mermaid: support ascii rendering

Cargo.toml

@@ -25,7 +25,7 @@ term_size = "0.3.2"
 resvg = { version = "0.48.1", default-features = false, features = ["text", "system-fonts", "raster-images"] }
 unicode-width = "0.2.2"
 htmlentity = "1.3.2"
-merman = { version = "0.8.0-alpha.3", default-features = false, features = ["render"] }
+merman = { version = "0.8.0-alpha.3", default-features = false, features = ["render", "ascii"] }
 # Pulled in directly only to enable label/URL sanitization, which strips external
 # resource references (e.g. file:// image hrefs) before the SVG reaches resvg.
 merman-core = { version = "0.8.0-alpha.3", default-features = false, features = ["full-sanitization"] }

README.md

@@ -25,7 +25,7 @@ Pipe your llm output through it, or a markdown file from the web, and it will di
 
 - display images referenced in markdown in your terminal that supports the kitty protocol (kitty, ghostty, alacritty, and others)
 - syntax-highlight fenced code blocks for many languages using [syntect](https://github.com/trishume/syntect)
-- render [mermaid diagrams](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams) into images and display them in your terminal
+- render [mermaid diagrams](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams) into images and display them in your terminal, or as box-drawing text diagrams when image output isn't available
 - parse a subset of HTML and display it sensibly in the terminal
 - accept input from stdin, files, or URLs
 - Use [OSC8 hyperlinks](https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda) to make links clickable

src/lib.rs

@@ -1490,9 +1490,13 @@ impl StreamingParser {
     }
 
     fn format_code_block(&self, lines: &[String], info: &str) -> String {
-        // Attempt to render mermaid diagrams as images when image protocol is enabled
-        if info.eq_ignore_ascii_case("mermaid") && self.image_protocol != ImageProtocol::None {
-            if let Some(rendered) = self.try_render_mermaid(lines) {
+        if info.eq_ignore_ascii_case("mermaid") {
+            if self.image_protocol != ImageProtocol::None {
+                if let Some(rendered) = self.try_render_mermaid(lines) {
+                    return rendered;
+                }
+            }
+            if let Some(rendered) = self.try_render_mermaid_ascii(lines) {
                 return rendered;
             }
             // Fall through to normal code block rendering on failure
@@ -1584,6 +1588,32 @@ impl StreamingParser {
         }
     }
 
+    /// Try to render a mermaid diagram as a box-drawing text diagram.
+    /// Returns `None` when the diagram family has no ASCII renderer, rendering fails,
+    /// or the result is too wide for the output width (where terminal wrapping would
+    /// garble the drawing).
+    fn try_render_mermaid_ascii(&self, lines: &[String]) -> Option<String> {
+        let source = lines.join("\n");
+
+        let diagram = merman::ascii::HeadlessAsciiRenderer::new()
+            .with_charset(merman::ascii::AsciiCharset::Unicode)
+            .render_ascii_sync(&source)
+            .ok()??;
+
+        let diagram = diagram.trim_end_matches('\n');
+        if diagram.is_empty() {
+            return None;
+        }
+        if diagram
+            .lines()
+            .any(|line| self.display_width(line) > self.width)
+        {
+            return None;
+        }
+
+        Some(format!("{}\n\n", diagram))
+    }
+
     /// Parse a task list item marker at the start of content.
     /// Returns Some((is_checked, remaining_content)) if a task list marker is found.
     /// Task list markers are: [ ] (unchecked) or [x]/[X] (checked), followed by whitespace.

tests/unit.rs

@@ -1589,19 +1589,50 @@ mod mermaid_rendering {
     }
 
     #[test]
-    fn test_mermaid_renders_as_code_without_images() {
+    fn test_mermaid_renders_as_ascii_without_images() {
         let mut p = plain_parser();
         let output = feed_all(&mut p, "```mermaid\nflowchart LR\n    A-->B-->C\n```\n");
-        // Should NOT contain kitty protocol
         assert!(
             !output.contains("\x1b_G"),
             "should not contain kitty graphics protocol"
         );
-        // Should contain the mermaid source as code
+        let stripped = super::strip_ansi(&output);
+        assert!(
+            !stripped.contains("flowchart LR"),
+            "should render a diagram, not the source, got: {:?}",
+            stripped
+        );
+        assert!(
+            stripped.contains('│') && stripped.contains('A'),
+            "should draw a box-drawing diagram, got: {:?}",
+            stripped
+        );
+    }
+
+    #[test]
+    fn test_mermaid_ascii_unsupported_falls_back_to_code() {
+        let mut p = plain_parser();
+        let output = feed_all(&mut p, "```mermaid\nnotADiagramType\n  a b c\n```\n");
+        let stripped = super::strip_ansi(&output);
+        assert!(
+            stripped.contains("notADiagramType"),
+            "fallback should show mermaid source as code, got: {:?}",
+            stripped
+        );
+    }
+
+    #[test]
+    fn test_mermaid_ascii_too_wide_falls_back_to_code() {
+        let mut p = StreamingParser::with_width("base16-ocean.dark", ImageProtocol::None, 20);
+        let output = feed_all(
+            &mut p,
+            "```mermaid\nflowchart LR\n    Alpha-->Beta-->Gamma-->Delta\n```\n",
+        );
         let stripped = super::strip_ansi(&output);
         assert!(
             stripped.contains("flowchart LR"),
-            "should show mermaid source as code"
+            "diagram wider than the output width should fall back to code, got: {:?}",
+            stripped
         );
     }