From 712f97e4ef6c83dca019b8d9c37a9a150a916e56 Mon Sep 17 00:00:00 2001
From: David Woodhouse <David.Woodhouse@intel.com>
Date: Mon, 7 Dec 2015 14:31:01 +0000
Subject: [PATCH] Add heuristic for detecting RFM01-framed FSK

---
 src/pulse_detect.c | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/src/pulse_detect.c b/src/pulse_detect.c
index 85aa3ed2..c9b58307 100644
--- a/src/pulse_detect.c
+++ b/src/pulse_detect.c
@@ -335,6 +335,40 @@ void histogram_print(const histogram_t *hist) {
 
 #define TOLERANCE (0.2)		// 20% tolerance should still discern between the pulse widths: 0.33, 0.66, 1.0
 
+// Detecting the RFM01/Si4320 framed FSK is non-trivial so do it in a separate function
+static int detect_framed_fsk(histogram_t *hist_pulses, histogram_t *hist_gaps, pulse_data_t *data)
+{
+	unsigned halfbit;
+
+	// It is theoretically possible for a valid data packet to contain
+	// no double-halfbit pulses, or no single-halfbit pulses, if the
+	// packet is either *all* zeroes or repeated 010101, respectively.
+	// As long the actual devices decoders *will* work for this unlikely
+	// case, it's probably OK for the heuristic not to guess it. This
+	// function is fun enough already without coping with that...
+	if (hist_pulses->bins_count < 3 || hist_gaps->bins_count < 3)
+		return 0;
+
+	// We use the gaps not the pulses because we *consistently* see
+	// a misdetected pulse of a few (5 or 6) samples, after a frame
+	// ends with a low frequency (i.e. last data bit is a 0). See
+	// discussion in Issue #218.
+	halfbit = min(hist_gaps->bins[0].mean, hist_gaps->bins[1].mean);
+
+	// Start of frame must be 3 halfbits high, followed by 3 or 4 low.
+	if (data->pulse[0] < halfbit * 5/2 || data->pulse[0] > halfbit * 7/2 ||
+	    data->gap[0] < halfbit * 5/2 || data->gap[0] > halfbit * 9/2)
+		return 0;
+
+	// We could do a lot here, to the extent of a full decoding to check
+	// the framing. That doesn't seem necessary, as long as we don't have
+	// too many false positives. This is just a heuristic, after all.
+	if (hist_gaps->bins[2].mean < halfbit * 5/2)
+		return 0;
+
+	return 1;
+}
+
 /// Analyze the statistics of a pulse data structure and print result
 void pulse_analyzer(pulse_data_t *data)
 {
@@ -400,6 +434,12 @@ void pulse_analyzer(pulse_data_t *data)
 		device.short_limit	= min(hist_pulses.bins[0].mean, hist_pulses.bins[1].mean);		// Assume shortest pulse is half period
 		device.long_limit	= 0; // Not used
 		device.reset_limit	= hist_gaps.bins[hist_gaps.bins_count-1].max + 1;				// Set limit above biggest gap
+	} else if(detect_framed_fsk(&hist_pulses, &hist_gaps, data)) {
+		fprintf(stderr, "RFM Framed Manchester coding\n");
+		device.modulation	= FSK_PULSE_MANCHESTER_FRAMED;
+		device.short_limit	= min(hist_gaps.bins[0].mean, hist_gaps.bins[1].mean);		// Assume shortest pulse is half period
+		device.long_limit	= 0; // Not used
+		device.reset_limit	= 0; // Not used
 	} else if(hist_pulses.bins_count == 3) {
 		fprintf(stderr, "Pulse Width Modulation with startbit/delimiter\n");
 		device.modulation	= OOK_PULSE_PWM_TERNARY;