new: postprocessing support

Add support for postprocessing of Markdown prior to writing converted
notes to disk.

Postprocessors may be used when making use of Obsidian export as a Rust
library to do the following:

1. Modify a note's `Context`, for example to change the destination
   filename or update its Frontmatter.
2. Change a note's contents by altering `MarkdownEvents`.
3. Prevent later postprocessors from running or cause a note to be
   skipped entirely.

Future releases of Obsidian export may come with built-in postprocessors
for users of the command-line tool to use, if general use-cases can be
identified.

For example, a future release might include functionality to make notes
more suitable for the Hugo static site generator. This functionality
would be implemented as a postprocessor that could be enabled through
command-line flags.

Cargo.lock

@@ -125,6 +125,12 @@ version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fea41bba32d969b513997752735605054bc0dfa92b4c56bf1189f2e174be7a10"
 
+[[package]]
+name = "dtoa"
+version = "0.4.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "88d7ed2934d741c6b37e33e3832298e8850b53fd2d2bea03873375596c7cea4e"
+
 [[package]]
 name = "either"
 version = "1.6.1"
@@ -245,6 +251,12 @@ version = "0.2.86"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b7282d924be3275cec7f6756ff4121987bc6481325397dde6ba3e7802b1a8b1c"
 
+[[package]]
+name = "linked-hash-map"
+version = "0.5.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7fb9b38af92608140b86b693604b9ffcc5824240a484d1ecd4795bacb2fe88f3"
+
 [[package]]
 name = "log"
 version = "0.4.14"
@@ -305,6 +317,7 @@ dependencies = [
  "pulldown-cmark-to-cmark",
  "rayon",
  "regex",
+ "serde_yaml",
  "slug",
  "snafu",
  "tempfile",
@@ -511,6 +524,24 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d29ab0c6d3fc0ee92fe66e2d99f700eab17a8d57d1c1d3b748380fb20baa78cd"
 
+[[package]]
+name = "serde"
+version = "1.0.123"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "92d5161132722baa40d802cc70b15262b98258453e85e5d1d365c757c73869ae"
+
+[[package]]
+name = "serde_yaml"
+version = "0.8.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "15654ed4ab61726bf918a39cb8d98a2e2995b002387807fa6ba58fdf7f59bb23"
+dependencies = [
+ "dtoa",
+ "linked-hash-map",
+ "serde",
+ "yaml-rust",
+]
+
 [[package]]
 name = "slug"
 version = "0.1.4"
@@ -649,3 +680,12 @@ name = "winapi-x86_64-pc-windows-gnu"
 version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
+
+[[package]]
+name = "yaml-rust"
+version = "0.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "56c1936c4cc7a1c9ab21a1ebb602eb942ba868cbd44a99cb7cdc5892335e1c85"
+dependencies = [
+ "linked-hash-map",
+]

Cargo.toml

@@ -35,6 +35,7 @@ pulldown-cmark = "0.8.0"
 pulldown-cmark-to-cmark = "6.0.0"
 rayon = "1.5.0"
 regex = "1.4.3"
+serde_yaml = "0.8.17"
 slug = "0.1.4"
 snafu = "0.6.10"
 

src/context.rs

@@ -0,0 +1,93 @@
+use crate::Frontmatter;
+use std::path::PathBuf;
+
+#[derive(Debug, Clone)]
+/// Context holds metadata about a note which is being parsed.
+///
+/// This is used internally to keep track of nesting and help with constructing proper references
+/// to other notes.
+///
+/// It is also passed to [postprocessors][crate::Postprocessor] to provide contextual information
+/// and allow modification of a note's frontmatter.
+pub struct Context {
+    file_tree: Vec<PathBuf>,
+
+    /// The path where this note will be written to when exported.
+    ///
+    /// Changing this path will result in the note being written to that new path instead, but
+    /// beware: links will not be updated automatically.  If this is changed by a
+    /// [postprocessor][crate::Postprocessor], it's up to that postprocessor to rewrite any
+    /// existing links to this new path.
+    pub destination: PathBuf,
+
+    /// The [Frontmatter] for this note. Frontmatter may be modified in-place (see
+    /// [serde_yaml::Mapping] for available methods) or replaced entirely.
+    ///
+    /// # Example
+    ///
+    /// Insert `foo: bar` into a note's frontmatter:
+    ///
+    /// ```
+    /// # use obsidian_export::Frontmatter;
+    /// # use obsidian_export::Context;
+    /// # use std::path::PathBuf;
+    /// use obsidian_export::serde_yaml::Value;
+    ///
+    /// # let mut context = Context::new(PathBuf::from("source"), PathBuf::from("destination"));
+    /// let key = Value::String("foo".to_string());
+    ///
+    /// context.frontmatter.insert(
+    ///     key.clone(),
+    ///     Value::String("bar".to_string()),
+    /// );
+    /// ```
+    pub frontmatter: Frontmatter,
+}
+
+impl Context {
+    /// Create a new `Context`
+    pub fn new(src: PathBuf, dest: PathBuf) -> Context {
+        Context {
+            file_tree: vec![src],
+            destination: dest,
+            frontmatter: Frontmatter::new(),
+        }
+    }
+
+    /// Create a new `Context` which inherits from a parent Context.
+    pub fn from_parent(context: &Context, child: &PathBuf) -> Context {
+        let mut context = context.clone();
+        context.file_tree.push(child.to_path_buf());
+        context
+    }
+
+    /// Return the path of the file currently being parsed.
+    pub fn current_file(&self) -> &PathBuf {
+        self.file_tree
+            .last()
+            .expect("Context not initialized properly, file_tree is empty")
+    }
+
+    /// Return the path of the root file.
+    ///
+    /// Typically this will yield the same element as `current_file`, but when a note is embedded
+    /// within another note, this will return the outer-most note.
+    pub fn root_file(&self) -> &PathBuf {
+        self.file_tree
+            .first()
+            .expect("Context not initialized properly, file_tree is empty")
+    }
+
+    /// Return the note depth (nesting level) for this context.
+    pub fn note_depth(&self) -> usize {
+        self.file_tree.len()
+    }
+
+    /// Return the list of files associated with this context.
+    ///
+    /// The first element corresponds to the root file, the final element corresponds to the file
+    /// which is currently being processed (see also `current_file`).
+    pub fn file_tree(&self) -> Vec<PathBuf> {
+        self.file_tree.clone()
+    }
+}

src/frontmatter.rs

@@ -0,0 +1,92 @@
+use serde_yaml::Result;
+
+/// YAML front matter from an Obsidian note.
+///
+/// This is essentially an alias of [serde_yaml::Mapping] so all the methods available on that type
+/// are available with `Frontmatter` as well.
+///
+/// # Examples
+///
+/// ```
+/// # use obsidian_export::Frontmatter;
+/// use serde_yaml::Value;
+///
+/// let mut frontmatter = Frontmatter::new();
+/// let key = Value::String("foo".to_string());
+///
+/// frontmatter.insert(
+///     key.clone(),
+///     Value::String("bar".to_string()),
+/// );
+///
+/// assert_eq!(
+///     frontmatter.get(&key),
+///     Some(&Value::String("bar".to_string())),
+/// )
+/// ```
+pub type Frontmatter = serde_yaml::Mapping;
+
+pub fn frontmatter_from_str(mut s: &str) -> Result<Frontmatter> {
+    if s.is_empty() {
+        s = "{}";
+    }
+    let frontmatter: Frontmatter = serde_yaml::from_str(s)?;
+    Ok(frontmatter)
+}
+
+pub fn frontmatter_to_str(frontmatter: Frontmatter) -> Result<String> {
+    if frontmatter.is_empty() {
+        return Ok("---\n---\n".to_string());
+    }
+
+    let mut buffer = String::new();
+    buffer.push_str(&serde_yaml::to_string(&frontmatter)?);
+    buffer.push_str("---\n");
+    Ok(buffer)
+}
+
+#[derive(Debug, Clone, Copy)]
+/// Available strategies for the inclusion of frontmatter in notes.
+pub enum FrontmatterStrategy {
+    /// Copy frontmatter when a note has frontmatter defined.
+    Auto,
+    /// Always add frontmatter header, including empty frontmatter when none was originally
+    /// specified.
+    Always,
+    /// Never add any frontmatter to notes.
+    Never,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+    use serde_yaml::Value;
+
+    #[test]
+    fn empty_string_should_yield_empty_frontmatter() {
+        assert_eq!(frontmatter_from_str("").unwrap(), Frontmatter::new())
+    }
+
+    #[test]
+    fn empty_frontmatter_to_str() {
+        let frontmatter = Frontmatter::new();
+        assert_eq!(
+            frontmatter_to_str(frontmatter).unwrap(),
+            format!("---\n---\n")
+        )
+    }
+
+    #[test]
+    fn nonempty_frontmatter_to_str() {
+        let mut frontmatter = Frontmatter::new();
+        frontmatter.insert(
+            Value::String("foo".to_string()),
+            Value::String("bar".to_string()),
+        );
+        assert_eq!(
+            frontmatter_to_str(frontmatter).unwrap(),
+            format!("---\nfoo: bar\n---\n")
+        )
+    }
+}

src/lib.rs

@@ -1,16 +1,25 @@
+pub extern crate pulldown_cmark;
+pub extern crate serde_yaml;
+
 #[macro_use]
 extern crate lazy_static;
 
+mod context;
+mod frontmatter;
+mod references;
 mod walker;
 
+pub use context::Context;
+pub use frontmatter::{Frontmatter, FrontmatterStrategy};
 pub use walker::{vault_contents, WalkOptions};
 
+use frontmatter::{frontmatter_from_str, frontmatter_to_str};
 use pathdiff::diff_paths;
 use percent_encoding::{utf8_percent_encode, AsciiSet, CONTROLS};
 use pulldown_cmark::{CodeBlockKind, CowStr, Event, Options, Parser, Tag};
 use pulldown_cmark_to_cmark::cmark_with_options;
 use rayon::prelude::*;
-use regex::Regex;
+use references::*;
 use slug::slugify;
 use snafu::{ResultExt, Snafu};
 use std::ffi::OsString;
@@ -21,13 +30,98 @@ use std::io::ErrorKind;
 use std::path::{Path, PathBuf};
 use std::str;
 
-type Result<T, E = ExportError> = std::result::Result<T, E>;
+/// A series of markdown [Event]s that are generated while traversing an Obsidian markdown note.
-type MarkdownTree<'a> = Vec<Event<'a>>;
+pub type MarkdownEvents<'a> = Vec<Event<'a>>;
+
+/// A post-processing function that is to be called after an Obsidian note has been fully parsed and
+/// converted to regular markdown syntax.
+///
+/// Postprocessors are called in the order they've been added through [Exporter::add_postprocessor]
+/// just before notes are written out to their final destination.
+/// They may be used to achieve the following:
+///
+/// 1. Modify a note's [Context], for example to change the destination filename or update its [Frontmatter] (see [Context::frontmatter]).
+/// 2. Change a note's contents by altering [MarkdownEvents].
+/// 3. Prevent later postprocessors from running ([PostprocessorResult::StopHere]) or cause a note
+///    to be skipped entirely ([PostprocessorResult::StopAndSkipNote]).
+///
+/// # Examples
+///
+/// ## Update frontmatter
+///
+/// This example shows how to make changes a note's frontmatter. In this case, the postprocessor is
+/// defined inline as a closure.
+///
+/// ```
+/// use obsidian_export::{Context, Exporter, MarkdownEvents, PostprocessorResult};
+/// use obsidian_export::pulldown_cmark::{CowStr, Event};
+/// use obsidian_export::serde_yaml::Value;
+/// # use std::path::PathBuf;
+/// # use tempfile::TempDir;
+///
+/// # let tmp_dir = TempDir::new().expect("failed to make tempdir");
+/// # let source = PathBuf::from("tests/testdata/input/postprocessors");
+/// # let destination = tmp_dir.path().to_path_buf();
+/// let mut exporter = Exporter::new(source, destination);
+///
+/// // add_postprocessor registers a new postprocessor. In this example we use a closure.
+/// exporter.add_postprocessor(&|mut context, events| {
+///     // This is the key we'll insert into the frontmatter. In this case, the string "foo".
+///     let key = Value::String("foo".to_string());
+///     // This is the value we'll insert into the frontmatter. In this case, the string "bar".
+///     let value = Value::String("baz".to_string());
+///
+///     // Frontmatter can be updated in-place, so we can call insert on it directly.
+///     context.frontmatter.insert(key, value);
+///
+///     // Postprocessors must return their (modified) context, the markdown events that make
+///     // up the note and a next action to take.
+///     (context, events, PostprocessorResult::Continue)
+/// });
+///
+/// exporter.run().unwrap();
+/// ```
+///
+/// ## Change note contents
+///
+/// In this example a note's markdown content is changed by iterating over the [MarkdownEvents] and
+/// changing the text when we encounter a [text element][Event::Text].
+///
+/// Instead of using a closure like above, this example shows how to use a separate function
+/// definition.
+/// ```
+/// # use obsidian_export::{Context, Exporter, MarkdownEvents, PostprocessorResult};
+/// # use pulldown_cmark::{CowStr, Event};
+/// # use std::path::PathBuf;
+/// # use tempfile::TempDir;
+/// #
+/// /// This postprocessor replaces any instance of "foo" with "bar" in the note body.
+/// fn foo_to_bar(
+///     context: Context,
+///     events: MarkdownEvents,
+/// ) -> (Context, MarkdownEvents, PostprocessorResult) {
+///     let events = events
+///         .into_iter()
+///         .map(|event| match event {
+///             Event::Text(text) => Event::Text(CowStr::from(text.replace("foo", "bar"))),
+///             event => event,
+///         })
+///         .collect();
+///     (context, events, PostprocessorResult::Continue)
+/// }
+///
+/// # let tmp_dir = TempDir::new().expect("failed to make tempdir");
+/// # let source = PathBuf::from("tests/testdata/input/postprocessors");
+/// # let destination = tmp_dir.path().to_path_buf();
+/// # let mut exporter = Exporter::new(source, destination);
+/// exporter.add_postprocessor(&foo_to_bar);
+/// # exporter.run().unwrap();
+/// ```
+
+pub type Postprocessor =
+    dyn Fn(Context, MarkdownEvents) -> (Context, MarkdownEvents, PostprocessorResult) + Send + Sync;
+type Result<T, E = ExportError> = std::result::Result<T, E>;
 
-lazy_static! {
-    static ref OBSIDIAN_NOTE_LINK_RE: Regex =
-        Regex::new(r"^(?P<file>[^#|]+)??(#(?P<section>.+?))??(\|(?P<label>.+?))??$").unwrap();
-}
 const PERCENTENCODE_CHARS: &AsciiSet = &CONTROLS.add(b' ').add(b'(').add(b')').add(b'%').add(b'?');
 const NOTE_RECURSION_LIMIT: usize = 10;
 
@@ -80,21 +174,34 @@ pub enum ExportError {
         #[snafu(source(from(ExportError, Box::new)))]
         source: Box<ExportError>,
     },
+
+    #[snafu(display("Failed to decode YAML frontmatter in '{}'", path.display()))]
+    FrontMatterDecodeError {
+        path: PathBuf,
+        #[snafu(source(from(serde_yaml::Error, Box::new)))]
+        source: Box<serde_yaml::Error>,
+    },
+
+    #[snafu(display("Failed to encode YAML frontmatter for '{}'", path.display()))]
+    FrontMatterEncodeError {
+        path: PathBuf,
+        #[snafu(source(from(serde_yaml::Error, Box::new)))]
+        source: Box<serde_yaml::Error>,
+    },
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq)]
-/// FrontmatterStrategy determines how frontmatter is handled in Markdown files.
+/// Emitted by [Postprocessor]s to signal the next action to take.
-pub enum FrontmatterStrategy {
+pub enum PostprocessorResult {
-    /// Copy frontmatter when a note has frontmatter defined.
+    /// Continue with the next post-processor (if any).
-    Auto,
+    Continue,
-    /// Always add frontmatter header, including empty frontmatter when none was originally
+    /// Use this note, but don't run any more post-processors after this one.
-    /// specified.
+    StopHere,
-    Always,
+    /// Skip this note (don't export it) and don't run any more post-processors.
-    /// Never add any frontmatter to notes.
+    StopAndSkipNote,
-    Never,
 }
 
-#[derive(Debug, Clone)]
+#[derive(Clone)]
 /// Exporter provides the main interface to this library.
 ///
 /// Users are expected to create an Exporter using [`Exporter::new`], optionally followed by
@@ -108,164 +215,26 @@ pub struct Exporter<'a> {
     vault_contents: Option<Vec<PathBuf>>,
     walk_options: WalkOptions<'a>,
     process_embeds_recursively: bool,
+    postprocessors: Vec<&'a Postprocessor>,
 }
 
-#[derive(Debug, Clone)]
+impl<'a> fmt::Debug for Exporter<'a> {
-/// Context holds parser metadata for the file/note currently being parsed.
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-struct Context {
+        f.debug_struct("WalkOptions")
-    file_tree: Vec<PathBuf>,
+            .field("root", &self.root)
-    frontmatter_strategy: FrontmatterStrategy,
+            .field("destination", &self.destination)
-}
+            .field("frontmatter_strategy", &self.frontmatter_strategy)
-
+            .field("vault_contents", &self.vault_contents)
-#[derive(Debug, Clone, PartialEq)]
+            .field("walk_options", &self.walk_options)
-/// ObsidianNoteReference represents the structure of a `[[note]]` or `![[embed]]` reference.
+            .field(
-struct ObsidianNoteReference<'a> {
+                "process_embeds_recursively",
-    /// The file (note name or partial path) being referenced.
+                &self.process_embeds_recursively,
-    /// This will be None in the case that the reference is to a section within the same document
+            )
-    file: Option<&'a str>,
+            .field(
-    /// If specific, a specific section/heading being referenced.
+                "postprocessors",
-    section: Option<&'a str>,
+                &format!("<{} postprocessors active>", self.postprocessors.len()),
-    /// If specific, the custom label/text which was specified.
+            )
-    label: Option<&'a str>,
+            .finish()
-}
-
-#[derive(PartialEq)]
-/// RefParserState enumerates all the possible parsing states [RefParser] may enter.
-enum RefParserState {
-    NoState,
-    ExpectSecondOpenBracket,
-    ExpectRefText,
-    ExpectRefTextOrCloseBracket,
-    ExpectFinalCloseBracket,
-    Resetting,
-}
-
-/// RefType indicates whether a note reference is a link (`[[note]]`) or embed (`![[embed]]`).
-enum RefType {
-    Link,
-    Embed,
-}
-
-/// RefParser holds state which is used to parse Obsidian WikiLinks (`[[note]]`, `![[embed]]`).
-struct RefParser {
-    state: RefParserState,
-    ref_type: Option<RefType>,
-    // References sometimes come in through multiple events. One example of this is when notes
-    // start with an underscore (_), presumably because this is also the literal which starts
-    // italic and bold text.
-    //
-    // ref_text concatenates the values from these partial events so that there's a fully-formed
-    // string to work with by the time the final `]]` is encountered.
-    ref_text: String,
-}
-
-impl RefParser {
-    fn new() -> RefParser {
-        RefParser {
-            state: RefParserState::NoState,
-            ref_type: None,
-            ref_text: String::new(),
-        }
-    }
-
-    fn transition(&mut self, new_state: RefParserState) {
-        self.state = new_state;
-    }
-
-    fn reset(&mut self) {
-        self.state = RefParserState::NoState;
-        self.ref_type = None;
-        self.ref_text.clear();
-    }
-}
-
-impl Context {
-    /// Create a new `Context`
-    fn new(file: PathBuf) -> Context {
-        Context {
-            file_tree: vec![file],
-            frontmatter_strategy: FrontmatterStrategy::Auto,
-        }
-    }
-
-    /// Create a new `Context` which inherits from a parent Context.
-    fn from_parent(context: &Context, child: &PathBuf) -> Context {
-        let mut context = context.clone();
-        context.file_tree.push(child.to_path_buf());
-        context
-    }
-
-    /// Associate a new `FrontmatterStrategy` with this context.
-    fn set_frontmatter_strategy(&mut self, strategy: FrontmatterStrategy) -> &mut Context {
-        self.frontmatter_strategy = strategy;
-        self
-    }
-
-    /// Return the path of the file currently being parsed.
-    fn current_file(&self) -> &PathBuf {
-        self.file_tree
-            .last()
-            .expect("Context not initialized properly, file_tree is empty")
-    }
-
-    /// Return the path of the root file.
-    ///
-    /// Typically this will yield the same element as `current_file`, but when a note is embedded
-    /// within another note, this will return the outer-most note.
-    fn root_file(&self) -> &PathBuf {
-        self.file_tree
-            .first()
-            .expect("Context not initialized properly, file_tree is empty")
-    }
-
-    /// Return the note depth (nesting level) for this context.
-    fn note_depth(&self) -> usize {
-        self.file_tree.len()
-    }
-
-    /// Return the list of files associated with this context.
-    ///
-    /// The first element corresponds to the root file, the final element corresponds to the file
-    /// which is currently being processed (see also `current_file`).
-    fn file_tree(&self) -> Vec<PathBuf> {
-        self.file_tree.clone()
-    }
-}
-
-impl<'a> ObsidianNoteReference<'a> {
-    fn from_str(text: &str) -> ObsidianNoteReference {
-        let captures = OBSIDIAN_NOTE_LINK_RE
-            .captures(&text)
-            .expect("note link regex didn't match - bad input?");
-        let file = captures.name("file").map(|v| v.as_str());
-        let label = captures.name("label").map(|v| v.as_str());
-        let section = captures.name("section").map(|v| v.as_str());
-
-        ObsidianNoteReference {
-            file,
-            label,
-            section,
-        }
-    }
-
-    fn display(&self) -> String {
-        format!("{}", self)
-    }
-}
-
-impl<'a> fmt::Display for ObsidianNoteReference<'a> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let label =
-            self.label
-                .map(|v| v.to_string())
-                .unwrap_or_else(|| match (self.file, self.section) {
-                    (Some(file), Some(section)) => format!("{} > {}", file, section),
-                    (Some(file), None) => file.to_string(),
-                    (None, Some(section)) => section.to_string(),
-
-                    _ => panic!("Reference exists without file or section!"),
-                });
-        write!(f, "{}", label)
     }
 }
 
@@ -280,6 +249,7 @@ impl<'a> Exporter<'a> {
             walk_options: WalkOptions::default(),
             process_embeds_recursively: true,
             vault_contents: None,
+            postprocessors: vec![],
         }
     }
 
@@ -308,6 +278,12 @@ impl<'a> Exporter<'a> {
         self
     }
 
+    /// Append a function to the chain of [postprocessors][Postprocessor] to run on exported Obsidian Markdown notes.
+    pub fn add_postprocessor(&mut self, processor: &'a Postprocessor) -> &mut Exporter<'a> {
+        self.postprocessors.push(processor);
+        self
+    }
+
     /// Export notes using the settings configured on this exporter.
     pub fn run(&mut self) -> Result<()> {
         if !self.root.exists() {
@@ -372,58 +348,64 @@ impl<'a> Exporter<'a> {
 
     fn export_note(&self, src: &Path, dest: &Path) -> Result<()> {
         match is_markdown_file(src) {
-            true => self.parse_and_export_obsidian_note(src, dest, self.frontmatter_strategy),
+            true => self.parse_and_export_obsidian_note(src, dest),
             false => copy_file(src, dest),
         }
         .context(FileExportError { path: src })
     }
 
-    fn parse_and_export_obsidian_note(
+    fn parse_and_export_obsidian_note(&self, src: &Path, dest: &Path) -> Result<()> {
-        &self,
+        let mut context = Context::new(src.to_path_buf(), dest.to_path_buf());
-        src: &Path,
-        dest: &Path,
-        frontmatter_strategy: FrontmatterStrategy,
-    ) -> Result<()> {
-        let content = fs::read_to_string(&src).context(ReadError { path: src })?;
 
-        let (mut frontmatter, _content) =
+        let (frontmatter, mut markdown_events) = self.parse_obsidian_note(&src, &context)?;
-            matter::matter(&content).unwrap_or(("".to_string(), content.to_string()));
+        context.frontmatter = frontmatter;
-        frontmatter = frontmatter.trim().to_string();
+        for func in &self.postprocessors {
-        //let mut outfile = create_file(&dest).context(FileIOError { filename: dest })?;
+            let res = func(context, markdown_events);
+            context = res.0;
+            markdown_events = res.1;
+            match res.2 {
+                PostprocessorResult::StopHere => break,
+                PostprocessorResult::StopAndSkipNote => return Ok(()),
+                _ => (),
+            }
+        }
+
+        let dest = context.destination;
         let mut outfile = create_file(&dest)?;
-
+        let write_frontmatter = match self.frontmatter_strategy {
-        let write_frontmatter = match frontmatter_strategy {
             FrontmatterStrategy::Always => true,
             FrontmatterStrategy::Never => false,
-            FrontmatterStrategy::Auto => !frontmatter.is_empty(),
+            FrontmatterStrategy::Auto => !context.frontmatter.is_empty(),
         };
         if write_frontmatter {
-            if !frontmatter.is_empty() && !frontmatter.ends_with('\n') {
+            let mut frontmatter_str = frontmatter_to_str(context.frontmatter)
-                frontmatter.push('\n');
+                .context(FrontMatterEncodeError { path: src })?;
-            }
+            frontmatter_str.push('\n');
             outfile
-                .write_all(format!("---\n{}---\n\n", frontmatter).as_bytes())
+                .write_all(frontmatter_str.as_bytes())
                 .context(WriteError { path: &dest })?;
         }
-
-        let mut context = Context::new(src.to_path_buf());
-        context.set_frontmatter_strategy(frontmatter_strategy);
-        let markdown_tree = self.parse_obsidian_note(&src, &context)?;
         outfile
-            .write_all(render_mdtree_to_mdtext(markdown_tree).as_bytes())
+            .write_all(render_mdevents_to_mdtext(markdown_events).as_bytes())
             .context(WriteError { path: &dest })?;
         Ok(())
     }
 
-    fn parse_obsidian_note<'b>(&self, path: &Path, context: &Context) -> Result<MarkdownTree<'b>> {
+    fn parse_obsidian_note<'b>(
+        &self,
+        path: &Path,
+        context: &Context,
+    ) -> Result<(Frontmatter, MarkdownEvents<'b>)> {
         if context.note_depth() > NOTE_RECURSION_LIMIT {
             return Err(ExportError::RecursionLimitExceeded {
                 file_tree: context.file_tree(),
             });
         }
         let content = fs::read_to_string(&path).context(ReadError { path })?;
-        let (_frontmatter, content) =
+        let (frontmatter, content) =
             matter::matter(&content).unwrap_or(("".to_string(), content.to_string()));
+        let frontmatter =
+            frontmatter_from_str(&frontmatter).context(FrontMatterDecodeError { path })?;
 
         let mut parser_options = Options::empty();
         parser_options.insert(Options::ENABLE_TABLES);
@@ -432,13 +414,13 @@ impl<'a> Exporter<'a> {
         parser_options.insert(Options::ENABLE_TASKLISTS);
 
         let mut ref_parser = RefParser::new();
-        let mut tree = vec![];
+        let mut events = vec![];
         // Most of the time, a reference triggers 5 events: [ or ![, [, <text>, ], ]
         let mut buffer = Vec::with_capacity(5);
 
         for event in Parser::new_ext(&content, parser_options) {
             if ref_parser.state == RefParserState::Resetting {
-                tree.append(&mut buffer);
+                events.append(&mut buffer);
                 buffer.clear();
                 ref_parser.reset();
             }
@@ -455,7 +437,7 @@ impl<'a> Exporter<'a> {
                             ref_parser.transition(RefParserState::ExpectSecondOpenBracket);
                         }
                         _ => {
-                            tree.push(event);
+                            events.push(event);
                             buffer.clear();
                         },
                     };
@@ -500,7 +482,7 @@ impl<'a> Exporter<'a> {
                                 ),
                                 context,
                             );
-                            tree.append(&mut elements);
+                            events.append(&mut elements);
                             buffer.clear();
                             ref_parser.transition(RefParserState::Resetting);
                         }
@@ -509,7 +491,7 @@ impl<'a> Exporter<'a> {
                                 ref_parser.ref_text.clone().as_ref(),
                                 context
                             )?;
-                            tree.append(&mut elements);
+                            events.append(&mut elements);
                             buffer.clear();
                             ref_parser.transition(RefParserState::Resetting);
                         }
@@ -523,9 +505,12 @@ impl<'a> Exporter<'a> {
             }
         }
         if !buffer.is_empty() {
-            tree.append(&mut buffer);
+            events.append(&mut buffer);
         }
-        Ok(tree.into_iter().map(event_to_owned).collect())
+        Ok((
+            frontmatter,
+            events.into_iter().map(event_to_owned).collect(),
+        ))
     }
 
     // Generate markdown elements for a file that is embedded within another note.
@@ -533,7 +518,11 @@ impl<'a> Exporter<'a> {
     // - If the file being embedded is a note, it's content is included at the point of embed.
     // - If the file is an image, an image tag is generated.
     // - For other types of file, a regular link is created instead.
-    fn embed_file<'b>(&self, link_text: &'a str, context: &'a Context) -> Result<MarkdownTree<'b>> {
+    fn embed_file<'b>(
+        &self,
+        link_text: &'a str,
+        context: &'a Context,
+    ) -> Result<MarkdownEvents<'b>> {
         let note_ref = ObsidianNoteReference::from_str(link_text);
 
         let path = match note_ref.file {
@@ -561,7 +550,7 @@ impl<'a> Exporter<'a> {
         let child_context = Context::from_parent(context, path);
         let no_ext = OsString::new();
 
-        if !self.process_embeds_recursively && context.file_tree.contains(path) {
+        if !self.process_embeds_recursively && context.file_tree().contains(path) {
             return Ok([
                 vec![Event::Text(CowStr::Borrowed("→ "))],
                 self.make_link_to_file(note_ref, &child_context),
@@ -569,13 +558,13 @@ impl<'a> Exporter<'a> {
             .concat());
         }
 
-        let tree = match path.extension().unwrap_or(&no_ext).to_str() {
+        let events = match path.extension().unwrap_or(&no_ext).to_str() {
             Some("md") => {
-                let mut tree = self.parse_obsidian_note(&path, &child_context)?;
+                let (_frontmatter, mut events) = self.parse_obsidian_note(&path, &child_context)?;
                 if let Some(section) = note_ref.section {
-                    tree = reduce_to_section(tree, section);
+                    events = reduce_to_section(events, section);
                 }
-                tree
+                events
             }
             Some("png") | Some("jpg") | Some("jpeg") | Some("gif") | Some("webp") => {
                 self.make_link_to_file(note_ref, &child_context)
@@ -605,14 +594,14 @@ impl<'a> Exporter<'a> {
             }
             _ => self.make_link_to_file(note_ref, &child_context),
         };
-        Ok(tree)
+        Ok(events)
     }
 
     fn make_link_to_file<'b, 'c>(
         &self,
         reference: ObsidianNoteReference<'b>,
         context: &Context,
-    ) -> MarkdownTree<'c> {
+    ) -> MarkdownEvents<'c> {
         let target_file = reference
             .file
             .map(|file| lookup_filename_in_vault(file, &self.vault_contents.as_ref().unwrap()))
@@ -687,7 +676,7 @@ fn lookup_filename_in_vault<'a>(
     })
 }
 
-fn render_mdtree_to_mdtext(markdown: MarkdownTree) -> String {
+fn render_mdevents_to_mdtext(markdown: MarkdownEvents) -> String {
     let mut buffer = String::new();
     cmark_with_options(
         markdown.iter(),
@@ -736,25 +725,25 @@ fn is_markdown_file(file: &Path) -> bool {
     ext == "md"
 }
 
-/// Reduce a given `MarkdownTree` to just those elements which are children of the given section
+/// Reduce a given `MarkdownEvents` to just those elements which are children of the given section
 /// (heading name).
-fn reduce_to_section<'a, 'b>(tree: MarkdownTree<'a>, section: &'b str) -> MarkdownTree<'a> {
+fn reduce_to_section<'a, 'b>(events: MarkdownEvents<'a>, section: &'b str) -> MarkdownEvents<'a> {
-    let mut new_tree = Vec::with_capacity(tree.len());
+    let mut filtered_events = Vec::with_capacity(events.len());
     let mut target_section_encountered = false;
     let mut currently_in_target_section = false;
     let mut section_level = 0;
     let mut last_level = 0;
     let mut last_tag_was_heading = false;
 
-    for event in tree.into_iter() {
+    for event in events.into_iter() {
-        new_tree.push(event.clone());
+        filtered_events.push(event.clone());
         match event {
             Event::Start(Tag::Heading(level)) => {
                 last_tag_was_heading = true;
                 last_level = level;
                 if currently_in_target_section && level <= section_level {
                     currently_in_target_section = false;
-                    new_tree.pop();
+                    filtered_events.pop();
                 }
             }
             Event::Text(cowstr) => {
@@ -769,20 +758,20 @@ fn reduce_to_section<'a, 'b>(tree: MarkdownTree<'a>, section: &'b str) -> Markdo
                     currently_in_target_section = true;
                     section_level = last_level;
 
-                    let current_event = new_tree.pop().unwrap();
+                    let current_event = filtered_events.pop().unwrap();
-                    let heading_start_event = new_tree.pop().unwrap();
+                    let heading_start_event = filtered_events.pop().unwrap();
-                    new_tree.clear();
+                    filtered_events.clear();
-                    new_tree.push(heading_start_event);
+                    filtered_events.push(heading_start_event);
-                    new_tree.push(current_event);
+                    filtered_events.push(current_event);
                 }
             }
             _ => {}
         }
         if target_section_encountered && !currently_in_target_section {
-            return new_tree;
+            return filtered_events;
         }
     }
-    new_tree
+    filtered_events
 }
 
 fn event_to_owned<'a>(event: Event) -> Event<'a> {
@@ -839,101 +828,3 @@ fn codeblock_kind_to_owned<'a>(codeblock_kind: CodeBlockKind) -> CodeBlockKind<'
         CodeBlockKind::Fenced(cowstr) => CodeBlockKind::Fenced(CowStr::from(cowstr.into_string())),
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn parse_note_refs_from_strings() {
-        assert_eq!(
-            ObsidianNoteReference::from_str("Just a note"),
-            ObsidianNoteReference {
-                file: Some("Just a note"),
-                label: None,
-                section: None,
-            }
-        );
-        assert_eq!(
-            ObsidianNoteReference::from_str("A note?"),
-            ObsidianNoteReference {
-                file: Some("A note?"),
-                label: None,
-                section: None,
-            }
-        );
-        assert_eq!(
-            ObsidianNoteReference::from_str("Note#with heading"),
-            ObsidianNoteReference {
-                file: Some("Note"),
-                label: None,
-                section: Some("with heading"),
-            }
-        );
-        assert_eq!(
-            ObsidianNoteReference::from_str("Note#Heading|Label"),
-            ObsidianNoteReference {
-                file: Some("Note"),
-                label: Some("Label"),
-                section: Some("Heading"),
-            }
-        );
-        assert_eq!(
-            ObsidianNoteReference::from_str("#Heading|Label"),
-            ObsidianNoteReference {
-                file: None,
-                label: Some("Label"),
-                section: Some("Heading"),
-            }
-        );
-    }
-
-    #[test]
-    fn test_display_of_note_refs() {
-        assert_eq!(
-            "Note",
-            ObsidianNoteReference {
-                file: Some("Note"),
-                label: None,
-                section: None,
-            }
-            .display()
-        );
-        assert_eq!(
-            "Note > Heading",
-            ObsidianNoteReference {
-                file: Some("Note"),
-                label: None,
-                section: Some("Heading"),
-            }
-            .display()
-        );
-        assert_eq!(
-            "Heading",
-            ObsidianNoteReference {
-                file: None,
-                label: None,
-                section: Some("Heading"),
-            }
-            .display()
-        );
-        assert_eq!(
-            "Label",
-            ObsidianNoteReference {
-                file: Some("Note"),
-                label: Some("Label"),
-                section: Some("Heading"),
-            }
-            .display()
-        );
-        assert_eq!(
-            "Label",
-            ObsidianNoteReference {
-                file: None,
-                label: Some("Label"),
-                section: Some("Heading"),
-            }
-            .display()
-        );
-    }
-}

src/references.rs

@@ -0,0 +1,204 @@
+use regex::Regex;
+use std::fmt;
+
+lazy_static! {
+    static ref OBSIDIAN_NOTE_LINK_RE: Regex =
+        Regex::new(r"^(?P<file>[^#|]+)??(#(?P<section>.+?))??(\|(?P<label>.+?))??$").unwrap();
+}
+
+#[derive(Debug, Clone, PartialEq)]
+/// ObsidianNoteReference represents the structure of a `[[note]]` or `![[embed]]` reference.
+pub struct ObsidianNoteReference<'a> {
+    /// The file (note name or partial path) being referenced.
+    /// This will be None in the case that the reference is to a section within the same document
+    pub file: Option<&'a str>,
+    /// If specific, a specific section/heading being referenced.
+    pub section: Option<&'a str>,
+    /// If specific, the custom label/text which was specified.
+    pub label: Option<&'a str>,
+}
+
+#[derive(PartialEq)]
+/// RefParserState enumerates all the possible parsing states [RefParser] may enter.
+pub enum RefParserState {
+    NoState,
+    ExpectSecondOpenBracket,
+    ExpectRefText,
+    ExpectRefTextOrCloseBracket,
+    ExpectFinalCloseBracket,
+    Resetting,
+}
+
+/// RefType indicates whether a note reference is a link (`[[note]]`) or embed (`![[embed]]`).
+pub enum RefType {
+    Link,
+    Embed,
+}
+
+/// RefParser holds state which is used to parse Obsidian WikiLinks (`[[note]]`, `![[embed]]`).
+pub struct RefParser {
+    pub state: RefParserState,
+    pub ref_type: Option<RefType>,
+    // References sometimes come in through multiple events. One example of this is when notes
+    // start with an underscore (_), presumably because this is also the literal which starts
+    // italic and bold text.
+    //
+    // ref_text concatenates the values from these partial events so that there's a fully-formed
+    // string to work with by the time the final `]]` is encountered.
+    pub ref_text: String,
+}
+
+impl RefParser {
+    pub fn new() -> RefParser {
+        RefParser {
+            state: RefParserState::NoState,
+            ref_type: None,
+            ref_text: String::new(),
+        }
+    }
+
+    pub fn transition(&mut self, new_state: RefParserState) {
+        self.state = new_state;
+    }
+
+    pub fn reset(&mut self) {
+        self.state = RefParserState::NoState;
+        self.ref_type = None;
+        self.ref_text.clear();
+    }
+}
+
+impl<'a> ObsidianNoteReference<'a> {
+    pub fn from_str(text: &str) -> ObsidianNoteReference {
+        let captures = OBSIDIAN_NOTE_LINK_RE
+            .captures(&text)
+            .expect("note link regex didn't match - bad input?");
+        let file = captures.name("file").map(|v| v.as_str());
+        let label = captures.name("label").map(|v| v.as_str());
+        let section = captures.name("section").map(|v| v.as_str());
+
+        ObsidianNoteReference {
+            file,
+            label,
+            section,
+        }
+    }
+
+    pub fn display(&self) -> String {
+        format!("{}", self)
+    }
+}
+
+impl<'a> fmt::Display for ObsidianNoteReference<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let label =
+            self.label
+                .map(|v| v.to_string())
+                .unwrap_or_else(|| match (self.file, self.section) {
+                    (Some(file), Some(section)) => format!("{} > {}", file, section),
+                    (Some(file), None) => file.to_string(),
+                    (None, Some(section)) => section.to_string(),
+
+                    _ => panic!("Reference exists without file or section!"),
+                });
+        write!(f, "{}", label)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_note_refs_from_strings() {
+        assert_eq!(
+            ObsidianNoteReference::from_str("Just a note"),
+            ObsidianNoteReference {
+                file: Some("Just a note"),
+                label: None,
+                section: None,
+            }
+        );
+        assert_eq!(
+            ObsidianNoteReference::from_str("A note?"),
+            ObsidianNoteReference {
+                file: Some("A note?"),
+                label: None,
+                section: None,
+            }
+        );
+        assert_eq!(
+            ObsidianNoteReference::from_str("Note#with heading"),
+            ObsidianNoteReference {
+                file: Some("Note"),
+                label: None,
+                section: Some("with heading"),
+            }
+        );
+        assert_eq!(
+            ObsidianNoteReference::from_str("Note#Heading|Label"),
+            ObsidianNoteReference {
+                file: Some("Note"),
+                label: Some("Label"),
+                section: Some("Heading"),
+            }
+        );
+        assert_eq!(
+            ObsidianNoteReference::from_str("#Heading|Label"),
+            ObsidianNoteReference {
+                file: None,
+                label: Some("Label"),
+                section: Some("Heading"),
+            }
+        );
+    }
+
+    #[test]
+    fn test_display_of_note_refs() {
+        assert_eq!(
+            "Note",
+            ObsidianNoteReference {
+                file: Some("Note"),
+                label: None,
+                section: None,
+            }
+            .display()
+        );
+        assert_eq!(
+            "Note > Heading",
+            ObsidianNoteReference {
+                file: Some("Note"),
+                label: None,
+                section: Some("Heading"),
+            }
+            .display()
+        );
+        assert_eq!(
+            "Heading",
+            ObsidianNoteReference {
+                file: None,
+                label: None,
+                section: Some("Heading"),
+            }
+            .display()
+        );
+        assert_eq!(
+            "Label",
+            ObsidianNoteReference {
+                file: Some("Note"),
+                label: Some("Label"),
+                section: Some("Heading"),
+            }
+            .display()
+        );
+        assert_eq!(
+            "Label",
+            ObsidianNoteReference {
+                file: None,
+                label: Some("Label"),
+                section: Some("Heading"),
+            }
+            .display()
+        );
+    }
+}

tests/export_test.rs

@@ -1,6 +1,10 @@
-use obsidian_export::{ExportError, Exporter, FrontmatterStrategy};
+use obsidian_export::{
+    Context, ExportError, Exporter, FrontmatterStrategy, MarkdownEvents, PostprocessorResult,
+};
 use pretty_assertions::assert_eq;
-use std::fs::{create_dir, read_to_string, set_permissions, File, Permissions};
+use pulldown_cmark::{CowStr, Event};
+use serde_yaml::Value;
+use std::fs::{create_dir, read_to_string, remove_file, set_permissions, File, Permissions};
 use std::io::prelude::*;
 use std::path::PathBuf;
 use tempfile::TempDir;
@@ -350,3 +354,108 @@ fn test_same_filename_different_directories() {
     let actual = read_to_string(tmp_dir.path().clone().join(PathBuf::from("Note.md"))).unwrap();
     assert_eq!(expected, actual);
 }
+
+/// This postprocessor replaces any instance of "foo" with "bar" in the note body.
+fn foo_to_bar(
+    ctx: Context,
+    events: MarkdownEvents,
+) -> (Context, MarkdownEvents, PostprocessorResult) {
+    let events = events
+        .into_iter()
+        .map(|event| match event {
+            Event::Text(text) => Event::Text(CowStr::from(text.replace("foo", "bar"))),
+            event => event,
+        })
+        .collect();
+    (ctx, events, PostprocessorResult::Continue)
+}
+
+/// This postprocessor appends "bar: baz" to frontmatter.
+fn append_frontmatter(
+    mut ctx: Context,
+    events: MarkdownEvents,
+) -> (Context, MarkdownEvents, PostprocessorResult) {
+    ctx.frontmatter.insert(
+        Value::String("bar".to_string()),
+        Value::String("baz".to_string()),
+    );
+    (ctx, events, PostprocessorResult::Continue)
+}
+
+// The purpose of this test to verify the `append_frontmatter` postprocessor is called to extend
+// the frontmatter, and the `foo_to_bar` postprocessor is called to replace instances of "foo" with
+// "bar" (only in the note body).
+#[test]
+fn test_postprocessors() {
+    let tmp_dir = TempDir::new().expect("failed to make tempdir");
+    let mut exporter = Exporter::new(
+        PathBuf::from("tests/testdata/input/postprocessors"),
+        tmp_dir.path().to_path_buf(),
+    );
+    exporter.add_postprocessor(&foo_to_bar);
+    exporter.add_postprocessor(&append_frontmatter);
+
+    exporter.run().unwrap();
+
+    let expected = read_to_string("tests/testdata/expected/postprocessors/Note.md").unwrap();
+    let actual = read_to_string(tmp_dir.path().clone().join(PathBuf::from("Note.md"))).unwrap();
+    assert_eq!(expected, actual);
+}
+
+#[test]
+fn test_postprocessor_stophere() {
+    let tmp_dir = TempDir::new().expect("failed to make tempdir");
+    let mut exporter = Exporter::new(
+        PathBuf::from("tests/testdata/input/postprocessors"),
+        tmp_dir.path().to_path_buf(),
+    );
+
+    exporter.add_postprocessor(&|ctx, mdevents| (ctx, mdevents, PostprocessorResult::StopHere));
+    exporter.add_postprocessor(&|_, _| panic!("should not be called due to above processor"));
+    exporter.run().unwrap();
+}
+
+#[test]
+fn test_postprocessor_stop_and_skip() {
+    let tmp_dir = TempDir::new().expect("failed to make tempdir");
+    let note_path = tmp_dir.path().clone().join(PathBuf::from("Note.md"));
+
+    let mut exporter = Exporter::new(
+        PathBuf::from("tests/testdata/input/postprocessors"),
+        tmp_dir.path().to_path_buf(),
+    );
+    exporter.run().unwrap();
+
+    assert!(note_path.exists());
+    remove_file(&note_path).unwrap();
+
+    exporter
+        .add_postprocessor(&|ctx, mdevents| (ctx, mdevents, PostprocessorResult::StopAndSkipNote));
+    exporter.run().unwrap();
+
+    assert!(!note_path.exists());
+}
+
+#[test]
+fn test_postprocessor_change_destination() {
+    let tmp_dir = TempDir::new().expect("failed to make tempdir");
+    let original_note_path = tmp_dir.path().clone().join(PathBuf::from("Note.md"));
+    let mut exporter = Exporter::new(
+        PathBuf::from("tests/testdata/input/postprocessors"),
+        tmp_dir.path().to_path_buf(),
+    );
+    exporter.run().unwrap();
+
+    assert!(original_note_path.exists());
+    remove_file(&original_note_path).unwrap();
+
+    exporter.add_postprocessor(&|mut ctx, mdevents| {
+        ctx.destination.set_file_name("MovedNote.md");
+        (ctx, mdevents, PostprocessorResult::Continue)
+    });
+    exporter.run().unwrap();
+
+    let new_note_path = tmp_dir.path().clone().join(PathBuf::from("MovedNote.md"));
+    assert!(!original_note_path.exists());
+    assert!(new_note_path.exists());
+}

tests/testdata/expected/postprocessors/Note.md

@@ -0,0 +1,8 @@
+---
+foo: bar
+bar: baz
+---
+
+# Title
+
+Sentence containing bar.

tests/testdata/input/postprocessors/Note.md

@@ -0,0 +1,7 @@
+---
+foo: bar
+---
+
+# Title
+
+Sentence containing foo.