使用go切片应注意的一个小问题：copy函数的重要性

起因是在写一个函数的时候

func (g *Grammar) extractCommonFactors() {
    flag := false
    newNonTerminalCounter := 0
    for i, production := range g.Productions {
        newAlternatives := []Alternative{}
        prefixMap := make(map[Symbol][]Alternative)
        for _, alternative := range production.Right {
            if len(alternative.Symbols) > 0 {
                firstSymbol := alternative.Symbols[0]
                if _, ok := prefixMap[firstSymbol]; !ok {
                    prefixMap[firstSymbol] = []Alternative{}
                }
                prefixMap[firstSymbol] = append(prefixMap[firstSymbol], alternative)
            }
        }
        for _, alternatives := range prefixMap {
            if len(alternatives) > 1 {
                flag = true
                commonPrefix := findLongestCommonPrefix(alternatives)
                newNonTerminalCounter++
                newValue := fmt.Sprintf("A%d", newNonTerminalCounter)
                newNonTerminal := Symbol{
                    Value:      newValue,
                    IsTerminal: false,
                }
                newRight := removeCommonPrefix(alternatives, commonPrefix)
                newProduction := Production{
                    Left:  newNonTerminal,
                    Right: newRight,
                }
                g.Productions = append(g.Productions, newProduction)
                var newAlternative Alternative
                newAlternative.Symbols = commonPrefix
                //bug出现的地方
                newAlternative.Symbols = append(newAlternative.Symbols, newNonTerminal)
                newAlternatives = append(newAlternatives, newAlternative)
            } else {
                newAlternatives = append(newAlternatives, alternatives[0])
            }
        }
        g.Productions[i].Right = newAlternatives
    }
    if flag {
        fmt.Println("extractCommonFactors grammar:")
        for _, prod := range g.Productions {
            rightParts := make([]string, len(prod.Right))
            for i, alt := range prod.Right {
                symbols := make([]string, len(alt.Symbols))
                for j, sym := range alt.Symbols {
                    symbols[j] = sym.Value
                }
                rightParts[i] = strings.Join(symbols, "")
            }
            fmt.Printf("%s -> %sn", prod.Left.Value, strings.Join(rightParts, "|"))
        }
    }
}

发现每次在执行

            newAlternative.Symbols = append(newAlternative.Symbols, newNonTerminal)                

时总会莫名其妙修改掉g.Productions 中的数据
原因就是因为

            var newAlternative Alternative
            newAlternative.Symbols = commonPrefix
            //bug出现的地方
            newAlternative.Symbols = append(newAlternative.Symbols, newNonTerminal)

中的newAlternative.Symbols是一个数组切片，这里直接对切片赋值，本质上是将newAlternative.Symbols的内容修改为commonPrefix切片所指向的数组，所以后续在修改newAlternative.Symbols时对别的内容也会产生影响
改正为

var newAlternative Alternative
commonPrefixCopy := make([]Symbol, len(commonPrefix))
copy(commonPrefixCopy, commonPrefix)
newAlternative.Symbols = commonPrefixCopy
newAlternative.Symbols = append(newAlternative.Symbols, newNonTerminal)

之后问题就消失了
因为copy函数是在内存中开辟一个新的数组给复制的切片使用，这样虽然消耗了更多的内存，但也更安全，是个更好的开发习惯

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