Add `pageRangesOverlap` utility

This will be used for testing whether an EPUB chapter's page range ovelaps a
page range focus filter.

src/sidebar/util/page-range.ts

+/**
+ * Parse a page number or range into a `[start, end]` integer pair. The `start`
+ * or `end` may be `null` if the range is open.
+ *
+ * Returns `null` if the page range could not be parsed.
+ */
+function parseRange(range: string): [number | null, number | null] | null {
+  let start;
+  let end;
+
+  if (range.includes('-')) {
+    [start, end] = range.split('-');
+  } else {
+    start = range;
+    end = range;
+  }
+
+  let startInt = null;
+  if (start) {
+    startInt = parseInt(start);
+    if (!Number.isInteger(startInt)) {
+      return null;
+    }
+  }
+
+  let endInt = null;
+  if (end) {
+    endInt = parseInt(end);
+    if (!Number.isInteger(endInt)) {
+      return null;
+    }
+  }
+
+  if (startInt === null || endInt === null) {
+    return [startInt, endInt];
+  }
+
+  if (startInt <= endInt) {
+    return [startInt, endInt];
+  } else {
+    return [endInt, startInt];
+  }
+}
+
 /**
  * Return true if the page number `label` is within `range`.
  *
+ * Returns `false` if the label is outside `range` or the relation between the
+ * label and the range could not be determined.
+ *
  * @param label - A page number such as "10", "iv"
  * @param range - A page range expressed as a single page number, or a hyphen
  *   separated range (eg. "10-12"). Page ranges are inclusive, so the page
@@ -8,29 +55,54 @@
  *   specify an empty range.
  */
 export function pageLabelInRange(label: string, range: string): boolean {
-  if (!range.includes('-')) {
-    return label === range;
-  }
+  return pageRangesOverlap(label, range) === true;
+}
 
-  let [start, end] = range.split('-');
-  if (!start) {
-    start = label;
-  }
-  if (!end) {
-    end = label;
+/** Convert an open range into an integer range. */
+function normalizeRange(
+  range: [number | null, number | null],
+  min: number,
+  max: number,
+): [number, number] {
+  return [range[0] ?? min, range[1] ?? max];
+}
+
+/**
+ * Return true if two page ranges overlap.
+ *
+ * Each range may be specified as a single page number, or a hyphen-separated
+ * range.
+ *
+ * Returns true if the ranges overlap, false if the ranges do not overlap, or
+ * `null` if the relation could not be determined.
+ */
+export function pageRangesOverlap(
+  rangeA: string,
+  rangeB: string,
+): boolean | null {
+  const intRangeA = parseRange(rangeA);
+  const intRangeB = parseRange(rangeB);
+
+  if (!intRangeA || !intRangeB) {
+    if (rangeA && rangeB && rangeA === rangeB) {
+      // As a special case for non-numeric ranges, we consider them overlapping
+      // if both are equal. This means `pageRangesOverlap("iv", "iv")` is true
+      // for example.
+      return true;
+    }
+
+    // We could not determine whether the ranges overlap.
+    return null;
   }
-  const [startInt, endInt, labelInt] = [
-    parseInt(start),
-    parseInt(end),
-    parseInt(label),
-  ];
-  if (
-    Number.isInteger(startInt) &&
-    Number.isInteger(endInt) &&
-    Number.isInteger(labelInt)
-  ) {
-    return labelInt >= startInt && labelInt <= endInt;
+
+  const minPage = 1;
+  const maxPage = 2 ** 31;
+  const [aStart, aEnd] = normalizeRange(intRangeA, minPage, maxPage);
+  const [bStart, bEnd] = normalizeRange(intRangeB, minPage, maxPage);
+
+  if (aStart <= bStart) {
+    return bStart <= aEnd;
   } else {
-    return false;
+    return aStart <= bEnd;
   }
 }

src/sidebar/util/test/page-range-test.js

-import { pageLabelInRange } from '../page-range';
+import { pageLabelInRange, pageRangesOverlap } from '../page-range';
 
 describe('pageLabelInRange', () => {
   [
@@ -94,3 +94,80 @@ describe('pageLabelInRange', () => {
     });
   });
 });
+
+describe('pageRangesOverlap', () => {
+  [
+    // Matching single page
+    {
+      rangeA: '1',
+      rangeB: '1',
+      overlap: true,
+    },
+    // Non-matching single page
+    {
+      rangeA: '1',
+      rangeB: '2',
+      overlap: false,
+    },
+    // Overlapping numeric ranges
+    {
+      rangeA: '1-2',
+      rangeB: '2-4',
+      overlap: true,
+    },
+    {
+      rangeA: '2-4',
+      rangeB: '1-2',
+      overlap: true,
+    },
+    // Inverted numeric ranges. These are implicitly normalized.
+    {
+      rangeA: '2-1',
+      rangeB: '4-2',
+      overlap: true,
+    },
+    // Half-open ranges
+    {
+      rangeA: '1-',
+      rangeB: '-3',
+      overlap: true,
+    },
+    // Non-overlapping numeric ranges
+    {
+      rangeA: '1-2',
+      rangeB: '3-4',
+      overlap: false,
+    },
+    {
+      rangeA: '3-4',
+      rangeB: '1-2',
+      overlap: false,
+    },
+    // Relation is undefined if either of the ranges is non-numeric.
+    {
+      rangeA: 'ii',
+      rangeB: '3-4',
+      overlap: null,
+    },
+    {
+      rangeA: 'i-iii',
+      rangeB: 'iii-iv',
+      overlap: null,
+    },
+    {
+      rangeA: '1-ii',
+      rangeB: 'ii-3',
+      overlap: null,
+    },
+    // As a special case, matching non-numeric ranges overlap.
+    {
+      rangeA: 'ii',
+      rangeB: 'ii',
+      overlap: true,
+    },
+  ].forEach(({ rangeA, rangeB, overlap }) => {
+    it('returns true if the two ranges overlap', () => {
+      assert.strictEqual(pageRangesOverlap(rangeA, rangeB), overlap);
+    });
+  });
+});