Search inside messages, across every conversation

🔴 There is no index, and that is a decision rather than an omission. An
index derived from decrypted history is plaintext by another name: it
would have to be sealed as carefully as the vault, taken by
forgetEverything, and kept consistent with the threads — a second copy
of everything anybody ever wrote, with its own ways to go wrong. This
reads what already exists and throws the plaintext away.

⚠️ It costs more here than on iOS, and the code says so. The iPhone app
keeps the whole decrypted history in memory; Android decrypts each
thread on demand. Hence a two-character minimum, a bound, a 250ms
debounce, and Dispatchers.IO.

🔴 Folding is length-preserving on purpose: the snippet slices the
ORIGINAL string at an index found in the folded one, so a fold that
dropped characters would cut in the wrong place — and for an accented
character, mid-character. There is a test with accents before the match.

Accent-insensitive because somebody typing "cafe" means "café", and a
search that knows the difference quietly fails in every language with
accents — Spanish is next on the localisation list.

Refs paulhitt/flamenet-messenger-ios#25

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/kotlin/io/flamenet/messenger/app/MessagesScreen.kt

@@ -19,6 +19,7 @@ import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.OutlinedTextField
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
@@ -131,6 +132,11 @@ fun ColumnScope.MessagesScreen(
     )
     Spacer(Modifier.height(Space.md))
 
+    // ⚠️ Above the conversation list. Somebody searching for a word they
+    // remember is looking for the message, not the person — name matches are
+    // already covered by the list underneath.
+    MessageHits(model, query.value)
+
     BuddyCirclesRow(model, phase)
     Spacer(Modifier.height(Space.sm))
     Divider()
@@ -294,3 +300,52 @@ private fun UnreadDot() {
             .background(MaterialTheme.colorScheme.primary),
     )
 }
+
+/**
+ * What a search found inside conversations, rather than in their names.
+ *
+ * ⚠️ Debounced. Each keystroke would otherwise decrypt every thread on the
+ * device — the cost the iPhone app does not pay, because it keeps the whole
+ * history in memory.
+ */
+@Composable
+private fun MessageHits(model: MessengerViewModel, query: String) {
+    var hits by remember { mutableStateOf<List<io.flamenet.e2e.MessageSearch.Hit>>(emptyList()) }
+
+    LaunchedEffect(query) {
+        if (query.trim().length < io.flamenet.e2e.MessageSearch.MINIMUM_QUERY) {
+            hits = emptyList()
+            return@LaunchedEffect
+        }
+        kotlinx.coroutines.delay(250)
+        hits = model.searchMessages(query)
+    }
+
+    if (hits.isEmpty()) return
+
+    Text(
+        "IN MESSAGES",
+        fontSize = 12.sp,
+        fontWeight = FontWeight.Medium,
+        color = Palette.SecondaryText,
+        letterSpacing = 0.8.sp,
+        modifier = Modifier.padding(top = Space.md, bottom = Space.xs),
+    )
+    for (hit in hits) {
+        Column(
+            Modifier.fillMaxWidth()
+                .padding(horizontal = Space.md, vertical = Space.sm)
+                .testTag("search-hit")
+        ) {
+            Text(
+                hit.snippet,
+                style = MaterialTheme.typography.bodyMedium,
+                maxLines = 2,
+                overflow = TextOverflow.Ellipsis,
+            )
+        }
+    }
+    Spacer(Modifier.height(Space.sm))
+    Divider()
+    Spacer(Modifier.height(Space.sm))
+}

app/src/main/kotlin/io/flamenet/messenger/app/MessengerViewModel.kt

@@ -2310,6 +2310,29 @@ class MessengerViewModel(app: Application) : AndroidViewModel(app) {
         viewModelScope.launch(Dispatchers.IO) { vault.saveDraft(chat.vaultThreadId(), text) }
     }
 
+    /**
+     * Messages whose words match, across every conversation on this device.
+     *
+     * 🔴 **Scanned, not indexed.** An index over the decrypted history is
+     * plaintext by another name and would have to be sealed and taken by
+     * `forgetEverything` like everything else in the vault. See
+     * [io.flamenet.e2e.MessageSearch] for the condition under which that stops
+     * being the right call.
+     *
+     * ⚠️ Off the main thread, because unlike the iPhone app this decrypts each
+     * thread on demand rather than reading one already in memory.
+     *
+     * ⚠️ The query is never persisted. What somebody searched for is often more
+     * revealing than what they found.
+     */
+    suspend fun searchMessages(query: String): List<io.flamenet.e2e.MessageSearch.Hit> {
+        val vault = vault ?: return emptyList()
+        return withContext(Dispatchers.IO) {
+            runCatching { io.flamenet.e2e.MessageSearch.hits(query, vault) }
+                .getOrDefault(emptyList())
+        }
+    }
+
     fun sendMessage(text: String, mentions: List<Int> = emptyList()) = viewModelScope.launch {
         val current = session ?: return@launch
         val chat = signedIn()?.openChat ?: return@launch