rustdoc-search: compressed bitmap to sort, then load desc

This adds a bit more data than "pure sharding" by
including information about which items have no description
at all. This way, it can sort the results, then truncate,
then finally download the description.

With the "e" bitmap: 2380KiB

Without the "e" bitmap: 2364KiB

src/librustdoc/html/render/search_index.rs

@@ -1,6 +1,7 @@
 use std::collections::hash_map::Entry;
 use std::collections::{BTreeMap, VecDeque};
 
+use base64::prelude::*;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_middle::ty::TyCtxt;
 use rustc_span::def_id::DefId;
@@ -21,14 +22,14 @@ use crate::html::render::{self, IndexItem, IndexItemFunctionType, RenderType, Re
 ///
 /// The `index` is a JSON-encoded list of names and other information.
 ///
-/// The desc has newlined descriptions, split up by size into 1MiB shards.
+/// The desc has newlined descriptions, split up by size into 128KiB shards.
 /// For example, `(4, "foo\nbar\nbaz\nquux")`.
 pub(crate) struct SerializedSearchIndex {
     pub(crate) index: String,
     pub(crate) desc: Vec<(usize, String)>,
 }
 
-const DESC_INDEX_SHARD_LEN: usize = 1024 * 1024;
+const DESC_INDEX_SHARD_LEN: usize = 128 * 1024;
 
 /// Builds the search index from the collected metadata
 pub(crate) fn build_index<'tcx>(
@@ -342,6 +343,8 @@ pub(crate) fn build_index<'tcx>(
         // A list of shard lengths encoded as vlqhex. See the comment in write_vlqhex_to_string
         // for information on the format.
         descindex: String,
+        // A list of items with no description. This is eventually turned into a bitmap.
+        emptydesc: Vec<u32>,
     }
 
     struct Paths {
@@ -456,7 +459,8 @@ pub(crate) fn build_index<'tcx>(
                 }
 
                 if item.deprecation.is_some() {
-                    deprecated.push(index);
+                    // bitmasks always use 1-indexing for items, with 0 as the crate itself
+                    deprecated.push(u32::try_from(index + 1).unwrap());
                 }
             }
 
@@ -473,9 +477,18 @@ pub(crate) fn build_index<'tcx>(
             crate_data.serialize_field("i", &parents)?;
             crate_data.serialize_field("f", &functions)?;
             crate_data.serialize_field("D", &self.descindex)?;
-            crate_data.serialize_field("c", &deprecated)?;
             crate_data.serialize_field("p", &paths)?;
             crate_data.serialize_field("b", &self.associated_item_disambiguators)?;
+            let mut buf = Vec::new();
+            let mut strbuf = String::new();
+            write_bitmap_to_bytes(&deprecated, &mut buf).unwrap();
+            BASE64_STANDARD.encode_string(&buf, &mut strbuf);
+            crate_data.serialize_field("c", &strbuf)?;
+            strbuf.clear();
+            buf.clear();
+            write_bitmap_to_bytes(&self.emptydesc, &mut buf).unwrap();
+            BASE64_STANDARD.encode_string(&buf, &mut strbuf);
+            crate_data.serialize_field("e", &strbuf)?;
             if has_aliases {
                 crate_data.serialize_field("a", &self.aliases)?;
             }
@@ -483,11 +496,18 @@ pub(crate) fn build_index<'tcx>(
         }
     }
 
-    let desc = {
+    let (emptydesc, desc) = {
+        let mut emptydesc = Vec::new();
         let mut result = Vec::new();
         let mut set = String::new();
         let mut len: usize = 0;
+        let mut itemindex: u32 = 0;
         for desc in std::iter::once(&crate_doc).chain(crate_items.iter().map(|item| &item.desc)) {
+            if desc == "" {
+                emptydesc.push(itemindex);
+                itemindex += 1;
+                continue;
+            }
             if set.len() >= DESC_INDEX_SHARD_LEN {
                 result.push((len, std::mem::replace(&mut set, String::new())));
                 len = 0;
@@ -496,9 +516,10 @@ pub(crate) fn build_index<'tcx>(
             }
             set.push_str(&desc);
             len += 1;
+            itemindex += 1;
         }
         result.push((len, std::mem::replace(&mut set, String::new())));
-        result
+        (emptydesc, result)
     };
 
     let descindex = {
@@ -509,7 +530,10 @@ pub(crate) fn build_index<'tcx>(
         descindex
     };
 
-    assert_eq!(crate_items.len() + 1, desc.iter().map(|(len, _)| *len).sum::<usize>());
+    assert_eq!(
+        crate_items.len() + 1,
+        desc.iter().map(|(len, _)| *len).sum::<usize>() + emptydesc.len()
+    );
 
     // The index, which is actually used to search, is JSON
     // It uses `JSON.parse(..)` to actually load, since JSON
@@ -523,6 +547,7 @@ pub(crate) fn build_index<'tcx>(
             aliases: &aliases,
             associated_item_disambiguators: &associated_item_disambiguators,
             descindex,
+            emptydesc,
         })
         .expect("failed serde conversion")
         // All these `replace` calls are because we have to go through JS string for JSON content.
@@ -571,6 +596,200 @@ pub(crate) fn write_vlqhex_to_string(n: i32, string: &mut String) {
     }
 }
 
+// checked against roaring-rs in
+// https://gitlab.com/notriddle/roaring-test
+pub fn write_bitmap_to_bytes(domain: &[u32], mut out: impl std::io::Write) -> std::io::Result<()> {
+    // https://arxiv.org/pdf/1603.06549.pdf
+    let mut keys = Vec::<u16>::new();
+    let mut containers = Vec::<Container>::new();
+    enum Container {
+        /// number of ones, bits
+        Bits(Box<[u64; 1024]>),
+        /// list of entries
+        Array(Vec<u16>),
+        /// list of (start, len-1)
+        Run(Vec<(u16, u16)>),
+    }
+    impl Container {
+        fn popcount(&self) -> u32 {
+            match self {
+                Container::Bits(bits) => bits.iter().copied().map(|x| x.count_ones()).sum(),
+                Container::Array(array) => {
+                    array.len().try_into().expect("array can't be bigger than 2**32")
+                }
+                Container::Run(runs) => {
+                    runs.iter().copied().map(|(_, lenm1)| u32::from(lenm1) + 1).sum()
+                }
+            }
+        }
+        fn push(&mut self, value: u16) {
+            match self {
+                Container::Bits(bits) => bits[value as usize >> 6] |= 1 << (value & 0x3F),
+                Container::Array(array) => {
+                    array.push(value);
+                    if array.len() >= 4096 {
+                        let array = std::mem::replace(array, Vec::new());
+                        *self = Container::Bits(Box::new([0; 1024]));
+                        for value in array {
+                            self.push(value);
+                        }
+                    }
+                }
+                Container::Run(runs) => {
+                    if let Some(r) = runs.last_mut()
+                        && r.0 + r.1 + 1 == value
+                    {
+                        r.1 += 1;
+                    } else {
+                        runs.push((value, 0));
+                    }
+                }
+            }
+        }
+        fn try_make_run(&mut self) -> bool {
+            match self {
+                Container::Bits(bits) => {
+                    let mut r: u64 = 0;
+                    for (i, chunk) in bits.iter().copied().enumerate() {
+                        let next_chunk =
+                            i.checked_add(1).and_then(|i| bits.get(i)).copied().unwrap_or(0);
+                        r += !chunk & u64::from((chunk << 1).count_ones());
+                        r += !next_chunk & u64::from((chunk >> 63).count_ones());
+                    }
+                    if (2 + 4 * r) < 8192 {
+                        let bits = std::mem::replace(bits, Box::new([0; 1024]));
+                        *self = Container::Run(Vec::new());
+                        for (i, bits) in bits.iter().copied().enumerate() {
+                            if bits == 0 {
+                                continue;
+                            }
+                            for j in 0..64 {
+                                let value = (u16::try_from(i).unwrap() << 6) | j;
+                                if bits & (1 << j) != 0 {
+                                    self.push(value);
+                                }
+                            }
+                        }
+                        true
+                    } else {
+                        false
+                    }
+                }
+                Container::Array(array) if array.len() <= 5 => false,
+                Container::Array(array) => {
+                    let mut r = 0;
+                    let mut prev = None;
+                    for value in array.iter().copied() {
+                        if value.checked_sub(1) != prev {
+                            r += 1;
+                        }
+                        prev = Some(value);
+                    }
+                    if 2 + 4 * r < 2 * array.len() + 2 {
+                        let array = std::mem::replace(array, Vec::new());
+                        *self = Container::Run(Vec::new());
+                        for value in array {
+                            self.push(value);
+                        }
+                        true
+                    } else {
+                        false
+                    }
+                }
+                Container::Run(_) => true,
+            }
+        }
+    }
+    let mut key: u16;
+    let mut domain_iter = domain.into_iter().copied().peekable();
+    let mut has_run = false;
+    while let Some(entry) = domain_iter.next() {
+        key = (entry >> 16).try_into().expect("shifted off the top 16 bits, so it should fit");
+        let value: u16 = (entry & 0x00_00_FF_FF).try_into().expect("AND 16 bits, so it should fit");
+        let mut container = Container::Array(vec![value]);
+        while let Some(entry) = domain_iter.peek().copied() {
+            let entry_key: u16 =
+                (entry >> 16).try_into().expect("shifted off the top 16 bits, so it should fit");
+            if entry_key != key {
+                break;
+            }
+            domain_iter.next().expect("peeking just succeeded");
+            container
+                .push((entry & 0x00_00_FF_FF).try_into().expect("AND 16 bits, so it should fit"));
+        }
+        keys.push(key);
+        has_run = container.try_make_run() || has_run;
+        containers.push(container);
+    }
+    // https://github.com/RoaringBitmap/RoaringFormatSpec
+    use byteorder::{WriteBytesExt, LE};
+    const SERIAL_COOKIE_NO_RUNCONTAINER: u32 = 12346;
+    const SERIAL_COOKIE: u32 = 12347;
+    const NO_OFFSET_THRESHOLD: u32 = 4;
+    let size: u32 = containers.len().try_into().unwrap();
+    let start_offset = if has_run {
+        out.write_u32::<LE>(SERIAL_COOKIE | ((size - 1) << 16))?;
+        for set in containers.chunks(8) {
+            let mut b = 0;
+            for (i, container) in set.iter().enumerate() {
+                if matches!(container, &Container::Run(..)) {
+                    b |= 1 << i;
+                }
+            }
+            out.write_u8(b)?;
+        }
+        if size < NO_OFFSET_THRESHOLD {
+            4 + 4 * size + ((size + 7) / 8)
+        } else {
+            4 + 8 * size + ((size + 7) / 8)
+        }
+    } else {
+        out.write_u32::<LE>(SERIAL_COOKIE_NO_RUNCONTAINER)?;
+        out.write_u32::<LE>(containers.len().try_into().unwrap())?;
+        4 + 4 + 4 * size + 4 * size
+    };
+    for (&key, container) in keys.iter().zip(&containers) {
+        // descriptive header
+        let key: u32 = key.into();
+        let count: u32 = container.popcount() - 1;
+        out.write_u32::<LE>((count << 16) | key)?;
+    }
+    if !has_run || size >= NO_OFFSET_THRESHOLD {
+        // offset header
+        let mut starting_offset = start_offset;
+        for container in &containers {
+            out.write_u32::<LE>(starting_offset)?;
+            starting_offset += match container {
+                Container::Bits(_) => 8192u32,
+                Container::Array(array) => u32::try_from(array.len()).unwrap() * 2,
+                Container::Run(runs) => 2 + u32::try_from(runs.len()).unwrap() * 4,
+            };
+        }
+    }
+    for container in &containers {
+        match container {
+            Container::Bits(bits) => {
+                for chunk in bits.iter() {
+                    out.write_u64::<LE>(*chunk)?;
+                }
+            }
+            Container::Array(array) => {
+                for value in array.iter() {
+                    out.write_u16::<LE>(*value)?;
+                }
+            }
+            Container::Run(runs) => {
+                out.write_u16::<LE>((runs.len()).try_into().unwrap())?;
+                for (start, lenm1) in runs.iter().copied() {
+                    out.write_u16::<LE>(start)?;
+                    out.write_u16::<LE>(lenm1)?;
+                }
+            }
+        }
+    }
+    Ok(())
+}
+
 pub(crate) fn get_function_type_for_search<'tcx>(
     item: &clean::Item,
     tcx: TyCtxt<'tcx>,