119 lines
4 KiB
Rust
119 lines
4 KiB
Rust
//! See [`HtmlWithLimit`].
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use std::fmt::Write;
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use std::ops::ControlFlow;
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use crate::html::escape::Escape;
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/// A buffer that allows generating HTML with a length limit.
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///
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/// This buffer ensures that:
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///
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/// * all tags are closed,
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/// * tags are closed in the reverse order of when they were opened (i.e., the correct HTML order),
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/// * no tags are left empty (e.g., `<em></em>`) due to the length limit being reached,
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/// * all text is escaped.
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#[derive(Debug)]
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pub(super) struct HtmlWithLimit {
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buf: String,
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len: usize,
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limit: usize,
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/// A list of tags that have been requested to be opened via [`Self::open_tag()`]
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/// but have not actually been pushed to `buf` yet. This ensures that tags are not
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/// left empty (e.g., `<em></em>`) due to the length limit being reached.
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queued_tags: Vec<&'static str>,
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/// A list of all tags that have been opened but not yet closed.
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unclosed_tags: Vec<&'static str>,
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}
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impl HtmlWithLimit {
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/// Create a new buffer, with a limit of `length_limit`.
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pub(super) fn new(length_limit: usize) -> Self {
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let buf = if length_limit > 1000 {
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// If the length limit is really large, don't preallocate tons of memory.
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String::new()
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} else {
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// The length limit is actually a good heuristic for initial allocation size.
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// Measurements showed that using it as the initial capacity ended up using less memory
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// than `String::new`.
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// See https://github.com/rust-lang/rust/pull/88173#discussion_r692531631 for more.
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String::with_capacity(length_limit)
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};
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Self {
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buf,
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len: 0,
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limit: length_limit,
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unclosed_tags: Vec::new(),
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queued_tags: Vec::new(),
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}
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}
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/// Finish using the buffer and get the written output.
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/// This function will close all unclosed tags for you.
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pub(super) fn finish(mut self) -> String {
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self.close_all_tags();
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self.buf
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}
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/// Write some plain text to the buffer, escaping as needed.
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///
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/// This function skips writing the text if the length limit was reached
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/// and returns [`ControlFlow::Break`].
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pub(super) fn push(&mut self, text: &str) -> ControlFlow<(), ()> {
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if self.len + text.len() > self.limit {
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return ControlFlow::Break(());
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}
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self.flush_queue();
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write!(self.buf, "{}", Escape(text)).unwrap();
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self.len += text.len();
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ControlFlow::Continue(())
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}
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/// Open an HTML tag.
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///
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/// **Note:** HTML attributes have not yet been implemented.
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/// This function will panic if called with a non-alphabetic `tag_name`.
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pub(super) fn open_tag(&mut self, tag_name: &'static str) {
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assert!(
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tag_name.chars().all(|c| ('a'..='z').contains(&c)),
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"tag_name contained non-alphabetic chars: {:?}",
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tag_name
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);
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self.queued_tags.push(tag_name);
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}
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/// Close the most recently opened HTML tag.
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pub(super) fn close_tag(&mut self) {
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match self.unclosed_tags.pop() {
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// Close the most recently opened tag.
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Some(tag_name) => write!(self.buf, "</{}>", tag_name).unwrap(),
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// There are valid cases where `close_tag()` is called without
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// there being any tags to close. For example, this occurs when
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// a tag is opened after the length limit is exceeded;
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// `flush_queue()` will never be called, and thus, the tag will
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// not end up being added to `unclosed_tags`.
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None => {}
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}
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}
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/// Write all queued tags and add them to the `unclosed_tags` list.
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fn flush_queue(&mut self) {
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for tag_name in self.queued_tags.drain(..) {
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write!(self.buf, "<{}>", tag_name).unwrap();
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self.unclosed_tags.push(tag_name);
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}
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}
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/// Close all unclosed tags.
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fn close_all_tags(&mut self) {
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while !self.unclosed_tags.is_empty() {
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self.close_tag();
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}
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}
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}
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#[cfg(test)]
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mod tests;
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