test(sidecar): pin the constant-cost erase and its residue check

sidecar/src/test/kotlin/dev/sanderling/sidecar/EraseTextTest.kt

@@ -0,0 +1,110 @@
+package dev.sanderling.sidecar
+
+import org.junit.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class EraseTextTest {
+
+    // maestro's eraseText sends one delete per character through its
+    // instrumentation, measured at 29.6 ms/char on the API 34 emulator: the
+    // 4096-character string the corpus types cost ~121s to clear, a fifth of a
+    // 20 minute run for one step. Selecting the field and deleting the
+    // selection is the same two key events whatever the field holds.
+    @Test fun aClearedFieldCostsTwoKeyEventsWhateverItsLength() {
+        for (length in listOf(1, 21, 512, 4096)) {
+            val sent = mutableListOf<String>()
+            eraseFocusedField(length, { sent.add(it) }) { 0 }
+            assertEquals(
+                listOf(SELECT_ALL_COMMAND, DELETE_KEY_COMMAND),
+                sent,
+                "length $length must not scale the erase",
+            )
+        }
+    }
+
+    // The dangerous failure is a fast erase that leaves characters behind: the
+    // next InputText appends to the residue and every reading downstream is
+    // wrong with nothing to catch it. A field the select-all did not clear is
+    // finished off per character rather than assumed empty.
+    @Test fun aFieldTheSelectAllMissedIsFinishedOffPerCharacter() {
+        val sent = mutableListOf<String>()
+        eraseFocusedField(4096, { sent.add(it) }) { 4096 }
+
+        assertEquals(SELECT_ALL_COMMAND, sent.first())
+        assertEquals(DELETE_KEY_COMMAND, sent[1])
+        assertEquals(
+            4096,
+            sent.drop(2).sumOf { command ->
+                command.removePrefix("input keyevent ").split(" ").size
+            },
+            "every character must still be deleted",
+        )
+    }
+
+    // Unknown is not empty. A tree that cannot name the focused field is no
+    // evidence the erase worked, and the safe way to be wrong is the delete
+    // that costs time rather than the one that leaves residue.
+    @Test fun aFieldThatCannotBeReadIsFinishedOffRatherThanAssumedEmpty() {
+        val sent = mutableListOf<String>()
+        eraseFocusedField(8, { sent.add(it) }) { null }
+        assertTrue(sent.size > 2, "an unverified erase must not stop at two")
+    }
+
+    @Test fun nothingToEraseIssuesNoKeysAtAll() {
+        val sent = mutableListOf<String>()
+        eraseFocusedField(0, { sent.add(it) }) { 0 }
+        eraseFocusedField(-1, { sent.add(it) }) { 0 }
+        assertEquals(emptyList(), sent)
+    }
+
+    // Batching is what keeps the fallback affordable: one round trip per batch
+    // rather than one per character, measured 2.3 ms/char against maestro's
+    // 29.6. The count must survive the batching exactly.
+    @Test fun deleteKeyCommandsBatchesWithoutLosingACharacter() {
+        for (count in listOf(1, 199, 200, 201, 4096)) {
+            val commands = deleteKeyCommands(count, DELETE_BATCH_KEYS)
+            val keys = commands.flatMap {
+                it.removePrefix("input keyevent ").split(" ")
+            }
+            assertEquals(count, keys.size, "count $count")
+            assertTrue(keys.all { it == "67" }, "only KEYCODE_DEL")
+            assertTrue(
+                commands.size <= (count + DELETE_BATCH_KEYS - 1) /
+                    DELETE_BATCH_KEYS,
+                "count $count used ${commands.size} round trips",
+            )
+        }
+    }
+
+    // The erase targets the field the runner just tapped, so the length that
+    // decides whether it worked is the focused editable node's, not some other
+    // field that legitimately still holds text.
+    @Test fun focusedEditableTextLengthReadsTheFocusedFieldOnly() {
+        assertEquals(4096, focusedEditableTextLength(TREE_WITH_FULL_FIELD))
+        assertEquals(0, focusedEditableTextLength(TREE_WITH_EMPTY_FIELD))
+    }
+
+    @Test fun aTreeWithNoFocusedFieldReadsAsUnknown() {
+        assertEquals(null, focusedEditableTextLength(TREE_WITH_NO_FOCUS))
+        assertEquals(null, focusedEditableTextLength(""))
+        assertEquals(null, focusedEditableTextLength("not json"))
+    }
+}
+
+private fun tree(focusedText: String?, otherText: String): String {
+    val focused = focusedText?.let {
+        """{"attributes":{"resource-id":"AccountNameField",
+           "editable":"true","focused":"true","text":"$it"},"children":[]},"""
+    } ?: ""
+    return """
+    {"attributes":{"resource-id":"AddAccountScreen"},"children":[
+      $focused
+      {"attributes":{"resource-id":"OtherField","editable":"true",
+       "focused":"false","text":"$otherText"},"children":[]}]}
+    """.trimIndent()
+}
+
+private val TREE_WITH_FULL_FIELD = tree("a".repeat(4096), "keep me")
+private val TREE_WITH_EMPTY_FIELD = tree("", "keep me")
+private val TREE_WITH_NO_FOCUS = tree(null, "keep me")