Introduce a lot of facet string helper iterators

milli/src/heed_codec/facet/facet_level_value_u32_codec.rs

@@ -0,0 +1,52 @@
+use std::borrow::Cow;
+use std::convert::TryInto;
+use std::num::NonZeroU8;
+
+use crate::FieldId;
+
+/// A codec that stores the field id, level 1 and higher and the groups ids.
+///
+/// It can only be used to encode the facet string of the level 1 or higher.
+pub struct FacetLevelValueU32Codec;
+
+impl<'a> heed::BytesDecode<'a> for FacetLevelValueU32Codec {
+    type DItem = (FieldId, NonZeroU8, u32, u32);
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let (field_id, bytes) = bytes.split_first()?;
+        let (level, bytes) = bytes.split_first()?;
+        let level = NonZeroU8::new(*level)?;
+        let left = bytes[16..20].try_into().ok().map(u32::from_be_bytes)?;
+        let right = bytes[20..].try_into().ok().map(u32::from_be_bytes)?;
+        Some((*field_id, level, left, right))
+    }
+}
+
+impl heed::BytesEncode<'_> for FacetLevelValueU32Codec {
+    type EItem = (FieldId, NonZeroU8, u32, u32);
+
+    fn bytes_encode((field_id, level, left, right): &Self::EItem) -> Option<Cow<[u8]>> {
+        let mut buffer = [0u8; 16];
+
+        // Write the big-endian integers.
+        let bytes = left.to_be_bytes();
+        buffer[..4].copy_from_slice(&bytes[..]);
+
+        let bytes = right.to_be_bytes();
+        buffer[4..8].copy_from_slice(&bytes[..]);
+
+        // Then the u32 values just to be able to read them back.
+        let bytes = left.to_be_bytes();
+        buffer[8..12].copy_from_slice(&bytes[..]);
+
+        let bytes = right.to_be_bytes();
+        buffer[12..].copy_from_slice(&bytes[..]);
+
+        let mut bytes = Vec::with_capacity(buffer.len() + 2);
+        bytes.push(*field_id);
+        bytes.push(level.get());
+        bytes.extend_from_slice(&buffer);
+
+        Some(Cow::Owned(bytes))
+    }
+}

milli/src/heed_codec/facet/facet_string_level_zero_codec.rs

@@ -0,0 +1,49 @@
+use std::borrow::Cow;
+use std::str;
+
+use crate::FieldId;
+
+/// A codec that stores the field id, level 0, and facet string.
+///
+/// It can only be used to encode the facet string of the level 0,
+/// as it hardcodes the level.
+///
+/// We encode the level 0 to not break the lexicographical ordering of the LMDB keys,
+/// and make sure that the levels are not mixed-up. The level 0 is special, the key
+/// are strings, other levels represent groups and keys are simply two integers.
+pub struct FacetStringLevelZeroCodec;
+
+impl FacetStringLevelZeroCodec {
+    pub fn serialize_into(field_id: FieldId, value: &str, out: &mut Vec<u8>) {
+        out.reserve(value.len() + 2);
+        out.push(field_id);
+        out.push(0); // the level zero (for LMDB ordering only)
+        out.extend_from_slice(value.as_bytes());
+    }
+}
+
+impl<'a> heed::BytesDecode<'a> for FacetStringLevelZeroCodec {
+    type DItem = (FieldId, &'a str);
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let (field_id, bytes) = bytes.split_first()?;
+        let (level, bytes) = bytes.split_first()?;
+
+        if *level != 0 {
+            return None;
+        }
+
+        let value = str::from_utf8(bytes).ok()?;
+        Some((*field_id, value))
+    }
+}
+
+impl<'a> heed::BytesEncode<'a> for FacetStringLevelZeroCodec {
+    type EItem = (FieldId, &'a str);
+
+    fn bytes_encode((field_id, value): &Self::EItem) -> Option<Cow<[u8]>> {
+        let mut bytes = Vec::new();
+        FacetStringLevelZeroCodec::serialize_into(*field_id, value, &mut bytes);
+        Some(Cow::Owned(bytes))
+    }
+}

milli/src/heed_codec/facet/facet_string_zero_bounds_value_codec.rs

@@ -0,0 +1,80 @@
+use std::borrow::Cow;
+use std::convert::TryInto;
+use std::{marker, str};
+
+/// A codec that encodes two strings in front of the value.
+///
+/// The usecase is for the facet string levels algorithm where we must
+/// know the origin of a group, the group left and right bounds are stored
+/// in the value to not break the lexicographical ordering of the LMDB keys.
+pub struct FacetStringZeroBoundsValueCodec<C>(marker::PhantomData<C>);
+
+impl<'a, C> heed::BytesDecode<'a> for FacetStringZeroBoundsValueCodec<C>
+where
+    C: heed::BytesDecode<'a>,
+{
+    type DItem = (Option<(&'a str, &'a str)>, C::DItem);
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let (contains_bounds, tail_bytes) = bytes.split_first()?;
+
+        if *contains_bounds != 0 {
+            let (left_len, bytes) = try_split_at(bytes, 2)?;
+            let (right_len, bytes) = try_split_at(bytes, 2)?;
+
+            let left_len = left_len.try_into().ok().map(u16::from_be_bytes)?;
+            let right_len = right_len.try_into().ok().map(u16::from_be_bytes)?;
+
+            let (left, bytes) = try_split_at(bytes, left_len as usize)?;
+            let (right, bytes) = try_split_at(bytes, right_len as usize)?;
+
+            let left = str::from_utf8(left).ok()?;
+            let right = str::from_utf8(right).ok()?;
+
+            C::bytes_decode(bytes).map(|item| (Some((left, right)), item))
+        } else {
+            C::bytes_decode(tail_bytes).map(|item| (None, item))
+        }
+    }
+}
+
+impl<'a, C> heed::BytesEncode<'a> for FacetStringZeroBoundsValueCodec<C>
+where
+    C: heed::BytesEncode<'a>,
+{
+    type EItem = (Option<(&'a str, &'a str)>, C::EItem);
+
+    fn bytes_encode((bounds, value): &'a Self::EItem) -> Option<Cow<[u8]>> {
+        let mut bytes = Vec::new();
+
+        match bounds {
+            Some((left, right)) => {
+                let left_len: u16 = left.len().try_into().ok()?;
+                let right_len: u16 = right.len().try_into().ok()?;
+                bytes.extend_from_slice(&left_len.to_be_bytes());
+                bytes.extend_from_slice(&right_len.to_be_bytes());
+
+                let value_bytes = C::bytes_encode(&value)?;
+                bytes.extend_from_slice(&value_bytes[..]);
+
+                Some(Cow::Owned(bytes))
+            }
+            None => {
+                bytes.push(0);
+                let value_bytes = C::bytes_encode(&value)?;
+                bytes.extend_from_slice(&value_bytes[..]);
+                Some(Cow::Owned(bytes))
+            }
+        }
+    }
+}
+
+/// Tries to split a slice in half at the given middle point,
+/// `None` if the slice is too short.
+fn try_split_at(slice: &[u8], mid: usize) -> Option<(&[u8], &[u8])> {
+    if slice.len() >= mid {
+        Some(slice.split_at(mid))
+    } else {
+        None
+    }
+}

milli/src/heed_codec/facet/mod.rs

@@ -1,9 +1,15 @@
 mod facet_level_value_f64_codec;
+mod facet_level_value_u32_codec;
+mod facet_string_level_zero_codec;
+mod facet_string_zero_bounds_value_codec;
 mod facet_value_string_codec;
 mod field_doc_id_facet_f64_codec;
 mod field_doc_id_facet_string_codec;
 
 pub use self::facet_level_value_f64_codec::FacetLevelValueF64Codec;
+pub use self::facet_level_value_u32_codec::FacetLevelValueU32Codec;
+pub use self::facet_string_level_zero_codec::FacetStringLevelZeroCodec;
+pub use self::facet_string_zero_bounds_value_codec::FacetStringZeroBoundsValueCodec;
 pub use self::facet_value_string_codec::FacetValueStringCodec;
 pub use self::field_doc_id_facet_f64_codec::FieldDocIdFacetF64Codec;
 pub use self::field_doc_id_facet_string_codec::FieldDocIdFacetStringCodec;