Add first draft (untested).

src/parseRangeHeader.ts

@@ -0,0 +1,57 @@
+export type Range = {
+  start: number;
+  end: number;
+};
+
+const rangeRegEx = /bytes=([0-9]*)-([0-9]*)/;
+
+export class RangeParserError extends Error {
+  constructor(start: any, end: any) {
+    super(`Invalid start and end values: ${start}-${end}.`);
+  }
+}
+
+export function parseRangeHeader(range: string, totalSize: number): Range | null {
+  // 1. If range is not specified or the file is empty, return null.
+  if (range === null || range.length === 0 || totalSize === 0) {
+    return null;
+  }
+
+  const [startValue, endValue] = range.split(rangeRegEx);
+  let start = Number.parseInt(startValue);
+  let end = Number.parseInt(endValue);
+
+  // 2. Parse start and end values and ensure they are within limits.
+  // 2.1. start: >= 0.
+  // 2.2. end: >= 0, <= totalSize - 1
+
+  let result = {
+    start: Number.isNaN(start) ? 0 : Math.max(start, 0),
+    end: Number.isNaN(end) ? totalSize - 1 : Math.min(Math.max(end, 0), totalSize - 1)
+  };
+
+  // 3.1. If end is not provided, set end to the last byte (totalSize - 1).
+  if (!Number.isNaN(start) && Number.isNaN(end)) {
+    result.start = start;
+    result.end = totalSize - 1;
+
+    return result;
+  }
+
+  // 3.2. If start is not provided, set it to the offset of last "end" bytes from the end of the file.
+  //      And set end to the last byte.
+  //      This way we return the last "end" bytes.
+  if (Number.isNaN(start) && !Number.isNaN(end)) {
+    result.start = totalSize - end;
+    result.end = totalSize - 1;
+
+    return result;
+  }
+
+  // 4. Handle invalid ranges.
+  if (start > end) {
+    throw new RangeParserError(start, end);
+  }
+
+  return result;
+}