rustdoc-search: better hashing, faster unification

The hash changes are based on some tests with `arti` and various
specific queries, aimed at reducing the false positive rate.

Sorting the query elements so that generics always come first is
instead aimed at reducing the number of Map operations on mgens,
assuming if the bloom filter does find a false positive, it'll
be able to reject the row without having to track a mapping.

- https://hur.st/bloomfilter/?n=3&p=&m=96&k=6

  Different functions have different amounts of inputs, and
  unification isn't very slow anyway, so figuring out a single
  ideal number of hash functions is nasty, but 6 keeps things
  low even up to 10 inputs.

- https://web.archive.org/web/20210927123933/https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.72.2442&rep=rep1&type=pdf

  This is the `h1` and `h2`, both derived from `h0`.

Author: notriddle

src/librustdoc/html/static/js/search.js

@@ -2261,6 +2261,22 @@ function initSearch(rawSearchIndex) {
                     }
                 }
             } else if (parsedQuery.foundElems > 0) {
+                // Sort input and output so that generic type variables go first and
+                // types with generic parameters go last.
+                // That's because of the way unification is structured: it eats off
+                // the end, and hits a fast path if the last item is a simple atom.
+                const sortQ = (a, b) => {
+                    const ag = a.generics.length === 0 && a.bindings.size === 0;
+                    const bg = b.generics.length === 0 && b.bindings.size === 0;
+                    if (ag !== bg) {
+                        return ag - bg;
+                    }
+                    const ai = a.id > 0;
+                    const bi = b.id > 0;
+                    return ai - bi;
+                };
+                parsedQuery.elems.sort(sortQ);
+                parsedQuery.returned.sort(sortQ);
                 for (let i = 0, nSearchWords = searchWords.length; i < nSearchWords; ++i) {
                     handleArgs(searchIndex[i], i, results_others);
                 }
@@ -2823,23 +2839,44 @@ ${item.displayPath}<span class="${type}">${name}</span>\
      * @param {Set<number>} fps - Set of distinct items
      */
     function buildFunctionTypeFingerprint(type, output, fps) {
-
         let input = type.id;
         // All forms of `[]` get collapsed down to one thing in the bloom filter.
         // Differentiating between arrays and slices, if the user asks for it, is
         // still done in the matching algorithm.
         if (input === typeNameIdOfArray || input === typeNameIdOfSlice) {
             input = typeNameIdOfArrayOrSlice;
         }
+        // http://burtleburtle.net/bob/hash/integer.html
+        // ~~ is toInt32. It's used before adding, so
+        // the number stays in safe integer range.
+        const hashint1 = k => {
+            k = (~~k + 0x7ed55d16) + (k << 12);
+            k = (k ^ 0xc761c23c) ^ (k >>> 19);
+            k = (~~k + 0x165667b1) + (k << 5);
+            k = (~~k + 0xd3a2646c) ^ (k << 9);
+            k = (~~k + 0xfd7046c5) + (k << 3);
+            return (k ^ 0xb55a4f09) ^ (k >>> 16);
+        };
+        const hashint2 = k => {
+            k = ~k + (k << 15);
+            k ^= k >>> 12;
+            k += k << 2;
+            k ^= k >>> 4;
+            k = Math.imul(k, 2057);
+            return k ^ (k >> 16);
+        };
         if (input !== null) {
-            // https://docs.rs/rustc-hash/1.1.0/src/rustc_hash/lib.rs.html#60
-            // Rotate is skipped because we're only doing one cycle anyway.
-            const h0 = Math.imul(input, 0x9e3779b9);
-            const h1 = Math.imul(479001599 ^ input, 0x9e3779b9);
-            const h2 = Math.imul(433494437 ^ input, 0x9e3779b9);
-            output[0] |= 1 << (h0 % 32);
-            output[1] |= 1 << (h1 % 32);
-            output[2] |= 1 << (h2 % 32);
+            const h0a = hashint1(input);
+            const h0b = hashint2(input);
+            // Less Hashing, Same Performance: Building a Better Bloom Filter
+            // doi=10.1.1.72.2442
+            const h1a = ~~(h0a + Math.imul(h0b, 2));
+            const h1b = ~~(h0a + Math.imul(h0b, 3));
+            const h2a = ~~(h0a + Math.imul(h0b, 4));
+            const h2b = ~~(h0a + Math.imul(h0b, 5));
+            output[0] |= (1 << (h0a % 32)) | (1 << (h1b % 32));
+            output[1] |= (1 << (h1a % 32)) | (1 << (h2b % 32));
+            output[2] |= (1 << (h2a % 32)) | (1 << (h0b % 32));
             fps.add(input);
         }
         for (const g of type.generics) {
@@ -2868,7 +2905,6 @@ ${item.displayPath}<span class="${type}">${name}</span>\
      *                          This function might return 0!
      */
     function compareTypeFingerprints(fullId, queryFingerprint) {
-
         const fh0 = functionTypeFingerprint[fullId * 4];
         const fh1 = functionTypeFingerprint[(fullId * 4) + 1];
         const fh2 = functionTypeFingerprint[(fullId * 4) + 2];