fastalp 0.1.5

use std::ptr::read_unaligned;

use crate::{
  constants::{AlpFloat, bit_mask_u64},
  error::{Error, Result},
};

/// 计算 N 个 W-bit 整数打包所需的总字节数
#[inline(always)]
pub const fn packed_byte_size(count: usize, bit_width: u8) -> usize {
  (count * (bit_width as usize)).div_ceil(8)
}

/// 高速位打包：将 `values` 打包入 `dst`
pub fn bitpack_u64(values: &[u64], bit_width: u8, dst: &mut Vec<u8>) {
  if values.is_empty() || bit_width == 0 {
    return;
  }

  let total_bytes = packed_byte_size(values.len(), bit_width);
  let old_len = dst.len();
  dst.reserve(total_bytes);

  if bit_width == 8 {
    // SAFETY: dst 已 reserve(total_bytes)，且循环严格写入 values.len() 个字节，写入完成后调用 set_len 确保内存全部初始化完毕。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len);
      for &v in values {
        *dst_ptr = v as u8;
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 16 {
    // SAFETY: dst 已 reserve(total_bytes)，[u8; 2] 对齐为 1 字节，写入 values.len() 个 2-byte 小端序列后安全更新长度。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len).cast::<[u8; 2]>();
      for &v in values {
        *dst_ptr = (v as u16).to_le_bytes();
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 32 {
    // SAFETY: dst 已 reserve(total_bytes)，[u8; 4] 对齐为 1 字节，写入 values.len() 个 4-byte 小端序列后安全更新长度。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len).cast::<[u8; 4]>();
      for &v in values {
        *dst_ptr = (v as u32).to_le_bytes();
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 64 {
    // SAFETY: dst 已 reserve(total_bytes)，[u8; 8] 对齐为 1 字节，写入 values.len() 个 8-byte 小端序列后安全更新长度。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len).cast::<[u8; 8]>();
      for &v in values {
        *dst_ptr = v.to_le_bytes();
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  }

  let mask = bit_mask_u64(bit_width);
  let mut acc: u128 = 0;
  let mut bits: u32 = 0;

  for &val in values {
    acc |= ((val & mask) as u128) << bits;
    bits += bit_width as u32;
    if bits >= 64 {
      dst.extend_from_slice(&(acc as u64).to_le_bytes());
      acc >>= 64;
      bits -= 64;
    }
  }

  while bits > 0 {
    dst.push(acc as u8);
    acc >>= 8;
    bits = bits.saturating_sub(8);
  }
}

/// 通用高速位打包已编码的浮点差值整数并直接写入 `dst`
pub fn bitpack_encoded<F: AlpFloat>(
  encoded_ints: &[F::Int],
  base: F::Int,
  bit_width: u8,
  dst: &mut Vec<u8>,
) {
  if encoded_ints.is_empty() || bit_width == 0 {
    return;
  }

  let total_bytes = packed_byte_size(encoded_ints.len(), bit_width);
  let old_len = dst.len();
  dst.reserve(total_bytes);

  if bit_width == 1 {
    let (chunks, rem) = encoded_ints.as_chunks::<8>();
    // SAFETY: dst 已 reserve(total_bytes)，按 8 个整数一组打包写入 chunks.len() 字节，余数最多写入 1 字节，刚好填满 total_bytes，无越界与未初始化。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len);
      for chunk in chunks {
        let o0 = (F::int_diff_to_u64(chunk[0], base) as u8) & 0x01;
        let o1 = (F::int_diff_to_u64(chunk[1], base) as u8) & 0x01;
        let o2 = (F::int_diff_to_u64(chunk[2], base) as u8) & 0x01;
        let o3 = (F::int_diff_to_u64(chunk[3], base) as u8) & 0x01;
        let o4 = (F::int_diff_to_u64(chunk[4], base) as u8) & 0x01;
        let o5 = (F::int_diff_to_u64(chunk[5], base) as u8) & 0x01;
        let o6 = (F::int_diff_to_u64(chunk[6], base) as u8) & 0x01;
        let o7 = (F::int_diff_to_u64(chunk[7], base) as u8) & 0x01;
        *dst_ptr =
          o0 | (o1 << 1) | (o2 << 2) | (o3 << 3) | (o4 << 4) | (o5 << 5) | (o6 << 6) | (o7 << 7);
        dst_ptr = dst_ptr.add(1);
      }
      if !rem.is_empty() {
        let mut b = 0u8;
        for (i, &val) in rem.iter().enumerate() {
          let o = (F::int_diff_to_u64(val, base) as u8) & 0x01;
          b |= o << i;
        }
        *dst_ptr = b;
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 2 {
    let (chunks, rem) = encoded_ints.as_chunks::<4>();
    // SAFETY: dst 已 reserve(total_bytes)，按 4 个整数一组打包写入 chunks.len() 字节，余数最多写入 1 字节，刚好填满 total_bytes。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len);
      for chunk in chunks {
        let o0 = (F::int_diff_to_u64(chunk[0], base) as u8) & 0x03;
        let o1 = (F::int_diff_to_u64(chunk[1], base) as u8) & 0x03;
        let o2 = (F::int_diff_to_u64(chunk[2], base) as u8) & 0x03;
        let o3 = (F::int_diff_to_u64(chunk[3], base) as u8) & 0x03;
        *dst_ptr = o0 | (o1 << 2) | (o2 << 4) | (o3 << 6);
        dst_ptr = dst_ptr.add(1);
      }
      if !rem.is_empty() {
        let mut b = 0u8;
        for (i, &val) in rem.iter().enumerate() {
          let o = (F::int_diff_to_u64(val, base) as u8) & 0x03;
          b |= o << (i * 2);
        }
        *dst_ptr = b;
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 4 {
    let (chunks, rem) = encoded_ints.as_chunks::<2>();
    // SAFETY: dst 已 reserve(total_bytes)，按 2 个整数一组打包写入 chunks.len() 字节，余数最多写入 1 字节，刚好填满 total_bytes。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len);
      for chunk in chunks {
        let o0 = (F::int_diff_to_u64(chunk[0], base) as u8) & 0x0f;
        let o1 = (F::int_diff_to_u64(chunk[1], base) as u8) & 0x0f;
        *dst_ptr = o0 | (o1 << 4);
        dst_ptr = dst_ptr.add(1);
      }
      if let Some(&last) = rem.first() {
        let o0 = (F::int_diff_to_u64(last, base) as u8) & 0x0f;
        *dst_ptr = o0;
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 8 {
    // SAFETY: dst 已 reserve(encoded_ints.len())，逐元素写入 u8，完全覆盖 total_bytes。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len);
      for &v in encoded_ints {
        *dst_ptr = F::int_diff_to_u64(v, base) as u8;
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 16 {
    // SAFETY: dst 已 reserve(encoded_ints.len() * 2)，逐元素写入 2-byte 小端序列，完全覆盖 total_bytes。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len).cast::<[u8; 2]>();
      for &v in encoded_ints {
        *dst_ptr = (F::int_diff_to_u64(v, base) as u16).to_le_bytes();
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 32 {
    // SAFETY: dst 已 reserve(encoded_ints.len() * 4)，逐元素写入 4-byte 小端序列，完全覆盖 total_bytes。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len).cast::<[u8; 4]>();
      for &v in encoded_ints {
        *dst_ptr = (F::int_diff_to_u64(v, base) as u32).to_le_bytes();
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  } else if bit_width == 64 {
    // SAFETY: dst 已 reserve(encoded_ints.len() * 8)，逐元素写入 8-byte 小端序列，完全覆盖 total_bytes。
    unsafe {
      let mut dst_ptr = dst.as_mut_ptr().add(old_len).cast::<[u8; 8]>();
      for &v in encoded_ints {
        *dst_ptr = F::int_diff_to_u64(v, base).to_le_bytes();
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + total_bytes);
    }
    return;
  }

  let mask = bit_mask_u64(bit_width);
  let mut acc: u128 = 0;
  let mut bits: u32 = 0;

  for &val in encoded_ints {
    let offset = F::int_diff_to_u64(val, base) & mask;
    acc |= (offset as u128) << bits;
    bits += bit_width as u32;
    if bits >= 64 {
      dst.extend_from_slice(&(acc as u64).to_le_bytes());
      acc >>= 64;
      bits -= 64;
    }
  }

  while bits > 0 {
    dst.push(acc as u8);
    acc >>= 8;
    bits = bits.saturating_sub(8);
  }
}

/// 高速位打包已编码的 f64 差值整数并直接写入 `dst`
#[inline]
pub fn bitpack_encoded_f64(encoded_ints: &[i64], base: i64, bit_width: u8, dst: &mut Vec<u8>) {
  bitpack_encoded::<f64>(encoded_ints, base, bit_width, dst);
}

/// 高速位打包已编码的 f32 差值整数并直接写入 `dst`
#[inline]
pub fn bitpack_encoded_f32(encoded_ints: &[i32], base: i32, bit_width: u8, dst: &mut Vec<u8>) {
  bitpack_encoded::<f32>(encoded_ints, base, bit_width, dst);
}

/// 高速位解包：从 `src` 解包出 `count` 个 `bit_width` 位的整数至 `dst`
pub fn bitunpack_u64(src: &[u8], count: usize, bit_width: u8, dst: &mut Vec<u64>) -> Result<()> {
  if count == 0 {
    return Ok(());
  }
  if bit_width == 0 {
    dst.resize(dst.len() + count, 0);
    return Ok(());
  }

  let required_bytes = packed_byte_size(count, bit_width);
  if src.len() < required_bytes {
    return Err(Error::UnexpectedEof {
      needed: required_bytes,
      available: src.len(),
    });
  }

  let old_len = dst.len();
  dst.reserve(count);

  // SAFETY:
  // 1. 上方已校验 src.len() >= required_bytes，保证读指针与 read_unaligned 严格在 src 有效内存边界内；
  // 2. dst 已预分配 dst.reserve(count)，写入 old_len..old_len+count 空间完全充足且无越界风险；
  // 3. 循环严格写入并初始化 count 个元素后，调用 dst.set_len(old_len + count) 安全更新长度。
  unsafe {
    let mut dst_ptr = dst.as_mut_ptr().add(old_len);

    if bit_width == 8 {
      for i in 0..count {
        *dst_ptr.add(i) = *src.get_unchecked(i) as u64;
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 16 {
      let src_ptr = src.as_ptr().cast::<[u8; 2]>();
      for i in 0..count {
        let bytes = read_unaligned(src_ptr.add(i));
        *dst_ptr.add(i) = u16::from_le_bytes(bytes) as u64;
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 32 {
      let src_ptr = src.as_ptr().cast::<[u8; 4]>();
      for i in 0..count {
        let bytes = read_unaligned(src_ptr.add(i));
        *dst_ptr.add(i) = u32::from_le_bytes(bytes) as u64;
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 64 {
      let src_ptr = src.as_ptr().cast::<[u8; 8]>();
      for i in 0..count {
        let bytes = read_unaligned(src_ptr.add(i));
        *dst_ptr.add(i) = u64::from_le_bytes(bytes);
      }
      dst.set_len(old_len + count);
      return Ok(());
    }

    let mask = bit_mask_u64(bit_width);
    let mut acc: u128 = 0;
    let mut bits_in_acc: u32 = 0;
    let mut src_ptr = src.as_ptr();
    let src_end = src.as_ptr().add(src.len());

    let mut i = 0;
    while i < count && src_ptr.add(8) <= src_end {
      if bits_in_acc < bit_width as u32 {
        let bytes = read_unaligned(src_ptr.cast::<[u8; 8]>());
        let chunk = u64::from_le_bytes(bytes);
        acc |= (chunk as u128) << bits_in_acc;
        bits_in_acc += 64;
        src_ptr = src_ptr.add(8);
      }
      let val = (acc as u64) & mask;
      acc >>= bit_width;
      bits_in_acc -= bit_width as u32;
      *dst_ptr = val;
      dst_ptr = dst_ptr.add(1);
      i += 1;
    }

    while i < count {
      while bits_in_acc < bit_width as u32 && src_ptr < src_end {
        acc |= (*src_ptr as u128) << bits_in_acc;
        bits_in_acc += 8;
        src_ptr = src_ptr.add(1);
      }
      let val = (acc as u64) & mask;
      acc >>= bit_width;
      bits_in_acc = bits_in_acc.saturating_sub(bit_width as u32);
      *dst_ptr = val;
      dst_ptr = dst_ptr.add(1);
      i += 1;
    }

    dst.set_len(old_len + count);
  }

  Ok(())
}

/// 通用零拷贝直接解包并重构浮点数据至 `dst`
#[inline(always)]
pub fn bitunpack_into<F: AlpFloat>(
  src: &[u8],
  count: usize,
  bit_width: u8,
  base: F::Int,
  fac_int: i64,
  frac_flt: F,
  dst: &mut Vec<F>,
) -> Result<()> {
  if count == 0 {
    return Ok(());
  }

  let required_bytes = packed_byte_size(count, bit_width);
  if src.len() < required_bytes {
    return Err(Error::UnexpectedEof {
      needed: required_bytes,
      available: src.len(),
    });
  }

  if bit_width == 0 {
    let val = F::decode_from_offset(0, base, fac_int, frac_flt);
    dst.resize(dst.len() + count, val);
    return Ok(());
  }

  let old_len = dst.len();
  dst.reserve(count);

  // SAFETY:
  // 1. 上方已校验 src.len() >= required_bytes，保证读指针与各 bit_width 分支的 read_unaligned / 查表访问严格在合法内存范围内；
  // 2. dst 已预分配 dst.reserve(count)，写入 old_len..old_len+count 空间完全充足且无越界风险；
  // 3. 循环严格解码并初始化 count 个浮点元素后，调用 dst.set_len(old_len + count) 安全更新长度。
  unsafe {
    let mut dst_ptr = dst.as_mut_ptr().add(old_len);

    if bit_width == 1 {
      let lut = F::build_lut::<2>(base, fac_int, frac_flt);
      let full_bytes = count / 8;
      for &b in &src[..full_bytes] {
        *dst_ptr.add(0) = *lut.get_unchecked((b & 0x01) as usize);
        *dst_ptr.add(1) = *lut.get_unchecked(((b >> 1) & 0x01) as usize);
        *dst_ptr.add(2) = *lut.get_unchecked(((b >> 2) & 0x01) as usize);
        *dst_ptr.add(3) = *lut.get_unchecked(((b >> 3) & 0x01) as usize);
        *dst_ptr.add(4) = *lut.get_unchecked(((b >> 4) & 0x01) as usize);
        *dst_ptr.add(5) = *lut.get_unchecked(((b >> 5) & 0x01) as usize);
        *dst_ptr.add(6) = *lut.get_unchecked(((b >> 6) & 0x01) as usize);
        *dst_ptr.add(7) = *lut.get_unchecked(((b >> 7) & 0x01) as usize);
        dst_ptr = dst_ptr.add(8);
      }
      let rem = count % 8;
      if rem > 0 {
        let b = *src.get_unchecked(full_bytes);
        for shift in 0..rem {
          let idx = ((b >> shift) & 0x01) as usize;
          *dst_ptr = *lut.get_unchecked(idx);
          dst_ptr = dst_ptr.add(1);
        }
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 2 {
      let lut = F::build_lut::<4>(base, fac_int, frac_flt);
      let full_bytes = count / 4;
      for &b in &src[..full_bytes] {
        *dst_ptr.add(0) = *lut.get_unchecked((b & 0x03) as usize);
        *dst_ptr.add(1) = *lut.get_unchecked(((b >> 2) & 0x03) as usize);
        *dst_ptr.add(2) = *lut.get_unchecked(((b >> 4) & 0x03) as usize);
        *dst_ptr.add(3) = *lut.get_unchecked(((b >> 6) & 0x03) as usize);
        dst_ptr = dst_ptr.add(4);
      }
      let rem = count % 4;
      if rem > 0 {
        let b = *src.get_unchecked(full_bytes);
        for i in 0..rem {
          let idx = ((b >> (i * 2)) & 0x03) as usize;
          *dst_ptr = *lut.get_unchecked(idx);
          dst_ptr = dst_ptr.add(1);
        }
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 4 {
      let lut = F::build_lut::<16>(base, fac_int, frac_flt);
      let full_bytes = count / 2;
      for &b in &src[..full_bytes] {
        *dst_ptr.add(0) = *lut.get_unchecked((b & 0x0f) as usize);
        *dst_ptr.add(1) = *lut.get_unchecked((b >> 4) as usize);
        dst_ptr = dst_ptr.add(2);
      }
      if !count.is_multiple_of(2) {
        let b = *src.get_unchecked(full_bytes);
        *dst_ptr = *lut.get_unchecked((b & 0x0f) as usize);
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 8 {
      let lut = F::build_lut::<256>(base, fac_int, frac_flt);
      for &b in &src[..count] {
        *dst_ptr = *lut.get_unchecked(b as usize);
        dst_ptr = dst_ptr.add(1);
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 16 {
      let src_ptr = src.as_ptr().cast::<[u8; 2]>();
      for i in 0..count {
        let bytes = read_unaligned(src_ptr.add(i));
        let off = u16::from_le_bytes(bytes) as u64;
        *dst_ptr.add(i) = F::decode_from_offset(off, base, fac_int, frac_flt);
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 32 {
      let src_ptr = src.as_ptr().cast::<[u8; 4]>();
      for i in 0..count {
        let bytes = read_unaligned(src_ptr.add(i));
        let off = u32::from_le_bytes(bytes) as u64;
        *dst_ptr.add(i) = F::decode_from_offset(off, base, fac_int, frac_flt);
      }
      dst.set_len(old_len + count);
      return Ok(());
    } else if bit_width == 64 {
      let src_ptr = src.as_ptr().cast::<[u8; 8]>();
      for i in 0..count {
        let bytes = read_unaligned(src_ptr.add(i));
        let off = u64::from_le_bytes(bytes);
        *dst_ptr.add(i) = F::decode_from_offset(off, base, fac_int, frac_flt);
      }
      dst.set_len(old_len + count);
      return Ok(());
    }

    let mask = bit_mask_u64(bit_width);
    let mut acc: u128 = 0;
    let mut bits_in_acc: u32 = 0;
    let mut src_ptr = src.as_ptr();
    let src_end = src.as_ptr().add(src.len());

    let mut i = 0;
    while i < count && src_ptr.add(8) <= src_end {
      if bits_in_acc < bit_width as u32 {
        let bytes = read_unaligned(src_ptr.cast::<[u8; 8]>());
        let chunk = u64::from_le_bytes(bytes);
        acc |= (chunk as u128) << bits_in_acc;
        bits_in_acc += 64;
        src_ptr = src_ptr.add(8);
      }
      let off = (acc as u64) & mask;
      acc >>= bit_width;
      bits_in_acc -= bit_width as u32;
      *dst_ptr = F::decode_from_offset(off, base, fac_int, frac_flt);
      dst_ptr = dst_ptr.add(1);
      i += 1;
    }

    while i < count {
      while bits_in_acc < bit_width as u32 && src_ptr < src_end {
        acc |= (*src_ptr as u128) << bits_in_acc;
        bits_in_acc += 8;
        src_ptr = src_ptr.add(1);
      }
      let off = (acc as u64) & mask;
      acc >>= bit_width;
      bits_in_acc = bits_in_acc.saturating_sub(bit_width as u32);
      *dst_ptr = F::decode_from_offset(off, base, fac_int, frac_flt);
      dst_ptr = dst_ptr.add(1);
      i += 1;
    }

    dst.set_len(old_len + count);
  }

  Ok(())
}

/// 零拷贝直接解包并重构 f64 数据至 `dst`
#[inline(always)]
pub fn bitunpack_f64_into(
  src: &[u8],
  count: usize,
  bit_width: u8,
  base: i64,
  fac_int: i64,
  frac_flt: f64,
  dst: &mut Vec<f64>,
) -> Result<()> {
  bitunpack_into::<f64>(src, count, bit_width, base, fac_int, frac_flt, dst)
}

/// 零拷贝直接解包并重构 f32 数据至 `dst`
#[inline(always)]
pub fn bitunpack_f32_into(
  src: &[u8],
  count: usize,
  bit_width: u8,
  base: i32,
  fac_int: i64,
  frac_flt: f32,
  dst: &mut Vec<f32>,
) -> Result<()> {
  bitunpack_into::<f32>(src, count, bit_width, base, fac_int, frac_flt, dst)
}