feat: compose TechLog static and mock adapters

src/features/tech-log/adapters/mock/cursor.ts

@@ -0,0 +1,39 @@
+import { StudioGatewayError } from "../../application/ports/studio-gateway-error.ts";
+import type { ProblemDetails } from "../../contracts/studio/contract.ts";
+import { stableStringify } from "./stable-stringify.ts";
+
+export type CursorPayload = { binding: string; lastValue: string; lastId: string };
+const UUID = /^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;
+
+function invalid(detail: string) {
+  const problem: ProblemDetails = {
+    type: "https://techlog.local/problems/request-validation-failed", title: "Request validation failed",
+    status: 422, detail, code: "REQUEST_VALIDATION_FAILED", retryable: false,
+    fieldErrors: [{ path: "/cursor", message: detail }],
+  };
+  return new StudioGatewayError(problem);
+}
+
+export function cursorBinding(value: unknown) { return stableStringify(value); }
+
+export function encodeCursor(payload: CursorPayload): string {
+  const bytes = new TextEncoder().encode(stableStringify(payload));
+  let binary = "";
+  for (const byte of bytes) binary += String.fromCharCode(byte);
+  return btoa(binary).replaceAll("+", "-").replaceAll("/", "_").replace(/=+$/, "");
+}
+
+export function decodeCursor(cursor: string, binding: string): CursorPayload {
+  try {
+    const base64 = cursor.replaceAll("-", "+").replaceAll("_", "/").padEnd(Math.ceil(cursor.length / 4) * 4, "=");
+    const bytes = Uint8Array.from(atob(base64), (character) => character.charCodeAt(0));
+    const parsed = JSON.parse(new TextDecoder().decode(bytes)) as Partial<CursorPayload>;
+    if (parsed.binding !== binding || typeof parsed.lastValue !== "string" || typeof parsed.lastId !== "string" || !UUID.test(parsed.lastId)) {
+      throw invalid("Cursor does not match the normalized filters and sort.");
+    }
+    return parsed as CursorPayload;
+  } catch (error) {
+    if (error instanceof StudioGatewayError) throw error;
+    throw invalid("Cursor is malformed.");
+  }
+}