1 files changed, 53 insertions, 0 deletions
diff --git a/activestorage/app/javascript/activestorage/file_checksum.js b/activestorage/app/javascript/activestorage/file_checksum.js
new file mode 100644
index 0000000000..ffaec1a128
--- /dev/null
+++ b/activestorage/app/javascript/activestorage/file_checksum.js
@@ -0,0 +1,53 @@
+import SparkMD5 from "spark-md5"
+
+const fileSlice = File.prototype.slice || File.prototype.mozSlice || File.prototype.webkitSlice
+
+export class FileChecksum {
+  static create(file, callback) {
+    const instance = new FileChecksum(file)
+    instance.create(callback)
+  }
+
+  constructor(file) {
+    this.file = file
+    this.chunkSize = 2097152 // 2MB
+    this.chunkCount = Math.ceil(this.file.size / this.chunkSize)
+    this.chunkIndex = 0
+  }
+
+  create(callback) {
+    this.callback = callback
+    this.md5Buffer = new SparkMD5.ArrayBuffer
+    this.fileReader = new FileReader
+    this.fileReader.addEventListener("load", event => this.fileReaderDidLoad(event))
+    this.fileReader.addEventListener("error", event => this.fileReaderDidError(event))
+    this.readNextChunk()
+  }
+
+  fileReaderDidLoad(event) {
+    this.md5Buffer.append(event.target.result)
+
+    if (!this.readNextChunk()) {
+      const binaryDigest = this.md5Buffer.end(true)
+      const base64digest = btoa(binaryDigest)
+      this.callback(null, base64digest)
+    }
+  }
+
+  fileReaderDidError(event) {
+    this.callback(`Error reading ${this.file.name}`)
+  }
+
+  readNextChunk() {
+    if (this.chunkIndex < this.chunkCount) {
+      const start = this.chunkIndex * this.chunkSize
+      const end = Math.min(start + this.chunkSize, this.file.size)
+      const bytes = fileSlice.call(this.file, start, end)
+      this.fileReader.readAsArrayBuffer(bytes)
+      this.chunkIndex++
+      return true
+    } else {
+      return false
+    }
+  }
+}