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//! Tokens.
/// What we expected to get, but did not get.
#[derive(Debug, PartialEq, Eq)]
pub enum Expect {
/// ` ` or `]` or `,`
Value,
/// `[` or `{`
ValueOrEnd,
/// `[1` or `[1 2`
CommaOrEnd,
/// `{0: 1}`
String,
/// `{"a" 1}`
Colon,
/// `true false` (when parsing exactly one value)
Eof,
}
impl core::fmt::Display for Expect {
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
use Expect::*;
match self {
Value => "value".fmt(f),
ValueOrEnd => "value or end of sequence".fmt(f),
CommaOrEnd => "comma or end of sequence".fmt(f),
String => "string".fmt(f),
Colon => "colon".fmt(f),
Eof => "end of file".fmt(f),
}
}
}
/// JSON lexer token.
#[derive(Debug, PartialEq, Eq)]
pub enum Token {
/// `null`
Null,
/// `true`
True,
/// `false`
False,
/// `,`
Comma,
/// `:`
Colon,
/// `[`
LSquare,
/// `]`
RSquare,
/// `{`
LCurly,
/// `}`
RCurly,
/// `"`
Quote,
/// a digit (0-9) or a minus (`-`)
DigitOrMinus,
/// anything else
Error,
}
impl core::fmt::Display for Token {
fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
use Token::*;
match self {
Null => "null".fmt(f),
True => "true".fmt(f),
False => "false".fmt(f),
Comma => ",".fmt(f),
Colon => ":".fmt(f),
LSquare => "[".fmt(f),
RSquare => "]".fmt(f),
LCurly => "{".fmt(f),
RCurly => "}".fmt(f),
Quote => '"'.fmt(f),
DigitOrMinus => "number".fmt(f),
Error => "unknown token".fmt(f),
}
}
}
impl Token {
/// Return `Ok(())` if `self` equals `token`, else return `Err(err)`.
pub fn equals_or<E>(&self, token: Token, err: E) -> Result<(), E> {
if *self == token {
Ok(())
} else {
Err(err)
}
}
}
/// Lexing that does not require allocation.
pub trait Lex: crate::Read {
/// Skip input until the earliest non-whitespace character.
fn eat_whitespace(&mut self) {
self.skip_next_until(|c| !matches!(c, b' ' | b'\t' | b'\r' | b'\n'))
}
/// Skip potential whitespace and return the following token if there is some.
fn ws_token(&mut self) -> Option<Token> {
self.eat_whitespace();
Some(self.token(*self.peek_next()?))
}
/// Return `out` if the input matches `s`, otherwise return an error.
fn exact<const N: usize>(&mut self, s: [u8; N], out: Token) -> Token {
// we are calling this function without having advanced before
self.take_next();
if self.strip_prefix(s) {
out
} else {
Token::Error
}
}
/// Convert a character to a token, such as '`:`' to `Token::Colon`.
///
/// When the token consists of several characters, such as
/// `null`, `true`, or `false`,
/// also consume the following characters.
fn token(&mut self, c: u8) -> Token {
let token = match c {
// it is important to `return` here in order not to read a byte,
// like we do for the regular, single-character tokens
b'n' => return self.exact([b'u', b'l', b'l'], Token::Null),
b't' => return self.exact([b'r', b'u', b'e'], Token::True),
b'f' => return self.exact([b'a', b'l', b's', b'e'], Token::False),
b'0'..=b'9' | b'-' => return Token::DigitOrMinus,
b'"' => Token::Quote,
b'[' => Token::LSquare,
b']' => Token::RSquare,
b'{' => Token::LCurly,
b'}' => Token::RCurly,
b',' => Token::Comma,
b':' => Token::Colon,
_ => Token::Error,
};
self.take_next();
token
}
/// Parse a string with given function, followed by a colon.
fn str_colon<T, E: From<Expect>, F>(&mut self, token: Token, f: F) -> Result<T, E>
where
F: FnOnce(&mut Self) -> Result<T, E>,
{
token.equals_or(Token::Quote, Expect::String)?;
let key = f(self)?;
let colon = self.ws_token().filter(|t| *t == Token::Colon);
colon.ok_or(Expect::Colon)?;
Ok(key)
}
/// Execute `f` for every item in the comma-separated sequence until `end`.
fn seq<E: From<Expect>, F>(&mut self, end: Token, mut f: F) -> Result<(), E>
where
F: FnMut(Token, &mut Self) -> Result<(), E>,
{
let mut token = self.ws_token().ok_or(Expect::ValueOrEnd)?;
if token == end {
return Ok(());
};
loop {
f(token, self)?;
token = self.ws_token().ok_or(Expect::CommaOrEnd)?;
if token == end {
return Ok(());
} else if token == Token::Comma {
token = self.ws_token().ok_or(Expect::Value)?;
} else {
return Err(Expect::CommaOrEnd)?;
}
}
}
/// Parse once using given function and assure that the function has consumed all tokens.
fn exactly_one<T, E: From<Expect>, F>(&mut self, f: F) -> Result<T, E>
where
F: FnOnce(Token, &mut Self) -> Result<T, E>,
{
let token = self.ws_token().ok_or(Expect::Value)?;
let v = f(token, self)?;
self.eat_whitespace();
match self.peek_next() {
None => Ok(v),
Some(_) => Err(Expect::Eof)?,
}
}
}
impl<T> Lex for T where T: crate::Read {}