write the library

.build.yml

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+image: alpine/latest
+packages:
+  - rust
+  - cargo
+sources:
+  - https://git.sr.ht/~boringcactus/md2gemtext
+tasks:
+  - test: |
+      cd md2gemtext
+      cargo test

.gitignore

@@ -0,0 +1 @@
+/target

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "bitflags"
+version = "1.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "gemtext"
+version = "0.2.0"
+source = "git+https://tulpa.dev/boringcactus/maj.git?branch=make-gemtext-node-clone#c07d81077a46f618edb38f4c3b482802d377818c"
+
+[[package]]
+name = "md2gemtext"
+version = "0.1.0"
+dependencies = [
+ "gemtext 0.2.0 (git+https://tulpa.dev/boringcactus/maj.git?branch=make-gemtext-node-clone)",
+ "pulldown-cmark 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "memchr"
+version = "2.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "pulldown-cmark"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "bitflags 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "memchr 2.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "unicase 2.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "unicase"
+version = "2.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "version_check 0.9.2 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "version_check"
+version = "0.9.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[metadata]
+"checksum bitflags 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "cf1de2fe8c75bc145a2f577add951f8134889b4795d47466a54a5c846d691693"
+"checksum gemtext 0.2.0 (git+https://tulpa.dev/boringcactus/maj.git?branch=make-gemtext-node-clone)" = "<none>"
+"checksum memchr 2.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "3728d817d99e5ac407411fa471ff9800a778d88a24685968b36824eaf4bee400"
+"checksum pulldown-cmark 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ffade02495f22453cd593159ea2f59827aae7f53fa8323f756799b670881dcf8"
+"checksum unicase 2.6.0 (registry+https://github.com/rust-lang/crates.io-index)" = "50f37be617794602aabbeee0be4f259dc1778fabe05e2d67ee8f79326d5cb4f6"
+"checksum version_check 0.9.2 (registry+https://github.com/rust-lang/crates.io-index)" = "b5a972e5669d67ba988ce3dc826706fb0a8b01471c088cb0b6110b805cc36aed"

Cargo.toml

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+[package]
+name = "md2gemtext"
+version = "0.1.0"
+authors = ["Melody Horn <melody@boringcactus.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+gemtext = { version = "0.2.0", git = "https://tulpa.dev/boringcactus/maj.git", branch = "make-gemtext-node-clone" }
+pulldown-cmark = { version = "0.8", default-features = false }

README.md

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+# md2gemtext
+
+[![builds.sr.ht status](https://builds.sr.ht/~boringcactus/md2gemtext.svg)](https://builds.sr.ht/~boringcactus/md2gemtext?)
+
+for converting Markdown into [gemtext](https://gemini.circumlunar.space/docs/specification.html).
+
+## standalone usage
+
+```shell script
+cargo install md2gemtext
+md2gemtext /path/to/some/file.md /path/to/some/file.gmi
+```
+
+## library usage
+
+```rust
+let gemtext = md2gemtext::convert("some markdown")?;
+```
+
+## translation rules
+
+- "thematic breaks" (hr tags) are translated to `-----` on a line by itself
+- headings turn into headings, levels beyond 3 get capped at 3
+- code blocks get turned into code blocks (info strings are discarded)
+- something happens to HTML, i forget what
+- paragraphs get empty lines between them, because i think that looks better
+- block quotes get turned into quotes
+- lists get turned into lists ("loose lists" probably misbehave, nested lists *definitely* misbehave, numbering is not preserved)
+- `code spans` turn into `\`code spans\``
+- *italics* turn into `_italics_`
+- **bold** turns into `**bold**`
+- ~~strikethrough~~ turns into `~~strikethrough~~`
+- `a [link](a://url) with context` turns into `a link with context` followed by `=> a://url link`
+- `an ![inline](a://url) image` turns into `an [image: inline] image` followed by `=> a://url [image: inline]`
+- if a link or image is its own paragraph, it becomes just the gemtext link, to not be redundant

src/lib.rs

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+use pulldown_cmark as md;
+
+/// Converts a given string of Markdown to semi-equivalent gemtext.
+///
+/// # Panics
+///
+/// Will panic if gemtext::render somehow produces invalid UTF-8.
+/// Since gemtext::render only produces valid UTF-8, this should never happen.
+pub fn convert(markdown_text: &str) -> String {
+    let mut options = md::Options::empty();
+    options.insert(md::Options::ENABLE_STRIKETHROUGH);
+    let parser = md::Parser::new_ext(markdown_text, options);
+    let mut state = State::new();
+
+    for event in parser {
+        println!("{:?}", event);
+        match event {
+            md::Event::Start(md::Tag::Paragraph) => (),
+            md::Event::Start(md::Tag::Heading(level)) => state.start_heading(level),
+            md::Event::Start(md::Tag::BlockQuote) => state.start_block_quote(),
+            md::Event::Start(md::Tag::CodeBlock(_)) => state.start_code_block(),
+            md::Event::Start(md::Tag::List(_)) => (),
+            md::Event::Start(md::Tag::Item) => state.start_list_item(),
+            md::Event::Start(md::Tag::FootnoteDefinition(_)) => unimplemented!("footnotes disabled"),
+            md::Event::Start(md::Tag::Table(_)) => unimplemented!("tables disabled"),
+            md::Event::Start(md::Tag::TableHead) => unimplemented!("tables disabled"),
+            md::Event::Start(md::Tag::TableRow) => unimplemented!("tables disabled"),
+            md::Event::Start(md::Tag::TableCell) => unimplemented!("tables disabled"),
+            md::Event::Start(md::Tag::Emphasis) => state.toggle_emphasis(),
+            md::Event::Start(md::Tag::Strong) => state.toggle_strong(),
+            md::Event::Start(md::Tag::Strikethrough) => state.toggle_strikethrough(),
+            md::Event::Start(md::Tag::Link(_, _, _)) => state.start_link(),
+            md::Event::Start(md::Tag::Image(_, _, _)) => state.start_image(),
+
+            md::Event::End(md::Tag::Paragraph) => state.finish_node(),
+            md::Event::End(md::Tag::Heading(_)) => state.finish_node(),
+            md::Event::End(md::Tag::BlockQuote) => state.finish_node(),
+            md::Event::End(md::Tag::CodeBlock(_)) => state.finish_node(),
+            md::Event::End(md::Tag::List(_)) => state.finish_list(),
+            md::Event::End(md::Tag::Item) => state.finish_node(),
+            md::Event::End(md::Tag::FootnoteDefinition(_)) => unimplemented!("footnotes disabled"),
+            md::Event::End(md::Tag::Table(_)) => unimplemented!("tables disabled"),
+            md::Event::End(md::Tag::TableHead) => unimplemented!("tables disabled"),
+            md::Event::End(md::Tag::TableRow) => unimplemented!("tables disabled"),
+            md::Event::End(md::Tag::TableCell) => unimplemented!("tables disabled"),
+            md::Event::End(md::Tag::Emphasis) => state.toggle_emphasis(),
+            md::Event::End(md::Tag::Strong) => state.toggle_strong(),
+            md::Event::End(md::Tag::Strikethrough) => state.toggle_strikethrough(),
+            md::Event::End(md::Tag::Link(_, href, _)) => state.end_link(&href),
+            md::Event::End(md::Tag::Image(_, src, _)) => state.end_image(&src),
+
+            md::Event::Text(text) => state.add_text(&text),
+            md::Event::Code(code) => state.add_inline_code(&code),
+            md::Event::Html(html) => state.add_text(&html),
+            md::Event::FootnoteReference(_) => unimplemented!("footnotes disabled"),
+            md::Event::SoftBreak => state.add_text(" "),
+            md::Event::HardBreak => state.finish_node(),
+            md::Event::Rule => state.add_rule(),
+            md::Event::TaskListMarker(_) => unimplemented!("task lists disabled"),
+        }
+    }
+
+    let nodes = state.nodes
+        .into_iter()
+        .map(condense)
+        .collect::<Vec<_>>()
+        .join(&gemtext::Node::blank());
+    let mut result: Vec<u8> = vec![];
+    gemtext::render(nodes, &mut result).expect("gemtext::render somehow failed");
+    String::from_utf8(result).expect("gemtext::render somehow produced invalid UTF-8")
+}
+
+type NodeCluster = Vec<gemtext::Node>;
+
+fn condense(original: NodeCluster) -> NodeCluster {
+    match original.as_slice() {
+        [gemtext::Node::Text(text), gemtext::Node::Link { name: Some(name), .. }] if text == name => vec![original[1].clone()],
+        _ => original,
+    }
+}
+
+enum NodeType {
+    Text,
+    Preformatted,
+    Heading { level: u8 },
+    ListItem,
+    Quote,
+}
+
+impl NodeType {
+    fn take(&mut self) -> Self {
+        std::mem::replace(self, NodeType::Text)
+    }
+
+    fn construct(self, body: String) -> gemtext::Node {
+        use NodeType::*;
+        match self {
+            Text => gemtext::Node::Text(body),
+            Preformatted => gemtext::Node::Preformatted(body),
+            Heading { level } => gemtext::Node::Heading { level, body },
+            ListItem => gemtext::Node::ListItem(body),
+            Quote => gemtext::Node::Quote(body),
+        }
+    }
+}
+
+struct State {
+    nodes: Vec<NodeCluster>,
+    pending_node_content: String,
+    pending_node_type: NodeType,
+    pending_other: Vec<gemtext::Node>,
+    link_text_stack: Vec<String>,
+}
+
+impl State {
+    fn new() -> Self {
+        State {
+            nodes: vec![],
+            pending_node_content: String::new(),
+            pending_node_type: NodeType::Text,
+            pending_other: vec![],
+            link_text_stack: vec![],
+        }
+    }
+
+    fn start_heading(&mut self, level: u32) {
+        let level = match level {
+            1 => 1,
+            2 => 2,
+            _ => 3,
+        };
+        self.pending_node_type = NodeType::Heading { level };
+    }
+
+    fn start_block_quote(&mut self) {
+        self.pending_node_type = NodeType::Quote;
+    }
+
+    fn start_code_block(&mut self) {
+        self.pending_node_type = NodeType::Preformatted;
+    }
+
+    fn start_list_item(&mut self) {
+        self.pending_node_type = NodeType::ListItem;
+    }
+
+    fn toggle_emphasis(&mut self) {
+        self.add_text("_");
+    }
+
+    fn toggle_strong(&mut self) {
+        self.add_text("**");
+    }
+
+    fn toggle_strikethrough(&mut self) {
+        self.add_text("~~");
+    }
+
+    fn start_link(&mut self) {
+        self.link_text_stack.push(String::new());
+    }
+
+    fn start_image(&mut self) {
+        self.link_text_stack.push(String::new());
+        self.pending_node_content += "[image: ";
+    }
+
+    fn finish_list(&mut self) {
+        self.nodes.push(vec![]);
+    }
+
+    fn end_link(&mut self, href: &str) {
+        let text = self.link_text_stack.pop().unwrap_or_else(|| href.to_string());
+        self.pending_other.push(gemtext::Node::Link { to: href.to_string(), name: Some(text) });
+    }
+
+    fn end_image(&mut self, src: &str) {
+        let text = self.link_text_stack.pop().unwrap_or_else(|| src.to_string());
+        let text = format!("[image: {}]", text);
+        self.pending_other.push(gemtext::Node::Link { to: src.to_string(), name: Some(text) });
+        self.pending_node_content += "]";
+    }
+
+    // will create an empty paragraph if pending_text is empty
+    fn finish_node(&mut self) {
+        match (&self.pending_node_type, self.nodes.last().and_then(|cluster| cluster.last())) {
+            (NodeType::ListItem, Some(gemtext::Node::ListItem(_))) => (),
+            _ => self.nodes.push(vec![]),
+        }
+        let node_text = self.pending_node_content.trim().to_string();
+        let new_node = self.pending_node_type.take().construct(node_text);
+        let last_cluster = self.nodes.last_mut().expect("empty cluster list??");
+        last_cluster.push(new_node);
+        last_cluster.extend(self.pending_other.drain(..));
+
+        self.pending_node_content = String::new();
+    }
+
+    fn add_text(&mut self, text: &str) {
+        for link_text in &mut self.link_text_stack {
+            *link_text += text;
+        }
+        self.pending_node_content += text;
+    }
+
+    fn add_inline_code(&mut self, code: &str) {
+        self.pending_node_content += "`";
+        self.pending_node_content += code;
+        self.pending_node_content += "`";
+    }
+
+    fn add_rule(&mut self) {
+        self.add_text("-----");
+    }
+}

src/main.rs

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+use std::env;
+use std::fs;
+
+fn main() {
+    let args = env::args_os().collect::<Vec<_>>();
+    match args.as_slice() {
+        [_, source_file, dest_file] => {
+            let source_text = fs::read_to_string(source_file).expect("couldn't read source file");
+            let result_text = md2gemtext::convert(&source_text);
+            fs::write(dest_file, result_text).expect("couldn't write dest file");
+        }
+        _ => {
+            eprintln!("usage: md2gemtext <source (markdown) file path> <dest (gemtext) file path>")
+        }
+    }
+}