# Implementing the Advent of Code 2022 Day 13 Packet Parser

In this post we are going to run through a lightning fast introduction to parsing by writing a parser for the Advent of Code 2022 day 13 signal packet data using Python. The day 13 problem describes the signal packet data as follows:

Packet data consists of lists and integers. Each list starts with `[`, ends with `]`, and contains zero or more comma-separated values (either integers or other lists). Each packet is always a list and appears on its own line.

With some example packet pairs provided in the sample test input:

``````[1,1,3,1,1]
[1,1,5,1,1]

[[1],[2,3,4]]
[[1],4]

[9]
[[8,7,6]]

[[4,4],4,4]
[[4,4],4,4,4]

[7,7,7,7]
[7,7,7]

[]
[3]

[[[]]]
[[]]

[1,[2,[3,[4,[5,6,7]]]],8,9]
[1,[2,[3,[4,[5,6,0]]]],8,9]
``````

The parser that we are going to write will parse one of these packets (i.e. one line), and return the parsed packet as a Python `list` containing nested `list` and `int` values. So parsing the input string `"[1,2,3,[4,5],[[6]],[7,[8,9]]]"`, should produce a Python list that would print as `[1, 2, 3, [4, 5], [[6]], [7, [8, 9]]]`.

## Quick Parsing Overview

A parser is a tool which transforms some input text into an equivalent abstract data structure according to a set of rules known as a grammar. A grammar may be described informally, as seen by the day 13 description of a packet, or formally using some sort of grammar notation:

``````PACKET  = LIST
VALUE   = LIST | INTEGER
LIST    = "[" (VALUE ("," VALUE)*)? "]"
INTEGER = [1-9]+
``````

Parsing usually consists of two phases:

1. Transforming the input text into a stream of tokens.
2. Building data structures from the stream of tokens according to rules based the next token(s) in the stream.

These two phases are usually called the lexing and parsing phases1. In this context, lexing refers to the phase where a stream of tokens is created from text, and parsing refers to the phase where data structures are built from the token stream according to the rules of a grammar.

A token is an atomic unit of the language we are parsing. In English, tokens are the individual words and punctuation that make up a sentence. The English sentence `"Alice walked to the store."` contains the tokens:

``````Alice
walked
to
the
store
.
``````

In the case of English, each word is one token, because the sentence would lose meaning if you broke the sentence down into smaller components2. In a programming language such as Python, tokens tend to be things like keywords, identifiers, punctuation, and literals such as integers and strings. The Python expression `print("hello, world", 123)` contains the tokens:

``````print
(
"hello, world"
,
123
)
``````

The day 13 packet language consists of the punctuation tokens `,`, `[`, and `]` as well as integer tokens. The packet `[123,456,[789]]` contains the tokens:

``````[
123
,
456
,
[
789
]
]
``````

Once input text is lexed into individual tokens by a lexer, a parser will attempt to build data structures from those tokens based on the rules of a grammar. English grammar is incredibly complicated3 and full of exceptions, but we (generally) know a sentence is valid if our brains can interpret the sentence (i.e. parse the sentence) as something that makes sense. In the English sentence `"Alice walked to the store."` we can identify `Alice` as the subject, `walk (past tense)` as the action associated with the subject, `to` indicates that the next noun in the sentence was Alice's destination, `the` is a determiner relating to the destination noun, and `store` is the destination noun we expected. The period indicates that the sentence has concluded.

We can construct a parser that will perform the same kind of analysis for our packet grammar, reading in tokens one at a time and seeing if the order of those tokens makes sense for a packet. We will first start by writing a lexer. Then, we will use that lexer to write a parser. Finally, we will wrap our lexer and parser into one function that will accept a packet input string and print the list parsed from that input string.

## Writing the Lexer

Our lexer is going to take some input as a string and then spit out tokens one at a time whenever we call the `next_token` method on the lexer. Speaking of tokens, they are pretty important, so we should probably define some sort of `Token` class for our lexer and parser to use:

``````from enum import Enum

class TokenKind(Enum):
EOI = "end-of-input"
COMMA = ","
INTEGER = "integer"
LBRACKET = "["
RBRACKET = "]"

class Token:
def __init__(self, kind, text):
self.kind = kind
self.text = text

def __str__(self):
if self.kind == TokenKind.INTEGER:
return f"{self.kind.value}({self.text})"
return f"{self.kind.value}"
``````

Here we have a class called `Token` which contains two fields: `kind` and `text`. The `kind` field of `Token` will be one of the values of the `TokenKind` enum, indicating what sort of token we have. Earlier we identified that the packet grammar consisted of the punctuation tokens `,`, `[`, and `]` represented by `TokenKind.COMMA`, `TokenKind.LBRACKET`, `TokenKind.RBRACKET`, as well as integer tokens represented by `TokenKind.INTEGER`. Observant readers will notice the `TokenKind.EOI` token kind not previously mentioned. This is a special token that indicates that the end of the input string has been reached and no more tokens can be lexed. It is useful to have this special kind of token so that the parser has a way to know that the entire input string has been consumed. The `text` field of `Token` will be the slice of characters from the input associated with the token. For the comma, left square bracket, and right square bracket tokens, the `kind` and `text` fields will end up being identical strings. For integer tokens, the `kind` field will indicate that the token is an integer token, and the `text` field will contain the digits of the parsed integer. The end-of-input token is typically represented using the empty string.

``````print(Token(TokenKind.COMMA, ","))
print(Token(TokenKind.LBRACKET, "["))
print(Token(TokenKind.RBRACKET, "]"))
print(Token(TokenKind.INTEGER, "123"))
print(Token(TokenKind.EOI, ""))
``````
``````\$ python3 parser.py
,
[
]
integer(123)
end-of-input
``````

Our `Lexer` class will need two pieces of information to operate: the input string and an integer keeping track of the current lexing position within the input string. Since lexing always begins at the start of the input string, our lexer only needs to accept the input string (called `source` in the `Lexer` class) as an argument to the lexer's `__init__` function:

``````class Lexer:
def __init__(self, source):
self.source = source
self.position = 0

lexer = Lexer("[1,2,3,[4,5],[[6]],[7,[8,9]]]")
print(f"source=\"{lexer.source}\", position={lexer.position}")
``````
``````\$ python3 parser.py
source="[1,2,3,[4,5],[[6]],[7,[8,9]]]", position=0
``````

Next I want to create two helper methods: `is_eoi` and `current_character`. The `is_eoi` method will return `True` if the lexer's position is past the end of the input string (i.e. we have reached the end-of-input) and `False` otherwise:

``````class Lexer:
def __init__(self, source):
self.source = source
self.position = 0

def is_eoi(self):
return self.position >= len(self.source)
``````

The `current_character` method will return the character of the input string corresponding to the lexer's current position. Crucially, if the lexer has reached the end-of-input then attempting to index the input string will produce an `IndexError`, so we use `is_eoi` to check if the lexer has reached the end-of-input and return the empty string in that case:

``````class Lexer:
EOI = ""

def __init__(self, source):
self.source = source
self.position = 0

def is_eoi(self):
return self.position >= len(self.source)

def current_character(self):
if self.is_eoi():
return Lexer.EOI
return self.source[self.position]
``````

Believe it or not, with just these two helper methods we have everything we need to write the `next_token` method. When lexing tokens there are five cases to consider:

1. The lexer has reached the end-of-input → return an `TokenKind.EOI` token
2. The current character is `,` → return a `TokenKind.COMMA` token
3. The current character is `[` → return a `TokenKind.LBRACKET` token
4. The current character is `]` → return a `TokenKind.RBRACKET` token
5. The current character is a digit → advance the position of the lexer until the current character is no longer a digit and return a `TokenKind.INTEGER` containing the digits as the token text

Our `next_token` method will essentially check for each of these cases using an `if` statement. If the current character does not match any of our cases then we will raise an exception to indicate that a lexing error has occurred:

``````class Lexer:
EOI = ""

def __init__(self, source):
self.source = source
self.position = 0

def is_eoi(self):
return self.position >= len(self.source)

def current_character(self):
if self.is_eoi():
return Lexer.EOI
return self.source[self.position]

def next_token(self):
start = self.position

if self.is_eoi():

if self.current_character() == TokenKind.COMMA.value:
self.position += len(TokenKind.COMMA.value)

if self.current_character() == TokenKind.LBRACKET.value:
self.position += len(TokenKind.LBRACKET.value)

if self.current_character() == TokenKind.RBRACKET.value:
self.position += len(TokenKind.RBRACKET.value)

if self.current_character().isdigit():
while self.current_character().isdigit():
self.position += 1

raise Exception(f"invalid character `{self.current_character()}`")
``````

Testing our lexer on some example input, we can see that the lexer correctly produces a valid stream of tokens for valid input and will raise an exception for invalid input:

``````print("VALID INPUT")
print("===========")
lexer = Lexer("[123,456,[789]]")
while (token := lexer.next_token()).kind != TokenKind.EOI:
print(token)

print("\n", end="")

print("INVALID INPUT")
print("=============")
try:
lexer = Lexer("[123,456,[\$]]")
while (token := lexer.next_token()).kind != TokenKind.EOI:
print(token)
except Exception as e:
print(e)
``````
``````\$ python3 parser.py
VALID INPUT
===========
[
integer(123)
,
integer(456)
,
[
integer(789)
]
]

INVALID INPUT
=============
[
integer(123)
,
integer(456)
,
[
invalid character `\$`
``````

With the addition of the `next_token` method, our lexer is complete. Not bad for less than fifty lines of code!

## Writing the Parser

Our parser is going to take a lexer and use the lexer's `next_token` method to parse a packet according to the rules of our packet grammar. The type of parser we will be implementing today is what is known as a recursive descent parser. We will take our list of rules and implement a method for each rule. Based on the day 13 description of the packet data, it appears grammar consists of four rules:

1. A packet is a list followed by the end-of-input token
2. A value is either a list or an integer
3. A list starts with `[` token, ends with `]` token and contains zero or more values separated by `,` tokens
4. An integer consists of a single integer token

In each of our rules, we only ever need to look one token ahead to know what to parse next. When parsing an integer we need to check one token ahead to make sure that token is an integer. When parsing a list we need to check one token ahead as the beginning of the rule to make sure the first token is a `[`, we need to check one token ahead when parsing the values of the list in order to see if we have reached the end of the list as indicated by a `]` token or if there are still more values to parse as indicated by a `,` token. We need to check one token ahead when parsing a value to know if we need to parse a list as indicated by a `[` token or parse an integer as indicated by an integer token. And finally we need to check one token ahead when parsing a packet in order make sure we have reached the end-of-input after parsing the list part of the packet.

Our `Parser` class will need two pieces of information to operate: the lexer instance and the current token in the token stream (a.k.a the token one ahead from all the tokens that we have already consumed). Since parsing always starts with the first token produced by the lexer, our parser only needs to accept the lexer instance (called `lexer` in the `Parser` class) as an argument to the parser's `__init__` function. The current token in the token stream (called `current` in the `Parser` class) is initialized in the parser's `__init__` function:

``````class Parser:
def __init__(self, lexer):
self.lexer = lexer
self.next = lexer.next_token()
``````

Next we will create a method for each of our four rules. For now each method will raise a TODO exception, but we will replace each of those TODOs with a parsing rule as we flesh out the parser:

``````class Parser:
def __init__(self, lexer):
self.lexer = lexer
self.next = lexer.next_token()

def parse_packet(self):
raise Exception("TODO")

def parse_value(self):
raise Exception("TODO")

def parse_list(self):
raise Exception("TODO")

def parse_integer(self):
raise Exception("TODO")
``````

Parsing an integer is the easiest of the rules to implement. An integer consists of a single integer token, so all we need to do is check to see if the next token is of kind `TokenKind.INTEGER`:

``````    def parse_integer(self):
if self.current.kind != TokenKind.INTEGER:
raise Exception(f"expected {TokenKind.INTEGER.value} (found {self.current})")
result = int(self.current.text)
self.current = self.lexer.next_token()
return result
``````

If the current token is not an integer then we raise an exception to indicate that we did not encounter the expected token. Otherwise we obtain the value of the integer as a python `int` with `result = int(self.current.text)` and advance past the integer token with `self.current = self.lexer.next_token()` before returning the parsed integer. Testing the `parse_integer` method with some example input shows that our parser will return the expected integer when an integer is encountered, and produce an error when a non-integer token is encountered:

``````lexer = Lexer("123")
parser = Parser(lexer)
print(parser.parse_integer())

try:
lexer = Lexer("[")
parser = Parser(lexer)
print(parser.parse_integer())
except Exception as e:
print(e)
``````
``````\$ python3 parser.py
123
expected integer (found `[`)
``````

The `parse_list` method is a bit trickier. For the opening `[` and closing `]` we can use the same pattern seen in the `parse_integer` method where we check the current token and raise an exception if the current token is not a token we were expecting:

``````    def parse_list(self):
if self.current.kind != TokenKind.LBRACKET:
raise Exception(f"expected `{TokenKind.LBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()

result = []
# TODO: Parse the comma-separated values...

if self.current.kind != TokenKind.RBRACKET:
raise Exception(f"expected `{TokenKind.RBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
return result
``````

But how do we parse the list of zero or more comma-separated values? We should try to parse either a list or a integer depending on the current token. We also need to make sure that we parse the comma separating the values, but only if at least one value has already been parsed. The trick to parse zero or more of something is to use a `while` loop that will only stop once the ending token (in this case a `]`) becomes the current token in the parser:

``````    def parse_list(self):
if self.current.kind != TokenKind.LBRACKET:
raise Exception(f"expected `{TokenKind.LBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()

result = []
while self.current.kind != TokenKind.RBRACKET:
# TODO: Parse the next comma-separated value...
pass

if self.current.kind != TokenKind.RBRACKET:
raise Exception(f"expected `{TokenKind.RBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
return result
``````

Every time the loop condition is checked, anything other than a `]` indicates that there are still more elements to parse. Inside the loop we add an additional check for the `,` separator, but only if at least one value has already been parsed:

``````    def parse_list(self):
if self.current.kind != TokenKind.LBRACKET:
raise Exception(f"expected `{TokenKind.LBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()

result = []
while self.current.kind != TokenKind.RBRACKET:
if len(result) != 0:
if self.current.kind != TokenKind.COMMA:
raise Exception(f"expected `{TokenKind.COMMA.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
# TODO: Parse the next comma-separated value...

if self.current.kind != TokenKind.RBRACKET:
raise Exception(f"expected `{TokenKind.RBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
return result
``````

And finally, after maybe-parsing the `,` separator, we attempt to parse a value and append the value to our parse result list:

``````    def parse_list(self):
if self.current.kind != TokenKind.LBRACKET:
raise Exception(f"expected `{TokenKind.LBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()

result = []
while self.current.kind != TokenKind.RBRACKET:
if len(result) != 0:
if self.current.kind != TokenKind.COMMA:
raise Exception(f"expected `{TokenKind.COMMA.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
result.append(self.parse_value())

if self.current.kind != TokenKind.RBRACKET:
raise Exception(f"expected `{TokenKind.RBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
return result
``````

Of course if we were to test this right now we would run into an errors, since we have not yet implemented `parse_value`:

``````try:
lexer = Lexer("[123,456,[789]]")
parser = Parser(lexer)
print(parser.parse_list())
except Exception as e:
print(e)
``````
``````\$ python3 parser.py
TODO
``````

A value is either a list or an integer, so our `parse_value` method will forward to either `parse_list` or `parse_integer` depending on the current token. If the current token is a `[` token then we know that the value being parsed is a list. If the current token is an integer token then we know that the value being parsed is an integer. If the current token is not a `[` token or an integer token then we will raise an exception to indicate that a parse error has occurred:

``````    def parse_value(self):
if self.current.kind == TokenKind.LBRACKET:
return self.parse_list()

if self.current.kind == TokenKind.INTEGER:
return self.parse_integer()

raise Exception(f"expected value (found `{self.current}`)")
``````

We can see that calls to the `parse_value` and `parse_list` methods will both produce the expected results now that both methods have been defined:

``````lexer = Lexer("[123,456,[789]]")
parser = Parser(lexer)
print(parser.parse_list())

try:
lexer = Lexer("[123,]")
parser = Parser(lexer)
print(parser.parse_list())
except Exception as e:
print(e)

lexer = Lexer("[123]")
parser = Parser(lexer)
print(parser.parse_value())

lexer = Lexer("123")
parser = Parser(lexer)
print(parser.parse_value())
``````
``````\$ python3 parser.py
[123, 456, [789]]
expected value (found `]`)
[123]
123
``````

The last method we have to implement is `parse_packet`. This method is similar to `parse_list` with an additional check for the end-of-input token after the list is parsed:

``````    def parse_packet(self):
value = self.parse_list()
if self.current.kind != TokenKind.EOI:
raise Exception(f"expected {TokenKind.EOI.value} (found `{self.current}`)")
return value
``````

The additional end-of-input check is used to make sure that a packet string consists of only one list. If we were to omit this check then a packet string such as `"[123,456][789]"` would be parsed into the Python list `[123, 456]` and be considered valid even though the erroneous tokens `[`, `789`, and `]` would still be unconsumed from the token stream. By checking for the end-of-input token, any extra tokens after the packet list will produce a parse error:

``````lexer = Lexer("[123,456,[789]]")
parser = Parser(lexer)
print(parser.parse_packet())

try:
lexer = Lexer("[123,456][789]")
parser = Parser(lexer)
print(parser.parse_packet())
except Exception as e:
print(e)
``````
``````\$ python3 parser.py
[123, 456, [789]]
expected end-of-input (found `[`)
``````

All together our `Parser` class takes the form:

``````class Parser:
def __init__(self, lexer):
self.lexer = lexer
self.current = lexer.next_token()

def parse_packet(self):
value = self.parse_list()
if self.current.kind != TokenKind.EOI:
raise Exception(f"expected {TokenKind.EOI.value} (found `{self.current}`)")
return value

def parse_value(self):
if self.current.kind == TokenKind.LBRACKET:
return self.parse_list()

if self.current.kind == TokenKind.INTEGER:
return self.parse_integer()

raise Exception(f"expected value (found `{self.current}`)")

def parse_list(self):
if self.current.kind != TokenKind.LBRACKET:
raise Exception(f"expected `{TokenKind.LBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()

result = []
while self.current.kind != TokenKind.RBRACKET:
if len(result) != 0:
if self.current.kind != TokenKind.COMMA:
raise Exception(f"expected `{TokenKind.COMMA.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
result.append(self.parse_value())

if self.current.kind != TokenKind.RBRACKET:
raise Exception(f"expected `{TokenKind.RBRACKET.value}` (found `{self.current}`)")
self.current = self.lexer.next_token()
return result

def parse_integer(self):
if self.current.kind != TokenKind.INTEGER:
raise Exception(f"expected {TokenKind.INTEGER.value} (found `{self.current}`)")
result = int(self.current.text)
self.current = self.lexer.next_token()
return result
``````

## Wrapping Up

Believe it or not, we are basically finished with our Advent of Code day 13 packet parser. We can add a nice `main` function to show how the argument parser might be used in practice:

``````def main(args):
try:
lexer = Lexer(args.packet)
parser = Parser(lexer)
print(parser.parse_packet())
except Exception as e:
print(f'"{s}" raised exception: {e}')

if __name__ == "__main__":
ap = ArgumentParser(description="Parse a packet from an input string")
main(ap.parse_args())
``````
``````\$ python3 parser.py [1,2,3,[4,5],[[6]],[7,[8,9]]]
[1, 2, 3, [4, 5], [[6]], [7, [8, 9]]]
``````

In this `main` function we create a lexer using the input string, then create a parser using the lexer, then we call the `parse_packet` method on the parser to transform the string into tokens into a list data structure containing the values of the packet. There are about a bajillion things that were glossed over and/or elided for the sake of simplicity in this blog post, but the lexer and parser that we wrote is still relatively close in structure to the lexers and parsers that one might find in the wild!

We went through things pretty quickly here, so I am going to recommend some additional exercises for those who would like to explore further:

1. Currently our lexer and parser do not allow whitespace as part of the input string. In many real life applications (e.g. programming languages) we want to allow arbitrary whitespace between tokens. So the input strings `"[123,456]"`, `"[123, 456]"` and `"[ 123, 456 ]"` would all be parsed into the same data structure. Modify the lexer to skip over whitespace when parsing the next token. Hint: the lexer should be modified without needing to introduce a `TokenKind.WHITESPACE` token kind.
2. Modify the lexer and parser to allow strings as values. For the purposes of this exercise, a string consists of an opening single quote, followed by zero or more characters, followed by a closing single quote (e.g `'hello'`). After modifying the lexer and parser, your program should be able to parse the packet `"[123,'foo',[456,'bar','baz']]"`. Hint: traditionally a string is considered a single token, so start by adding `TokenKind.STRING` to the `TokenKind` enum and modify `Lexer.next_token` to parse a string if the current character is `'`.

The full source code for `parser.py` can be found here.

## Footnotes

1. The term "parsing" has a somewhat overloaded meaning here, referring to the process of as a whole as well as the second phase of a lexing-parsing pipeline.

2. One would not typically read a sentence letter by letter such as "A-L-I-C-E W-A-L-K-E-D T-O T-H-E S-T-O-R-E." Our human brains essentially have a builtin lexer that automatically tokenizes English text into individual words (tokens).

3. "Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo" is an English sentence that is grammatically correct, but incredibly difficult to understand. Similarly, "Colorless green ideas sleep furiously" is an English sentence that is grammatically correct, but semantically nonsensical.