istrain-public: from-scratch build of a passive RF train detector

Public community release. The complete working system — DSP + decoders (scripts/), web dashboard + API (dashboard/), container stack (docker/), config templates (config/) — plus APOCALYPSE-EDITION.md, the full build guide for a human or a coding agent. GPLv3 (PyEOT dependency). Scrubbed of all secrets, internal IPs, hostnames, and location detail. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
2026-07-24 23:19:16 -04:00
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+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+PyEOT End-of-Train Device Decoder
+Copyright (c) 2018 Eric Reuter
+
+This source file is subject of the GNU general public license.
+
+history:    2018-08-09 Initial Version
+
+purpose:    Misc. functions used by eot_decoder.py
+
+            Function XOR() and mod2div() adapted from:
+            https://www.geeksforgeeks.org/cyclic-redundancy-check-python/
+"""
+
+
+# XOR two strings of bytes representing binary symbols
+def xor(a, b):
+    result = []
+    for i in range(len(b)):
+        if a[i] == b[i]:
+            result.append('0')
+        else:
+            result.append('1')
+    return ''.join(result)
+
+
+# Reverse string
+def reverse(data):
+    return ''.join(data[::-1])
+
+
+# Perform modulo-2 division on two strings of binary symbols
+def mod2div(dividend, divisor):
+
+    # Number of bits to be XORed at a time.
+    pick = len(divisor)
+
+    # Slicing the dividend to appropriate
+    # length for particular step
+    tmp = dividend[0:pick]
+
+    while pick < len(dividend):
+
+        if tmp[0] == '1':
+
+            # replace the dividend by the result
+            # of XOR and pull 1 bit down
+            tmp = xor(divisor[1:], tmp[1:]) + dividend[pick]
+
+        else:   # If leftmost bit is '0'
+            # If the leftmost bit of the dividend (or the
+            # part used in each step) is 0, the step cannot
+            # use the regular divisor; we need to use an
+            # all-0s divisor.
+            tmp = xor(('0'*pick)[1:], tmp[1:]) + dividend[pick]
+
+        # increment pick to move further
+        pick += 1
+
+    # For the last n bits, we have to carry it out
+    # normally as increased value of pick will cause
+    # Index Out of Bounds.
+    if tmp[0] == '1':
+        tmp = xor(divisor[1:], tmp[1:])
+    else:
+        tmp = xor(('0'*pick)[1:], tmp[1:])
+
+    remainder = tmp
+    return remainder
+
+
+# Calculate BCH checkbits
+def checkbits(data, key):
+    appended_data = data + '0'*(len(key)-1)  # Appends n-1 zeros at end of data
+    remainder = mod2div(appended_data, key)
+    return ''.join(remainder)