title: How to parse hid report descriptor?
body: <p>All the tutorials I've read about usb hid report descriptors use numbers to represent data. Where do these numbers come from?</p>
  
  <p>Example:</p>
  
  <pre><code>0x05, 0x01,                    // USAGE_PAGE (Generic Desktop)
  0x09, 0x02,                    // USAGE (Mouse)
  0xa1, 0x01,                    // COLLECTION (Application)
  0x09, 0x01,                    //   USAGE (Pointer)
  0xa1, 0x00,                    //   COLLECTION (Physical)
  0x05, 0x09,                    //     USAGE_PAGE (Button)
  0x19, 0x01,                    //     USAGE_MINIMUM (Button 1)
  0x29, 0x03,                    //     USAGE_MAXIMUM (Button 3)
  0x15, 0x00,                    //     LOGICAL_MINIMUM (0)
  0x25, 0x01,                    //     LOGICAL_MAXIMUM (1)
  0x95, 0x03,                    //     REPORT_COUNT (3)
  0x75, 0x01,                    //     REPORT_SIZE (1)
  0x81, 0x02,                    //     INPUT (Data,Var,Abs)
  0x95, 0x01,                    //     REPORT_COUNT (1)
  0x75, 0x05,                    //     REPORT_SIZE (5)
  0x81, 0x03,                    //     INPUT (Cnst,Var,Abs)
  0x05, 0x01,                    //     USAGE_PAGE (Generic Desktop)
  0x09, 0x30,                    //     USAGE (X)
  0x09, 0x31,                    //     USAGE (Y)
  0x15, 0x81,                    //     LOGICAL_MINIMUM (-127)
  0x25, 0x7f,                    //     LOGICAL_MAXIMUM (127)
  0x75, 0x08,                    //     REPORT_SIZE (8)
  0x95, 0x02,                    //     REPORT_COUNT (2)
  0x81, 0x06,                    //     INPUT (Data,Var,Rel)
  0xc0,                          //   END_COLLECTION
  0xc0                           // END_COLLECTION
  </code></pre>
  
  <p>For example, where does the 0x05 and 0x01 in the first line come from? How does that transalate to // USAGE_PAGE (Generic Desktop)? What are valid values? How are these ordered? How is nesting defined? </p>