Introduce the TreeLevel struct

milli/src/heed_codec/str_level_position_codec.rs

@@ -1,12 +1,14 @@
 use std::borrow::Cow;
-use std::convert::TryInto;
+use std::convert::{TryFrom, TryInto};
 use std::mem::size_of;
 use std::str;
 
+use crate::TreeLevel;
+
 pub struct StrLevelPositionCodec;
 
 impl<'a> heed::BytesDecode<'a> for StrLevelPositionCodec {
-    type DItem = (&'a str, u8, u32, u32);
+    type DItem = (&'a str, TreeLevel, u32, u32);
 
     fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
         let footer_len = size_of::<u8>() + size_of::<u32>() * 2;
@@ -19,13 +21,14 @@ impl<'a> heed::BytesDecode<'a> for StrLevelPositionCodec {
         let (level, bytes) = bytes.split_first()?;
         let left = bytes[..4].try_into().map(u32::from_be_bytes).ok()?;
         let right = bytes[4..].try_into().map(u32::from_be_bytes).ok()?;
+        let level = TreeLevel::try_from(*level).ok()?;
 
-        Some((word, *level, left, right))
+        Some((word, level, left, right))
     }
 }
 
 impl<'a> heed::BytesEncode<'a> for StrLevelPositionCodec {
-    type EItem = (&'a str, u8, u32, u32);
+    type EItem = (&'a str, TreeLevel, u32, u32);
 
     fn bytes_encode((word, level, left, right): &Self::EItem) -> Option<Cow<[u8]>> {
         let left = left.to_be_bytes();
@@ -33,7 +36,7 @@ impl<'a> heed::BytesEncode<'a> for StrLevelPositionCodec {
 
         let mut bytes = Vec::with_capacity(word.len() + 1 + left.len() + right.len());
         bytes.extend_from_slice(word.as_bytes());
-        bytes.push(*level);
+        bytes.push((*level).into());
         bytes.extend_from_slice(&left[..]);
         bytes.extend_from_slice(&right[..]);
 

milli/src/lib.rs

@@ -9,6 +9,7 @@ pub mod facet;
 pub mod heed_codec;
 pub mod index;
 pub mod proximity;
+pub mod tree_level;
 pub mod update;
 
 use std::borrow::Cow;
@@ -27,6 +28,7 @@ pub use self::heed_codec::{RoaringBitmapCodec, BoRoaringBitmapCodec, CboRoaringB
 pub use self::heed_codec::{RoaringBitmapLenCodec, BoRoaringBitmapLenCodec, CboRoaringBitmapLenCodec};
 pub use self::index::Index;
 pub use self::search::{Search, FacetDistribution, FacetCondition, SearchResult, MatchingWords};
+pub use self::tree_level::TreeLevel;
 pub use self::update_store::UpdateStore;
 
 pub type FastMap4<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher32>>;

milli/src/tree_level.rs

@@ -0,0 +1,47 @@
+use std::convert::TryFrom;
+use std::fmt;
+
+/// This is just before the lowest printable character (space, sp, 32)
+const MAX_VALUE: u8 = 31;
+
+#[derive(Debug, Copy, Clone)]
+pub enum Error {
+    LevelTooHigh(u8),
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[repr(transparent)]
+pub struct TreeLevel(u8);
+
+impl TreeLevel {
+    pub const fn max_value() -> TreeLevel {
+        TreeLevel(MAX_VALUE)
+    }
+
+    pub const fn min_value() -> TreeLevel {
+        TreeLevel(0)
+    }
+}
+
+impl Into<u8> for TreeLevel {
+    fn into(self) -> u8 {
+        self.0
+    }
+}
+
+impl TryFrom<u8> for TreeLevel {
+    type Error = Error;
+
+    fn try_from(value: u8) -> Result<TreeLevel, Error> {
+        match value {
+            0..=MAX_VALUE => Ok(TreeLevel(value)),
+            _ => Err(Error::LevelTooHigh(value)),
+        }
+    }
+}
+
+impl fmt::Display for TreeLevel {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}

milli/src/update/words_level_positions.rs

@@ -1,4 +1,5 @@
 use std::cmp;
+use std::convert::TryFrom;
 use std::fs::File;
 use std::num::NonZeroUsize;
 
@@ -9,9 +10,9 @@ use log::debug;
 use roaring::RoaringBitmap;
 
 use crate::heed_codec::{StrLevelPositionCodec, CboRoaringBitmapCodec};
-use crate::Index;
 use crate::update::index_documents::WriteMethod;
 use crate::update::index_documents::{create_writer, writer_into_reader, write_into_lmdb_database};
+use crate::{Index, TreeLevel};
 
 pub struct WordsLevelPositions<'t, 'u, 'i> {
     wtxn: &'t mut heed::RwTxn<'i, 'u>,
@@ -105,8 +106,8 @@ fn compute_positions_levels(
         let (word, ()) = result?;
 
         let level_0_range = {
-            let left = (word, 0, u32::min_value(), u32::min_value());
-            let right = (word, 0, u32::max_value(), u32::max_value());
+            let left = (word, TreeLevel::min_value(), u32::min_value(), u32::min_value());
+            let right = (word, TreeLevel::max_value(), u32::max_value(), u32::max_value());
             left..=right
         };
 
@@ -117,7 +118,7 @@ fn compute_positions_levels(
         // Groups sizes are always a power of the original level_group_size and therefore a group
         // always maps groups of the previous level and never splits previous levels groups in half.
         let group_size_iter = (1u8..)
-            .map(|l| (l, level_group_size.get().pow(l as u32)))
+            .map(|l| (TreeLevel::try_from(l).unwrap(), level_group_size.get().pow(l as u32)))
             .take_while(|(_, s)| first_level_size / *s >= min_level_size.get());
 
         // As specified in the documentation, we also write the level 0 entries.
@@ -163,7 +164,7 @@ fn compute_positions_levels(
 fn write_level_entry(
     writer: &mut Writer<File>,
     word: &str,
-    level: u8,
+    level: TreeLevel,
     left: u32,
     right: u32,
     ids: &RoaringBitmap,