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+# threejs 源码阅读
+
+## Math
+
+生成全局唯一id
+
+```js
+var generateUUID = (function() {
+  // http://stackoverflow.com/questions/105034/how-to-create-a-guid-uuid-in-javascript/21963136#21963136
+
+  var lut = [];
+
+  for (var i = 0; i < 256; i++) {
+    lut[i] = (i < 16 ? "0" : "") + i.toString(16);
+  }
+
+  return function generateUUID() {
+    var d0 = (Math.random() * 0xffffffff) | 0;
+    var d1 = (Math.random() * 0xffffffff) | 0;
+    var d2 = (Math.random() * 0xffffffff) | 0;
+    var d3 = (Math.random() * 0xffffffff) | 0;
+    var uuid =
+      lut[d0 & 0xff] +
+      lut[(d0 >> 8) & 0xff] +
+      lut[(d0 >> 16) & 0xff] +
+      lut[(d0 >> 24) & 0xff] +
+      "-" +
+      lut[d1 & 0xff] +
+      lut[(d1 >> 8) & 0xff] +
+      "-" +
+      lut[((d1 >> 16) & 0x0f) | 0x40] +
+      lut[(d1 >> 24) & 0xff] +
+      "-" +
+      lut[(d2 & 0x3f) | 0x80] +
+      lut[(d2 >> 8) & 0xff] +
+      "-" +
+      lut[(d2 >> 16) & 0xff] +
+      lut[(d2 >> 24) & 0xff] +
+      lut[d3 & 0xff] +
+      lut[(d3 >> 8) & 0xff] +
+      lut[(d3 >> 16) & 0xff] +
+      lut[(d3 >> 24) & 0xff];
+
+    // .toUpperCase() here flattens concatenated strings to save heap memory space.
+    return uuid.toUpperCase();
+  };
+})();
+```