OP_TOALTSTACK OP_FROMALTSTACK opcodes

src/script/engine.zig

@@ -116,6 +116,8 @@ pub const Engine = struct {
             Opcode.OP_NOP => try self.opNop(),
             Opcode.OP_VERIFY => try self.opVerify(),
             Opcode.OP_RETURN => try self.opReturn(),
+            Opcode.OP_TOALTSTACK => try self.opToAltStack(),
+            Opcode.OP_FROMALTSTACK => try self.opFromAltStack(),
             Opcode.OP_2DROP => try self.op2Drop(),
             Opcode.OP_2DUP => try self.op2Dup(),
             Opcode.OP_3DUP => try self.op3Dup(),
@@ -269,6 +271,18 @@ pub const Engine = struct {
         return error.EarlyReturn;
     }
 
+    /// OP_TOALTSTACK: Puts the value onto the top of the alt stack, and removes it from the main stack.
+    fn opToAltStack(self: *Engine) EngineError!void {
+        const value = try self.stack.pop();
+        try self.alt_stack.pushElement(value);
+    }
+
+    /// OP_FROMALTSTACK: Puts the value onto the top of the main stack, and removes it from the alt stack.
+    fn opFromAltStack(self: *Engine) EngineError!void {
+        const value = try self.alt_stack.pop();
+        try self.stack.pushElement(value);
+    }
+
     /// OP_2DROP: Drops top 2 stack items
     fn op2Drop(self: *Engine) !void {
         const first = try self.stack.pop();
@@ -527,6 +541,28 @@ test "Script execution - OP_RETURN" {
     }
 }
 
+test "Script execution - OP_TOALTSTACK OP_FROMALTSTACK" {
+    const allocator = std.testing.allocator;
+
+    // Simple script: OP_1 OP_TOALTSTACK OP_FROMALTSTACK
+    const script_bytes = [_]u8{
+        Opcode.OP_1.toBytes(),
+        Opcode.OP_2.toBytes(),
+        Opcode.OP_TOALTSTACK.toBytes(),
+        Opcode.OP_TOALTSTACK.toBytes(),
+        Opcode.OP_FROMALTSTACK.toBytes(),
+    };
+    const script = Script.init(&script_bytes);
+
+    var engine = Engine.init(allocator, script, .{});
+    defer engine.deinit();
+
+    try engine.execute();
+
+    try std.testing.expectEqual(1, try engine.stack.peekInt(0));
+    try std.testing.expectEqual(2, try engine.alt_stack.peekInt(0));
+}
+
 test "Script execution - OP_1 OP_1 OP_1 OP_2Drop" {
     const allocator = std.testing.allocator;