From 809ab3690be0a38041c39b1bb7e95796496b599e Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Tue, 9 Jun 2020 14:36:19 +0200
Subject: [PATCH] return -1 if unsure

---
 short_bch_code_decoder.hh         | 16 +++++++++++++++-
 tests/soft_bch_regression_test.cc |  4 ++--
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/short_bch_code_decoder.hh b/short_bch_code_decoder.hh
index fa292fc..775f25d 100644
--- a/short_bch_code_decoder.hh
+++ b/short_bch_code_decoder.hh
@@ -62,15 +62,20 @@ public:
 	{
 		// maximum likelihood
 		if (0) {
-			int word = 0, best = 0;
+			int word = 0, best = -1, next = -1;
 			for (int msg = 0; msg < W; ++msg) {
 				int enc = (msg << P) | par[msg];
 				int met = metric(code, enc);
 				if (met > best) {
+					next = best;
 					best = met;
 					word = enc;
+				} else if (met > next) {
+					next = met;
 				}
 			}
+			if (best == next)
+				return -1;
 			return word;
 		}
 		int cw = 0;
@@ -82,6 +87,7 @@ public:
 			return word;
 		// metric of hard decision
 		int best = metric(code, word);
+		int next = -1;
 		// flip each bit and see ..
 		if (0) {
 			for (int j = 0; j < N; ++j) {
@@ -89,8 +95,11 @@ public:
 				int dec = (*this)(cw ^ tmp);
 				int met = metric(code, dec);
 				if (met > best) {
+					next = best;
 					best = met;
 					word = dec;
+				} else if (word != dec && met > next) {
+					next = met;
 				}
 			}
 		}
@@ -122,11 +131,16 @@ public:
 				int dec = (*this)(cw ^ tmp);
 				int met = metric(code, dec);
 				if (met > best) {
+					next = best;
 					best = met;
 					word = dec;
+				} else if (word != dec && met > next) {
+					next = met;
 				}
 			}
 		}
+		if (best == next)
+			return -1;
 		return word;
 	}
 };
diff --git a/tests/soft_bch_regression_test.cc b/tests/soft_bch_regression_test.cc
index f47b8ee..e291b6e 100644
--- a/tests/soft_bch_regression_test.cc
+++ b/tests/soft_bch_regression_test.cc
@@ -91,8 +91,8 @@ int main()
 				awgn_errors += noisy[i] * orig[i] < 0;
 			for (int i = 0; i < CODE_LEN; ++i)
 				quantization_erasures += !noisy[i];
-			uncorrected_errors += popcnt(enc^dec);
-			for (int i = 0; i < CODE_LEN; ++i)
+			uncorrected_errors += dec < 0 ? CODE_LEN : popcnt(enc^dec);
+			for (int i = 0; dec >= 0 && i < CODE_LEN; ++i)
 				decoder_errors += ((enc^dec)&(1<<i)) && orig[i] * noisy[i] > 0;
 		}
 		float bit_error_rate = (float)uncorrected_errors / (float)(CODE_LEN * LOOPS);