From d4b836fb4ebb80359ec99b75e7a4ab4e130c657b Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Wed, 6 Mar 2024 09:30:29 +0100
Subject: [PATCH] replaced nth_element with insertion sort

---
 osd.hh | 52 +++++++++++++++++++++++++++-------------------------
 1 file changed, 27 insertions(+), 25 deletions(-)

diff --git a/osd.hh b/osd.hh
index 91b9dcf..27bc9b3 100644
--- a/osd.hh
+++ b/osd.hh
@@ -243,10 +243,10 @@ class OrderedStatisticsListDecoder
 	static const int S = sizeof(size_t);
 	static const int W = (N+S-1) & ~(S-1);
 	int8_t G[W*K];
-	int8_t codeword[W], candidate[W*2*L];
+	int8_t codeword[W], candidate[W*L];
 	int8_t softperm[W];
 	int16_t perm[W];
-	int score[2*L], scoreperm[2*L];
+	int score[L], cperm[L];
 	void row_echelon()
 	{
 		for (int k = 0; k < K; ++k) {
@@ -302,13 +302,29 @@ class OrderedStatisticsListDecoder
 		for (int i = 0; i < W; ++i)
 			codeword[i] ^= G[W*j+i];
 	}
-	static int metric(const int8_t *hard, const int8_t *soft)
+	int metric()
 	{
 		int sum = 0;
 		for (int i = 0; i < W; ++i)
-			sum += (1 - 2 * hard[i]) * soft[i];
+			sum += (1 - 2 * codeword[i]) * softperm[i];
 		return sum;
 	}
+	void update()
+	{
+		int j = L-1;
+		int met = metric();
+		if (met <= score[j])
+			return;
+		int pos = cperm[j];
+		for (int i = 0; i < W; ++i)
+			candidate[pos*W+i] = codeword[i];
+		for (; j > 0 && met > score[j-1]; --j) {
+			score[j] = score[j-1];
+			cperm[j] = cperm[j-1];
+		}
+		score[j] = met;
+		cperm[j] = pos;
+	}
 public:
 	void operator()(int *rank, uint8_t *hard, const int8_t *soft, const int8_t *genmat)
 	{
@@ -331,24 +347,11 @@ public:
 		encode();
 		for (int i = 0; i < W; ++i)
 			candidate[i] = codeword[i];
-		score[0] = metric(codeword, softperm);
-		int count = 1;
-		int worst = -1;
-		for (int i = 0; i < 2*L; ++i)
-			scoreperm[i] = i;
-		auto update = [this, &count, &worst]() {
-			int met = metric(codeword, softperm);
-			if (met > worst) {
-				score[scoreperm[count]] = met;
-				for (int i = 0; i < W; ++i)
-					candidate[scoreperm[count]*W+i] = codeword[i];
-				if (++count >= 2*L) {
-					std::nth_element(scoreperm, scoreperm+(L-1), scoreperm+2*L, [this](int a, int b){ return score[a] > score[b]; });
-					worst = score[scoreperm[L-1]];
-					count = L;
-				}
-			}
-		};
+		score[0] = metric();
+		for (int i = 1; i < L; ++i)
+			score[i] = -1;
+		for (int i = 0; i < L; ++i)
+			cperm[i] = i;
 		for (int a = 0; O >= 1 && a < K; ++a) {
 			flip(a);
 			update();
@@ -379,13 +382,12 @@ public:
 			}
 			flip(a);
 		}
-		std::sort(scoreperm, scoreperm+count, [this](int a, int b){ return score[a] > score[b]; });
 		for (int j = 0, r = 0; j < L; ++j) {
-			if (j > 0 && score[scoreperm[j-1]] != score[scoreperm[j]])
+			if (j > 0 && score[j-1] != score[j])
 				++r;
 			rank[j] = r;
 			for (int i = 0; i < N; ++i)
-				set_be_bit(hard+j*((N+7)/8), perm[i], candidate[scoreperm[j]*W+i]);
+				set_be_bit(hard+j*((N+7)/8), perm[i], candidate[cperm[j]*W+i]);
 		}
 	}
 };