffer copy
        unpack_block_n(&self.term_freqs, self.tf_bit_width, output, count);

        // TF is stored as tf-1, so add 1 back
        simd::add_one(output, count);

        count
    }
}

/// Bitpacked posting list with block-level skip info
#[derive(Debug, Clone)]
pub struct HorizontalBP128PostingList {
    /// Blocks of postings
    pub blocks: Vec<HorizontalBP128Block>,
    /// Total document count
    pub doc_count: u32,
    /// Maximum score across all blocks (for MaxScore pruning)
    pub max_score: f32,
}

impl HorizontalBP128PostingList {
    /// Create from raw doc_ids and term frequencies
    pub fn from_postings(doc_ids: &[u32], term_freqs: &[u32], idf: f32) -> Self {
        assert_eq!(doc_ids.len(), term_freqs.len());

        if doc_ids.is_empty() {
            return Self {
                blocks: Vec::new(),
                doc_count: 0,
                max_score: 0.0,
            };
        }

        let mut blocks = Vec::new();
        let mut max_score = 0.0f32;
        let mut i = 0;

        while i < doc_ids.len() {
            let block_end = (i + HORIZONTAL_BP128_BLOCK_SIZE).min(doc_ids.len());
            let block_docs = &doc_ids[i..block_end];
            let block_tfs = &term_freqs[i..block_end];

            let block = Self::create_block(block_docs, block_tfs, idf);
            max_score = max_score.max(block.max_block_score);
            blocks.push(block);

            i = block_end;
        }

        Self {
            blocks,
            doc_count: doc_ids.len() as u32,
            max_score,
        }
    }

    fn create_block(doc_ids: &[u32], term_freqs: &[u32], idf: f32) -> HorizontalBP128Block {
        use crate::query::bm25_upper_bound;

        let num_docs = doc_ids.len();
        let first_doc_id = doc_ids[0];
        let last_doc_id = *doc_ids.last().unwrap();

        // Compute deltas (delta - 1 to save one bit since deltas are always >= 1)
        let mut deltas = [0u32; HORIZONTAL_BP128_BLOCK_SIZE];
        let mut max_delta = 0u32;
        for j in 1..num_docs {
            let delta = doc_ids[j] - doc_ids[j - 1] - 1;
            deltas[j - 1] = delta;
            max_delta = max_delta.max(delta);
        }

        // Compute max TF and prepare TF array (store tf-1)
        let mut tfs = [0u32; HORIZONTAL_BP128_BLOCK_SIZE];
        let mut max_tf = 0u32;

        for (j, &tf) in term_freqs.iter().enumerate() {
            tfs[j] = tf - 1; // Store tf-1
            max_tf = max_tf.max(tf);
        }

        // BM25 upper bound score using conservative length normalization
        let max_block_score = bm25_upper_bound(max_tf as f32, idf);

        let doc_bit_width = simd::bits_needed(max_delta);
        let tf_bit_width = simd::bits_needed(max_tf.saturating_sub(1)); // Store tf-1

        let mut doc_deltas = Vec::new();
        pack_block(&deltas, doc_bit_width, &mut doc_deltas);

        let mut term_freqs_packed = Vec::new();
        pack_block(&tfs, tf_bit_width, &mut term_freqs_packed);

        HorizontalBP128Block {
            doc_deltas,
            doc_bit_width,
            term_freqs: term_freqs_packed,
            tf_bit_width,
            first_doc_id,
            last_doc_id,
            num_docs: num_docs as u16,
            max_tf,
            max_block_score,
        }
    }

    /// Serialize the posting list
    pub fn serialize<W: Write>(&self, writer: &mut W) -> io::Result<()> {
        writer.write_u32::<LittleEndian>(self.doc_count)?;
        writer.write_f32::<LittleEndian>(self.max_score)?;
        writer.write_u32::<LittleEndian>(self.blocks.len() as u32)?;

        for block in &self.blocks {
            block.serialize(writer)?;
        }

        Ok(())
    }

    /// Deserialize a posting list
    pub fn deserialize<R: Read>(reader: &mut R) -> io::Result<Self> {
        let doc_count = reader.read_u32::<LittleEndian>()?;
        let max_score = reader.read_f32::<LittleEndian>()?;
        let num_blocks = reader.read_u32::<LittleEndian>()? as usize;

        let mut blocks = Vec::with_capacity(num_blocks);
        for _ in 0..num_blocks {
            blocks.push(HorizontalBP128Block::deserialize(reader)?);
        }

        Ok(Self {
            blocks,
            doc_count,
            max_score,
        })
    }

    /// Create an iterator
    pub fn iterator(&self) -> HorizontalBP128Iterator<'_> {
        HorizontalBP128Iterator::new(self)
    }
}

/// Iterator over bitpacked posting list with block skipping support
pub struct HorizontalBP128Iterator<'a> {
    posting_list: &'a HorizontalBP128PostingList,
    /// Current block index
    current_block: usize,
    /// Number of valid elements in current block
    current_block_len: usize,
    /// Pre-allocated buffer for decoded doc_ids (avoids allocation per block)
    block_doc_ids: Vec<u32>,
    /// Pre-allocated buffer for decoded term freqs
    block_term_freqs: Vec<u32>,
    /// Position within current block
    pos_in_block: usize,
    /// Whether we've exhausted all postings
    exhausted: bool,
}

impl<'a> HorizontalBP128Iterator<'a> {
    pub fn new(posting_list: &'a HorizontalBP128PostingList) -> Self {
        // Pre-allocate buffers to block size to avoid allocations during iteration
        let mut iter = Self {
            posting_list,
            current_block: 0,
            current_block_len: 0,
            block_doc_ids: vec![0u32; HORIZONTAL_BP128_BLOCK_SIZE],
            block_term_freqs: vec![0u32; HORIZONTAL_BP128_BLOCK_SIZE],
            pos_in_block: 0,
            exhausted: posting_list.blocks.is_empty(),
        };

        if !iter.exhausted {
            iter.decode_current_block();
        }

        iter
    }

    #[inline]
    fn decode_current_block(&mut self) {
        let block = &self.posting_list.blocks[self.current_block];
        // Decode into pre-allocated buffers (no allocation!)
        self.current_block_len = block.decode_doc_ids_into(&mut self.block_doc_ids);
        block.decode_term_freqs_into(&mut self.block_term_freqs);
        self.pos_in_block = 0;
    }

    /// Current document ID
    #[inline]
    pub fn doc(&self) -> u32 {
        if self.exhausted {
            u32::MAX
        } else {
            self.block_doc_ids[self.pos_in_block]
        }
    }

    /// Current term frequency
    #[inline]
    pub fn term_freq(&self) -> u32 {
        if self.exhausted {
            0
        } else {
            self.block_term_freqs[self.pos_in_block]
        }
    }

    /// Advance to next document
    #[inline]
    pub fn advance(&mut self) -> u32 {
        if self.exhausted {
            return u32::MAX;
        }

        self.pos_in_block += 1;

        if self.pos_in_block >= self.current_block_len {
            self.current_block += 1;
            if self.current_block >= self.posting_list.blocks.len() {
                self.exhausted = true;
                return u32::MAX;
            }
            self.decode_current_block();
        }

        self.doc()
    }

    /// Seek to first doc >= target (with block skipping and binary search)
    pub fn seek(&mut self, target: u32) -> u32 {
        if self.exhausted {
            return u32::MAX;
        }

        // Binary search to find the right block
        let block_idx = self.posting_list.blocks[self.current_block..].binary_search_by(|block| {
            if block.last_doc_id < target {
                std::cmp::Ordering::Less
            } else if block.first_doc_id > target {
                std::cmp::Ordering::Greater
            } else {
                std::cmp::Ordering::Equal
            }
        });

        let target_block = match block_idx {
            Ok(idx) => self.current_block + idx,
            Err(idx) => {
                if self.current_block + idx >= self.posting_list.blocks.len() {
                    self.exhausted = true;
                    return u32::MAX;
                }
                self.current_block + idx
            }
        };

        // Move to target block if different
        if target_block != self.current_block {
            self.current_block = target_block;
            self.decode_current_block();
        } else if self.current_block_len == 0 {
            self.decode_current_block();
        }

        // Binary search within the block
        let pos = binary_search_block(
            &self.block_doc_ids[self.pos_in_block..self.current_block_len],
            target,
        );
        self.pos_in_block += pos;

        if self.pos_in_block >= self.current_block_len {
            // Target not in this block, move to next
            self.current_block += 1;
            if self.current_block >= self.posting_list.blocks.len() {
                self.exhausted = true;
                return u32::MAX;
            }
            self.decode_current_block();
        }

        self.doc()
    }

    /// Get max score for remaining blocks (for MaxScore optimization)
    pub fn max_remaining_score(&self) -> f32 {
        if self.exhausted {
            return 0.0;
        }

        self.posting_list.blocks[self.current_block..]
            .iter()
            .map(|b| b.max_block_score)
            .fold(0.0f32, |a, b| a.max(b))
    }

    /// Skip to next block (for block-max pruning)
    pub fn skip_to_block_with_doc(&mut self, target: u32) -> Option<(u32, f32)> {
        while self.current_block < self.posting_list.blocks.len() {
            let block = &self.posting_list.blocks[self.current_block];
            if block.last_doc_id >= target {
                return Some((block.first_doc_id, block.max_block_score));
            }
            self.current_block += 1;
        }
        self.exhausted = true;
        None
    }

    /// Get current block's max score
    pub fn current_block_max_score(&self) -> f32 {
        if self.exhausted {
            0.0
        } else {
            self.posting_list.blocks[self.current_block].max_block_score
        }
    }

    /// Get current block's max term frequency (for BM25F upper bound recalculation)
    pub fn current_block_max_tf(&self) -> u32 {
        if self.exhausted {
            0
        } else {
            self.posting_list.blocks[self.current_block].max_tf
        }
    }
}

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

    #[cfg(all(unix, feature = "native"))]
    #[test]
    fn exact_width_unpack_never_reads_past_the_encoded_slice() {
        const CHILD: &str = "SUMMA_CODEC_GUARD_CHILD";
        const COMPLETE: &str = "guard-page decode verified";
        if std::env::var_os(CHILD).is_none() {
            let child = std::process::Command::new(std::env::current_exe().unwrap())
                .args([
                    "--exact",
                    "structures::postings::horizontal_bp128::tests::exact_width_unpack_never_reads_past_the_encoded_slice",
                    "--nocapture",
                    "--test-threads=1",
                ])
                .env(CHILD, "1")
                .output()
                .unwrap();
            assert!(
                child.status.success(),
                "exact-width decoder crossed the protected boundary: {}\n{}\n{}",
                child.status,
                String::from_utf8_lossy(&child.stdout),
                String::from_utf8_lossy(&child.stderr)
            );
            assert!(String::from_utf8_lossy(&child.stdout).contains(COMPLETE));
            return;
        }
        // Isolate a potential SIGBUS/SIGSEGV in the child test process.
        let page = unsafe { libc::sysconf(libc::_SC_PAGESIZE) };
        assert!(page >= 512);
        let page = page as usize;
        let mapping = unsafe {
            libc::mmap(
                std::ptr::null_mut(),
                page * 2,
                libc::PROT_READ | libc::PROT_WRITE,
                libc::MAP_PRIVATE | libc::MAP_ANON,
                -1,
                0,
            )
        };
        assert_ne!(mapping, libc::MAP_FAILED);
        struct Mapping(*mut libc::c_void, usize);
        impl Drop for Mapping {
            fn drop(&mut self) {
                unsafe {
                    libc::munmap(self.0, self.1);
                }
            }
        }
        let _mapping = Mapping(mapping, page * 2);
        let boundary = unsafe { mapping.cast::<u8>().add(page) };
        assert_eq!(
            unsafe { libc::mprotect(boundary.cast(), page, libc::PROT_NONE) },
            0
        );
        for width in 0..=32u8 {
            let mask = if width == 32 {
                u32::MAX
            } else {
                (1u32 << width) - 1
            };
            let values = std::array::from_fn(|i| {
                if width == 0 {
                    0
                } else {
                    (i as u32).wrapping_mul(2_654_435_761) & mask
                }
            });
            let mut packed = Vec::new();
            pack_block(&values, width, &mut packed);
            // Independent bit-at-a-time oracle for the persisted layout.
            let mut expected_bytes = vec![0u8; (values.len() * usize::from(width)).div_ceil(8)];
            for (i, &value) in values.iter().enumerate() {
                for bit in 0..usize::from(width) {
                    let at = i * usize::from(width) + bit;
                    expected_bytes[at / 8] |= (((value >> bit) & 1) as u8) << (at % 8);
                }
            }
            assert_eq!(packed, expected_bytes);
            for count in 0..=HORIZONTAL_BP128_BLOCK_SIZE {
                let len = (count * usize::from(width)).div_ceil(8);
                let start = unsafe { boundary.sub(len) };
                unsafe {
                    std::ptr::copy_nonoverlapping(packed.as_ptr(), start, len);
                }
                let input = unsafe { std::slice::from_raw_parts(start, len) };
                let mut decoded = vec![0xDEADBEEF; count + 4];
                unpack_block_n(input, width, &mut decoded[..count], count);
                assert_eq!(
                    &decoded[..count],
                    &values[..count],
                    "width={width} count={count}"
                );
                assert_eq!(&decoded[count..], &[0xDEADBEEF; 4]);
                if count == HORIZONTAL_BP128_BLOCK_SIZE {
                    let mut full = [0; HORIZONTAL_BP128_BLOCK_SIZE];
                    unpack_block(input, width, &mut full);
                    assert_eq!(full, values);
                }
            }
        }
        println!("{COMPLETE}");
    }

    #[test]
    fn test_bits_needed() {
        assert_eq!(simd::bits_needed(0), 0);
        assert_eq!(simd::bits_needed(1), 1);
        assert_eq!(simd::bits_needed(2), 2);
        assert_eq!(simd::bits_needed(3), 2);
        assert_eq!(simd::bits_needed(255), 8);
        assert_eq!(simd::bits_needed(256), 9);
    }

    #[test]
    fn test_pack_unpack() {
        let mut values = [0u32; HORIZONTAL_BP128_BLOCK_SIZE];
        for (i, value) in values.iter_mut().enumerate() {
            *value = (i * 3) as u32;
        }

        let max_val = values.iter().max().copied().unwrap();
        let bit_width = simd::bits_needed(max_val);

        let mut packed = Vec::new();
        pack_block(&values, bit_width, &mut packed);

        let mut unpacked = [0u32; HORIZONTAL_BP128_BLOCK_SIZE];
        unpack_block(&packed, bit_width, &mut unpacked);

        assert_eq!(values, unpacked);
    }

    #[test]
    fn test_bitpacked_posting_list() {
        let doc_ids: Vec<u32> = (0..200).map(|i| i * 2).collect();
        let term_freqs: Vec<u32> = (0..200).map(|i| (i % 10) + 1).collect();

        let posting_list = HorizontalBP128PostingList::from_postings(&doc_ids, &term_freqs, 1.0);

        assert_eq!(posting_list.doc_count, 200);
        assert_eq!(posting_list.blocks.len(), 2); // 128 + 72

        // Test iteration
        let mut iter = posting_list.iterator();
        for (i, &expected_doc) in doc_ids.iter().enumerate() {
            assert_eq!(iter.doc(), expected_doc, "Mismatch at position {}", i);
            assert_eq!(iter.term_freq(), term_freqs[i]);
            if i < doc_ids.len() - 1 {
                iter.advance();
            }
        }
    }

    #[test]
    fn test_bitpacked_seek() {
        let doc_ids: Vec<u32> = vec![10, 20, 30, 100, 200, 300, 1000, 2000];
        let term_freqs: Vec<u32> = vec![1, 2, 3, 4, 5, 6, 7, 8];

        let posting_list = HorizontalBP128PostingList::from_postings(&doc_ids, &term_freqs, 1.0);
        let mut iter = posting_list.iterator();

        assert_eq!(iter.seek(25), 30);
        assert_eq!(iter.seek(100), 100);
        assert_eq!(iter.seek(500), 1000);
        assert_eq!(iter.seek(3000), u32::MAX);
    }

    #[test]
    fn test_serialization() {
        let doc_ids: Vec<u32> = (0..50).map(|i| i * 3).collect();
        let term_freqs: Vec<u32> = (0..50).map(|_| 1).collect();

        let posting_list = HorizontalBP128PostingList::from_postings(&doc_ids, &term_freqs, 1.5);

        let mut buffer = Vec::new();
        posting_list.serialize(&mut buffer).unwrap();

        let restored = HorizontalBP128PostingList::deserialize(&mut &buffer[..]).unwrap();

        assert_eq!(restored.doc_count, posting_list.doc_count);
        assert_eq!(restored.blocks.len(), posting_list.blocks.len());

        // Verify iteration produces same results
        let mut iter1 = posting_list.iterator();
        let mut iter2 = restored.iterator();

        while iter1.doc() != u32::MAX {
            assert_eq!(iter1.doc(), iter2.doc());
            assert_eq!(iter1.term_freq(), iter2.term_freq());
            iter1.advance();
            iter2.advance();
        }
    }

    #[test]
    fn test_simd_delta_decode() {
        // Test simd::delta_decode
        let deltas2 = [0u32; 16]; // gaps of 1 (stored as 0)
        let mut output2 = [0u32; 16];
        simd::delta_decode(&mut output2, &deltas2, 100, 8);
        // first_doc_id=100, then +1 each
        assert_eq!(&output2[..8], &[100, 101, 102, 103, 104, 105, 106, 107]);

        // Test with varying deltas (stored as gap-1)
        // gaps: 2, 1, 3, 1, 5, 1, 1 → stored as: 1, 0, 2, 0, 4, 0, 0
        let deltas3 = [1u32, 0, 2, 0, 4, 0, 0, 0];
        let mut output3 = [0u32; 8];
        simd::delta_decode(&mut output3, &deltas3, 10, 8);
        // 10, 10+2=12, 12+1=13, 13+3=16, 16+1=17, 17+5=22, 22+1=23, 23+1=24
        assert_eq!(&output3[..8], &[10, 12, 13, 16, 17, 22, 23, 24]);
    }

    #[test]
    fn test_delta_decode_large_block() {
        // Test with a full 128-element block
        let doc_ids: Vec<u32> = (0..128).map(|i| i * 5 + 100).collect();
        let term_freqs: Vec<u32> = vec![1; 128];

        let posting_list = HorizontalBP128PostingList::from_postings(&doc_ids, &term_freqs, 1.0);
        let decoded = posting_list.blocks[0].decode_doc_ids();

        assert_eq!(decoded.len(), 128);
        for (i, (&expected, &actual)) in doc_ids.iter().zip(decoded.iter()).enumerate() {
            assert_eq!(expected, actual, "Mismatch at position {}", i);
        }
    }
}