CustomAdt.parseInt(String) has been implemented.

src/main/kotlin/CustomAdt.kt

@@ -6,10 +6,48 @@ sealed interface ParseResult {
     data class Failure(val pos: Int, val char: Char) : ParseResult
 }
 
+fun String.firstNonDigitCharOrNull(): IndexedValue<Char>? =
+    this
+        .asSequence()
+        .withIndex()
+        .filterNot { isCorrectIntStringChar(it.index, it.value, this.first()) }
+        .firstOrNull()
+
+fun isCorrectIntStringChar(pos: Int, char: Char, firstChar: Char): Boolean {
+    val maxLengthOfPositiveIntString = Int.MAX_VALUE.toString().length
+    return when {
+        pos == 0 && char == '-' -> true
+        pos == 1 && char == '0' && firstChar == '-' -> false
+        pos == 1 && char == '0' && firstChar == '0' -> false
+        pos < maxLengthOfPositiveIntString && char.isDigit() -> true
+        pos == maxLengthOfPositiveIntString && char.isDigit() && firstChar == '-' -> true
+        else -> false
+    }
+}
+
+fun parseCorrectInt(str: String): Int =
+    when (str.first()) {
+        '-' -> parseNegativeInt(str)
+        else -> parsePositiveInt(str)
+    }
+
+fun parseNegativeInt(str: String): Int =
+    -1 * parsePositiveInt(str.drop(1))
+
+fun parsePositiveInt(str: String): Int =
+    str
+        .map { it - '0' }
+        .fold(0) { accumulator, digit -> accumulator * 10 + digit }
+
 object CustomAdt {
 
     fun parseInt(str: String): ParseResult {
-        TODO()
+        return when {
+            str.isEmpty() -> ParseResult.Failure(-1, '\u0000')
+            else -> str.firstNonDigitCharOrNull()
+                ?.let { ParseResult.Failure(it.index, it.value) }
+                ?: ParseResult.Success(parseCorrectInt(str))
+        }
     }
 
 }