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//! Low-level char-based scanner.
use std::fmt::{self, Debug, Formatter};
use std::slice::SliceIndex;
use std::str::Chars;
/// A low-level featureful char scanner.
pub struct Scanner<'s> {
src: &'s str,
iter: Chars<'s>,
index: usize,
}
impl<'s> Scanner<'s> {
/// Create a new char scanner.
pub fn new(src: &'s str) -> Self {
Self { src, iter: src.chars(), index: 0 }
}
/// Consume the next char.
pub fn eat(&mut self) -> Option<char> {
let next = self.iter.next();
if let Some(c) = next {
self.index += c.len_utf8();
}
next
}
/// Consume the next char if it is the given one.
///
/// Returns whether the char was consumed.
pub fn eat_if(&mut self, c: char) -> bool {
// Don't decode the char twice through peek() and eat().
//
// TODO: Benchmark this vs. the naive version.
if self.iter.next() == Some(c) {
self.index += c.len_utf8();
true
} else {
self.reset();
false
}
}
/// Consume the next char, debug-asserting that it is the given one.
pub fn eat_assert(&mut self, c: char) {
let next = self.eat();
debug_assert_eq!(next, Some(c));
}
/// Consume the next char, coalescing `\r\n` to just `\n`.
pub fn eat_merging_crlf(&mut self) -> Option<char> {
let c = self.eat();
if c == Some('\r') && self.eat_if('\n') {
Some('\n')
} else {
c
}
}
/// Eat chars while the condition is true.
pub fn eat_while(&mut self, mut f: impl FnMut(char) -> bool) -> &'s str {
self.eat_until(|c| !f(c))
}
/// Eat chars until the condition is true.
pub fn eat_until(&mut self, mut f: impl FnMut(char) -> bool) -> &'s str {
let start = self.index;
while let Some(c) = self.iter.next() {
if f(c) {
// Undo the previous `next()` without peeking all the time
// during iteration.
//
// TODO: Benchmark this vs. the naive peeking version.
self.reset();
break;
}
self.index += c.len_utf8();
}
&self.src[start .. self.index]
}
/// Uneat the last eaten character.
pub fn uneat(&mut self) {
self.index = self.prev_index();
self.reset();
}
/// Peek at the next char without consuming it.
pub fn peek(&self) -> Option<char> {
self.iter.clone().next()
}
/// Peek at the nth-next char without consuming anything.
pub fn peek_nth(&self, n: usize) -> Option<char> {
self.iter.clone().nth(n)
}
/// Checks whether the next character fulfills a condition.
///
/// Returns `false` is there is no next character.
pub fn check(&self, f: impl FnMut(char) -> bool) -> bool {
self.peek().map(f).unwrap_or(false)
}
/// Go back to the where the index says.
fn reset(&mut self) {
self.iter = self.src[self.index ..].chars();
}
}
impl<'s> Scanner<'s> {
/// The current index in the string.
pub fn index(&self) -> usize {
self.index
}
/// The previous index in the string.
pub fn prev_index(&self) -> usize {
self.src[.. self.index]
.chars()
.next_back()
.map(|c| self.index - c.len_utf8())
.unwrap_or(0)
}
/// Slice a part out of the source string.
pub fn get<I>(&self, index: I) -> &'s str
where
I: SliceIndex<str, Output = str>,
{
&self.src[index]
}
/// The full source string.
pub fn src(&self) -> &'s str {
self.src
}
/// The full source string up to the current index.
pub fn eaten(&self) -> &'s str {
&self.src[.. self.index]
}
/// The source string from `start` to the current index.
pub fn eaten_from(&self, start: usize) -> &'s str {
&self.src[start .. self.index]
}
/// The remaining source string after the current index.
pub fn rest(&self) -> &'s str {
&self.src[self.index ..]
}
}
impl Debug for Scanner<'_> {
fn fmt(&self, f: &mut Formatter) -> fmt::Result {
write!(f, "Scanner({}|{})", self.eaten(), self.rest())
}
}
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