Memoize hashes to FS

.gitignore

@@ -4,6 +4,7 @@ sample/img/
 sample/.cache/
 package-lock.json
 .cache
+.image-hash-cache.js
 
 # generated by tests
 test/generated-*

.npmignore

@@ -1,3 +1,4 @@
 test
 sample
-.cache
+.cache
+.image-hash-cache.js

img.js

@@ -122,10 +122,24 @@ function getValidWidths(originalWidth, widths = [], allowUpscale = false) {
   return filtered.sort((a, b) => a - b);
 }
 
+// Try to read memoized hashes from filesystem
 let imgHashCache = {};
+try {
+  imgHashCache = require(__dirname+"/.image-hash-cache.js");
+} catch(err) {
+  if(err.code !== 'MODULE_NOT_FOUND') {console.log(err);}
+}
+// Save hashes before exiting.
+process.on("exit", () => {
+  fs.writeFileSync(__dirname+"/.image-hash-cache.js", "module.exports = " + JSON.stringify(imgHashCache));
+});
 function getHash(src, imgOptions={}, length=10) {
-  if(src in imgHashCache) return imgHashCache[src];
-
+  if(src in imgHashCache) {
+    if(imgHashCache[src]["type"] === "notFile") return imgHashCache[src]["hash"];
+    if(imgHashCache[src]["mtime"] === fs.statSync(src).mtimeMs) return imgHashCache[src]["hash"];
+  }
+  imgHashCache[src] = {};
+
   const hash = createHash("sha256");
 
   let opts = Object.assign({
@@ -149,14 +163,18 @@ function getHash(src, imgOptions={}, length=10) {
   if(fs.existsSync(src)) {
     const fileContent = fs.readFileSync(src);
     hash.update(fileContent);
+    imgHashCache[src]["type"] = "file";
+    imgHashCache[src]["mtime"] = fs.statSync(src).mtimeMs;
   } else {
     hash.update(src);
+    imgHashCache[src]["type"] = "notFile";
+    imgHashCache[src]["mtime"] = 0;
   }
 
   hash.update(JSON.stringify(opts));
-  imgHashCache[src] = hash.digest("base64url").substring(0, length);
+  imgHashCache[src]["hash"] = hash.digest("base64url").substring(0, length);
 
-  return imgHashCache[src];
+  return imgHashCache[src]["hash"];
 }
 
 function getFilename(src, width, format, options = {}) {

package.json

@@ -52,6 +52,7 @@
     ],
     "ignoredByWatcher": [
       "./.cache/*",
+      "./.image-hash-cache.js",
       "./img/*",
       "./test/img/*",
       "./test/**/generated*"