test(engine): add unit tests for parseLine in parser.ts

Added a test suite for `parseLine` covering basic arithmetic,
assignments, noise word filtering, known variable shadowing,
goal seeking, and timezone conversion target retention.

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

src/engine/parser.test.ts

@@ -0,0 +1,95 @@
+import { describe, it, expect } from "vitest";
+import { parseLine } from "./parser";
+import { tokenize } from "./tokenizer";
+import { buildRegistry } from "./registry";
+import { Decimal } from "./types";
+
+const reg = buildRegistry();
+
+function parse(line: string, knownVars = new Set<string>()) {
+  const tokens = tokenize(line, reg);
+  return parseLine(tokens, knownVars, line);
+}
+
+describe("parseLine", () => {
+  it("parses basic expressions", () => {
+    const res = parse("2 + 2");
+    expect(res.assign).toBeUndefined();
+    expect(res.expr).toMatchObject({
+      k: "bin",
+      op: "plus",
+      l: { k: "num", v: new Decimal(2) },
+      r: { k: "num", v: new Decimal(2) }
+    });
+  });
+
+  it("parses variable assignment with '='", () => {
+    const res = parse("x = 5 + 3");
+    expect(res.assign).toBe("x");
+    expect(res.expr).toMatchObject({
+      k: "bin",
+      op: "plus",
+      l: { k: "num", v: new Decimal(5) },
+      r: { k: "num", v: new Decimal(3) }
+    });
+  });
+
+  it("parses variable assignment with 'is'", () => {
+    const res = parse("myVar is 10");
+    expect(res.assign).toBe("myVar");
+    expect(res.expr).toMatchObject({
+      k: "num",
+      v: new Decimal(10)
+    });
+  });
+
+  it("rejects invalid variable names for assignment", () => {
+    const res = parse("123 = 5");
+    expect(res.assign).toBeUndefined();
+  });
+
+  it("handles known variables shadowing built-ins", () => {
+    // "m" is normally a unit (meters), but if it's a known variable, it's treated as a word
+    const res = parse("m = 5", new Set(["m"]));
+    expect(res.assign).toBe("m");
+    expect(res.expr).toMatchObject({
+      k: "num",
+      v: new Decimal(5)
+    });
+  });
+
+  it("filters noise words", () => {
+    // "what is 2 + 2" -> "what" and "is" are noise, leaving "2 + 2"
+    const res = parse("what is 2 + 2");
+    expect(res.expr).toMatchObject({
+      k: "bin",
+      op: "plus"
+    });
+  });
+
+  it("keeps words after a conversion operator (e.g., timezones)", () => {
+    // "now in Europe/Berlin"
+    // "Europe/Berlin" should be kept as a timezone, not dropped as noise.
+    const res = parse("now in Europe/Berlin");
+    expect(res.expr).toMatchObject({
+      k: "conv",
+      target: {
+        type: "tz"
+      }
+    });
+  });
+
+  it("parses goal seek expressions", () => {
+    const res = parse("? * 1.2 = 1000");
+    expect(res.expr).toMatchObject({
+      k: "goalseek",
+      lhs: { k: "bin", op: "mul", l: { k: "unknown" }, r: { k: "num", v: new Decimal(1.2) } },
+      rhs: { k: "num", v: new Decimal(1000) }
+    });
+  });
+
+  it("returns null expression for empty or noise-only lines", () => {
+    const res = parse("just some noise");
+    expect(res.expr).toBeNull();
+  });
+});