From 1898ff35ddbc717304c2afd1e25257013919b995 Mon Sep 17 00:00:00 2001
From: Lars Wirzenius <liw@liw.fi>
Date: Sat, 3 Sep 2022 11:19:47 +0300
Subject: refactor: expose functionality to extract embedded YAML from md

Sponsored-by: author
---
 src/ast.rs | 88 +++++++++++++++++++++++++-------------------------------------
 src/lib.rs |  2 +-
 2 files changed, 36 insertions(+), 54 deletions(-)

diff --git a/src/ast.rs b/src/ast.rs
index 4fba52e..afa8b3b 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -51,27 +51,33 @@ impl std::str::FromStr for AbstractSyntaxTree {
 
     /// Create an abstract syntax tree from a string.
     fn from_str(markdown: &str) -> Result<Self, Self::Err> {
-        trace!("Parsing markdown");
-        let ast = if let Some((yaml, markdown)) = get_yaml(&LEADING_YAML_PATTERN, markdown) {
-            trace!("Found leading YAML: {:?}", yaml);
-            let meta = YamlMetadata::new(yaml)?;
-            let blocks = parse_blocks(markdown);
-            AbstractSyntaxTree::new(meta, blocks)
-        } else if let Some((yaml, _markdown)) = get_yaml(&TRAILING_YAML_PATTERN, markdown) {
-            trace!("Found trailing YAML: {:?}", yaml);
-            let meta = YamlMetadata::new(yaml)?;
-            let blocks = parse_blocks(markdown);
-            AbstractSyntaxTree::new(meta, blocks)
-        } else {
-            trace!("No YAML to be found");
-            let blocks = parse_blocks(markdown);
-            AbstractSyntaxTree::new(YamlMetadata::default(), blocks)
-        };
+        trace!("Parsing markdown into AST");
+        let (meta, markdown) = extract_metadata(markdown)?;
+        let blocks = parse_blocks(markdown);
+        let ast = AbstractSyntaxTree::new(meta, blocks);
         trace!("Parsing markdown: OK");
         Ok(ast)
     }
 }
 
+/// Extract YAML metadata from a Markdown document.
+pub fn extract_metadata(markdown: &str) -> Result<(YamlMetadata, &str), Error> {
+    trace!("Extracting YAML from Markdown");
+    let (yaml, md) = if let Some((yaml, markdown)) = get_yaml(&LEADING_YAML_PATTERN, markdown) {
+        trace!("Found leading YAML: {:?}", yaml);
+        (yaml, markdown)
+    } else if let Some((yaml, _markdown)) = get_yaml(&TRAILING_YAML_PATTERN, markdown) {
+        trace!("Found trailing YAML: {:?}", yaml);
+        (yaml, markdown)
+    } else {
+        trace!("No YAML to be found");
+        return Err(Error::NoMetadata);
+    };
+    let meta = YamlMetadata::new(yaml)?;
+    trace!("Parsing markdown: OK");
+    Ok((meta, md))
+}
+
 // Extract a YAML metadata block using a given regex.
 fn get_yaml<'a>(pat: &Regex, markdown: &'a str) -> Option<(&'a str, &'a str)> {
     trace!("Markdown: {:?}", markdown);
@@ -272,22 +278,25 @@ pub enum Error {
     #[error(transparent)]
     Regex(#[from] regex::Error),
 
+    #[error("Markdown doesn't contain a YAML block for document metadata")]
+    NoMetadata,
+
     #[error(transparent)]
     Yaml(#[from] serde_yaml::Error),
 }
 
-// Document metadata.
-//
-// This is expressed in the Markdown input file as an embedded YAML
-// block.
-//
-// Note that this structure needs to be able to capture any metadata
-// block we can work with, in any input file. By being strict here we
-// make it easier to tell the user when a metadata block has, say, a
-// misspelled field.
+/// Document metadata.
+///
+/// This is expressed in the Markdown input file as an embedded YAML
+/// block.
+///
+/// Note that this structure needs to be able to capture any metadata
+/// block we can work with, in any input file. By being strict here we
+/// make it easier to tell the user when a metadata block has, say, a
+/// misspelled field.
 #[derive(Debug, Default, Deserialize)]
 #[serde(deny_unknown_fields)]
-struct YamlMetadata {
+pub struct YamlMetadata {
     title: String,
     subtitle: Option<String>,
     author: Option<String>,
@@ -399,33 +408,6 @@ mod test {
         );
     }
 
-    #[test]
-    fn empty_input() {
-        let ast = AbstractSyntaxTree::from_str("").unwrap();
-        let doc = ast.to_pandoc();
-        assert!(doc.blocks.is_empty());
-        assert!(!doc.pandoc_api_version.is_empty());
-    }
-
-    #[test]
-    fn simple() {
-        let ast = AbstractSyntaxTree::from_str(
-            "\
-            # Introduction \n\
-            \n\
-            First paragraph.\n\
-            ",
-        )
-        .unwrap();
-        let doc = ast.to_pandoc();
-        assert!(!doc.pandoc_api_version.is_empty());
-
-        let attr = ("".to_string(), vec![], vec![]);
-        let h = Block::Header(1, attr, vec![Inline::Str("Introduction".to_string())]);
-        let para = Block::Para(vec![Inline::Str("First paragraph.".to_string())]);
-        assert_eq!(doc.blocks, &[h, para]);
-    }
-
     #[test]
     fn parses_leading_meta() {
         let markdown = "\n\n---\ntitle: Foo Bar\n...\nfoobar\n";
diff --git a/src/lib.rs b/src/lib.rs
index 1d0060b..725e49c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -73,4 +73,4 @@ mod codegen;
 pub use codegen::generate_test_program;
 
 mod ast;
-pub use ast::AbstractSyntaxTree;
+pub use ast::{extract_metadata, AbstractSyntaxTree, YamlMetadata};
-- 
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