diff --git a/src/mermaidFlowchartDiff.ts b/src/mermaidFlowchartDiff.ts
index fef5f2d..0ac4a66 100644
--- a/src/mermaidFlowchartDiff.ts
+++ b/src/mermaidFlowchartDiff.ts
@@ -1,4 +1,102 @@
-// stub, replaced in Tasks 2-3
+/**
+ * mermaidFlowchartDiff — F7.1 (#22). Pure flowchart parser + diff/emit. Parses a
+ * `graph`/`flowchart` source into nodes (by id) and edges (by declaration order),
+ * diffs baseline vs current, and re-emits the CURRENT source augmented with
+ * `classDef`/`class`/`linkStyle` directives coloring added/changed elements and
+ * ghosting removed ones (INV-29/31). Deterministic; no vscode, no DOM.
+ */
+import { CW_COLORS } from "./mermaidDiff";
+
+export interface FlowNode {
+  id: string;
+  label?: string;
+  open?: string;
+  close?: string;
+  /** Verbatim declaration token, e.g. `A[Start]`, for ghost re-injection. */
+  decl?: string;
+}
+export interface FlowEdge {
+  from: string;
+  to: string;
+  label?: string;
+  index: number;
+  /** Verbatim edge statement for ghost re-injection. */
+  raw: string;
+}
+export interface FlowGraph {
+  header: string;
+  nodes: Map<string, FlowNode>;
+  edges: FlowEdge[];
+}
+
+const NODE_TOKEN = /^([A-Za-z0-9_]+)(\[\[[^\]]*\]\]|\[[^\]]*\]|\(\([^)]*\)\)|\([^)]*\)|\{[^}]*\}|>[^\]]*\])?/;
+// link operators: -->, ---, -.->, -.-, ==>, ===, --x, --o, ==x, ==o, optionally |label|
+const LINK = /^(-->|---|-\.->|-\.-|==>|===|--[xo]|==[xo])(\|([^|]*)\|)?/;
+
+function shapeOf(bracket: string): { label: string; open: string; close: string } {
+  if (bracket.startsWith("[[")) return { label: bracket.slice(2, -2), open: "[[", close: "]]" };
+  if (bracket.startsWith("((")) return { label: bracket.slice(2, -2), open: "((", close: "))" };
+  const open = bracket[0];
+  const close = bracket[bracket.length - 1];
+  return { label: bracket.slice(1, -1), open, close };
+}
+
+function recordNode(nodes: Map<string, FlowNode>, token: string): string {
+  const m = token.match(NODE_TOKEN)!;
+  const id = m[1];
+  const existing = nodes.get(id) ?? { id };
+  if (m[2]) {
+    const s = shapeOf(m[2]);
+    existing.label = s.label;
+    existing.open = s.open;
+    existing.close = s.close;
+    existing.decl = `${id}${m[2]}`;
+  }
+  nodes.set(id, existing);
+  return id;
+}
+
+export function parseFlowchart(source: string): FlowGraph {
+  const lines = source.split(/\r?\n/);
+  let header = "";
+  const nodes = new Map<string, FlowNode>();
+  const edges: FlowEdge[] = [];
+  for (const rawLine of lines) {
+    const line = rawLine.trim();
+    if (line === "" || line.startsWith("%%")) continue;
+    if (!header && /^(flowchart|graph)\b/.test(line)) {
+      header = line;
+      continue;
+    }
+    let rest = line;
+    const firstM = rest.match(NODE_TOKEN);
+    if (!firstM || firstM[0] === "") continue;
+    let leftId = recordNode(nodes, firstM[0]);
+    rest = rest.slice(firstM[0].length).trimStart();
+    // Walk a (possibly chained) edge statement: A[..] --> B -->|x| C
+    while (rest.length > 0) {
+      const linkM = rest.match(LINK);
+      if (!linkM) break;
+      rest = rest.slice(linkM[0].length).trimStart();
+      const rhsM = rest.match(NODE_TOKEN);
+      if (!rhsM || rhsM[0] === "") break;
+      const rightId = recordNode(nodes, rhsM[0]);
+      edges.push({
+        from: leftId,
+        to: rightId,
+        label: linkM[3] || undefined,
+        index: edges.length,
+        raw: `${leftId} ${linkM[1]}${linkM[2] ?? ""} ${rightId}`,
+      });
+      rest = rest.slice(rhsM[0].length).trimStart();
+      leftId = rightId;
+    }
+  }
+  return { header: header || "flowchart TD", nodes, edges };
+}
+
+// Replaced in Task 3 with the real diff/emit.
 export function diffFlowchart(_beforeSrc: string, currentSrc: string): string {
+  void CW_COLORS;
   return currentSrc;
 }
diff --git a/test/mermaidFlowchartDiff.test.ts b/test/mermaidFlowchartDiff.test.ts
new file mode 100644
index 0000000..1faf5b1
--- /dev/null
+++ b/test/mermaidFlowchartDiff.test.ts
@@ -0,0 +1,37 @@
+import { describe, it, expect } from "vitest";
+import { parseFlowchart } from "../src/mermaidFlowchartDiff";
+
+describe("parseFlowchart", () => {
+  it("captures the header and node declarations with labels + shapes", () => {
+    const g = parseFlowchart("flowchart LR\n  A[Start] --> B(Middle)\n  B --> C{End}");
+    expect(g.header).toBe("flowchart LR");
+    expect(g.nodes.get("A")).toMatchObject({ id: "A", label: "Start", open: "[", close: "]" });
+    expect(g.nodes.get("B")).toMatchObject({ id: "B", label: "Middle", open: "(", close: ")" });
+    expect(g.nodes.get("C")).toMatchObject({ id: "C", label: "End", open: "{", close: "}" });
+  });
+
+  it("captures edges in declaration order with from/to and index", () => {
+    const g = parseFlowchart("graph TD\n A-->B\n B-->C");
+    expect(g.edges.map((e) => [e.from, e.to, e.index])).toEqual([
+      ["A", "B", 0],
+      ["B", "C", 1],
+    ]);
+  });
+
+  it("records a node referenced only in an edge (no explicit declaration)", () => {
+    const g = parseFlowchart("flowchart LR\n A-->B");
+    expect(g.nodes.has("A")).toBe(true);
+    expect(g.nodes.has("B")).toBe(true);
+    expect(g.nodes.get("A")?.label).toBeUndefined();
+  });
+
+  it("handles a labelled edge `A -->|yes| B`", () => {
+    const g = parseFlowchart("flowchart LR\n A -->|yes| B");
+    expect(g.edges[0]).toMatchObject({ from: "A", to: "B", label: "yes" });
+  });
+
+  it("ignores comments and blank lines", () => {
+    const g = parseFlowchart("flowchart LR\n%% a comment\n\n A-->B");
+    expect(g.edges).toHaveLength(1);
+  });
+});