tag-validator.py

# Time:  O(n)
# Space: O(n)

# Given a string representing a code snippet,
# you need to implement a tag validator to parse the code and return whether it is valid.
# A code snippet is valid if all the following rules hold:
#
# 1. The code must be wrapped in a valid closed tag. Otherwise, the code is invalid.
# 2. A closed tag (not necessarily valid) has exactly the following format :
#    <TAG_NAME>TAG_CONTENT</TAG_NAME>. Among them, <TAG_NAME> is the start tag,
#    and </TAG_NAME> is the end tag. The TAG_NAME in start and end tags should be the same.
#    A closed tag is valid if and only if the TAG_NAME and TAG_CONTENT are valid.
# 3. A valid TAG_NAME only contain upper-case letters, and has length in range [1,9]. 
#   Otherwise, the TAG_NAME is invalid.
# 4. A valid TAG_CONTENT may contain other valid closed tags,
#    cdata and any characters (see note1) EXCEPT unmatched <,
#    unmatched start and end tag, and unmatched or closed tags with invalid TAG_NAME.
#    Otherwise, the TAG_CONTENT is invalid.
# 5. A start tag is unmatched if no end tag exists with the same TAG_NAME,
#    and vice versa. However, you also need to consider the issue of unbalanced when tags are nested.
# 6. A < is unmatched if you cannot find a subsequent >.
#    And when you find a < or </, all the subsequent characters until the next > should be
#    parsed as TAG_NAME (not necessarily valid).
# 7. The cdata has the following format : <![CDATA[CDATA_CONTENT]]>.
#    The range of CDATA_CONTENT is defined as the characters between <![CDATA[ and the first subsequent ]]>.
# 8. CDATA_CONTENT may contain any characters.
#    The function of cdata is to forbid the validator to parse CDATA_CONTENT,
#    so even it has some characters that can be parsed as tag (no matter valid or invalid),
#    you should treat it as regular characters.
#
# Valid Code Examples:
# Input: "<DIV>This is the first line <![CDATA[<div>]]></DIV>"
# Output: True
# Explanation: 
# The code is wrapped in a closed tag : <DIV> and </DIV>. 
# The TAG_NAME is valid, the TAG_CONTENT consists of some characters and cdata. 
# Although CDATA_CONTENT has unmatched start tag with invalid TAG_NAME,
# it should be considered as plain text, not parsed as tag.
# So TAG_CONTENT is valid, and then the code is valid. Thus return true.
#
# Input: "<DIV>>>  ![cdata[]] <![CDATA[<div>]>]]>]]>>]</DIV>"
# Output: True
# Explanation:
# We first separate the code into : start_tag|tag_content|end_tag.
# start_tag -> "<DIV>"
# end_tag -> "</DIV>"
# tag_content could also be separated into : text1|cdata|text2.
# text1 -> ">>  ![cdata[]] "
# cdata -> "<![CDATA[<div>]>]]>", where the CDATA_CONTENT is "<div>]>"
# text2 -> "]]>>]"
#
# The reason why start_tag is NOT "<DIV>>>" is because of the rule 6.
# The reason why cdata is NOT "<![CDATA[<div>]>]]>]]>" is because of the rule 7.
# Invalid Code Examples:
# Input: "<A>  <B> </A>   </B>"
# Output: False
# Explanation: Unbalanced. If "<A>" is closed, then "<B>" must be unmatched, and vice versa.
#
# Input: "<DIV>  div tag is not closed  <DIV>"
# Output: False
#
# Input: "<DIV>  unmatched <  </DIV>"
# Output: False
#
# Input: "<DIV> closed tags with invalid tag name  <b>123</b> </DIV>"
# Output: False
#
# Input: "<DIV> unmatched tags with invalid tag name  </1234567890> and <CDATA[[]]>  </DIV>"
# Output: False
#
# Input: "<DIV>  unmatched start tag <B>  and unmatched end tag </C>  </DIV>"
# Output: False
# Note:
# For simplicity, you could assume the input code (including the any characters mentioned above)
# only contain letters, digits, '<','>','/','!','[',']' and ' '.

class Solution(object):
    def isValid(self, code):
        """
        :type code: str
        :rtype: bool
        """
        def validText(s, i):
            j = i
            i = s.find("<", i)
            return i != j, i

        def validCData(s, i):
            if s.find("<![CDATA[", i) != i:
                return False, i
            j = s.find("]]>", i)
            if j == -1:
                return False, i
            return True, j+3

        def parseTagName(s, i):
            if s[i] != '<':
                return "", i
            j = s.find('>', i)
            if j == -1 or not (1 <= (j-1-i) <= 9):
                return "", i
            tag = s[i+1:j]
            for c in tag:
                if not (ord('A') <= ord(c) <= ord('Z')):
                    return "", i
            return tag, j+1

        def parseContent(s, i):
            while i < len(s):
                result, i = validText(s, i)
                if result:
                    continue
                result, i = validCData(s, i)
                if result:
                    continue
                result, i = validTag(s, i)
                if result:
                    continue
                break
            return i

        def validTag(s, i):
            tag, j = parseTagName(s, i)
            if not tag:
                return False, i
            j = parseContent(s, j)
            k = j + len(tag) + 2 
            if k >= len(s) or s[j:k+1] != "</" + tag + ">":
                return False, i
            return True, k+1
    
        result, i = validTag(code, 0)
        return result and i == len(code)