Optimize scanning for in-buffer settings by scanning forward for possible keywords.

Previously we stepped through the document character by character which involved a lot of extra processing inside OrgSource. By scanning for possible keywords, we can skip many of the intermediate steps.

src/parser/in_buffer_settings.rs

@@ -1,17 +1,15 @@
 use nom::branch::alt;
 use nom::bytes::complete::is_not;
 use nom::bytes::complete::tag_no_case;
-use nom::character::complete::anychar;
+use nom::bytes::complete::take_until;
 use nom::character::complete::space1;
-use nom::combinator::map;
-use nom::multi::many0;
-use nom::multi::many_till;
 use nom::multi::separated_list0;
 
 use super::keyword::filtered_keyword;
 use super::keyword_todo::todo_keywords;
 use super::OrgSource;
 use crate::context::HeadlineLevelFilter;
+use crate::error::CustomError;
 use crate::error::Res;
 use crate::types::Keyword;
 use crate::GlobalSettings;
@@ -20,13 +18,40 @@ use crate::GlobalSettings;
 pub(crate) fn scan_for_in_buffer_settings<'s>(
     input: OrgSource<'s>,
 ) -> Res<OrgSource<'s>, Vec<Keyword<'s>>> {
-    // TODO: Optimization idea: since this is slicing the OrgSource at each character, it might be more efficient to do a parser that uses a search function like take_until, and wrap it in a function similar to consumed but returning the input along with the normal output, then pass all of that into a verify that confirms we were at the start of a line using the input we just returned.
+    // TODO: Write some tests to make sure this is functioning properly.
 
-    let keywords = many0(map(
-        many_till(anychar, filtered_keyword(in_buffer_settings_key)),
-        |(_, kw)| kw,
-    ))(input);
-    keywords
+    let mut keywords = Vec::new();
+    let mut remaining = input;
+    loop {
+        // Skip text until possible in_buffer_setting
+        let start_of_pound = take_until::<_, _, CustomError<_>>("#+")(remaining);
+        let start_of_pound = if let Ok((start_of_pound, _)) = start_of_pound {
+            start_of_pound
+        } else {
+            break;
+        };
+        // Go backwards to the start of the line and run the filtered_keyword parser
+        let start_of_line = start_of_pound.get_start_of_line();
+
+        let (remain, maybe_kw) = match filtered_keyword(in_buffer_settings_key)(start_of_line) {
+            Ok((remain, kw)) => (remain, Some(kw)),
+            Err(_) => {
+                let end_of_line = take_until::<_, _, CustomError<_>>("\n")(start_of_pound);
+                if let Ok((end_of_line, _)) = end_of_line {
+                    (end_of_line, None)
+                } else {
+                    break;
+                }
+            }
+        };
+
+        if let Some(kw) = maybe_kw {
+            keywords.push(kw);
+        }
+        remaining = remain;
+    }
+
+    Ok((remaining, keywords))
 }
 
 #[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]

src/parser/org_source.rs

@@ -1,6 +1,7 @@
 use std::ops::RangeBounds;
 
 use nom::Compare;
+use nom::FindSubstring;
 use nom::InputIter;
 use nom::InputLength;
 use nom::InputTake;
@@ -77,6 +78,55 @@ impl<'s> OrgSource<'s> {
         self.slice(..(other.start - self.start))
     }
 
+    pub(crate) fn get_start_of_line(&self) -> OrgSource<'s> {
+        let skipped_text = self.text_since_line_break();
+        let mut bracket_depth = self.bracket_depth;
+        let mut brace_depth = self.brace_depth;
+        let mut parenthesis_depth = self.parenthesis_depth;
+        // Since we're going backwards, this does the opposite.
+        for byte in skipped_text.bytes() {
+            match byte {
+                b'\n' => {
+                    panic!("Should not hit a line break when only going back to the start of the line.");
+                }
+                b'[' => {
+                    bracket_depth -= 1;
+                }
+                b']' => {
+                    bracket_depth += 1;
+                }
+                b'{' => {
+                    brace_depth -= 1;
+                }
+                b'}' => {
+                    brace_depth += 1;
+                }
+                b'(' => {
+                    parenthesis_depth -= 1;
+                }
+                b')' => {
+                    parenthesis_depth += 1;
+                }
+                _ => {}
+            };
+        }
+
+        OrgSource {
+            full_source: self.full_source,
+            start: self.start_of_line,
+            end: self.end,
+            start_of_line: self.start_of_line,
+            preceding_character: if self.start_of_line > 0 {
+                Some('\n')
+            } else {
+                None
+            },
+            bracket_depth,
+            brace_depth,
+            parenthesis_depth,
+        }
+    }
+
     pub(crate) fn get_bracket_depth(&self) -> BracketDepth {
         self.bracket_depth
     }
@@ -310,6 +360,12 @@ impl<'s> InputTakeAtPosition for OrgSource<'s> {
     }
 }
 
+impl<'n, 's> FindSubstring<&'n str> for OrgSource<'s> {
+    fn find_substring(&self, substr: &'n str) -> Option<usize> {
+        Into::<&str>::into(self).find(substr)
+    }
+}
+
 pub(crate) fn convert_error<'a, I: Into<CustomError<&'a str>>>(
     err: nom::Err<I>,
 ) -> nom::Err<CustomError<&'a str>> {