Somewhat simplified black/white balance penalty calculation in all language versions.

c/qrcodegen.c

@@ -619,10 +619,11 @@ static long getPenaltyScore(const uint8_t qrcode[]) {
 				black++;
 		}
 	}
+	// Note that size is odd, so black/total != 1/2
 	int total = qrsize * qrsize;
-	// Find smallest k such that (45-5k)% <= dark/total <= (55+5k)%
-	for (int k = 0; black*20L < (9L-k)*total || black*20L > (11L+k)*total; k++)
-		result += PENALTY_N4;
+	// Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)%
+	int k = (int)((labs(black * 20L - total * 10L) + total - 1) / total) - 1;
+	result += k * PENALTY_N4;
 	return result;
 }
 

cpp/QrCode.cpp

@@ -485,10 +485,11 @@ long QrCode::getPenaltyScore() const {
 				black++;
 		}
 	}
+	// Note that size is odd, so black/total != 1/2
 	int total = size * size;
-	// Find smallest k such that (45-5k)% <= dark/total <= (55+5k)%
-	for (int k = 0; black*20L < (9L-k)*total || black*20L > (11L+k)*total; k++)
-		result += PENALTY_N4;
+	// Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)%
+	int k = static_cast<int>((std::abs(black * 20L - total * 10L) + total - 1) / total) - 1;
+	result += k * PENALTY_N4;
 	return result;
 }
 

java/io/nayuki/qrcodegen/QrCode.java

@@ -632,10 +632,11 @@ public final class QrCode {
 					black++;
 			}
 		}
+		// Note that size is odd, so black/total != 1/2
 		int total = size * size;
-		// Find smallest k such that (45-5k)% <= dark/total <= (55+5k)%
-		for (int k = 0; black*20 < (9-k)*total || black*20 > (11+k)*total; k++)
-			result += PENALTY_N4;
+		// Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)%
+		int k = (Math.abs(black * 20 - total * 10) + total - 1) / total - 1;
+		result += k * PENALTY_N4;
 		return result;
 	}
 	

javascript/qrcodegen.js

@@ -480,10 +480,11 @@ var qrcodegen = new function() {
 						black++;
 				});
 			});
+			// Note that size is odd, so black/total != 1/2
 			var total = size * size;
-			// Find smallest k such that (45-5k)% <= dark/total <= (55+5k)%
-			for (var k = 0; black*20 < (9-k)*total || black*20 > (11+k)*total; k++)
-				result += QrCode.PENALTY_N4;
+			// Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)%
+			var k = Math.ceil(Math.abs(black * 20 - total * 10) / total) - 1;
+			result += k * QrCode.PENALTY_N4;
 			return result;
 		}
 		

python/qrcodegen.py

@@ -476,12 +476,11 @@ class QrCode(object):
 		
 		# Balance of black and white modules
 		black = sum((1 if cell else 0) for row in modules for cell in row)
+		# Note that size is odd, so black/total != 1/2
 		total = size**2
-		# Find smallest k such that (45-5k)% <= dark/total <= (55+5k)%
-		for k in itertools.count():
-			if (9-k)*total <= black*20 <= (11+k)*total:
-				break
-			result += QrCode._PENALTY_N4
+		# Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)%
+		k = (abs(black * 20 - total * 10) + total - 1) // total - 1
+		result += k * QrCode._PENALTY_N4
 		return result
 	
 	

rust/src/lib.rs

@@ -609,13 +609,11 @@ impl QrCode {
 		for color in &self.modules {
 			black += *color as i32;
 		}
+		// Note that size is odd, so black/total != 1/2
 		let total: i32 = size * size;
-		// Find smallest k such that (45-5k)% <= dark/total <= (55+5k)%
-		let mut k: i32 = 0;
-		while black*20 < (9-k)*total || black*20 > (11+k)*total {
-			result += PENALTY_N4;
-			k += 1;
-		}
+		// Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)%
+		let k: i32 = ((black * 20 - total * 10).abs() + total - 1) / total - 1;
+		result += k * PENALTY_N4;
 		result
 	}
 	

typescript/qrcodegen.ts

@@ -548,10 +548,11 @@ namespace qrcodegen {
 						black++;
 				});
 			});
+			// Note that size is odd, so black/total != 1/2
 			let total: int = this.size * this.size;
-			// Find smallest k such that (45-5k)% <= dark/total <= (55+5k)%
-			for (let k = 0; black*20 < (9-k)*total || black*20 > (11+k)*total; k++)
-				result += QrCode.PENALTY_N4;
+			// Compute the smallest integer k >= 0 such that (45-5k)% <= black/total <= (55+5k)%
+			let k: int = Math.ceil(Math.abs(black * 20 - total * 10) / total) - 1;
+			result += k * QrCode.PENALTY_N4;
 			return result;
 		}