From d73eb1b20544071415bda7a12e31a35833e40c70 Mon Sep 17 00:00:00 2001
From: Steve Pinkham <spinkham@users.noreply.github.com>
Date: Tue, 28 Jul 2026 11:39:51 -0400
Subject: [PATCH] feat: seed the PRNG (--seed N, default random, printed for
 reproducibility)

rand() was never seeded, so every pathsim invocation produced
bit-identical fading and noise (verified byte-identical across separate
process runs): N runs intended as independent Monte-Carlo trials replay
one realization N times, silently. Seed from std::random_device by
default, accept --seed N for exact reproduction, and print the seed used
to stderr (stderr so a future stdout streaming mode stays clean).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 main.cpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/main.cpp b/main.cpp
index 9aac205..e3811df 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1,7 +1,9 @@
 #include "PathSimProcessor.h"
 #include "PathSimParams.h"
 
+#include <cstdlib>
 #include <iostream>
+#include <random>
 #include "cxxopts.h"
 #include "AudioFile.h"
 
@@ -37,6 +39,9 @@ from a Windows GUI application by Moe Wheatley, AE4JY, 2000\n\
 		}
 
 	    options.add_options("Propagation")
+	        ("seed", "PRNG seed for the fading/noise processes (default: random; "
+	                 "the seed used is printed so any run can be reproduced exactly)",
+	                 cxxopts::value<unsigned int>())
 	        ("snr", "Signal to Noise Ratio (SNR)", cxxopts::value<double>())
 	        ("spread", "Frequency spread of the 1st path [Hz]", cxxopts::value<double>())
 	        ("offset", "Frequency offset of the 1st path [Hz]", cxxopts::value<double>())
@@ -56,6 +61,20 @@ from a Windows GUI application by Moe Wheatley, AE4JY, 2000\n\
 			exit(0);
 		}
 
+        // Seed the PRNG behind the Rayleigh fading and AWGN stages. Without
+        // this, rand() runs from its default seed and EVERY invocation
+        // produces bit-identical "random" fading and noise — N runs give one
+        // realization N times, silently defeating ensemble statistics.
+        // Default is a fresh random seed; it is printed (stderr, so pipes
+        // stay clean) so any run can be reproduced with --seed.
+        {
+            unsigned int seed = result.count("seed")
+                ? result["seed"].as<unsigned int>()
+                : std::random_device{}();
+            std::srand(seed);
+            std::cerr << "pathsim: seed " << seed << std::endl;
+        }
+
         if (argc > 1 || 
             ((result.count("input_file") != 1 || result.count("output_file") != 1) &&
              result.arguments().size() != 2))  {