GH-4950 LMDB: use additional zigzag encoding for double exponents

Author: kenwenzel

core/sail/lmdb/src/main/java/org/eclipse/rdf4j/sail/lmdb/inlined/Decimals.java

@@ -16,6 +16,7 @@
 import org.eclipse.rdf4j.model.Literal;
 import org.eclipse.rdf4j.model.ValueFactory;
 import org.eclipse.rdf4j.sail.lmdb.ValueIds;
+import org.eclipse.rdf4j.sail.lmdb.Varint;
 
 public class Decimals {
 
@@ -68,12 +69,11 @@ public static int encodeExponent10Bits(int exponent11) {
 
 		// Normal number, unbiased exponent in [-1022, 1023]
 		int unbiasedExp = exponent11 - 1023;
-		int encoded = unbiasedExp + 511; // Shift range to [0, 1023]
-		if (encoded < 1 || encoded > 1022) {
+		if (unbiasedExp < 1 || unbiasedExp > 1022) {
 			// Out of range for 10 bits (excluding reserved 0 and 0x3FF)
 			return -1;
 		}
-		return encoded;
+		return (int) Integers.encodeZigZag(unbiasedExp);
 	}
 
 	/**
@@ -91,8 +91,7 @@ public static int decodeExponent10Bits(int encoded) {
 			// Reserved for NaN/Inf
 			return 0x7FF;
 		}
-		// Normal
-		int unbiased = encoded - 511;
+		int unbiased = (int) Integers.decodeZigZag(encoded);
 		return unbiased + 1023;
 	}