22: Implement AnchorIdlJupiterPerpsCustodyDecoder.decodeCurrentCumulativeInterestRate() method

src/main/tjava/com/r35157/libs/jupiter/perps/impl/anchoridl/AnchorIdlJupiterPerpsCustodyDecoder.tjava

@@ -23,8 +23,21 @@ class AnchorIdlJupiterPerpsCustodyDecoder {
         return readPublicKey(data, MINT_OFFSET);
     }
 
-    BigInteger decodeCurrentCumulativeInterestRate(SolanaAccountInfo custodyAccountInfo) {
+    BigInteger decodeCurrentCumulativeInterestRate(
-        return null;
+            SolanaAccountInfo custodyAccountInfo
+    ) {
+        byte[] data = Base64.getDecoder().decode(custodyAccountInfo.dataBase64());
+
+        if (data.length < CUMULATIVE_INTEREST_RATE_OFFSET + U128_LENGTH) {
+            throw new IllegalArgumentException(
+                    "Jupiter Perps custody account data is too short: " + data.length
+            );
+        }
+
+        return readU128(
+                data,
+                CUMULATIVE_INTEREST_RATE_OFFSET
+        );
     }
 
     private ΩSPLMintAddressΩ readPublicKey(
@@ -44,6 +57,23 @@ class AnchorIdlJupiterPerpsCustodyDecoder {
         return base58.encode(publicKeyBytes);
     }
 
+    private static BigInteger readU128(
+            byte[] data,
+            int offset
+    ) {
+        byte[] bytes = new byte[U128_LENGTH];
+
+        for (int i = 0; i < U128_LENGTH; i++) {
+            bytes[i] = data[offset + U128_LENGTH - 1 - i];
+        }
+
+        return new BigInteger(
+                1,
+                bytes
+        );
+    }
+
+    private static final int U128_LENGTH = 16;
     private static final int ANCHOR_DISCRIMINATOR_LENGTH = 8;
     private static final int PUBLIC_KEY_LENGTH = 32;