fix(nostr): do not pick relays around (0, 0) for an undecodable geohash

app/src/main/java/com/bitchat/android/geohash/Geohash.kt

@@ -66,6 +66,13 @@ object Geohash {
         return geohash.toString()
     }
 
+    /**
+     * True when every character of [geohash] is a base32 geohash digit and the
+     * string is non-empty, i.e. when it names an actual cell.
+     */
+    fun isValid(geohash: String): Boolean =
+        geohash.isNotEmpty() && geohash.lowercase().all { charToValue.containsKey(it) }
+
     /**
      * Decodes a geohash string to the center latitude/longitude of its cell.
      * @return Pair(latitude, longitude)
@@ -77,8 +84,31 @@ object Geohash {
         return latCenter to lonCenter
     }
 
+    /**
+     * Decodes a geohash string to the center latitude/longitude of its cell, or
+     * null when the string does not name a cell.
+     *
+     * Prefer this over [decodeToCenter] wherever the string can come from
+     * outside: an empty or malformed geohash decodes to the (0, 0) box, and
+     * (0, 0) is a real place in the Gulf of Guinea, so a caller that cannot
+     * tell the two apart silently acts on a location the user never chose.
+     */
+    fun decodeToCenterOrNull(geohash: String): Pair<Double, Double>? =
+        if (isValid(geohash)) decodeToCenter(geohash) else null
+
+    /**
+     * Decodes a geohash string to a bounding box, or null when the string does
+     * not name a cell.  See [decodeToCenterOrNull].
+     */
+    fun decodeToBoundsOrNull(geohash: String): Bounds? =
+        if (isValid(geohash)) decodeToBounds(geohash) else null
+
     /**
      * Decodes a geohash string to bounding box (lat/lon min/max).
+     *
+     * An empty or malformed geohash yields the degenerate box at (0, 0); use
+     * [decodeToBoundsOrNull] when that has to be distinguishable from a real
+     * cell there.
      */
     fun decodeToBounds(geohash: String): Bounds {
         if (geohash.isEmpty()) return Bounds(0.0, 0.0, 0.0, 0.0)

app/src/main/java/com/bitchat/android/nostr/RelayDirectory.kt

@@ -88,10 +88,10 @@ object RelayDirectory {
     fun closestRelaysForGeohash(geohash: String, nRelays: Int): List<String> {
         val snapshot = synchronized(relaysLock) { relays.toList() }
         if (snapshot.isEmpty()) return emptyList()
-        val center = try {
-            val c = com.bitchat.android.geohash.Geohash.decodeToCenter(geohash)
-            c
-        } catch (e: Exception) {
+        // An empty or malformed geohash decodes to (0, 0), which would pick the
+        // relays closest to the Gulf of Guinea rather than declining to choose.
+        val center = com.bitchat.android.geohash.Geohash.decodeToCenterOrNull(geohash)
+        if (center == null) {
             Log.e(TAG, "Failed to decode geohash")
             return emptyList()
         }

app/src/test/java/com/bitchat/android/geohash/GeohashValidityTest.kt

@@ -0,0 +1,61 @@
+package com.bitchat.android.geohash
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * A geohash that names no cell has to be distinguishable from one that names a
+ * cell at (0, 0): the origin is a real place in the Gulf of Guinea, so anything
+ * that picks relays or geocodes a name from a decoded centre would otherwise
+ * act on a location the user never chose.
+ */
+class GeohashValidityTest {
+
+    @Test
+    fun `base32 strings are valid regardless of case`() {
+        assertTrue(Geohash.isValid("u4pruydqqvj"))
+        assertTrue(Geohash.isValid("U4PRUYDQQVJ"))
+        assertTrue(Geohash.isValid("9"))
+    }
+
+    @Test
+    fun `empty and malformed strings are invalid`() {
+        assertFalse(Geohash.isValid(""))
+        // a, i, l and o are not geohash digits
+        assertFalse(Geohash.isValid("hello"))
+        assertFalse(Geohash.isValid("u4pr!"))
+        assertFalse(Geohash.isValid("café"))
+    }
+
+    @Test
+    fun `nullable decoders reject what the plain ones map onto the origin`() {
+        assertEquals(0.0 to 0.0, Geohash.decodeToCenter(""))
+        assertEquals(0.0 to 0.0, Geohash.decodeToCenter("hello"))
+
+        assertNull(Geohash.decodeToCenterOrNull(""))
+        assertNull(Geohash.decodeToCenterOrNull("hello"))
+        assertNull(Geohash.decodeToBoundsOrNull(""))
+        assertNull(Geohash.decodeToBoundsOrNull("hello"))
+    }
+
+    @Test
+    fun `nullable decoders agree with the plain ones on real cells`() {
+        val geohash = "u4pruydqqvj"
+        assertEquals(Geohash.decodeToCenter(geohash), Geohash.decodeToCenterOrNull(geohash))
+        assertEquals(Geohash.decodeToBounds(geohash), Geohash.decodeToBoundsOrNull(geohash))
+        assertNotNull(Geohash.decodeToCenterOrNull("s000"))
+    }
+
+    @Test
+    fun `a cell at the origin still decodes`() {
+        // "s000..." is the cell containing (0, 0) — a valid geohash, not a failure.
+        val center = Geohash.decodeToCenterOrNull("s0000")
+        assertNotNull(center)
+        assertTrue(center!!.first in -1.0..1.0)
+        assertTrue(center.second in -1.0..1.0)
+    }
+}