7c709f0ffd
## What changed Add `codex-mermaid`, a standalone crate that renders supported subsets of flowchart, sequence, state, class, and ER diagrams as Unicode text. Expose `render` for plain text and `render_spans` for semantic node, edge, and text spans that callers can style. Enforce source, diagram, canvas, and display-width limits. Return errors for unsupported input or exceeded limits without producing partial diagrams, leaving source fallback to callers. Include documentation and a stdin rendering example. ## Testing Add snapshots for each diagram family, checks for relationship endpoints, Unicode labels, semantic spans, truncated input, and size limits, plus edge reconstruction for all 512 directed three-node graphs in all four layout directions. GitOrigin-RevId: 5b4e65d9152abce5d833e4e6b12113a5a21f700e
codex-mermaid
Standalone, bounded Mermaid text renderer.
It uses the existing unicode-width dependency; no Mermaid runtime or new external
production dependency is needed.
Supported subsets
| Family | Supported syntax |
|---|---|
flowchart, graph |
TD/TB, BT, LR, RL; rectangle and decision labels; directed and labeled --> edges; chains, branches, merges, loops |
sequenceDiagram |
Implicit participants, participant/actor, aliases, ->, ->>, -->, -->>, -x, --x, self-messages, Note over A[,B], nested loop/alt/opt/critical/break, one labeled else per alt |
stateDiagram-v2, stateDiagram |
Flat states, state "label" as ID, descriptions, directed transitions with optional labels, initial/final [*], direction declarations |
classDiagram |
class ID, multiline member bodies, ID : member, solid/dashed links, association, inheritance, composition, aggregation, dependency, realization, quoted endpoint cardinalities, relationship labels, direction declarations |
erDiagram |
Entities, multiline attribute bodies, type name [PK, FK, UK] ["comment"], all four endpoint cardinalities, identifying/non-identifying relationships, relationship labels, direction declarations |
Identifiers are ASCII letters followed by letters, digits, or underscores. Text
supports ordinary Unicode and CJK. Full-line %% comments and semicolon-separated
statements are supported; semicolons inside labels are not. Member/attribute bodies
need a separate statement for each opening brace, member, and closing brace (for
example class Order { followed by member lines and a final }). Class member text
is retained in one compartment, including visibility, signatures, and return types.
ER attribute types and names use the identifier grammar above.
These are explicit subsets, not complete Mermaid compatibility. Compound states, flowchart subgraphs, other shapes, sequence activation and parallel fragments, styling, front matter, directives, HTML, escapes, combining/zero-width characters, and ligatures with non-additive widths return errors. Callers should retain source on any error; the library never returns a partial diagram.
Layout and notation
Graph nodes appear in declaration/first-reference order, in the requested direction.
Each edge gets its own lane and endpoint positions. Crossings use ╪ and never
join routes. Decisions use ◇ inside a box. Horizontal layouts
reserve a text gutter for every endpoint, which can make connected graphs wide;
the caller receives TooWide if the complete output does not fit.
Class links use arrows into the referenced class, ◁/△ for inheritance/realization,
◆ for composition, and ◇ for aggregation. Endpoint cardinalities appear in
parentheses next to the appropriate class or entity. ER cardinalities are written
as 1, 0..1, 1..many, or 0..many. Solid ER links are identifying; dashed links
are non-identifying. Class members and ER keys/comments appear verbatim as readable
text, rather than renderer metadata.
Sequence messages retain chronological order. ->>/-->> use ▶/◀ arrowheads,
->/--> have no arrowhead, and -x/--x use a cross endpoint. Solid and dashed
strokes remain distinct. Lifelines crossed by a message use ┼; this is not a recipient.
Control fragments have nested frames and explicit branch labels. Notes span their
named participants. States use separate ● initial and ◎ final nodes.
Limits and evaluation
All inputs are limited to 16 KiB. Graphs allow 16 nodes, 24 edges, and 16 members per node. Sequences allow 8 participants, 64 events (including fragment boundaries), and 4 fragment levels. Source labels and identifiers are limited to 40 display cells/ASCII bytes respectively. Rendered canvases are capped at 65,536 cells, independent of the caller's maximum width. The library performs no I/O.
From codex-rs, preview a file, optionally specifying the available width:
cargo run -p codex-mermaid --example render -- 180 < diagram.mmd
Run just test -p codex-mermaid --lib. Coverage includes complex snapshots for every
family, relationship endpoints, all ER cardinalities, width/error bounds,
truncated input, and reconstruction of every edge in all 512 directed three-node
graphs in each of the four layout directions.
render_spans returns the same layout as lines of semantic Node, Edge, and
Text spans. Callers apply their own theme; the crate never emits ANSI escapes.