From 7fb76ed7191ecad7ad7b100c35196e5424a5b3dc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bastian=20M=C3=BCller?= <bastian@turbolent.com>
Date: Mon, 2 Dec 2024 10:02:00 -0800
Subject: [PATCH] address at index is only valid in range [1 .. 2^45 - 1]

---
 contracts/LinearCodeAddressGenerator.cdc  |  7 +++-
 tests/LinearCodeAddressGenerator_test.cdc | 39 ++++++++++++++++++++++-
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/contracts/LinearCodeAddressGenerator.cdc b/contracts/LinearCodeAddressGenerator.cdc
index d868ef06..b9b9003a 100644
--- a/contracts/LinearCodeAddressGenerator.cdc
+++ b/contracts/LinearCodeAddressGenerator.cdc
@@ -91,7 +91,12 @@ contract LinearCodeAddressGenerator {
 
     /// Returns the address at the given index, for the given chain.
     access(all)
-    fun address(at index: UInt64, chain: Chain): Address {
+    fun address(at index: UInt64, chain: Chain): Address? {
+        // The index must be in the range [1 .. 2^45 - 1]
+        // TODO: Use Cadence exponentiation operator, once available
+        if index < 1 || index > 35184372088831 {
+            return nil
+        }
         return Address(self.encodeWord(index) ^ self.invalidCodeWord(forChain: chain))
     }
 
diff --git a/tests/LinearCodeAddressGenerator_test.cdc b/tests/LinearCodeAddressGenerator_test.cdc
index c1b78373..969e1d05 100644
--- a/tests/LinearCodeAddressGenerator_test.cdc
+++ b/tests/LinearCodeAddressGenerator_test.cdc
@@ -22,7 +22,7 @@ fun generateAddresses(
             at: index,
             chain: chain
         )
-        addresses.append(address)
+        addresses.append(address!)
     }
 
     return addresses
@@ -127,3 +127,40 @@ fun testTransient() {
         ]
     )
 }
+
+access(all)
+fun testAddressAtIndex() {
+
+    let minIndex: UInt64 = 1
+    let maxIndex: UInt64 = 35184372088831
+
+    for chain in [
+        LinearCodeAddressGenerator.Chain.Mainnet,
+        LinearCodeAddressGenerator.Chain.Testnet,
+        LinearCodeAddressGenerator.Chain.Transient
+    ] {
+        let lessThanMinAddress = LinearCodeAddressGenerator.address(
+            at: minIndex - 1,
+            chain: chain
+        )
+        Test.assert(lessThanMinAddress == nil)
+
+        let minAddress = LinearCodeAddressGenerator.address(
+            at: minIndex,
+            chain: chain
+        )
+        Test.assert(minAddress != nil)
+
+        let maxAddress = LinearCodeAddressGenerator.address(
+            at: maxIndex,
+            chain: chain
+        )
+        Test.assert(maxAddress != nil)
+
+        let greaterThanMaxAddress = LinearCodeAddressGenerator.address(
+            at: 35184372088832,
+            chain: chain
+        )
+        Test.assert(greaterThanMaxAddress == nil)
+    }
+}
\ No newline at end of file