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/// This code was borrowed from @fasterthanlime.
use color_eyre::eyre::Result;
pub use xesite_types::Frontmatter as Data;
enum State {
SearchForStart,
ReadingMarker { count: usize, end: bool },
ReadingFrontMatter { buf: String, line_start: bool },
SkipNewline { end: bool },
}
#[derive(Debug, thiserror::Error)]
enum Error {
#[error("EOF while parsing frontmatter")]
EOF,
#[error("Error parsing yaml: {0:?}")]
Yaml(#[from] serde_yaml::Error),
}
pub fn parse(input: &str) -> Result<(Data, usize)> {
let mut state = State::SearchForStart;
let mut payload = None;
let offset;
let mut chars = input.char_indices();
'parse: loop {
let (idx, ch) = match chars.next() {
Some(x) => x,
None => return Err(Error::EOF)?,
};
match &mut state {
State::SearchForStart => match ch {
'-' => {
state = State::ReadingMarker {
count: 1,
end: false,
};
}
'\n' | '\t' | ' ' => {
// ignore whitespace
}
_ => {
panic!("Start of frontmatter not found");
}
},
State::ReadingMarker { count, end } => match ch {
'-' => {
*count += 1;
if *count == 3 {
state = State::SkipNewline { end: *end };
}
}
_ => {
panic!("Malformed frontmatter marker");
}
},
State::SkipNewline { end } => match ch {
'\n' => {
if *end {
offset = idx + 1;
break 'parse;
} else {
state = State::ReadingFrontMatter {
buf: String::new(),
line_start: true,
};
}
}
_ => panic!("Expected newline, got {:?}", ch),
},
State::ReadingFrontMatter { buf, line_start } => match ch {
'-' if *line_start => {
let mut state_temp = State::ReadingMarker {
count: 1,
end: true,
};
std::mem::swap(&mut state, &mut state_temp);
if let State::ReadingFrontMatter { buf, .. } = state_temp {
payload = Some(buf);
} else {
unreachable!();
}
}
ch => {
buf.push(ch);
*line_start = ch == '\n';
}
},
}
}
// unwrap justification: option set in state machine, Rust can't statically analyze it
let payload = payload.unwrap();
let fm: Data = serde_yaml::from_str(&payload)?;
Ok((fm, offset))
}
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