1 files changed, 108 insertions, 0 deletions
diff --git a/src/endpoint.ts b/src/endpoint.ts
new file mode 100644
index 0000000..d3f4b0d
--- /dev/null
+++ b/src/endpoint.ts
@@ -0,0 +1,108 @@
+/**
+ * Which browser this extension searches through.
+ *
+ * Deliberately the *same* configuration as pi-browser-harness, so one decision
+ * governs both packages and they never drive different browsers in the same
+ * session: `BU_CDP_HTTP`, then the `/browser-target` file that harness writes,
+ * then the built-in castle default.
+ *
+ * Resolution is pure — no I/O — so it can be unit-tested and reported without
+ * touching the network.
+ */
+
+/** Default CDP endpoint: castle's Chrome, exposed on the wg8 overlay. */
+export const DEFAULT_REMOTE_CDP = "10.88.0.25:9223";
+
+/** Values of BU_CDP_HTTP that mean "ignore the default, use this machine". */
+const LOCAL_ALIASES = new Set(["local", "localhost", "off", "none", "0", "no"]);
+
+/** Port a local Chrome is expected to serve CDP on when the target is "local". */
+export const LOCAL_CDP_PORT = 9222;
+
+/**
+ * Where a target came from. "env" is BU_CDP_HTTP, "stored" is the harness's
+ * `/browser-target`, "default" is the built-in.
+ */
+export type CdpTargetSource = "env" | "stored" | "default";
+
+export type CdpTarget = {
+  readonly host: string;
+  readonly port: number;
+  readonly source: CdpTargetSource;
+};
+
+export type CdpTargetResolution =
+  | { readonly kind: "ok"; readonly target: CdpTarget }
+  | { readonly kind: "invalid"; readonly raw: string; readonly reason: string; readonly source: CdpTargetSource };
+
+/**
+ * Precedence: BU_CDP_HTTP, then the stored `/browser-target` choice, then the
+ * built-in default. The environment wins so a one-off `BU_CDP_HTTP=… pi` is
+ * never silently overridden by a choice made days ago.
+ *
+ * Unlike pi-browser-harness this never falls back from the default to a local
+ * browser on its own. The harness can fall back because it drives a browser the
+ * operator is watching; here a silent fallback would mean *searching from the
+ * operator's own signed-in Chrome* without saying so. When the configured
+ * endpoint is unreachable the caller reports that, and the operator can say
+ * `local` explicitly.
+ *
+ * `stored` is passed in rather than read here so this stays pure and synchronous.
+ */
+export const resolveCdpTarget = (
+  env: NodeJS.ProcessEnv = process.env,
+  stored: string | null = null,
+): CdpTargetResolution => {
+  const fromEnv = env["BU_CDP_HTTP"]?.trim();
+  const fromStore = stored?.trim();
+
+  const hasEnv = fromEnv !== undefined && fromEnv !== "";
+  const raw = hasEnv ? fromEnv : fromStore;
+  const source: CdpTargetSource = hasEnv ? "env" : "stored";
+
+  if (raw === undefined || raw === "") {
+    const parsed = parseHostPort(DEFAULT_REMOTE_CDP);
+    // Unreachable in practice: the constant above is covered by its own test.
+    if (!parsed) {
+      return { kind: "invalid", raw: DEFAULT_REMOTE_CDP, source: "default", reason: "built-in default is malformed" };
+    }
+    return { kind: "ok", target: { ...parsed, source: "default" } };
+  }
+
+  if (LOCAL_ALIASES.has(raw.toLowerCase())) {
+    return { kind: "ok", target: { host: "127.0.0.1", port: LOCAL_CDP_PORT, source } };
+  }
+
+  const parsed = parseHostPort(raw);
+  if (!parsed) {
+    return {
+      kind: "invalid",
+      raw,
+      source,
+      reason: `expected "host:port" or one of ${[...LOCAL_ALIASES].join(", ")}`,
+    };
+  }
+  return { kind: "ok", target: { ...parsed, source } };
+};
+
+const parseHostPort = (value: string): { host: string; port: number } | null => {
+  const sep = value.lastIndexOf(":");
+  if (sep === -1) return null;
+  const host = value.slice(0, sep).trim();
+  const port = Number(value.slice(sep + 1).trim());
+  if (!host) return null;
+  if (!Number.isInteger(port) || port <= 0 || port >= 65536) return null;
+  return { host, port };
+};
+
+const sourceLabel = (source: CdpTargetSource): string =>
+  source === "env" ? "BU_CDP_HTTP" : source === "stored" ? "/browser-target" : "built-in default";
+
+/** One-line description for progress messages and errors. */
+export const describeCdpTarget = (target: CdpTarget): string =>
+  `${target.host}:${target.port} (${sourceLabel(target.source)})`;
+
+/** One-line description of a resolution failure. */
+export const describeResolutionFailure = (
+  failure: Extract<CdpTargetResolution, { kind: "invalid" }>,
+): string => `invalid CDP target "${failure.raw}" from ${sourceLabel(failure.source)} — ${failure.reason}`;