近期搞項目中遇到Bencode解碼的問題，就用Py寫了個Bencode解碼的代碼。作為筆記儲存參考。
BEncoding是BitTorrent用在傳輸資料結構的編碼方式，這種編碼方式支援四種類型的資料：string, int, Dictionary, List，各自的編碼規則如下：
string類型的編碼格式為[length]:[string]。以字元串的長度開頭，加一個冒号，并以字元串内容結束。示例："abc" => 3:abc
int類型的編碼格式為i[int]e。以i開頭，加上數字，以e結尾。 示例：123 => i123e
List類型的編碼格式為l[object]e。以l開頭，加上清單中各個元素的編碼（元素的類型同樣為BEncoding支援的類型），以e結尾。 示例：List<"abc", 123> => l3:abci123ee
Dictionary類型的編碼格式為d[Key-Value Pair]e。以d開頭，加上字典中每個鍵值對的編碼，以e結尾。
# 解析List類型，例：l3:abci123ee => List<"abc", 123>
def decode_list(str,p):
l = len(str)
# print(l)
# print(p)
list = []
while p < l:
if str[p + 1].isdigit():
f_m = str.index(":",p + 1,l)
f_n = int(str[p + 1:f_m])
f_e = f_m + f_n
data_str = str[f_m + 1:f_e + 1]
list.append(data_str)
p = f_e
elif str[p + 1] == "i":
end = str.index("e", p + 1, l)
data_int = str[p + 2: int(end)]
list.append(data_int)
p = end
elif str[p + 1] == "e":
p = p + 2
list.append("p")
list.append(p)
break
elif str[p + 1] == "l":
p = p + 1
return list
# 解析Dictionary類型，例：d4:name11:create chen3:agei23ee => Dictionary<{"name":"create chen"},{"age":23}>
def decode_dict(str,p):
test = decode_list(str,p)
data_seq = []
data_val = []
i = 0
while i < len(test):
data_seq.append(test[i])
data_val.append(test[i + 1])
i = i + 2
data_dict = dict.fromkeys(data_seq)
i = 0
while i < len(test) / 2:
data_dict[data_seq[i]] = data_val[i]
i = i + 1
return data_dict
data = "d4:name11:create chen3:agei23eel3:abci123eei23e3:age"
print(data)
# 定義全局指針，追蹤所查位置
global p
l = len(data)
p = 0
while p < l:
# 判斷是否為dict型
if data[p] == "d":
data_dict = decode_dict(data,p)
p = int(data_dict["p"])
del data_dict["p"]
print(data_dict)
f = open("text.txt","a+")
for k, v in data_dict.items():
f.write(k + ":" + str(v))
f.write("\n")
f.close()
# 判斷是否為list型
elif data[p] == "l":
data_list = decode_list(data,p)
p = int(data_list[-1])
del data_list[-1]
del data_list[-1]
print(data_list)
f = open("text.txt", "a+")
for i in data_list:
f.write(i)
f.write("\n")
f.close()
# 判斷是否為str型，解析
elif data[p] == "i":
f = data.index("e",p,l)
data_int = data[p + 1:f]
p = f + 1
print(data_int)
f = open("text.txt", "a+")
f.write(data_int)
f.write("\n")
f.close()
# 判斷是否為int型，解析（為了友善，暫未将其結果轉為int型，而是str型，例：“123”
elif data[p].isdigit():
f = data.index(":",p,l)
print(data[f])
data_str = data[f + 1:int(data[p:f]) + f + 1]
print(data_str)
p = int(data[p:f]) + f + 1
f = open("text.txt", "a+")
f.write(data_str)
f.write("\n")
f.close()
print("It's done")