使用 Swift 解析化学式

问：

我是编码新手，一直在通过反复试验自学。我一直在开发一个应用程序来计算化合物的摩尔质量，但似乎无法让括号正常工作。例如，Mg（NO3）2 返回具有两个 Mg 原子而不是一个原子的质量。我希望这个问题对某人来说是显而易见的，因为我已经研究了好几天了。任何建议将不胜感激！

func calculateMolarMass() {
        var totalMass = 0.0
        var i = compoundFormula.startIndex
        var massStack: [Double] = []
        var operatorStack: [Character] = []
        var openParenthesesCount = 0
        var openSquareBracketsCount = 0
        
        while i < compoundFormula.endIndex {
            let char = compoundFormula[i]
            
            if char.isLetter {
                var elementSymbol = String(char)
                i = compoundFormula.index(after: i)
                
                while i < compoundFormula.endIndex && compoundFormula[i].isLowercase {
                    elementSymbol.append(compoundFormula[i])
                    i = compoundFormula.index(after: i)
                }
                
                var subscriptValue = ""
                while i < compoundFormula.endIndex && compoundFormula[i].isNumber {
                    subscriptValue.append(compoundFormula[i])
                    i = compoundFormula.index(after: i)
                }
                
                let elementMass = atomicMasses[elementSymbol] ?? 0.0
                let multiplier = Double(subscriptValue) ?? 1.0
                
                totalMass += elementMass * multiplier
                
            } else if char == "(" || char == "[" {
                        operatorStack.append(char)
                        i = compoundFormula.index(after: i)

                        if char == "(" {
                            openParenthesesCount += 1
                        } else {
                            openSquareBracketsCount += 1
                        }

                    } else if char == ")" || char == "]" {
                        let openBracket: Character = char == ")" ? "(" : "["
                        i = compoundFormula.index(after: i)

                        while !operatorStack.isEmpty && operatorStack.last != openBracket {
                            applyOperator(&totalMass, &massStack, operatorStack.removeLast())
                        }

                        if operatorStack.isEmpty || operatorStack.last != openBracket {
                            fatalError("Mismatched parentheses/brackets")
                        }

                        if char == ")" {
                            openParenthesesCount -= 1
                        } else {
                            openSquareBracketsCount -= 1
                        }

                        operatorStack.removeLast() // Remove the open parenthesis

                        if i < compoundFormula.endIndex && compoundFormula[i].isNumber {
                            var numberString = ""
                            while i < compoundFormula.endIndex && compoundFormula[i].isNumber {
                                numberString.append(compoundFormula[i])
                                i = compoundFormula.index(after: i)
                            }
                            let multiplier = Double(numberString) ?? 1.0
                            totalMass *= multiplier
                        }
                    }

                    if openParenthesesCount < 0 || openSquareBracketsCount < 0 {
                        fatalError("Mismatched parentheses/brackets")
                    }
                }

                while !operatorStack.isEmpty {
                    if operatorStack.last == "(" || operatorStack.last == "[" {
                        fatalError("Mismatched parentheses/brackets")
                    }
                    applyOperator(&totalMass, &massStack, operatorStack.removeLast())
                }

                if openParenthesesCount != 0 || openSquareBracketsCount != 0 {
                    fatalError("Mismatched parentheses/brackets")
                }

                molarMass = totalMass
            } // Code for calculating Molar Mass