fix(decoder): bad state transition when decode map like value

Author: iseki0

src/commonMain/kotlin/space/iseki/bencoding/BencodeDecoder0.kt

@@ -203,7 +203,16 @@ internal class BencodeDecoder0(
                 if (s and C_READ == 0) {
                     error("call decodeElementIndex before decode*")
                 }
-                stack[sp] = s xor C_READ
+                stack[sp] = postReadStateOf(stack[sp])
+            }
+        }
+
+        private fun postReadStateOf(old: Int): Int {
+            assert(old and C_READ != 0)
+            return when (old) {
+                MAP_KEY_READ -> MAP_VAL_IDLE
+                MAP_VAL_READ -> MAP_KEY_IDLE
+                else -> old xor C_READ
             }
         }
 

src/commonTest/kotlin/space/iseki/bencoding/MapLikeTest.kt

@@ -0,0 +1,24 @@
+package space.iseki.bencoding
+
+import kotlinx.serialization.decodeFromByteArray
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class MapLikeTest {
+    val data = "d3:abci10e2:cdi-20ee"
+    val value = mapOf("abc" to 10, "cd" to -20)
+
+    @Test
+    fun testDecode() {
+        val decoded = Bencode.decodeFromByteArray<Map<String, Int>>(data.encodeToByteArray())
+        assertEquals(value, decoded)
+    }
+
+    @Test
+    fun testManyDecode() {
+        val data = (0..100).joinToString("", prefix = "d", postfix = "e") { val k = "$it"; "${k.length}:${k}i${k}e" }
+        val value = (0..100).associateBy { val k = "$it"; k }
+        val decoded = Bencode.decodeFromByteArray<Map<String, Int>>(data.encodeToByteArray())
+        assertEquals(value, decoded)
+    }
+}