Fix sidebar unit navigation to include reverse conversion pairs

hdyc-svelte/src/lib/components/Sidebar.svelte

@@ -23,35 +23,75 @@
 
   type UnitGroup = {
     label: string;
-    conversions: CalculatorDef[];
+    conversions: UnitConversionLink[];
   };
 
   type UnitBucket = {
     label: string;
-    conversions: CalculatorDef[];
+    conversions: UnitConversionLink[];
   };
 
-  const sortConversionsForUnit = (conversions: CalculatorDef[]) =>
+  type UnitConversionLink = {
+    name: string;
+    slug: string;
+    sortKey: string;
+  };
+
+  const sortConversionsForUnit = (conversions: UnitConversionLink[]) =>
     conversions.slice().sort((a, b) => a.name.localeCompare(b.name));
 
+  const toPairKey = (unitA: string, unitB: string) =>
+    [unitA.toLowerCase(), unitB.toLowerCase()].sort().join('::');
+
+  const toDirectionKey = (fromUnit: string, toUnit: string) =>
+    `${fromUnit.toLowerCase()}::${toUnit.toLowerCase()}`;
+
+  function addConversion(
+    buckets: Map<string, UnitBucket>,
+    fromUnit: string,
+    toUnit: string,
+    slug: string
+  ) {
+    const bucketKey = fromUnit.toLowerCase();
+    const directionKey = toDirectionKey(fromUnit, toUnit);
+    const conversion: UnitConversionLink = {
+      name: `${fromUnit} to ${toUnit}`,
+      slug,
+      sortKey: directionKey,
+    };
+
+    const existing = buckets.get(bucketKey);
+    if (existing) {
+      if (!existing.conversions.some(link => link.sortKey === directionKey)) {
+        existing.conversions.push(conversion);
+      }
+      return;
+    }
+
+    buckets.set(bucketKey, {
+      label: fromUnit,
+      conversions: [conversion],
+    });
+  }
+
   $: categoryUnitGroups = Object.entries(categories).map(([key, meta]) => {
     const buckets = new Map<string, UnitBucket>();
     const calcs = getCalculatorsByCategory(key);
+    const canonicalByPair = new Map<string, CalculatorDef>();
 
     calcs.forEach(calc => {
-      const unit = calc.labels.in1;
-      const bucketKey = unit.toLowerCase();
-      const existing = buckets.get(bucketKey);
-      if (existing) {
-        existing.conversions.push(calc);
-      } else {
-        buckets.set(bucketKey, {
-          label: unit,
-          conversions: [calc],
-        });
+      const pairKey = toPairKey(calc.labels.in1, calc.labels.in2);
+      const existing = canonicalByPair.get(pairKey);
+      if (!existing || calc.slug.localeCompare(existing.slug) < 0) {
+        canonicalByPair.set(pairKey, calc);
       }
     });
 
+    canonicalByPair.forEach(calc => {
+      addConversion(buckets, calc.labels.in1, calc.labels.in2, calc.slug);
+      addConversion(buckets, calc.labels.in2, calc.labels.in1, calc.slug);
+    });
+
     const units = [...buckets.entries()]
       .sort(([a], [b]) => a.localeCompare(b))
       .map(([, bucket]) => ({
@@ -178,14 +218,14 @@
                 </button>
                 {#if expandedUnits[group.key] === unit.label}
                   <ul class="unit-list">
-                    {#each unit.conversions as calc}
+                    {#each unit.conversions as conversion}
                       <li>
                         <a
-                          href="/{calc.slug}"
-                          class:current={currentPath === `/${calc.slug}`}
-                          aria-current={currentPath === `/${calc.slug}` ? 'page' : undefined}
+                          href="/{conversion.slug}"
+                          class:current={currentPath === `/${conversion.slug}`}
+                          aria-current={currentPath === `/${conversion.slug}` ? 'page' : undefined}
                         >
-                          {calc.name}
+                          {conversion.name}
                         </a>
                       </li>
                     {/each}