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The simplified molecular input line entry system (smiles) is a specification that uses ASCII strings to describe the molecular structure. It was developed by David Weininger and Arthur Weininger in the late 1980s, and modified and extended by others, especially sunlight Chemical Information System Co., Ltd. SMILES strings can be imported by most molecular editing software and converted into two-dimensional figures or three-dimensional models of molecules. To convert a 2D drawing, you can use hellson's structure diagram generation algorithms. Because smiles describes a three-dimensional chemical structure with a string of characters, it must transform the chemical structure into a spanning tree. This system adopts the longitudinal first traversal tree algorithm. In the process of conversion, the hydrogen must be removed first and the ring must be opened. In representation, the atoms at the removed bond end are marked with numbers and the branch chains are written in parentheses. The uniqueness of smiles for each structure depends on the normalization algorithm used to generate it and is called canonical smiles. A common application of the specification of smiles is to index and ensure the uniqueness of the molecules in the database.