Use memoization

.gitignore

@@ -1,2 +1,3 @@
 node_modules/
 _docpress
+cache

index.js

@@ -1,9 +1,11 @@
 'use strict'
 
 const ware = require('ware')
+const assign = Object.assign
 
 const useCache = require('./lib/helpers/use_cache')
 const buildJs = require('./lib/build_js')
+const memoize = require('./lib/memoize')
 
 module.exports = function search (options) {
   const ctx = {}
@@ -23,23 +25,32 @@ module.exports = function search (options) {
  */
 
 function updateIndex (files, ms, done) {
-  let idx = {}
+  const idx = {}
 
   Object.keys(files).forEach((fname) => {
     const file = files[fname]
-    if (file.$) index(fname, file, idx)
+    // memoize based on file contents
+    if (file.$) {
+      const cacheKey = [ ms.directory(), fname, file.contents ]
+      const newIndices = memoize(['index', cacheKey], () => {
+        return index(fname, file)
+      })
+      assign(idx, newIndices)
+    }
   })
 
   extendIndex(files['_docpress.json'], (file) => {
     file.searchIndex = idx
-    file.lunrIndex = JSON.stringify(lunrize(idx))
+    file.lunrIndex = memoize([ 'lunr', ms.directory(), idx ], () => {
+      return JSON.stringify(lunrize(idx))
+    })
   })
 
   done()
 }
 
 /**
- * Internal: adds the search.js file
+  Internal: adds the search.js file
  */
 
 function addJs (files, ms, done) {
@@ -104,9 +115,10 @@ function extendIndex (file, block) {
  * Internal: adds new indexes to `idx` from the file `file`/`fname`.
  */
 
-function index (fname, file, idx) {
+function index (fname, file) {
   let slug = fname
   let title = file.title
+  let idx = {}
 
   file.$(':root').each(function () {
     const $this = file.$(this)
@@ -127,6 +139,8 @@ function index (fname, file, idx) {
     }
   })
 
+  return idx
+
   function initial () {
     return {
       title: title,

lib/memoize.js

@@ -0,0 +1,36 @@
+'use strict'
+
+let cache = {}
+let hits = []
+
+function memoize (keyObject, fn) {
+  const key = JSON.stringify(keyObject)
+  if (cache[key]) return cache[key]
+  cache[key] = fn()
+  hits.push(key)
+  return cache[key]
+}
+
+// Ensure that the last X keys are always available
+memoize.keepKeys = 20
+
+// Truncate every now and then
+memoize.truncateInterval = 3 * 60 * 1000
+
+// truncate the cache to the last N keys used
+memoize.prune = function prune () {
+  const preserved = hits.slice(hits.length - memoize.keepKeys)
+  const newCache = {}
+
+  preserved.forEach((key) => {
+    newCache[key] = cache[key]
+  })
+
+  hits = preserved
+  cache = newCache
+}
+
+// truncate every few minutes
+memoize.timer = setInterval(memoize.prune, memoize.truncateInterval)
+
+module.exports = memoize