fix: perf regression on hot string munging path

This doesn't change any functionality, but it optimizes a few extremely
hot code paths for the input typically encountered during a large npm
project installation.

`String.normalize()` is cached, and trailing-slash removal is done with
a single `String.slice()`, rather than multiple slices and
`String.length` comparisons.

It is extremely rare that any code path is ever hot enough for this kind
of thing to be relevant enough to justify this sort of
microoptimization, but these two issues resulted in a 25-50% install
time increase in some cases, which is fairly dramatic.

Fix: npm/cli#3676

PR-URL: #286
Credit: @isaacs
Close: #286
Reviewed-by: @wraithgar

Author: isaacs

Diff for: lib/normalize-unicode.js

@@ -0,0 +1,11 @@
+// warning: extremely hot code path.
+// This has been meticulously optimized for use
+// within npm install on large package trees.
+// Do not edit without careful benchmarking.
+const normalizeCache = Object.create(null)
+const {hasOwnProperty} = Object.prototype
+module.exports = s => {
+  if (!hasOwnProperty.call(normalizeCache, s))
+    normalizeCache[s] = s.normalize('NFKD')
+  return normalizeCache[s]
+}

Diff for: lib/path-reservations.js

@@ -7,7 +7,7 @@
 // while still allowing maximal safe parallelization.
 
 const assert = require('assert')
-const normPath = require('./normalize-windows-path.js')
+const normalize = require('./normalize-unicode.js')
 const stripSlashes = require('./strip-trailing-slashes.js')
 const { join } = require('path')
 
@@ -28,7 +28,7 @@ module.exports = () => {
   const getDirs = path => {
     const dirs = path.split('/').slice(0, -1).reduce((set, path) => {
       if (set.length)
-        path = normPath(join(set[set.length - 1], path))
+        path = join(set[set.length - 1], path)
       set.push(path || '/')
       return set
     }, [])
@@ -116,9 +116,8 @@ module.exports = () => {
     // So, we just pretend that every path matches every other path here,
     // effectively removing all parallelization on windows.
     paths = isWindows ? ['win32 parallelization disabled'] : paths.map(p => {
-      return stripSlashes(normPath(join(p)))
-        .normalize('NFKD')
-        .toLowerCase()
+      // don't need normPath, because we skip this entirely for windows
+      return normalize(stripSlashes(join(p))).toLowerCase()
     })
 
     const dirs = new Set(

Diff for: lib/strip-trailing-slashes.js

@@ -1,24 +1,13 @@
-// this is the only approach that was significantly faster than using
-// str.replace(/\/+$/, '') for strings ending with a lot of / chars and
-// containing multiple / chars.
-const batchStrings = [
-  '/'.repeat(1024),
-  '/'.repeat(512),
-  '/'.repeat(256),
-  '/'.repeat(128),
-  '/'.repeat(64),
-  '/'.repeat(32),
-  '/'.repeat(16),
-  '/'.repeat(8),
-  '/'.repeat(4),
-  '/'.repeat(2),
-  '/',
-]
-
+// warning: extremely hot code path.
+// This has been meticulously optimized for use
+// within npm install on large package trees.
+// Do not edit without careful benchmarking.
 module.exports = str => {
-  for (const s of batchStrings) {
-    while (str.length >= s.length && str.slice(-1 * s.length) === s)
-      str = str.slice(0, -1 * s.length)
+  let i = str.length - 1
+  let slashesStart = -1
+  while (i > -1 && str.charAt(i) === '/') {
+    slashesStart = i
+    i--
   }
-  return str
+  return slashesStart === -1 ? str : str.slice(0, slashesStart)
 }

Diff for: lib/unpack.js

@@ -17,6 +17,7 @@ const pathReservations = require('./path-reservations.js')
 const stripAbsolutePath = require('./strip-absolute-path.js')
 const normPath = require('./normalize-windows-path.js')
 const stripSlash = require('./strip-trailing-slashes.js')
+const normalize = require('./normalize-unicode.js')
 
 const ONENTRY = Symbol('onEntry')
 const CHECKFS = Symbol('checkFs')
@@ -101,8 +102,7 @@ const uint32 = (a, b, c) =>
 // Note that on windows, we always drop the entire cache whenever a
 // symbolic link is encountered, because 8.3 filenames are impossible
 // to reason about, and collisions are hazards rather than just failures.
-const cacheKeyNormalize = path => stripSlash(normPath(path))
-  .normalize('NFKD')
+const cacheKeyNormalize = path => normalize(stripSlash(normPath(path)))
   .toLowerCase()
 
 const pruneCache = (cache, abs) => {

Diff for: test/normalize-unicode.js

@@ -0,0 +1,12 @@
+const t = require('tap')
+const normalize = require('../lib/normalize-unicode.js')
+
+// café
+const cafe1 = Buffer.from([0x63, 0x61, 0x66, 0xc3, 0xa9]).toString()
+
+// cafe with a `
+const cafe2 = Buffer.from([0x63, 0x61, 0x66, 0x65, 0xcc, 0x81]).toString()
+
+t.equal(normalize(cafe1), normalize(cafe2), 'matching unicodes')
+t.equal(normalize(cafe1), normalize(cafe2), 'cached')
+t.equal(normalize('foo'), 'foo', 'non-unicdoe string')

Diff for: test/strip-trailing-slashes.js

@@ -3,5 +3,6 @@ const stripSlash = require('../lib/strip-trailing-slashes.js')
 const short = '///a///b///c///'
 const long = short.repeat(10) + '/'.repeat(1000000)
 
+t.equal(stripSlash('no slash'), 'no slash')
 t.equal(stripSlash(short), '///a///b///c')
 t.equal(stripSlash(long), short.repeat(9) + '///a///b///c')