Auto-correction: short words with two swapped letters (teh → the)

app/src/main/java/helium314/keyboard/latin/ShortWordSwap.kt

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-3.0-only
+package helium314.keyboard.latin
+
+import java.util.Locale
+
+/**
+ * Exception to LeanType's rule that unknown words of up to 3 letters are not auto-corrected (see [Suggest]):
+ * two neighboring letters typed in the wrong order, like "dsa" for "das", "ihc" for "ich" or "teh" for "the",
+ * are corrected. Abbreviations like "db" or "yt" stay, their swapped form is no word.
+ */
+object ShortWordSwap {
+    const val LENGTH = 3
+
+    /** Whether [word] is [typed] with two neighboring (different) letters swapped, case is ignored */
+    fun isSwapOfNeighbors(typed: String, word: String): Boolean {
+        if (typed.length != LENGTH || word.length != LENGTH) return false
+        val a = typed.lowercase(Locale.ROOT)
+        val b = word.lowercase(Locale.ROOT)
+        val i = a.indices.firstOrNull { a[it] != b[it] } ?: return false
+        if (i + 1 >= a.length) return false
+        return a[i] == b[i + 1] && a[i + 1] == b[i] && a.substring(i + 2) == b.substring(i + 2)
+            && a.all { it.isLetter() }
+    }
+}

app/src/main/java/helium314/keyboard/latin/Suggest.kt

@@ -333,7 +333,9 @@ class Suggest(private val mDictionaryFacilitator: DictionaryFacilitator) {
                 val isUserHistory = firstSuggestion.mSourceDict?.mDictType == Dictionary.TYPE_USER_HISTORY
                 val isWhitelist = firstSuggestion.isKindOf(SuggestedWordInfo.KIND_WHITELIST)
                 val isShortcut = firstSuggestion.isKindOf(SuggestedWordInfo.KIND_SHORTCUT) && Settings.getValues().mAutoCorrectShortcuts
-                if (!isExactOrCaseMatch && !isUserHistory && !isWhitelist && !isShortcut && expectedContraction == null) {
+                // two neighboring letters typed in the wrong order, like "teh" for "the", are a typo, not an abbreviation
+                val isSwap = ShortWordSwap.isSwapOfNeighbors(consideredWord, firstSuggestion.mWord)
+                if (!isExactOrCaseMatch && !isUserHistory && !isWhitelist && !isShortcut && !isSwap && expectedContraction == null) {
                     return true to false
                 }
             }

app/src/test/java/helium314/keyboard/latin/ShortWordSwapTest.kt

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-3.0-only
+package helium314.keyboard.latin
+
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class ShortWordSwapTest {
+    @Test
+    fun swappedNeighborsAreFound() {
+        for ((typed, word) in listOf("dsa" to "das", "ihc" to "ich", "dei" to "die", "nud" to "und", "teh" to "the",
+                "adn" to "and", "Dsa" to "Das", "IHC" to "ich", "eni" to "ein")) {
+            assertTrue("$typed -> $word", ShortWordSwap.isSwapOfNeighbors(typed, word))
+        }
+    }
+
+    @Test
+    fun otherDifferencesAreNoSwap() {
+        assertFalse(ShortWordSwap.isSwapOfNeighbors("das", "das")) // same word
+        assertFalse(ShortWordSwap.isSwapOfNeighbors("dsa", "sad")) // not neighbors
+        assertFalse(ShortWordSwap.isSwapOfNeighbors("das", "dar")) // a different letter
+        assertFalse(ShortWordSwap.isSwapOfNeighbors("db", "bd")) // only 3 letters
+        assertFalse(ShortWordSwap.isSwapOfNeighbors("ders", "dres")) // only 3 letters
+        assertFalse(ShortWordSwap.isSwapOfNeighbors("a1b", "1ab")) // only letters
+        assertFalse(ShortWordSwap.isSwapOfNeighbors("abc", "bca"))
+    }
+}