numeric.rs

#![allow(unsafe_op_in_unsafe_fn)]
use std::fmt::Debug;
use std::mem::MaybeUninit;

use arrow::array::{Array, PrimitiveArray};
use arrow::bitmap::Bitmap;
use arrow::datatypes::ArrowDataType;
use arrow::types::NativeType;
use polars_utils::slice::*;
use polars_utils::total_ord::{canonical_f32, canonical_f64};

use crate::row::RowEncodingOptions;
pub(crate) trait FromSlice {
    fn from_slice(slice: &[u8]) -> Self;
}

impl<const N: usize> FromSlice for [u8; N] {
    #[inline]
    fn from_slice(slice: &[u8]) -> Self {
        slice.try_into().unwrap()
    }
}

/// Encodes a value of a particular fixed width type into bytes
pub trait FixedLengthEncoding: Copy + Debug {
    // 1 is validity 0 or 1
    // bit repr of encoding
    const ENCODED_LEN: usize = 1 + size_of::<Self::Encoded>();

    type Encoded: Sized + Copy + AsRef<[u8]> + AsMut<[u8]>;

    fn encode(self) -> Self::Encoded;

    fn decode(encoded: Self::Encoded) -> Self;

    fn decode_reverse(mut encoded: Self::Encoded) -> Self {
        for v in encoded.as_mut() {
            *v = !*v
        }
        Self::decode(encoded)
    }
}

// encode as big endian
macro_rules! encode_unsigned {
    ($n:expr, $t:ty) => {
        impl FixedLengthEncoding for $t {
            type Encoded = [u8; $n];

            fn encode(self) -> [u8; $n] {
                self.to_be_bytes()
            }

            fn decode(encoded: Self::Encoded) -> Self {
                Self::from_be_bytes(encoded)
            }
        }
    };
}

encode_unsigned!(1, u8);
encode_unsigned!(2, u16);
encode_unsigned!(4, u32);
encode_unsigned!(8, u64);

// toggle the sign bit and then encode as big indian
macro_rules! encode_signed {
    ($n:expr, $t:ty) => {
        impl FixedLengthEncoding for $t {
            type Encoded = [u8; $n];

            fn encode(self) -> [u8; $n] {
                #[cfg(target_endian = "big")]
                {
                    todo!()
                }

                let mut b = self.to_be_bytes();
                // Toggle top "sign" bit to ensure consistent sort order
                b[0] ^= 0x80;
                b
            }

            fn decode(mut encoded: Self::Encoded) -> Self {
                // Toggle top "sign" bit
                encoded[0] ^= 0x80;
                Self::from_be_bytes(encoded)
            }
        }
    };
}

encode_signed!(1, i8);
encode_signed!(2, i16);
encode_signed!(4, i32);
encode_signed!(8, i64);
encode_signed!(16, i128);

impl FixedLengthEncoding for f32 {
    type Encoded = [u8; 4];

    fn encode(self) -> [u8; 4] {
        // https://github.com/rust-lang/rust/blob/9c20b2a8cc7588decb6de25ac6a7912dcef24d65/library/core/src/num/f32.rs#L1176-L1260
        let s = canonical_f32(self).to_bits() as i32;
        let val = s ^ (((s >> 31) as u32) >> 1) as i32;
        val.encode()
    }

    fn decode(encoded: Self::Encoded) -> Self {
        let bits = i32::decode(encoded);
        let val = bits ^ (((bits >> 31) as u32) >> 1) as i32;
        Self::from_bits(val as u32)
    }
}

impl FixedLengthEncoding for f64 {
    type Encoded = [u8; 8];

    fn encode(self) -> [u8; 8] {
        // https://github.com/rust-lang/rust/blob/9c20b2a8cc7588decb6de25ac6a7912dcef24d65/library/core/src/num/f32.rs#L1176-L1260
        let s = canonical_f64(self).to_bits() as i64;
        let val = s ^ (((s >> 63) as u64) >> 1) as i64;
        val.encode()
    }

    fn decode(encoded: Self::Encoded) -> Self {
        let bits = i64::decode(encoded);
        let val = bits ^ (((bits >> 63) as u64) >> 1) as i64;
        Self::from_bits(val as u64)
    }
}

pub unsafe fn encode<T: NativeType + FixedLengthEncoding>(
    buffer: &mut [MaybeUninit<u8>],
    arr: &PrimitiveArray<T>,
    opt: RowEncodingOptions,
    offsets: &mut [usize],
) {
    if arr.null_count() == 0 {
        crate::fixed::numeric::encode_slice(buffer, arr.values().as_slice(), opt, offsets)
    } else {
        crate::fixed::numeric::encode_iter(
            buffer,
            arr.into_iter().map(|v| v.copied()),
            opt,
            offsets,
        )
    }
}

#[inline]
unsafe fn encode_value<T: FixedLengthEncoding>(
    value: &T,
    offset: &mut usize,
    descending: bool,
    buf: &mut [MaybeUninit<u8>],
) {
    let end_offset = *offset + T::ENCODED_LEN;
    let dst = unsafe { buf.get_unchecked_mut(*offset..end_offset) };
    // set valid
    dst[0] = MaybeUninit::new(1);
    let mut encoded = value.encode();

    // invert bits to reverse order
    if descending {
        for v in encoded.as_mut() {
            *v = !*v
        }
    }

    dst[1..].copy_from_slice(encoded.as_ref().as_uninit());
    *offset = end_offset;
}

unsafe fn encode_opt_value<T: FixedLengthEncoding>(
    opt_value: Option<T>,
    offset: &mut usize,
    opt: RowEncodingOptions,
    buffer: &mut [MaybeUninit<u8>],
) {
    let descending = opt.contains(RowEncodingOptions::DESCENDING);
    if let Some(value) = opt_value {
        encode_value(&value, offset, descending, buffer);
    } else {
        unsafe { *buffer.get_unchecked_mut(*offset) = MaybeUninit::new(opt.null_sentinel()) };
        let end_offset = *offset + T::ENCODED_LEN;

        // initialize remaining bytes
        let remainder = unsafe { buffer.get_unchecked_mut(*offset + 1..end_offset) };
        remainder.fill(MaybeUninit::new(0));

        *offset = end_offset;
    }
}

pub(crate) unsafe fn encode_slice<T: FixedLengthEncoding>(
    buffer: &mut [MaybeUninit<u8>],
    input: &[T],
    opt: RowEncodingOptions,
    row_starts: &mut [usize],
) {
    let descending = opt.contains(RowEncodingOptions::DESCENDING);
    for (offset, value) in row_starts.iter_mut().zip(input) {
        encode_value(value, offset, descending, buffer);
    }
}

pub(crate) unsafe fn encode_iter<I: Iterator<Item = Option<T>>, T: FixedLengthEncoding>(
    buffer: &mut [MaybeUninit<u8>],
    input: I,
    opt: RowEncodingOptions,
    row_starts: &mut [usize],
) {
    for (offset, opt_value) in row_starts.iter_mut().zip(input) {
        encode_opt_value(opt_value, offset, opt, buffer);
    }
}

pub(crate) unsafe fn decode_primitive<T: NativeType + FixedLengthEncoding>(
    rows: &mut [&[u8]],
    opt: RowEncodingOptions,
) -> PrimitiveArray<T>
where
    T::Encoded: FromSlice,
{
    let dtype: ArrowDataType = T::PRIMITIVE.into();
    let mut has_nulls = false;
    let descending = opt.contains(RowEncodingOptions::DESCENDING);
    let null_sentinel = opt.null_sentinel();

    let values = rows
        .iter()
        .map(|row| {
            has_nulls |= *row.get_unchecked(0) == null_sentinel;
            // skip null sentinel
            let start = 1;
            let end = start + T::ENCODED_LEN - 1;
            let slice = row.get_unchecked(start..end);
            let bytes = T::Encoded::from_slice(slice);

            if descending {
                T::decode_reverse(bytes)
            } else {
                T::decode(bytes)
            }
        })
        .collect::<Vec<_>>();

    let validity = if has_nulls {
        let null_sentinel = opt.null_sentinel();
        Some(decode_nulls(rows, null_sentinel))
    } else {
        None
    };

    // validity byte and data length
    let increment_len = T::ENCODED_LEN;

    increment_row_counter(rows, increment_len);
    PrimitiveArray::new(dtype, values.into(), validity)
}

unsafe fn increment_row_counter(rows: &mut [&[u8]], fixed_size: usize) {
    for row in rows {
        *row = row.get_unchecked(fixed_size..);
    }
}

pub(super) unsafe fn decode_nulls(rows: &[&[u8]], null_sentinel: u8) -> Bitmap {
    rows.iter()
        .map(|row| *row.get_unchecked(0) != null_sentinel)
        .collect()
}