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use nom::branch::alt;
use nom::bytes::streaming::{is_not, take_while_m_n};
use nom::character::streaming::{char, multispace1};
use nom::combinator::{map, map_opt, map_res, value, verify};
use nom::error::{FromExternalError, ParseError};
use nom::multi::fold_many0;
use nom::sequence::{delimited, preceded};
use nom::{IResult, Parser};
// parser combinators are constructed from the bottom up:
// first we write parsers for the smallest elements (escaped characters),
// then combine them into larger parsers.
/// Parse a unicode sequence, of the form u{XXXX}, where XXXX is 1 to 6
/// hexadecimal numerals. We will combine this later with parse_escaped_char
/// to parse sequences like \u{00AC}.
fn parse_unicode<'a, E>(input: &'a str) -> IResult<&'a str, char, E>
where
E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
{
// `take_while_m_n` parses between `m` and `n` bytes (inclusive) that match
// a predicate. `parse_hex` here parses between 1 and 6 hexadecimal numerals.
let parse_hex = take_while_m_n(1, 6, |c: char| c.is_ascii_hexdigit());
// `preceded` takes a prefix parser, and if it succeeds, returns the result
// of the body parser. In this case, it parses u{XXXX}.
let parse_delimited_hex = preceded(
char('u'),
// `delimited` is like `preceded`, but it parses both a prefix and a suffix.
// It returns the result of the middle parser. In this case, it parses
// {XXXX}, where XXXX is 1 to 6 hex numerals, and returns XXXX
delimited(char('{'), parse_hex, char('}')),
);
// `map_res` takes the result of a parser and applies a function that returns
// a Result. In this case we take the hex bytes from parse_hex and attempt to
// convert them to a u32.
let parse_u32 = map_res(parse_delimited_hex, move |hex| u32::from_str_radix(hex, 16));
// map_opt is like map_res, but it takes an Option instead of a Result. If
// the function returns None, map_opt returns an error. In this case, because
// not all u32 values are valid unicode code points, we have to fallibly
// convert to char with from_u32.
map_opt(parse_u32, std::char::from_u32).parse(input)
}
/// Parse an escaped character: \n, \t, \r, \u{00AC}, etc.
fn parse_escaped_char<'a, E>(input: &'a str) -> IResult<&'a str, char, E>
where
E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
{
preceded(
char('\\'),
// `alt` tries each parser in sequence, returning the result of
// the first successful match
alt((
parse_unicode,
// The `value` parser returns a fixed value (the first argument) if its
// parser (the second argument) succeeds. In these cases, it looks for
// the marker characters (n, r, t, etc) and returns the matching
// character (\n, \r, \t, etc).
value('\n', char('n')),
value('\r', char('r')),
value('\t', char('t')),
value('\u{08}', char('b')),
value('\u{0C}', char('f')),
value('\\', char('\\')),
value('/', char('/')),
value('"', char('"')),
)),
)
.parse(input)
}
/// Parse a backslash, followed by any amount of whitespace. This is used later
/// to discard any escaped whitespace.
fn parse_escaped_whitespace<'a, E: ParseError<&'a str>>(
input: &'a str,
) -> IResult<&'a str, &'a str, E> {
preceded(char('\\'), multispace1).parse(input)
}
/// Parse a non-empty block of text that doesn't include \ or "
fn parse_literal<'a, E: ParseError<&'a str>>(input: &'a str) -> IResult<&'a str, &'a str, E> {
// `is_not` parses a string of 0 or more characters that aren't one of the
// given characters.
let not_quote_slash = is_not("\"\\");
// `verify` runs a parser, then runs a verification function on the output of
// the parser. The verification function accepts out output only if it
// returns true. In this case, we want to ensure that the output of is_not
// is non-empty.
verify(not_quote_slash, |s: &str| !s.is_empty()).parse(input)
}
/// A string fragment contains a fragment of a string being parsed: either
/// a non-empty Literal (a series of non-escaped characters), a single
/// parsed escaped character, or a block of escaped whitespace.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum StringFragment<'a> {
Literal(&'a str),
EscapedChar(char),
EscapedWS,
}
/// Combine parse_literal, parse_escaped_whitespace, and parse_escaped_char
/// into a StringFragment.
fn parse_fragment<'a, E>(input: &'a str) -> IResult<&'a str, StringFragment<'a>, E>
where
E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
{
alt((
// The `map` combinator runs a parser, then applies a function to the output
// of that parser.
map(parse_literal, StringFragment::Literal),
map(parse_escaped_char, StringFragment::EscapedChar),
value(StringFragment::EscapedWS, parse_escaped_whitespace),
))
.parse(input)
}
/// Parse a string. Use a loop of parse_fragment and push all of the fragments
/// into an output string.
pub fn parse_string<'a, E>(input: &'a str) -> IResult<&'a str, String, E>
where
E: ParseError<&'a str> + FromExternalError<&'a str, std::num::ParseIntError>,
{
// fold is the equivalent of iterator::fold. It runs a parser in a loop,
// and for each output value, calls a folding function on each output value.
let build_string = fold_many0(
// Our parser function – parses a single string fragment
parse_fragment,
// Our init value, an empty string
String::new,
// Our folding function. For each fragment, append the fragment to the
// string.
|mut string, fragment| {
match fragment {
StringFragment::Literal(s) => string.push_str(s),
StringFragment::EscapedChar(c) => string.push(c),
StringFragment::EscapedWS => {}
}
string
},
);
// Finally, parse the string. Note that, if `build_string` could accept a raw
// " character, the closing delimiter " would never match. When using
// `delimited` with a looping parser (like fold), be sure that the
// loop won't accidentally match your closing delimiter!
delimited(char('"'), build_string, char('"')).parse(input)
}
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