Optimize encoding incompressible u8.

Author: caibear

src/derive/vec.rs

@@ -363,7 +363,7 @@ impl<'a, T: Decode<'a>> Decoder<'a, VecDeque<T>> for VecDecoder<'a, T> {
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use alloc::collections::*;
     use alloc::vec::Vec;
 
@@ -386,10 +386,24 @@ mod test {
         type T = BinaryHeap<u8>;
         let data: T = bench_data();
         let encoded = crate::encode(&data);
+
+        let mut buffer = crate::Buffer::new();
         b.iter(|| {
-            let decoded: T = crate::decode::<T>(&encoded).unwrap();
+            let decoded: T = buffer.decode::<T>(&encoded).unwrap();
             debug_assert!(data.iter().eq(decoded.iter()));
             decoded
         })
     }
 }
+
+#[cfg(test)]
+mod tests2 {
+    type T = [[u8; 32]; 32];
+    fn bench_data() -> Vec<Vec<T>> {
+        crate::random_data(1000)
+            .into_iter()
+            .map(|t: T| if t[0][0] & 1 == 0 { vec![t] } else { vec![] })
+            .collect()
+    }
+    crate::bench_encode_decode!(vec_zero_or_one_large_array_vec: Vec<Vec<T>>);
+}

src/lib.rs

@@ -29,6 +29,7 @@ mod length;
 mod nightly;
 mod pack;
 mod pack_ints;
+mod pack_shared;
 mod str;
 mod u8_char;
 

src/pack.rs

@@ -3,6 +3,7 @@ use crate::consume::{consume_byte, consume_byte_arrays, consume_bytes};
 use crate::error::err;
 use crate::fast::CowSlice;
 use crate::pack_ints::{Int, SizedInt};
+use crate::pack_shared::PackingTrait;
 use alloc::vec::Vec;
 
 /// Possible states per byte in descending order. Each packed byte will use `log2(states)` bits.
@@ -17,8 +18,9 @@ enum Packing {
     _2,
 }
 
-impl Packing {
-    fn new(max: u8) -> Self {
+impl PackingTrait for Packing {
+    fn new<T: crate::pack_ints::SizedUInt>(max: T) -> Self {
+        let max: u8 = bytemuck::must_cast(max);
         match max {
             // We could encode max 0 as nothing, but that could allocate unbounded memory when decoding.
             0..=1 => Self::_2,
@@ -30,12 +32,16 @@ impl Packing {
         }
     }
 
-    fn write(self, out: &mut Vec<u8>, offset_by_min: bool) {
+    fn write<T: crate::pack_ints::SizedUInt>(self, out: &mut Vec<u8>, offset_by_min: bool) {
+        // pack_ints::Packing needs generics, we only use this on u8 here.
+        assert_eq!(std::mem::size_of::<T>(), 1);
         // Encoded in such a way such that 0 is `Self::_256` and higher numbers are smaller packing.
         // Also makes `Self::_256` with offset_by_min = true is unrepresentable.
         out.push(self as u8 * 2 - offset_by_min as u8);
     }
+}
 
+impl Packing {
     fn read(input: &mut &[u8]) -> Result<(Self, bool)> {
         let v = consume_byte(input)?;
         let p_u8 = crate::nightly::div_ceil_u8(v, 2);
@@ -93,44 +99,26 @@ pub fn pack_bytes<T: Byte>(bytes: &mut [T], out: &mut Vec<u8>) {
         out.extend_from_slice(bytemuck::must_cast_slice(bytes));
         return;
     }
-    let (min, max) = crate::pack_ints::minmax(bytes);
-
-    // i8 packs as u8 if positive.
-    let basic_packing = if min >= T::default() {
-        Packing::new(bytemuck::must_cast(max))
-    } else {
-        Packing::_256 // Any negative i8 as u8 is > 15 and can't be packed without offset_packing.
-    };
 
-    // u8::wrapping_sub == i8::wrapping_sub, so we can use u8s from here onward.
-    let min: u8 = bytemuck::must_cast(min);
-    let max: u8 = bytemuck::must_cast(max);
+    let (basic_packing, min_max) =
+        crate::pack_shared::basic_packing_and_signed_min_max_cast_to_unsigned(bytes, out);
     let bytes: &mut [u8] = bytemuck::must_cast_slice_mut(bytes);
-    pack_bytes_unsigned(bytes, out, basic_packing, min, max);
+
+    // <T as SizedInt>::Unsigned for T: Byte is always u8, but we can't prove
+    // that here, so we perform this cast which doesn't change the underlying type.
+    let min_max = min_max.map(|(min, max)| (bytemuck::must_cast(min), bytemuck::must_cast(max)));
+
+    pack_bytes_unsigned(bytes, out, basic_packing, min_max);
 }
 
 /// [`pack_bytes`] but after i8s have been cast to u8s.
 fn pack_bytes_unsigned(
     bytes: &mut [u8],
     out: &mut Vec<u8>,
     basic_packing: Packing,
-    min: u8,
-    max: u8,
+    min_max: Option<(u8, u8)>,
 ) {
-    // If subtracting min from all bytes results in a better packing do it, otherwise don't bother.
-    let offset_packing = Packing::new(max.wrapping_sub(min));
-    let p = if offset_packing > basic_packing && bytes.len() > 5 {
-        for b in bytes.iter_mut() {
-            *b = b.wrapping_sub(min);
-        }
-        offset_packing.write(out, true);
-        out.push(min);
-        offset_packing
-    } else {
-        basic_packing.write(out, false);
-        basic_packing
-    };
-
+    let p = crate::pack_shared::offset_packing(bytes, out, basic_packing, min_max);
     match p {
         Packing::_256 => out.extend_from_slice(bytes),
         Packing::_16 => pack_arithmetic::<16>(bytes, out),

src/pack_ints.rs

@@ -3,6 +3,7 @@ use crate::consume::{consume_byte, consume_byte_arrays};
 use crate::error::error;
 use crate::fast::CowSlice;
 use crate::pack::{invalid_packing, pack_bytes, unpack_bytes};
+use crate::pack_shared::PackingTrait;
 use crate::Error;
 use alloc::vec::Vec;
 use bytemuck::Pod;
@@ -19,7 +20,7 @@ enum Packing {
     _8,
 }
 
-impl Packing {
+impl PackingTrait for Packing {
     fn new<T: SizedUInt>(max: T) -> Self {
         let max: u128 = max.try_into().unwrap_or_else(|_| unreachable!()); // From<usize> isn't implemented for u128.
         #[allow(clippy::match_overlapping_arm)] // Just make sure not to reorder them.
@@ -37,7 +38,9 @@ impl Packing {
         // Also makes no packing with offset_by_min = true is unrepresentable.
         out.push((self as u8 - Self::new(T::MAX) as u8) * 2 - offset_by_min as u8);
     }
+}
 
+impl Packing {
     fn read<T: SizedUInt>(input: &mut &[u8]) -> Result<(Self, bool)> {
         let v = consume_byte(input)?;
         let p_u8 = crate::nightly::div_ceil_u8(v, 2) + Self::new(T::MAX) as u8;
@@ -329,18 +332,8 @@ impl SizedUInt for u8 {
     }
 }
 
-pub fn minmax<T: SizedInt>(v: &[T]) -> (T, T) {
-    let mut min = T::MAX;
-    let mut max = T::MIN;
-    for &v in v.iter() {
-        min = min.min(v);
-        max = max.max(v);
-    }
-    (min, max)
-}
-
 fn skip_packing<T: SizedInt>(length: usize) -> bool {
-    // Be careful using size_of::<T> since usize can be 4 or 8.
+    // T is SizedInt, so it cannot be usize/isize, therefore comparisions against size are safe.
     if core::mem::size_of::<T>() == 1 {
         return true; // u8s can't be packed by pack_ints (only pack_bytes).
     }
@@ -370,31 +363,9 @@ fn pack_ints_sized<T: SizedInt>(ints: &mut [T], out: &mut Vec<u8>) {
     let (basic_packing, min_max) = if skip_packing::<T>(ints.len()) {
         (Packing::new(T::Unsigned::MAX), None)
     } else {
-        // Take a small sample to avoid wastefully scanning the whole slice.
-        let (sample, remaining) = ints.split_at(ints.len().min(16));
-        let (min, max) = minmax(sample);
-
-        // Only have to check packing(max - min) since it's always as good as packing(max).
-        let none = Packing::new(T::Unsigned::MAX);
-        if Packing::new(max.to_unsigned().wrapping_sub(min.to_unsigned())) == none {
-            none.write::<T::Unsigned>(out, false);
-            (none, None)
-        } else {
-            let (remaining_min, remaining_max) = minmax(remaining);
-            let min = min.min(remaining_min);
-            let max = max.max(remaining_max);
-
-            // Signed ints pack as unsigned ints if positive.
-            let basic_packing = if min >= T::default() {
-                Packing::new(max.to_unsigned())
-            } else {
-                none // Any negative can't be packed without offset_packing.
-            };
-            (basic_packing, Some((min, max)))
-        }
+        crate::pack_shared::basic_packing_and_signed_min_max_cast_to_unsigned(ints, out)
     };
     let ints = bytemuck::must_cast_slice_mut(ints);
-    let min_max = min_max.map(|(min, max)| (min.to_unsigned(), max.to_unsigned()));
     pack_ints_sized_unsigned::<T::Unsigned>(ints, out, basic_packing, min_max);
 }
 
@@ -405,24 +376,7 @@ fn pack_ints_sized_unsigned<T: SizedUInt>(
     basic_packing: Packing,
     min_max: Option<(T, T)>,
 ) {
-    let p = if let Some((min, max)) = min_max {
-        // If subtracting min from all ints results in a better packing do it, otherwise don't bother.
-        let offset_packing = Packing::new(max.wrapping_sub(min));
-        if offset_packing > basic_packing && ints.len() > 5 {
-            for b in ints.iter_mut() {
-                *b = b.wrapping_sub(min);
-            }
-            offset_packing.write::<T>(out, true);
-            T::write(min, out);
-            offset_packing
-        } else {
-            basic_packing.write::<T>(out, false);
-            basic_packing
-        }
-    } else {
-        basic_packing
-    };
-
+    let p = crate::pack_shared::offset_packing(ints, out, basic_packing, min_max);
     match p {
         Packing::_128 => T::pack128(ints, out),
         Packing::_64 => T::pack64(ints, out),

src/pack_shared.rs

@@ -0,0 +1,80 @@
+use crate::pack_ints::{SizedInt, SizedUInt};
+
+pub trait PackingTrait: Copy + PartialOrd {
+    fn new<T: SizedUInt>(max: T) -> Self;
+
+    fn write<T: SizedUInt>(self, out: &mut Vec<u8>, offset_by_min: bool);
+}
+
+fn minmax<T: SizedInt>(v: &[T]) -> (T, T) {
+    let mut min = T::MAX;
+    let mut max = T::MIN;
+    for &v in v.iter() {
+        min = min.min(v);
+        max = max.max(v);
+    }
+    (min, max)
+}
+
+// Writes a packing to `out` iff it returns None.
+pub fn basic_packing_and_signed_min_max_cast_to_unsigned<T: SizedInt, P: PackingTrait>(
+    ints: &[T],
+    out: &mut Vec<u8>,
+) -> (P, Option<(T::Unsigned, T::Unsigned)>) {
+    // Take a small sample to avoid wastefully scanning the whole slice.
+    // Note: This small sample is purely an optimization, it has no impact on the encoded result
+    // because we only use it to bail from scanning the entire slice if the first 16-32 elements
+    // cannot be packed.
+    let sample_size = (32 / std::mem::size_of::<T>()).max(16);
+    let (sample, remaining) = ints.split_at(ints.len().min(sample_size));
+    let (min, max) = minmax(sample);
+
+    // Only have to check packing(max - min) since it's always as good as packing(max).
+    let none = P::new(T::Unsigned::MAX);
+    if P::new(max.to_unsigned().wrapping_sub(min.to_unsigned())) == none {
+        none.write::<T::Unsigned>(out, false);
+        (none, None)
+    } else {
+        let (remaining_min, remaining_max) = minmax(remaining);
+        let min = min.min(remaining_min);
+        let max = max.max(remaining_max);
+
+        // Signed ints pack as unsigned ints if positive.
+        let basic_packing = if min >= T::default() {
+            P::new(max.to_unsigned())
+        } else {
+            none // Any negative can't be packed without offset_packing.
+        };
+
+        (basic_packing, Some((min.to_unsigned(), max.to_unsigned())))
+    }
+}
+
+// Writes a packing to `out` iff `min_max` is Some.
+pub fn offset_packing<T: SizedUInt, P: PackingTrait>(
+    ints: &mut [T],
+    out: &mut Vec<u8>,
+    basic_packing: P,
+    min_max: Option<(T, T)>,
+) -> P {
+    if let Some((min, max)) = min_max {
+        // If subtracting min from all ints results in a better packing do it, otherwise don't bother.
+        let offset_packing = P::new(max.wrapping_sub(min));
+        // TODO(breaking change) don't hardcode this as 5. Only perform offset_packing
+        // on a few elements if the added T::write(min, out) makes it still smaller.
+        let small_skip_offset_packing = 5;
+        if offset_packing > basic_packing && ints.len() > small_skip_offset_packing {
+            for b in ints.iter_mut() {
+                *b = b.wrapping_sub(min);
+            }
+            offset_packing.write::<T>(out, true);
+            T::write(min, out);
+            offset_packing
+        } else {
+            basic_packing.write::<T>(out, false);
+            basic_packing
+        }
+    } else {
+        basic_packing
+    }
+}