Always validate inputs and handle edge cases before performing algorithmic operations, especially when dealing with indices, state transitions, or data structure traversals. Check for boundary conditions, invalid states, and null/undefined values that could cause out-of-bounds access or undefined behavior. Key practices: - Validate array indices before access - Check for special sentinel values (like -1 or null states) - Ensure required fields exist before using them - Handle empty or invalid input gracefully Example from parse table operations: ```cpp // Before: Direct access without validation *state_index = new_state_ids[*state_index]; // After: Validate before accessing if (*state_index != (ParseStateId)(-1)) *state_index = new_state_ids[*state_index]; // Also ensure meaningful fields exist if ((action.type == ParseActionTypeShift && !action.extra) || action.type == ParseActionTypeRecover) fn(&action.state_index); ``` This prevents crashes from invalid indices and ensures algorithms only operate on valid data, making code more robust and maintainable.