fix(fw): max stack height calculation in __add__

Author: pdobacz

src/ethereum_test_vm/bytecode.py

@@ -148,12 +148,35 @@ def __add__(self, other: "Bytecode | int | None") -> "Bytecode":
         a_min, a_max = self.min_stack_height, self.max_stack_height
         b_pop, b_push = other.popped_stack_items, other.pushed_stack_items
         b_min, b_max = other.min_stack_height, other.max_stack_height
-        a_out = a_min - a_pop + a_push
 
-        c_pop = max(0, a_pop + (b_pop - a_push))
-        c_push = max(0, a_push + b_push - b_pop)
-        c_min = a_min if a_out >= b_min else (b_min - a_out) + a_min
-        c_max = max(a_max + max(0, b_min - a_out), b_max + max(0, a_out - b_min))
+        # NOTE: "_pop" is understood as the number of elements required by an instruction or
+        # bytecode to be popped off the stack before it starts returning (pushing).
+
+        # Auxiliary variables representing "stages" of the execution of `c = a + b` bytecode:
+        # Assume starting point 0 as reference:
+        a_start = 0
+        # A (potentially) pops some elements and reaches its "bottom", might be negative:
+        a_bottom = a_start - a_pop
+        # After this A pushes some elements, then B pops and reaches its "bottom":
+        b_bottom = a_bottom + a_push - b_pop
+
+        # C's bottom is either at the bottom of A or B:
+        c_bottom = min(a_bottom, b_bottom)
+        if c_bottom == a_bottom:
+            # C pops the same as A to reach its bottom, then the rest of A and B are C's "push"
+            c_pop = a_pop
+            c_push = a_push - b_pop + b_push
+        else:
+            # A and B are C's "pop" to reach its bottom, then pushes the same as B
+            c_pop = a_pop - a_push + b_pop
+            c_push = b_push
+
+        # C's minimum required stack is either A's or B's shifted by the net stack balance of A
+        c_min = max(a_min, b_min + a_pop - a_push)
+
+        # C starts from c_min, then reaches max either in the spot where A reached a_max or in the
+        # spot where B reached b_max, after A had completed.
+        c_max = max(c_min + a_max - a_min, c_min - a_pop + a_push + b_max - b_min)
 
         return Bytecode(
             bytes(self) + bytes(other),