LCHCAPITALHUMAIN · August 29, 2015 14:18 · May 6, 2011
diff --git a/gistfile1.js b/gistfile1.js
@@ -0,0 +1,55 @@
+// Converts an ArrayBuffer directly to base64, without any intermediate 'convert to string then
+// use window.btoa' step. According to my tests, this appears to be a faster approach:
+// http://jsperf.com/encoding-xhr-image-data/5
+
+function base64ArrayBuffer(arrayBuffer) {
+  var base64    = ''
+  var encodings = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
+
+  var bytes         = new Uint8Array(arrayBuffer)
+  var byteLength    = bytes.byteLength
+  var byteRemainder = byteLength % 3
+  var mainLength    = byteLength - byteRemainder
+
+  var a, b, c, d
+  var chunk
+
+  // Main loop deals with bytes in chunks of 3
+  for (var i = 0; i < mainLength; i = i + 3) {
+    // Combine the three bytes into a single integer
+    chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2]
+
+    // Use bitmasks to extract 6-bit segments from the triplet
+    a = (chunk & 16515072) >> 18 // 16515072 = (2^6 - 1) << 18
+    b = (chunk & 258048)   >> 12 // 258048   = (2^6 - 1) << 12
+    c = (chunk & 4032)     >>  6 // 4032     = (2^6 - 1) << 6
+    d = chunk & 63               // 63       = 2^6 - 1
+
+    // Convert the raw binary segments to the appropriate ASCII encoding
+    base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d]
+  }
+
+  // Deal with the remaining bytes and padding
+  if (byteRemainder == 1) {
+    chunk = bytes[mainLength]
+
+    a = (chunk & 252) >> 2 // 252 = (2^6 - 1) << 2
+
+    // Set the 4 least significant bits to zero
+    b = (chunk & 3)   << 4 // 3   = 2^2 - 1
+
+    base64 += encodings[a] + encodings[b] + '=='
+  } else if (byteRemainder == 2) {
+    chunk = (bytes[mainLength] << 8) | bytes[mainLength + 1]
+
+    a = (chunk & 64512) >> 10 // 64512 = (2^6 - 1) << 10
+    b = (chunk & 1008)  >>  4 // 1008  = (2^6 - 1) << 4
+
+    // Set the 2 least significant bits to zero
+    c = (chunk & 15)    <<  2 // 15    = 2^4 - 1
+
+    base64 += encodings[a] + encodings[b] + encodings[c] + '='
+  }
+
+  return base64
+}