struct SQLTableAlterationGenerator {
    private enum TableAlterationKind {
        case addColumn(SQLColumnGenerator)
        case addIndex(SQLIndexGenerator)
        case renameColumn(old: String, new: String)
        case dropColumn(String)
    }
    
    private var name: String
    private var alterations: [TableAlterationKind] = []
    
    func sql(_ db: Database) throws -> String {
        var statements: [String] = []
        
        for alteration in alterations {
            switch alteration {
            case let .addColumn(column):
                var chunks: [String] = []
                chunks.append("ALTER TABLE")
                chunks.append(name.quotedDatabaseIdentifier)
                chunks.append("ADD COLUMN")
                let sql = try column.sql(db, tableName: name, primaryKeyColumns: {
                    try db.primaryKey(name).columnInfos.map { columnInfos in
                        columnInfos.map { SQLColumnDescriptor($0) }
                    }
                })
                chunks.append(sql)
                let statement = chunks.joined(separator: " ")
                statements.append(statement)
                
            case let .addIndex(index):
                try statements.append(index.sql(db))
                
            case let .renameColumn(oldName, newName):
                var chunks: [String] = []
                chunks.append("ALTER TABLE")
                chunks.append(name.quotedDatabaseIdentifier)
                chunks.append("RENAME COLUMN")
                chunks.append(oldName.quotedDatabaseIdentifier)
                chunks.append("TO")
                chunks.append(newName.quotedDatabaseIdentifier)
                let statement = chunks.joined(separator: " ")
                statements.append(statement)
                
            case let .dropColumn(column):
                var chunks: [String] = []
                chunks.append("ALTER TABLE")
                chunks.append(name.quotedDatabaseIdentifier)
                chunks.append("DROP COLUMN")
                chunks.append(column.quotedDatabaseIdentifier)
                let statement = chunks.joined(separator: " ")
                statements.append(statement)
            }
        }
        
        return statements.joined(separator: "; ")
    }
}

extension SQLTableAlterationGenerator {
    init(_ tableAlteration: TableAlteration) {
        self.name = tableAlteration.name
        self.alterations = []
        
        for alteration in tableAlteration.alterations {
            switch alteration {
            case let .add(column):
                alterations.append(.addColumn(.columnDefinition(column)))
                if let indexDefinition = column.indexDefinition(in: name) {
                    alterations.append(.addIndex(SQLIndexGenerator(index: indexDefinition)))
                }
                
            case let .addColumnLiteral(sql):
                alterations.append(.addColumn(.columnLiteral(sql)))
                
            case let .rename(old: oldName, new: newName):
                alterations.append(.renameColumn(old: oldName, new: newName))
                
            case let .drop(column):
                alterations.append(.dropColumn(column))
            }
        }
    }
}