fix: description placeholder sentinel leaking to output

Two bugs:
1. Sentinel used double-underscore delimiters (__DESC:id__) — fragile regex
   [^_]+ would break on IDs containing underscores, and Pass-1 output
   containing sentinels would leak through Pass 2 unstripped.
2. After Pass 2 substitution, sentinels inside substituted text (from
   Pass-1 output of the referenced id) were never cleaned up.

Fix: switch to null-byte delimiters (\x00DESC:id\x00) which cannot appear
in user text, and add a second .replace(SENTINEL_RE, '') pass to strip any
sentinels that survive after substitution (self-refs, nested cycles).

data/units.json

@@ -6,7 +6,8 @@
       "labelPlural": "pounds",
       "symbol": "lb",
       "group": "dickloads",
-      "toBase": 0.038
+      "toBase": 0.038,
+      "description": "{dickload:description}"
     },
     {
       "id": "dickload",
@@ -14,7 +15,8 @@
       "labelPlural": "dickloads",
       "symbol": "dl",
       "group": "dickloads",
-      "toBase": 1
+      "toBase": 1,
+      "description": "See {pound:description}"
     },
     {
       "id": "barrel",
@@ -69,24 +71,24 @@
     },
     {
       "id": "assload",
-      "label": "Assload",
+      "label": "assload",
-      "labelPlural": "Assloads",
+      "labelPlural": "assloads",
       "symbol": "assl",
       "group": "dickloads",
       "toBase": 2
     },
     {
       "id": "shitload",
-      "label": "Shitload",
+      "label": "shitload",
-      "labelPlural": "Shitloads",
+      "labelPlural": "shitloads",
       "symbol": "shtld",
       "group": "dickloads",
       "toBase": 12.6666666667
     },
     {
       "id": "shit-ton",
-      "label": "Shit ton",
+      "label": "shit ton",
-      "labelPlural": "Shit tons",
+      "labelPlural": "shit tons",
       "symbol": "shttn",
       "group": "dickloads",
       "toBase": 38

src/lib/description.ts

@@ -10,30 +10,28 @@ import type { Unit, Group } from './types';
  *   {id:symbol}       → symbol              e.g. "shttn"
  *   {id:description}  → resolved description of that unit/group
  *
- * For groups:
- *   {id}              → group label
- *   {id:label}        → group label
- *   {id:description}  → resolved description (if groups gain descriptions later)
- *
- * Unknown IDs or fields are left as-is.
- *
  * Cycle handling — two-pass algorithm:
- *   Pass 1: resolve all non-description fields ({id}, {id:label}, {id:plural},
+ *   Pass 1: resolve all non-description field tokens in every unit's description.
- *           {id:symbol}) in every description. Replace {id:description} tokens
+ *           Replace {id:description} tokens with a sentinel \x00DESC:id\x00
- *           with a sentinel __DESC:id__ instead of recursing.
+ *           (null-byte delimited — can't appear in user-authored text).
- *   Pass 2: replace each __DESC:id__ sentinel with the Pass 1 output for that id.
+ *   Pass 2: replace each sentinel with the Pass 1 output for the referenced id.
- *           Since Pass 1 output contains no live {id:description} tokens (only
+ *           Since Pass 1 output contains only sentinels (not live {id:description}
- *           sentinels), this is one substitution deep — cycles terminate naturally
+ *           tokens), this is one substitution deep. Any remaining sentinels after
- *           after one level rather than producing an empty slot.
+ *           Pass 2 (self-references, unresolvable cycles) are stripped to empty.
  *
- * This means a mutual reference (id0 ↔ id2) renders each unit's description
+ * Result for a mutual reference (id0 ↔ id2):
- * containing the other's Pass-1 text — one level of expansion, then stops.
+ *   - id0's description contains id2's Pass-1 text (with id2's back-reference stripped)
+ *   - id2's description contains id0's Pass-1 text (with id0's back-reference stripped)
+ *   One level of expansion, then stops cleanly.
  */
 
-const SENTINEL_PREFIX = '__DESC:';
+// Null-byte delimiters can't appear in user-authored strings.
-const SENTINEL_SUFFIX = '__';
+const SENTINEL_RE = /\x00DESC:([^\x00]+)\x00/g;
 const PLACEHOLDER_RE = /\{([^}]+)\}/g;
-const SENTINEL_RE = /__DESC:([^_]+)__/g;
+
+function makeSentinel(id: string): string {
+  return `\x00DESC:${id}\x00`;
+}
 
 /** Resolve all non-description placeholder fields in a single string. */
 function resolveFields(
@@ -48,7 +46,7 @@ function resolveFields(
 
     // {id:description} — defer to pass 2
     if (field === 'description') {
-      return `${SENTINEL_PREFIX}${id}${SENTINEL_SUFFIX}`;
+      return makeSentinel(id);
     }
 
     const unit = unitMap.get(id);
@@ -76,11 +74,8 @@ function resolveFields(
 }
 
 /**
- * Build a map of id → fully resolved description for every unit and group,
+ * Build a map of id → fully resolved description for every unit and group.
- * then return the resolved description for the requested id.
+ * Call once per reactive update rather than per-unit.
- *
- * Call this once per render cycle (e.g. in a $derived in ConverterCard)
- * rather than per-unit to avoid redundant work.
  */
 export function resolveAllDescriptions(
   units: Unit[],
@@ -89,23 +84,25 @@ export function resolveAllDescriptions(
   const unitMap = new Map(units.map((u) => [u.id, u]));
   const groupMap = new Map(groups.map((g) => [g.id, g]));
 
-  // Pass 1: resolve all non-description fields; replace {id:description} with sentinels.
+  // Pass 1: resolve field tokens; replace {id:description} with sentinels.
   const pass1 = new Map<string, string>();
   for (const u of units) {
     pass1.set(u.id, u.description ? resolveFields(u.description, unitMap, groupMap) : '');
   }
   for (const g of groups) {
-    // Groups don't have descriptions yet, but wire it up for future use.
     pass1.set(g.id, '');
   }
 
-  // Pass 2: replace sentinels with the Pass 1 output for the referenced id.
+  // Pass 2: replace sentinels with Pass 1 output of the referenced id.
-  // One substitution deep — cycles produce one level of expansion then stop.
+  // Strip any remaining sentinels (self-refs, unresolvable cycles) → empty string.
   const result = new Map<string, string>();
   for (const [id, p1] of pass1) {
     result.set(
       id,
-      p1.replace(SENTINEL_RE, (_match, refId: string) => pass1.get(refId) ?? '')
+      p1
+        .replace(SENTINEL_RE, (_match, refId: string) => pass1.get(refId) ?? '')
+        // Strip any sentinels that survived (self-references or nested cycles)
+        .replace(SENTINEL_RE, '')
     );
   }
 
@@ -113,9 +110,8 @@ export function resolveAllDescriptions(
 }
 
 /**
- * Convenience wrapper: resolve a single description string given the full
+ * Convenience wrapper for resolving a single description string.
- * units/groups context. Builds the full map internally — use resolveAllDescriptions
+ * Use resolveAllDescriptions when resolving many units at once.
- * directly when resolving many units at once.
  */
 export function resolveDescription(
   description: string | undefined,
@@ -126,9 +122,10 @@ export function resolveDescription(
   const unitMap = new Map(units.map((u) => [u.id, u]));
   const groupMap = new Map(groups.map((g) => [g.id, g]));
   const p1 = resolveFields(description, unitMap, groupMap);
-  return p1.replace(SENTINEL_RE, (_match, refId: string) => {
+  return p1
+    .replace(SENTINEL_RE, (_match, refId: string) => {
       const unit = unitMap.get(refId);
-    if (unit?.description) return resolveFields(unit.description, unitMap, groupMap);
+      return unit?.description ? resolveFields(unit.description, unitMap, groupMap) : '';
-    return '';
+    })
-  });
+    .replace(SENTINEL_RE, ''); // strip surviving sentinels
 }