13: Decode Jupiter Perps position - entry price only

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

@@ -3,14 +3,55 @@ package com.r35157.libs.jupiter.perps.impl.anchoridl;
 import com.r35157.libs.jupiter.perps.JupiterPerpsPosition;
 import com.r35157.libs.solana.SolanaAccountInfo;
 
+import java.math.BigDecimal;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.Base64;
+
 class AnchorIdlJupiterPerpsPositionDecoder {
 
     JupiterPerpsPosition decode(
             ΩJupiterPerpsPositionAccountΩ positionAccount,
             SolanaAccountInfo accountInfo
     ) {
-        throw new UnsupportedOperationException(
+        byte[] data = Base64.getDecoder().decode(accountInfo.dataBase64());
-                "Jupiter Perps position decoding is not implemented yet."
+
+        if (data.length < PRICE_OFFSET + U64_LENGTH) {
+            throw new IllegalArgumentException(
+                    "Jupiter Perps position account data is too short: " + data.length
             );
         }
+
+        long rawEntryPrice = ByteBuffer
+                .wrap(data, PRICE_OFFSET, U64_LENGTH)
+                .order(ByteOrder.LITTLE_ENDIAN)
+                .getLong();
+
+        ΩUSDCPriceΩ entryPrice = BigDecimal
+                .valueOf(rawEntryPrice)
+                .movePointLeft(6);
+
+        JupiterPerpsPosition pos = new JupiterPerpsPosition(
+                positionAccount,
+                entryPrice
+        );
+
+        return pos;
+    }
+
+    private static final int ANCHOR_DISCRIMINATOR_LENGTH = 8;
+    private static final int PUBLIC_KEY_LENGTH = 32;
+    private static final int I64_LENGTH = 8;
+    private static final int U64_LENGTH = 8;
+    private static final int SIDE_ENUM_LENGTH = 1;
+
+    private static final int PRICE_OFFSET =
+            ANCHOR_DISCRIMINATOR_LENGTH
+                    + PUBLIC_KEY_LENGTH // owner
+                    + PUBLIC_KEY_LENGTH // pool
+                    + PUBLIC_KEY_LENGTH // custody
+                    + PUBLIC_KEY_LENGTH // collateralCustody
+                    + I64_LENGTH        // openTime
+                    + I64_LENGTH        // updateTime
+                    + SIDE_ENUM_LENGTH; // side
 }