Table based crc for computing message checksum

Author: ps2

analysis/table_crc.ipynb

@@ -0,0 +1,170 @@
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "import pandas as pd\n",
+    "%matplotlib inline\n",
+    "import matplotlib\n",
+    "import numpy as np\n",
+    "import matplotlib.pyplot as plt\n",
+    "import dateutil\n",
+    "import analysis\n",
+    "\n",
+    "packets = analysis.parse_packet_file('data/all.txt')\n",
+    "packets += analysis.parse_packet_file('data/temp_basals.txt')\n",
+    "\n",
+    "# Select valid packets of a particular length\n",
+    "packets = filter(lambda x: x.is_valid() and x.body_len == 10, packets)\n",
+    "\n",
+    "# Uncomment if you want to work with less data (faster while developing)\n",
+    "#packets = packets[0:500]"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 16,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def message_hash_split(data):\n",
+    "    # Everything except ID1, byte 4 (sequence & flags), packet crc, and 16bit chksum\n",
+    "    msg = data[5:-3]\n",
+    "    chksum = data[-3:-1]\n",
+    "    return (msg, chksum)\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 21,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'1f014829280a1d1802a82800002b7bff'"
+      ]
+     },
+     "execution_count": 21,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "msg, chksum = message_hash_split(packets[0].tx_data())\n",
+    "msg.encode('hex')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 25,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "'\\x1f\\x01H)\\xf9\\x1f\\x01H)(\\n\\x1d\\x18\\x02\\xa8(\\x00\\x00+{\\xff\\x814E'"
+      ]
+     },
+     "execution_count": 25,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "packets[0].tx_data()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 23,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "crc_table16 = [0, 32773, 32783, 10, 32795, 30, 20, 32785, 32819, 54, 60, 32825, 40, 32813, 32807, 34, 32867, 102, 108, 32873, 120, 32893, 32887, 114, 80, 32853, 32863, 90, 32843, 78, 68, 32833, 32963, 198, 204, 32969, 216, 32989, 32983, 210, 240, 33013, 33023, 250, 33003, 238, 228, 32993, 160, 32933, 32943, 170, 32955, 190, 180, 32945, 32915, 150, 156, 32921, 136, 32909, 32903, 130, 33155, 390, 396, 33161, 408, 33181, 33175, 402, 432, 33205, 33215, 442, 33195, 430, 420, 33185, 480, 33253, 33263, 490, 33275, 510, 500, 33265, 33235, 470, 476, 33241, 456, 33229, 33223, 450, 320, 33093, 33103, 330, 33115, 350, 340, 33105, 33139, 374, 380, 33145, 360, 33133, 33127, 354, 33059, 294, 300, 33065, 312, 33085, 33079, 306, 272, 33045, 33055, 282, 33035, 270, 260, 33025, 33539, 774, 780, 33545, 792, 33565, 33559, 786, 816, 33589, 33599, 826, 33579, 814, 804, 33569, 864, 33637, 33647, 874, 33659, 894, 884, 33649, 33619, 854, 860, 33625, 840, 33613, 33607, 834, 960, 33733, 33743, 970, 33755, 990, 980, 33745, 33779, 1014, 1020, 33785, 1000, 33773, 33767, 994, 33699, 934, 940, 33705, 952, 33725, 33719, 946, 912, 33685, 33695, 922, 33675, 910, 900, 33665, 640, 33413, 33423, 650, 33435, 670, 660, 33425, 33459, 694, 700, 33465, 680, 33453, 33447, 674, 33507, 742, 748, 33513, 760, 33533, 33527, 754, 720, 33493, 33503, 730, 33483, 718, 708, 33473, 33347, 582, 588, 33353, 600, 33373, 33367, 594, 624, 33397, 33407, 634, 33387, 622, 612, 33377, 544, 33317, 33327, 554, 33339, 574, 564, 33329, 33299, 534, 540, 33305, 520, 33293, 33287, 514]"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 47,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def crc16(msg):\n",
+    "    acc = 0x00\n",
+    "    for x in msg:\n",
+    "        acc = (acc >> 8) ^ crc_table16[(acc ^ ord(x)) & 0xff]\n",
+    "    return acc"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 48,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def test(packet):\n",
+    "    msg, chksum = message_hash_split(packet.tx_data())\n",
+    "    crc = crc16(msg)\n",
+    "    print \"0x%s, 0x%04x\" % (chksum.encode('hex'), crc)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 51,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "0x020a, 0x020a\n"
+     ]
+    }
+   ],
+   "source": [
+    "test(packets[6])"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.12"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}