カラム番号をスプレッドシートとかのカラム定義に変換するやつ（非推奨。R1C1形式を使うこと）

main.go

func columnIndexFromGenericToSpreadsheet(columnIndex int) (string, error) {
	return columnIndexFromGenericToSpreadsheetImpl(columnIndex, "ABCDEFGHIJKLMNOPQRSTUVWXYZ")
}

func columnIndexFromGenericToSpreadsheetImpl(columnIndex int, spreadsheetColumnIndexList string) (string, error) {
	var (
		genericColumnIndex = ""
		listLength         = len(spreadsheetColumnIndexList)
	)

	for {
		div := columnIndex / listLength
		if div > 0 {
			prefix, err := columnIndexFromGenericToSpreadsheetImpl(div, " ABCDEFGHIJKLMNOPQRSTUVWXYZ")
			if err != nil {
				return "", err
			}
			genericColumnIndex = prefix + genericColumnIndex
			columnIndex -= div * listLength
			spreadsheetColumnIndexList = strings.TrimSpace(spreadsheetColumnIndexList)
			continue
		}

		remain := columnIndex % listLength
		res := spreadsheetColumnIndexList[remain : remain+1]
		genericColumnIndex += res
		break
	}

	return genericColumnIndex, nil
}