feat(FN-4219): complete Step 1 — add METRIC parser

Fusion-Task-Id: FN-4219
Fusion-Task-Lineage: 8d9a9ba6-6729-4376-b935-549e28a7fa35

packages/engine/src/experiment/metric-parser.ts

@@ -0,0 +1,97 @@
+import type { ExperimentSecondaryMetric } from "@fusion/core";
+
+export const METRIC_LINE_REGEX =
+  /^METRIC\s+([A-Za-z_][A-Za-z0-9_.-]*)\s*=\s*(-?\d+(?:\.\d+)?(?:[eE][+-]?\d+)?)\s*(?:\(([^)]+)\))?\s*$/;
+
+export const PROTOTYPE_POLLUTION_DENYLIST = new Set([
+  "__proto__",
+  "constructor",
+  "prototype",
+]);
+
+export interface ParsedMetricLines {
+  primary?: { name: string; value: number; unit?: string };
+  secondary: ExperimentSecondaryMetric[];
+  warnings: string[];
+}
+
+interface ParsedMetricEntry {
+  name: string;
+  value: number;
+  unit?: string;
+}
+
+export function parseMetricLines(stdout: string): ParsedMetricLines {
+  const warnings: string[] = [];
+  const byName = new Map<string, ParsedMetricEntry>();
+  const firstSeenOrder: string[] = [];
+
+  for (const rawLine of stdout.split(/\r?\n/)) {
+    const line = rawLine.trim();
+    if (!line) {
+      continue;
+    }
+
+    const match = METRIC_LINE_REGEX.exec(line);
+    if (!match) {
+      const looseMatch = /^METRIC\s+([A-Za-z_][A-Za-z0-9_.-]*)\s*=\s*([^\s(]+).*$/.exec(
+        line,
+      );
+      if (looseMatch) {
+        const [, looseName, looseRawValue] = looseMatch;
+        const looseValue = Number(looseRawValue);
+        if (!Number.isFinite(looseValue)) {
+          warnings.push(
+            `Ignored non-finite metric value for ${looseName}: ${looseRawValue}`,
+          );
+        }
+      }
+      continue;
+    }
+
+    const [, name, rawValue, rawUnit] = match;
+    if (PROTOTYPE_POLLUTION_DENYLIST.has(name)) {
+      warnings.push(`Ignored denylisted metric name: ${name}`);
+      continue;
+    }
+
+    const value = Number(rawValue);
+    if (!Number.isFinite(value)) {
+      warnings.push(`Ignored non-finite metric value for ${name}: ${rawValue}`);
+      continue;
+    }
+
+    if (!byName.has(name)) {
+      firstSeenOrder.push(name);
+    }
+
+    byName.set(name, {
+      name,
+      value,
+      unit: rawUnit?.trim() || undefined,
+    });
+  }
+
+  const ordered = firstSeenOrder
+    .map((name) => byName.get(name))
+    .filter((entry): entry is ParsedMetricEntry => Boolean(entry));
+
+  const primaryEntry = ordered[0];
+  const secondary = ordered.slice(1).map((entry) => ({
+    name: entry.name,
+    value: entry.value,
+    unit: entry.unit,
+  }));
+
+  return {
+    primary: primaryEntry
+      ? {
+          name: primaryEntry.name,
+          value: primaryEntry.value,
+          unit: primaryEntry.unit,
+        }
+      : undefined,
+    secondary,
+    warnings,
+  };
+}