From 6d966107ffa565578ead1f2ccb9d5f5c9a449331 Mon Sep 17 00:00:00 2001
From: msinkec <sinkec.mihael@gmail.com>
Date: Fri, 17 Nov 2023 09:30:35 +0100
Subject: [PATCH] Check contract is being called from first input.

---
 src/contracts/bsv20SellLimitOrder.ts | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/contracts/bsv20SellLimitOrder.ts b/src/contracts/bsv20SellLimitOrder.ts
index b9586efe..b1fbf6b2 100644
--- a/src/contracts/bsv20SellLimitOrder.ts
+++ b/src/contracts/bsv20SellLimitOrder.ts
@@ -12,6 +12,8 @@ import {
     assert,
     len,
     toByteString,
+    slice,
+    int2ByteString,
 } from 'scrypt-ts'
 
 /**
@@ -54,6 +56,12 @@ export class BSV20SellLimitOrder extends BSV20V2 {
 
     @method()
     public buy(amount: bigint, buyerAddr: Addr) {
+        // Ensure the contract is being called via the first input of the call tx.
+        const outpoint: ByteString =
+            this.ctx.utxo.outpoint.txid +
+            int2ByteString(this.ctx.utxo.outpoint.outputIndex, 4n)
+        assert(outpoint == slice(this.prevouts, 0n, 36n))
+
         // Check token amount doesn't exceed total.
         assert(
             this.tokenAmtSold + amount < this.tokenAmt,