diff --git a/exercises/practice/acronym/src/main/kotlin/Acronym.kt b/exercises/practice/acronym/src/main/kotlin/Acronym.kt
index ceb1d767..1056bef3 100644
--- a/exercises/practice/acronym/src/main/kotlin/Acronym.kt
+++ b/exercises/practice/acronym/src/main/kotlin/Acronym.kt
@@ -1,5 +1,10 @@
 object Acronym {
-    fun generate(phrase: String) : String {
-        TODO("Implement the function to complete the task")
-    }
+    fun generate(phrase: String) = phrase.trim()
+        .split(" ", "-")
+        .map { str -> str.filter { it.isLetter() } }
+        .takeIf { it.isNotEmpty() }
+        ?.mapNotNull { it.firstOrNull() }
+        ?.map { it.uppercaseChar() }
+        ?.joinToString("")
+        ?: ""
 }
diff --git a/exercises/practice/difference-of-squares/src/main/kotlin/DifferenceOfSquares.kt b/exercises/practice/difference-of-squares/src/main/kotlin/DifferenceOfSquares.kt
index b0bccaad..7f9e42ad 100644
--- a/exercises/practice/difference-of-squares/src/main/kotlin/DifferenceOfSquares.kt
+++ b/exercises/practice/difference-of-squares/src/main/kotlin/DifferenceOfSquares.kt
@@ -1,15 +1,12 @@
-class Squares {
-    //TODO: implement proper constructor
+class Squares(val square: Int) {
 
-    fun sumOfSquares() {
-        TODO("Implement the function to complete the task")
-    }
+    fun sumOfSquares() = (1..square)
+        .map { it * it }
+        .sum()
 
-    fun squareOfSum() {
-        TODO("Implement the function to complete the task")
-    }
+    fun squareOfSum() = (1..square)
+        .sum()
+        .let { it * it }
 
-    fun difference() {
-        TODO("Implement the function to complete the task")
-    }
-}
+    fun difference() = squareOfSum() - sumOfSquares()
+}
\ No newline at end of file
diff --git a/exercises/practice/dnd-character/src/main/kotlin/DndCharacter.kt b/exercises/practice/dnd-character/src/main/kotlin/DndCharacter.kt
index 4c219d00..c4284af0 100644
--- a/exercises/practice/dnd-character/src/main/kotlin/DndCharacter.kt
+++ b/exercises/practice/dnd-character/src/main/kotlin/DndCharacter.kt
@@ -1,22 +1,25 @@
+import java.lang.Math.floorDiv
+import kotlin.random.Random
+
 class DndCharacter {
 
-    val strength: Int = TODO("Initialize value to complete the task")
-    val dexterity: Int = TODO("Initialize value to complete the task")
-    val constitution: Int = TODO("Initialize value to complete the task")
-    val intelligence: Int = TODO("Initialize value to complete the task")
-    val wisdom: Int = TODO("Initialize value to complete the task")
-    val charisma: Int = TODO("Initialize value to complete the task")
-    val hitpoints: Int = TODO("Initialize value to complete the task")
+    val strength = ability()
+    val dexterity = ability()
+    val constitution = ability()
+    val intelligence = ability()
+    val wisdom = ability()
+    val charisma = ability()
+    val hitpoints = 10 + modifier(constitution)
 
     companion object {
 
-        fun ability(): Int {
-            TODO("Implement the function to complete the task")
-        }
+        fun ability() = (1..4)
+            .map { Random.nextInt(1, 6) }
+            .sorted()
+            .drop(1)
+            .sum()
 
-        fun modifier(score: Int): Int {
-            TODO("Implement the function to complete the task")
-        }
+        fun modifier(score: Int) = floorDiv(score - 10, 2)
     }
 
 }
diff --git a/exercises/practice/flatten-array/.meta/src/reference/kotlin/Flattener.kt b/exercises/practice/flatten-array/.meta/src/reference/kotlin/Flattener.kt
index e58b761a..81b65311 100644
--- a/exercises/practice/flatten-array/.meta/src/reference/kotlin/Flattener.kt
+++ b/exercises/practice/flatten-array/.meta/src/reference/kotlin/Flattener.kt
@@ -4,6 +4,8 @@ object Flattener {
         return unflattened.filterNotNull().fold(mutableListOf()) { accum, element -> accum.addFlattened(element); accum }
     }
 
-    fun MutableCollection<Any>.addFlattened(item: Any) = if (item is Collection<*>) this.addAll(flatten(item)) else this.add(item)
+    fun MutableCollection<Any>.addFlattened(item: Any) = if (item is Collection<*>)
+        this.addAll(flatten(item))
+    else this.add(item)
 
 }
diff --git a/exercises/practice/flatten-array/src/main/kotlin/Flattener.kt b/exercises/practice/flatten-array/src/main/kotlin/Flattener.kt
index 17f24d67..338c23a7 100644
--- a/exercises/practice/flatten-array/src/main/kotlin/Flattener.kt
+++ b/exercises/practice/flatten-array/src/main/kotlin/Flattener.kt
@@ -1,5 +1,11 @@
 object Flattener {
-    fun flatten(source: Collection<Any?>): List<Any> {
-        TODO("Implement the function to complete the task")
-    }
+    fun flatten(source: Collection<Any?>): List<Any> = source
+        .filterNotNull()
+        .flatMap {
+            if (it is Collection<*>) {
+                return@flatMap flatten(it)
+            } else {
+                listOf(it)
+            }
+        }
 }
diff --git a/exercises/practice/gigasecond/.meta/src/reference/kotlin/Gigasecond.kt b/exercises/practice/gigasecond/.meta/src/reference/kotlin/Gigasecond.kt
index d353e1ec..cd9f8172 100644
--- a/exercises/practice/gigasecond/.meta/src/reference/kotlin/Gigasecond.kt
+++ b/exercises/practice/gigasecond/.meta/src/reference/kotlin/Gigasecond.kt
@@ -5,5 +5,5 @@ data class Gigasecond(val initialDateTime: LocalDateTime) {
 
     constructor(initialDate: LocalDate): this(initialDate.atTime(0, 0))
 
-    val date = initialDateTime.plusSeconds(1_000_000_000)
+    val date = ldt.plusSeconds(1_000_000_000)
 }
diff --git a/exercises/practice/gigasecond/src/main/kotlin/Gigasecond.kt b/exercises/practice/gigasecond/src/main/kotlin/Gigasecond.kt
index 362906ed..0dff5418 100644
--- a/exercises/practice/gigasecond/src/main/kotlin/Gigasecond.kt
+++ b/exercises/practice/gigasecond/src/main/kotlin/Gigasecond.kt
@@ -1,8 +1,9 @@
 import java.time.LocalDateTime
+import java.time.LocalDate
 
-class Gigasecond {
+class Gigasecond(val ldt: LocalDateTime) {
 
-    // TODO: Implement proper constructor
+    constructor(ld: LocalDate): this(ld.atTime(0, 0))
 
-    val date: LocalDateTime = TODO("Implement this getter to complete the task")
+    val date = ldt.plusSeconds(1_000_000_000)
 }
diff --git a/exercises/practice/hamming/src/main/kotlin/Hamming.kt b/exercises/practice/hamming/src/main/kotlin/Hamming.kt
index 20162550..f0326de4 100644
--- a/exercises/practice/hamming/src/main/kotlin/Hamming.kt
+++ b/exercises/practice/hamming/src/main/kotlin/Hamming.kt
@@ -1,6 +1,10 @@
 object Hamming {
 
     fun compute(leftStrand: String, rightStrand: String): Int {
-        TODO("Implement this function to complete the task")
+        if (leftStrand.length != rightStrand.length) throw IllegalArgumentException("left and right strands must be of equal length")
+
+        return leftStrand
+            .zip(rightStrand) { a, b -> a != b }
+            .count { it }
     }
 }
diff --git a/exercises/practice/hello-world/src/main/kotlin/HelloWorld.kt b/exercises/practice/hello-world/src/main/kotlin/HelloWorld.kt
index 6adfdeb0..fe21c76f 100644
--- a/exercises/practice/hello-world/src/main/kotlin/HelloWorld.kt
+++ b/exercises/practice/hello-world/src/main/kotlin/HelloWorld.kt
@@ -1,3 +1 @@
-fun hello(): String {
-    return "Goodbye, Mars!"
-}
+fun hello() = "Hello, World!"
diff --git a/exercises/practice/luhn/src/main/kotlin/Luhn.kt b/exercises/practice/luhn/src/main/kotlin/Luhn.kt
index f2df32ac..bd6199d4 100644
--- a/exercises/practice/luhn/src/main/kotlin/Luhn.kt
+++ b/exercises/practice/luhn/src/main/kotlin/Luhn.kt
@@ -1,6 +1,21 @@
+
 object Luhn {
 
     fun isValid(candidate: String): Boolean {
-        TODO("Implement this function to complete the task")
+        val stripped = candidate.replace(" ", "")
+        if (!stripped.all { it.isDigit() }) return false
+        if (stripped.length <= 1) return false
+
+        val sumOdd = (stripped.length - 2 downTo 0 step 2)
+            .map { stripped[it].digitToInt() }
+            .sumOf {
+                val sum = it + it
+                if (sum > 9) sum - 9
+                else sum
+            }
+        val sumEven = (stripped.length - 1 downTo 0 step 2)
+            .sumOf { stripped[it].digitToInt() }
+
+        return (sumOdd + sumEven) % 10 == 0
     }
-}
+}
\ No newline at end of file
diff --git a/exercises/practice/matrix/src/main/kotlin/Matrix.kt b/exercises/practice/matrix/src/main/kotlin/Matrix.kt
index 7ef62f50..815e5b1f 100644
--- a/exercises/practice/matrix/src/main/kotlin/Matrix.kt
+++ b/exercises/practice/matrix/src/main/kotlin/Matrix.kt
@@ -1,10 +1,18 @@
 class Matrix(private val matrixAsString: String) {
 
-    fun column(colNr: Int): List<Int> {
-        TODO("Implement this to complete the task")
-    }
+    private val matrix = matrixAsString.split("\n")
+        .withIndex()
+        .mapNotNull { indexedRow ->
+            indexedRow.value.split(" ")
+                .map { it.toInt() }
+                .takeIf { it.isNotEmpty() }
+                ?.let { it[0] to it }
+        }
 
-    fun row(rowNr: Int): List<Int> {
-        TODO("Implement this to complete the task")
-    }
+    fun column(colNr: Int) = matrix.map { it.second }
+        .mapNotNull { it.getOrNull(colNr - 1) }
+
+    fun row(rowNr: Int) = matrix.getOrNull(rowNr - 1)
+        ?.second
+        ?: emptyList()
 }
diff --git a/exercises/practice/nucleotide-count/src/main/kotlin/Dna.kt b/exercises/practice/nucleotide-count/src/main/kotlin/Dna.kt
index 7b264d0e..b7b054ef 100644
--- a/exercises/practice/nucleotide-count/src/main/kotlin/Dna.kt
+++ b/exercises/practice/nucleotide-count/src/main/kotlin/Dna.kt
@@ -1,9 +1,20 @@
-class Dna {
+class Dna(dna: String) {
 
-    // TODO: Implement proper constructor
+    private val map = mutableMapOf(
+        'A' to 0,
+        'C' to 0,
+        'T' to 0,
+        'G' to 0,
+    )
 
-    val nucleotideCounts: Map<Char, Int>
-        get() {
-            TODO("Implement this function to complete the task")
+    init {
+        if (dna.isNotEmpty()) {
+            require(dna.all { map.keys.contains(it) })
         }
-}
+        dna.forEach {
+            map[it] = map.getOrDefault(it, 0) + 1
+        }
+    }
+
+    val nucleotideCounts = map
+}
\ No newline at end of file
diff --git a/exercises/practice/phone-number/src/main/kotlin/PhoneNumber.kt b/exercises/practice/phone-number/src/main/kotlin/PhoneNumber.kt
index d60f1339..1329c102 100644
--- a/exercises/practice/phone-number/src/main/kotlin/PhoneNumber.kt
+++ b/exercises/practice/phone-number/src/main/kotlin/PhoneNumber.kt
@@ -1,6 +1,22 @@
-class PhoneNumber {
 
-    // TODO: Implement proper constructor
+class PhoneNumber(plainPhoneNumber: String) {
 
-    val number: String? = TODO("Implement this getter to complete the task")
-}
+    var number: String
+
+    init {
+        val nRange = 2..9
+
+        number = plainPhoneNumber.filter { it.isDigit() }
+            .takeIf {
+                it.length == 11 && it[0] == '1'
+                        || it.length == 10
+            }
+            ?.takeLast(10)
+            ?.takeIf {
+                it[0].digitToInt() in nRange
+                        && it[3].digitToInt() in nRange
+            }
+            ?: throw IllegalArgumentException()
+    }
+
+}
\ No newline at end of file
diff --git a/exercises/practice/rna-transcription/src/main/kotlin/RnaTranscription.kt b/exercises/practice/rna-transcription/src/main/kotlin/RnaTranscription.kt
index f895ae11..07383339 100644
--- a/exercises/practice/rna-transcription/src/main/kotlin/RnaTranscription.kt
+++ b/exercises/practice/rna-transcription/src/main/kotlin/RnaTranscription.kt
@@ -1 +1,9 @@
-fun transcribeToRna(dna: String): String = ""
+fun transcribeToRna(dna: String) = dna.map {
+    when (it) {
+        'A' -> 'U'
+        'G' -> 'C'
+        'C' -> 'G'
+        'T' -> 'A'
+        else -> it
+    }
+}.joinToString("")
diff --git a/exercises/practice/scrabble-score/src/main/kotlin/ScrabbleScore.kt b/exercises/practice/scrabble-score/src/main/kotlin/ScrabbleScore.kt
index 212bfda7..dda559c1 100644
--- a/exercises/practice/scrabble-score/src/main/kotlin/ScrabbleScore.kt
+++ b/exercises/practice/scrabble-score/src/main/kotlin/ScrabbleScore.kt
@@ -1,10 +1,36 @@
 object ScrabbleScore {
 
-    fun scoreLetter(c: Char): Int {
-        TODO("Implement this function to complete the task")
-    }
+    val scores = mapOf (
+        'a' to 1,
+        'e' to 1,
+        'i' to 1,
+        'o' to 1,
+        'u' to 1,
+        'l' to 1,
+        'n' to 1,
+        'r' to 1,
+        's' to 1,
+        't' to 1,
+        'd' to 2,
+        'g' to 2,
+        'b' to 3,
+        'c' to 3,
+        'm' to 3,
+        'p' to 3,
+        'f' to 4,
+        'h' to 4,
+        'v' to 4,
+        'w' to 4,
+        'y' to 4,
+        'k' to 5,
+        'j' to 8,
+        'x' to 8,
+        'q' to 10,
+        'z' to 10,
+    )
 
-    fun scoreWord(word: String): Int {
-        TODO("Implement this function to complete the task")
-    }
+
+    fun scoreLetter(c: Char) = scores.getOrDefault(c.lowercaseChar(), 0)
+
+    fun scoreWord(word: String) = word.sumOf { scoreLetter(it) }
 }
diff --git a/exercises/practice/secret-handshake/src/main/kotlin/HandshakeCalculator.kt b/exercises/practice/secret-handshake/src/main/kotlin/HandshakeCalculator.kt
index 24e7cf9b..d6dc2b7b 100644
--- a/exercises/practice/secret-handshake/src/main/kotlin/HandshakeCalculator.kt
+++ b/exercises/practice/secret-handshake/src/main/kotlin/HandshakeCalculator.kt
@@ -1,5 +1,10 @@
 object HandshakeCalculator {
+
     fun calculateHandshake(number: Int): List<Signal> {
-        TODO("Implement the function to complete the task")
+        val signals = Signal.values()
+            .filter { number shr it.ordinal and 1 == 1 }
+
+        return if (number shr 4 and 1 == 1) signals.asReversed()
+        else signals
     }
-}
+}
\ No newline at end of file
diff --git a/exercises/practice/triangle/src/main/kotlin/Triangle.kt b/exercises/practice/triangle/src/main/kotlin/Triangle.kt
index 91790f9d..af110fef 100644
--- a/exercises/practice/triangle/src/main/kotlin/Triangle.kt
+++ b/exercises/practice/triangle/src/main/kotlin/Triangle.kt
@@ -1,8 +1,16 @@
+import java.lang.IllegalArgumentException
+
 class Triangle<out T : Number>(val a: T, val b: T, val c: T) {
 
-    // TODO: Implement proper constructor
+    init {
+        require(a > 0 && b > 0 && c > 0) { "Sides must be > 0" }
+        require(a + b >= c && b + c >= a && c + a >= b) { "Sides must satisfy triangle inequality" }
+    }
+
+    val isEquilateral: Boolean = a == b && b == c
+    val isIsosceles: Boolean = a == b || b == c || a == c
+    val isScalene: Boolean = a != b && b != c && a != c
 
-    val isEquilateral: Boolean = TODO("Implement this getter to complete the task")
-    val isIsosceles: Boolean = TODO("Implement this getter to complete the task")
-    val isScalene: Boolean = TODO("Implement this getter to complete the task")
+    private infix operator fun <T: Number> T.compareTo(other: T): Int = this.toDouble().compareTo(other.toDouble())
+    private infix operator fun <T: Number> T.plus(b: T): Double = this.toDouble().plus(b.toDouble())
 }
diff --git a/exercises/practice/two-fer/src/main/kotlin/TwoFer.kt b/exercises/practice/two-fer/src/main/kotlin/TwoFer.kt
index b0496de0..32b9c6fb 100644
--- a/exercises/practice/two-fer/src/main/kotlin/TwoFer.kt
+++ b/exercises/practice/two-fer/src/main/kotlin/TwoFer.kt
@@ -1,3 +1 @@
-fun twofer(name: String): String {
-    TODO("Implement the function to complete the task")
-}
+fun twofer(name: String? = null) = "One for ${name ?: "you"}, one for me."