Feature: Implement Atomic Resume Patching API (#2692)

src/utils/resume/patch.ts

@@ -0,0 +1,109 @@
+import type { JsonPatchError, Operation } from "fast-json-patch";
+import jsonpatch from "fast-json-patch";
+import { type ResumeData, resumeDataSchema } from "@/schema/resume/data";
+
+export type { Operation };
+
+/**
+ * A structured error thrown when a JSON Patch operation fails.
+ * Contains only the relevant details -- never the full document tree.
+ */
+export class ResumePatchError extends Error {
+	/** The error code from `fast-json-patch`, e.g. `TEST_OPERATION_FAILED`. */
+	code: string;
+	/** The zero-based index of the failing operation in the operations array. */
+	index: number;
+	/** The operation object that caused the failure. */
+	operation: Operation;
+
+	constructor(code: string, message: string, index: number, operation: Operation) {
+		super(message);
+		this.name = "ResumePatchError";
+		this.code = code;
+		this.index = index;
+		this.operation = operation;
+	}
+}
+
+/**
+ * Human-readable messages for each `fast-json-patch` error code.
+ * These are returned to the API consumer instead of the raw library output.
+ */
+const patchErrorMessages: Record<string, string> = {
+	SEQUENCE_NOT_AN_ARRAY: "Patch sequence must be an array.",
+	OPERATION_NOT_AN_OBJECT: "Operation is not an object.",
+	OPERATION_OP_INVALID: "Operation `op` property is not one of the operations defined in RFC 6902.",
+	OPERATION_PATH_INVALID: "Operation `path` property is not a valid JSON Pointer string.",
+	OPERATION_FROM_REQUIRED: "Operation `from` property is required for `move` and `copy` operations.",
+	OPERATION_VALUE_REQUIRED: "Operation `value` property is required for `add`, `replace`, and `test` operations.",
+	OPERATION_VALUE_CANNOT_CONTAIN_UNDEFINED:
+		"Operation `value` contains an `undefined` value, which is not valid in JSON.",
+	OPERATION_PATH_CANNOT_ADD: "Cannot perform an `add` operation at the desired path.",
+	OPERATION_PATH_UNRESOLVABLE: "Cannot perform the operation at a path that does not exist.",
+	OPERATION_FROM_UNRESOLVABLE: "Cannot perform the operation from a path that does not exist.",
+	OPERATION_PATH_ILLEGAL_ARRAY_INDEX: "Array index in path must be an unsigned base-10 integer.",
+	OPERATION_VALUE_OUT_OF_BOUNDS: "The specified array index is greater than the number of elements in the array.",
+	TEST_OPERATION_FAILED: "Test operation failed -- the value at the given path did not match the expected value.",
+};
+
+/**
+ * Checks whether an error is a `JsonPatchError` from `fast-json-patch`.
+ * The library doesn't export the class directly, so we duck-type it.
+ */
+function isJsonPatchError(error: unknown): error is JsonPatchError {
+	return error instanceof Error && "index" in error && "operation" in error;
+}
+
+/**
+ * Converts a `JsonPatchError` into a clean `ResumePatchError` that omits the document tree.
+ */
+function toResumePatchError(error: JsonPatchError): ResumePatchError {
+	const code = error.name;
+	const message = patchErrorMessages[code] ?? error.message;
+	const index = error.index ?? 0;
+	const operation = error.operation as Operation;
+
+	return new ResumePatchError(code, message, index, operation);
+}
+
+/**
+ * Applies an array of JSON Patch (RFC 6902) operations to a `ResumeData` object.
+ *
+ * This function validates the operations before applying them, then validates the
+ * resulting document against the `resumeDataSchema` to ensure the patched data is
+ * still a valid resume.
+ *
+ * The original `data` object is not mutated; a deep clone is created internally.
+ *
+ * @see https://docs.rxresu.me/guides/using-the-patch-api — for usage examples and API details.
+ * @see https://datatracker.ietf.org/doc/html/rfc6902 — JSON Patch specification.
+ *
+ * @param data - The current resume data to patch.
+ * @param operations - An array of JSON Patch operations to apply.
+ * @returns The patched and validated `ResumeData` object.
+ * @throws {ResumePatchError} If the operations are structurally invalid or target non-existent paths.
+ * @throws {ResumePatchError} If a `test` operation does not match.
+ * @throws {Error} If the patched document does not conform to the `ResumeData` schema.
+ */
+export function applyResumePatches(data: ResumeData, operations: Operation[]): ResumeData {
+	// Validate operations structurally before applying.
+	const validationError = jsonpatch.validate(operations, data);
+	if (validationError) throw toResumePatchError(validationError);
+
+	// Apply operations. applyPatch throws on `test` failures.
+	let patched: ResumeData;
+
+	try {
+		const result = jsonpatch.applyPatch(data, operations, false, false);
+		patched = result.newDocument;
+	} catch (error: unknown) {
+		if (isJsonPatchError(error)) throw toResumePatchError(error);
+		throw error;
+	}
+
+	// Validate the result still conforms to ResumeData.
+	const parsed = resumeDataSchema.safeParse(patched);
+	if (!parsed.success) throw new Error(`Patch produced invalid resume data: ${parsed.error.message}`);
+
+	return parsed.data;
+}