Example 1 · atom
- Arguments
- text
" Route_7\t"
- Expected return
"Route_7"
Test data
- Input
" Route_7\t"
- Expected output
"Route_7"
One-pass string parser
One-pass string parser
MediumYour task
- Read one atom from a small input string and return its exact spelling.
- For example, Route_7 stays Route_7, and 007 stays a string.
- Do not evaluate or convert values.
Required API
- ParseError.Error
func (e *ParseError) Error() string - ParseSexp
func ParseSexp(text string) (any, error) - ParseError.Code
Code string - ParseError.Index
Index int
Behavior
- An atom contains one or more characters from [A-Za-z0-9_+-].
- Ignore leading and trailing space, tab, carriage return and newline.
- The input is ASCII and at most 4,096 characters long.
- Parentheses are not supplied in this part.
- Use one forward-moving cursor, without splitting or tokenizing the whole input, eval or a general parser.
- Aim for O(n) time.
- Each call starts fresh.
- Use only the standard library.
- Report the first violation reached, with a zero-based character index.
- Empty or whitespace-only input is expected_expression at EOF (len(text)).
- An unsupported character is invalid_character at its position.
- After the atom and optional whitespace, another atom is trailing_input at its first character; an unsupported character is still invalid_character.
- Stop there rather than scanning ahead.
- On failure return no partial result.
- Error messages are not graded.
Examples
The runner calls your functions directly; you do not need to parse JSON.
Example 2 · extra root
- Arguments
- text
"a b"
- Expected outcome
ParseError(code="trailing_input", index=2)
Test data
- Input
"a b"
- Expected output
ParseError(code="trailing_input", index=2)
Related guides
Part 1 only.
Solution guide
Reveal solution guide — spoilers
Approach
- Skip permitted whitespace using one forward cursor. Read the maximal atom and preserve its spelling.
- Skip trailing whitespace and classify the next character immediately. Return no partial value on an error.
Why it works
The cursor separates consumed characters from unread input. Each successful reader consumes exactly one value and leaves the next character for its caller.
Complexity
O(n) time. Read forward without building a separate token list; returned values account for output storage.
Edge cases to check
- Whitespace-only input reports expected_expression at EOF.
- Another atom after a complete root is trailing_input; stop at that first violation.
Restoring local drafts…
Checking execution availability…
Public examples
NO CODE EXECUTEDExplore the examples.
View inputs and expected outputs. Examples do not run your code.