diff --git a/lib/http.ts b/lib/http.ts
new file mode 100644
index 0000000000000000000000000000000000000000..1802a440af89e9d1a3de68bff553467210fc46bd
--- /dev/null
+++ b/lib/http.ts
@@ -0,0 +1,42 @@
+/**
+ * parses the [`Range`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Range_requests)
+ * header, returning `null` if it is invalid. the `fileSize`
+ */
+export function parseRangeHeader(rangeHeader: string, fileSize: number): Range[] | null {
+  const [unit, ranges] = rangeHeader.split("=");
+  if (!ranges) return null;
+  if (unit !== "bytes") return null;
+
+  const result: Range[] = [];
+  const rangeParts = ranges.split(",");
+
+  for (const range of rangeParts) {
+    const split = range.split("-");
+    const [start, end] = split;
+    if (!start || !end) return null;
+    if (start === "" && end === "") return null;
+    result.push({
+      start: start === "" ? fileSize - +end : +start,
+      end: end === "" ? fileSize - 1 : +end,
+    });
+  }
+
+  // validate that ranges do not intersect
+  result.sort((a, b) => a.start - b.start);
+  for (let i = 1; i < result.length; i++) {
+    if (UNWRAP(result[i]).start <= UNWRAP(result[i - 1]).end) {
+      return null;
+    }
+  }
+
+  return result;
+}
+
+export interface Range {
+  /** inclusive */
+  start: number;
+  /** inclusive */
+  end: number;
+}
+
+import { UNWRAP } from "./assert.ts";
diff --git a/src/file-viewer/backend.tsx b/src/file-viewer/backend.tsx
index cb84e7d1109156182990b797a24385fcb4d7ace6..32c9028e12e9a1aa1752506c37a4bc8ecf12c9ae 100644
--- a/src/file-viewer/backend.tsx
+++ b/src/file-viewer/backend.tsx
@@ -161,7 +161,7 @@ app.get("/file/*", async (c, next) => {
   // Range requests
   // https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests
   if (rangeHeader) {
-    const ranges = parseRange(rangeHeader, file.size);
+    const ranges = http.parseRangeHeader(rangeHeader, file.size);
     if (ranges?.[0] && ranges.length === 1) {
       start = ranges[0][0];
       end = ranges[0][1];
@@ -216,39 +216,6 @@ function ifRangeOutdated(file: MediaFile, ifRangeHeader: string) {
   return new Date(ifRangeHeader) < file.date;
 }
 
-/** The end is inclusive */
-type Ranges = Array<[start: number, end: number]>;
-
-function parseRange(rangeHeader: string, fileSize: number): Ranges | null {
-  const [unit, ranges] = rangeHeader.split("=");
-  if (!ranges) return null;
-  if (unit !== "bytes") return null;
-
-  const result: Array<[start: number, end: number]> = [];
-  const rangeParts = ranges.split(",");
-
-  for (const range of rangeParts) {
-    const split = range.split("-");
-    const [start, end] = split;
-    if (!start || !end) return null;
-    if (start === "" && end === "") return null;
-    const parsedRange: [number, number] = [
-      start === "" ? fileSize - +end : +start,
-      end === "" ? fileSize - 1 : +end,
-    ];
-    result.push(parsedRange);
-  }
-
-  // Validate that ranges do not intersect
-  result.sort((a, b) => a[0] - b[0]);
-  for (let i = 1; i < result.length; i++) {
-    if (UNWRAP(result[i])[0] <= UNWRAP(result[i - 1])[1]) {
-      return null;
-    }
-  }
-
-  return result;
-}
 
 function checkCotyledonCookie(c: Context) {
   const cookie = c.req.header("Cookie");
@@ -317,6 +284,7 @@ import { type Context, Hono } from "hono";
 import * as render from "@clo/lib/render";
 import { etagMatches, serveAsset } from "#sitegen/assets";
 import * as view from "#sitegen/view";
+import * as http from "@clo/lib/http";
 import * as mime from "@clo/lib/mime";
 
 import { getForFile, requireFriendAuth } from "#src/friend-auth.ts";