From a2125f33b6088acb1fb7a71e2a85c8c151da10f1 Mon Sep 17 00:00:00 2001
From: Scott Griffiths <dr.scottgriffiths@gmail.com>
Date: Wed, 7 Feb 2024 15:49:25 +0000
Subject: [PATCH] Making it so that build doesn't modify the value list
 supplied.

---
 bitformat/format.py | 27 ++++++++++++++-------------
 tests/test_field.py |  2 +-
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/bitformat/format.py b/bitformat/format.py
index 3153358..751cecf 100644
--- a/bitformat/format.py
+++ b/bitformat/format.py
@@ -71,11 +71,11 @@ def parse(self, b: Bits) -> int:
         return self._parse(b, {})
 
     @abc.abstractmethod
-    def _build(self, values: List[Any], vars_: Dict[str, Any]) -> Bits:
+    def _build(self, values: List[Any], index: int, vars_: Dict[str, Any]) -> Tuple[Bits, int]:
         ...
 
-    def build(self, values: List[Any] = []) -> Bits:
-        return self._build(values, {})
+    def build(self, values: List[Any]) -> Bits:
+        return self._build(values, 0, {})[0]
 
     @abc.abstractmethod
     def bits(self) -> Bits:
@@ -207,16 +207,15 @@ def _parse(self, b: Bits, vars_: Dict[str, Any]) -> int:
             self._setvalue(b[:self.dtype.bitlength * self.items])
             return self.dtype.bitlength * self.items
 
-    def _build(self, values: List[Any], vars_: Dict[str, Any]) -> Bits:
+    def _build(self, values: List[Any], index: int, vars_: Dict[str, Any]) -> Tuple[Bits, int]:
         if self.const or self._bits:
-            return self._bits
+            return self._bits, 0
         if self.items_expression is not None:
             self.items = self.items_expression.safe_eval(vars_)
-        self._setvalue(values[0])
+        self._setvalue(values[index])
         if self.name != '':
             vars_[self.name] = self.value
-        values.pop(0)
-        return self._bits
+        return self._bits, 1
 
     def bits(self) -> Bits:
         return self._bits if self._bits is not None else Bits()
@@ -372,10 +371,12 @@ class FieldListType(FieldType):
     def __init__(self) -> None:
         self.fieldtypes = []
 
-    def _build(self, values: List[Any] = [], _vars={}) -> Bits:
+    def _build(self, values: List[Any], index: int, _vars={}) -> Tuple[Bits, int]:
+        values_used = 0
         for fieldtype in self.fieldtypes:
-            fieldtype._build(values, _vars)
-        return self.bits()
+            _, v = fieldtype._build(values, index + values_used, _vars)
+            values_used += v
+        return self.bits(), values_used
 
     def _parse(self, b: Bits, vars_: Dict[str, Any]) -> int:
         pos = 0
@@ -504,8 +505,8 @@ def __init__(self, bits: Bits | str, bytealigned: bool = True, name: str = ''):
         self.bytealigned = bytealigned
         self.name = name
 
-    def _build(self, values: List[Any] = [], _vars={}) -> Bits:
-        return Bits()
+    def _build(self, values: List[Any], index: int, _vars: Dict[str, Any]) -> Tuple[Bits, int]:
+        return Bits(), 0
 
     def bits(self) -> Bits:
         return Bits()
diff --git a/tests/test_field.py b/tests/test_field.py
index d2ab3dd..b386480 100644
--- a/tests/test_field.py
+++ b/tests/test_field.py
@@ -70,4 +70,4 @@ def testStringCreationWithConst(self):
         f1.clear()
         f2.clear()
         self.assertEqual(f1.build([0]), '0b0')
-        self.assertEqual(f2.build(), '0b1')
\ No newline at end of file
+        self.assertEqual(f2.build([]), '0b1')
\ No newline at end of file