fix: links updated

Author: danil-pavlov

Builders/Builders implementation/task.md

@@ -3,4 +3,4 @@
 Complete the implementation of a simplified DSL for HTML.
 Implement `tr` and `td` functions.
 
-Learn more about [type-safe builders](https://kotlinlang.org/docs/reference/type-safe-builders.html#type-safe-builders).
+Learn more about [type-safe builders](https://kotlinlang.org/docs/type-safe-builders.html#type-safe-builders).

Builders/Function literals with receiver/task.md

@@ -1,6 +1,6 @@
 ## Function literals with receiver
 
-Learn about [function literals with receiver](https://kotlinlang.org/docs/reference/lambdas.html#function-literals-with-receiver).
+Learn about [function literals with receiver](https://kotlinlang.org/docs/lambdas.html#function-literals-with-receiver).
 
 You can declare `isEven` and `isOdd` as values, that can be called as extension functions.
 Complete the declarations in the code.

Builders/The function apply/task.md

@@ -4,5 +4,5 @@ The previous examples can be rewritten using the library function
 [`apply`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/apply.html).
 Write your own implementation of this function named `myApply`.
 
-Learn about the other [scope functions](https://kotlinlang.org/docs/reference/scope-functions.html)
+Learn about the other [scope functions](https://kotlinlang.org/docs/scope-functions.html)
 and how to use them.

Classes/Data classes/task.md

@@ -1,8 +1,8 @@
 ## Data classes
 
-Learn about [classes](https://kotlinlang.org/docs/reference/classes.html),
-[properties](https://kotlinlang.org/docs/reference/properties.html)
-and [data classes](https://kotlinlang.org/docs/reference/data-classes.html)
+Learn about [classes](https://kotlinlang.org/docs/classes.html),
+[properties](https://kotlinlang.org/docs/properties.html)
+and [data classes](https://kotlinlang.org/docs/data-classes.html)
 and then rewrite the following Java code to Kotlin:
 
 ```java

Classes/Extension functions/task.md

@@ -1,6 +1,6 @@
 ## Extension functions
 
-Learn about [extension functions](https://kotlinlang.org/docs/reference/extensions.html).
+Learn about [extension functions](https://kotlinlang.org/docs/extensions.html#extension-functions).
 Then implement the extension functions `Int.r()` and `Pair.r()` and make them convert `Int` and `Pair` to a `RationalNumber`.
 
 `Pair` is a class defined in the standard library:

Classes/Rename on import/task.md

@@ -1,7 +1,7 @@
 ## Rename on import
 
-When you [import](https://kotlinlang.org/docs/reference/packages.html#imports)
-a class or a function, you can specify a different name to use for it
+When you [import](https://kotlinlang.org/docs/packages.html#imports)
+a class or a function, you can specify a different name for it
 by adding `as NewName` after the import directive.
 This can be useful if you want to use two classes or functions with similar names
 from different libraries.

Classes/Sealed classes/task.md

@@ -1,5 +1,5 @@
 ## Sealed classes
 
 Reuse your solution from the previous task, but replace the interface
-with the [`sealed` class](https://kotlinlang.org/docs/reference/sealed-classes.html).
+with the [`sealed` class](https://kotlinlang.org/docs/sealed-classes.html).
 Then you no longer need the `else` branch in `when`. 

Classes/Smart casts/task.md

@@ -1,7 +1,7 @@
 ## Smart casts
 
-Rewrite the following Java code using [smart casts](https://kotlinlang.org/docs/reference/typecasts.html#smart-casts)
-and the [when](https://kotlinlang.org/docs/reference/control-flow.html#when-expression) expression:
+Rewrite the following Java code using [smart casts](https://kotlinlang.org/docs/typecasts.html#smart-casts)
+and the [when](https://kotlinlang.org/docs/control-flow.html#when-expression) expression:
 
 ```java
 public int eval(Expr expr) {

Collections/All Any and other predicates/task.md

@@ -1,7 +1,7 @@
 ## All, Any, and other predicates
 
-Learn about [testing predicates](https://kotlinlang.org/docs/reference/collection-filtering.html#testing-predicates)
-and [retrieving elements by condition](https://kotlinlang.org/docs/reference/collection-elements.html#retrieving-by-condition).
+Learn about [testing predicates](https://kotlinlang.org/docs/collection-filtering.html#test-predicates)
+and [retrieving elements by condition](https://kotlinlang.org/docs/collection-elements.html#retrieve-by-condition).
 
 Implement the following functions using
 [`all`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/all.html),

Collections/Associate/task.md

@@ -1,6 +1,6 @@
 ## Associate
 
-Learn about [association](https://kotlinlang.org/docs/reference/collection-transformations.html#associate).
+Learn about [association](https://kotlinlang.org/docs/collection-transformations.html#associate).
 Implement the following functions using 
 [`associateBy`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/associate-by.html),
 [`associateWith`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/associate-with.html), 

Collections/Filter map/task.md

@@ -1,7 +1,7 @@
 ## Filter; map
 
-Learn about [mapping](https://kotlinlang.org/docs/reference/collection-transformations.html#mapping) and 
-[filtering](https://kotlinlang.org/docs/reference/collection-filtering.html#filtering-by-predicate) a collection.
+Learn about [mapping](https://kotlinlang.org/docs/collection-transformations.html#mapping) and 
+[filtering](https://kotlinlang.org/docs/collection-filtering.html#filtering-by-predicate) a collection.
 
 Implement the following extension functions
 using the

Collections/FlatMap/task.md

@@ -1,6 +1,6 @@
 ## FlatMap
 
-Learn about [flattening](https://kotlinlang.org/docs/reference/collection-transformations.html#flattening)
+Learn about [flattening](https://kotlinlang.org/docs/collection-transformations.html#flatten)
 and implement two functions using
 [`flatMap`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/flat-map.html):
 - The first should return all products the given customer has ordered

Collections/Fold/task.md

@@ -1,6 +1,6 @@
 ## Fold
 
-Learn about [fold and reduce](https://kotlinlang.org/docs/reference/collection-aggregate.html#fold-and-reduce)
+Learn about [fold and reduce](https://kotlinlang.org/docs/collection-aggregate.html#fold-and-reduce)
 and implement a function that returns the set of products that were ordered by all the customers using
 [`fold`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/fold.html).
 

Collections/GroupBy/task.md

@@ -1,6 +1,6 @@
 ## Group By
 
-Learn about [grouping](https://kotlinlang.org/docs/reference/collection-grouping.html).
+Learn about [grouping](https://kotlinlang.org/docs/collection-grouping.html).
 Use
 [`groupBy`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/group-by.html)
 to implement the function to build a map that stores the customers living in a given city.

Collections/Introduction/task.md

@@ -4,7 +4,7 @@ This section was inspired by [GS Collections Kata](https://github.com/goldmansac
 
 Kotlin can be easily mixed with Java code.
 Default collections in Kotlin are all Java collections under the hood.
-Learn about [read-only and mutable views on Java collections](https://kotlinlang.org/docs/reference/collections-overview.html#collection-types).
+Learn about [read-only and mutable views on Java collections](https://kotlinlang.org/docs/collections-overview.html#collection-types).
 
 The [Kotlin standard library](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/)
 contains lots of extension functions that make working with collections more convenient.

Collections/Max min/task.md

@@ -1,6 +1,6 @@
 ## Max min
 
-Learn about [Collection Aggregate Operations](https://kotlinlang.org/docs/reference/collection-aggregate.html).
+Learn about [collection aggregate operations](https://kotlinlang.org/docs/collection-aggregate.html).
 
 Implement two functions:
 - the first should return the customer who has placed the most amount of orders in this shop 
@@ -18,7 +18,7 @@ listOf(1, 42, 4).maxOrNull() == 42
 listOf("a", "ab").minByOrNull(String::length) == "a"
 ```
 
-You can use [callable references](https://kotlinlang.org/docs/reference/lambdas.html#instantiating-a-function-type)
+You can use [callable references](https://kotlinlang.org/docs/lambdas.html#instantiating-a-function-type)
 instead of lambdas. This can be especially helpful in call chains, where
 `it` occurs in different lambdas and has different types.
 Implement the `getMostExpensiveProductBy` function using callable references.  

Collections/Partition/task.md

@@ -1,7 +1,7 @@
 ## Partition
 
-Learn about [partitioning](https://kotlinlang.org/docs/reference/collection-filtering.html#partitioning)
-and the [destructuring declaration](https://kotlinlang.org/docs/reference/multi-declarations.html)
+Learn about [partitioning](https://kotlinlang.org/docs/collection-filtering.html#partition)
+and the [destructuring declaration](https://kotlinlang.org/docs/destructuring-declarations.html)
 syntax that is often used together with `partition`.
 
 Then implement a function for returning customers who have more undelivered orders

Collections/Sequences/task.md

@@ -1,6 +1,6 @@
 ## Sequences
 
-Learn about [sequences](https://kotlinlang.org/docs/reference/sequences.html),
+Learn about [sequences](https://kotlinlang.org/docs/sequences.html),
 they allow you to perform operations lazily rather than eagerly.
 Copy the implementation from the previous task and modify it in a way
 that the operations on sequences are used. 

Collections/Sort/task.md

@@ -1,9 +1,9 @@
 ## Sort
 
 Learn about
-[collection ordering](https://kotlinlang.org/docs/reference/collection-ordering.html)
+[collection ordering](https://kotlinlang.org/docs/collection-ordering.html)
 and the
-[the difference](https://kotlinlang.org/docs/reference/collection-operations.html#write-operations)
+[the difference](https://kotlinlang.org/docs/collection-operations.html#write-operations)
 between operations in-place on mutable collections and operations returning new collections. 
 
 Implement a function for returning the list of customers,

Collections/Sum/task.md

@@ -6,11 +6,10 @@ Note that each product should be counted as many times as it was ordered.
 
 Use
 [`sum`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/sum.html) or
-[`sumBy`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/sum-by.html).
-If you want to sum up double values, use
-[`sumByDouble`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/sum-by-double.html).
+[`sumOf`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/sum-of.html).
+If you want to sum up double values, use `sumOfDouble`.
 
 ```kotlin
 listOf(1, 5, 3).sum() == 9
-listOf("a", "b", "cc").sumBy { it.length } == 4
+listOf("a", "b", "cc").sumOf { it.length } == 4
 ```

Conventions/Comparison/task.md

@@ -1,10 +1,10 @@
 ## Comparison
 
-Learn about [operator overloading](https://kotlinlang.org/docs/reference/operator-overloading.html)
+Learn about [operator overloading](https://kotlinlang.org/docs/operator-overloading.html)
 and how the different conventions for operations like `==`, `<`, `+` work in Kotlin.
 Add the function `compareTo` to the class `MyDate` to make it comparable.
 After this the code below `date1 < date2` should start to compile.
 
 Note that when you override a member in Kotlin, the
-[`override`](https://kotlinlang.org/docs/reference/classes.html#overriding-members)
+[`override`](https://kotlinlang.org/docs/inheritance.html#overriding-methods)
 modifier is mandatory.

Conventions/For loop/task.md

@@ -1,12 +1,12 @@
 ## For loop
 
-A Kotlin [for loop](https://kotlinlang.org/docs/reference/control-flow.html#for-loops)
+A Kotlin [for loop](https://kotlinlang.org/docs/control-flow.html#for-loops)
 can iterate through any object if the corresponding `iterator` member or extension function is available.
 
 Make the class `DateRange` implement [`Iterable<MyDate>`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-iterable/),
 so that it can be iterated over.
 Use the function `MyDate.followingDate()` defined in `DateUtil.kt`;
 you don't have to implement the logic for finding the following date on your own.
 
-Use an [object expression](https://kotlinlang.org/docs/reference/object-declarations.html)
+Use an [object expression](https://kotlinlang.org/docs/object-declarations.html#object-expressions)
 which plays the same role in Kotlin as an anonymous class in Java. 

Conventions/Invoke/task.md

@@ -1,6 +1,6 @@
 ## Invoke
 
-Objects with an [`invoke()`](https://kotlinlang.org/docs/reference/operator-overloading.html#invoke)
+Objects with an [`invoke()`](https://kotlinlang.org/docs/operator-overloading.html#invoke-operator)
 method can be invoked as a function.
 
 You can add an `invoke` extension for any class, but it's better not to overuse it:

Conventions/Ranges/task.md

@@ -1,13 +1,13 @@
 ## Ranges
 
-Using [ranges](https://kotlinlang.org/docs/reference/ranges.html)
+Using [ranges](https://kotlinlang.org/docs/ranges.html)
 implement a function that checks whether the date is in the range
 between the first date and the last date (inclusive).
 
 You can build a range of any comparable elements.
-In Kotlin [`in` checks](https://kotlinlang.org/docs/reference/operator-overloading.html#in)
+In Kotlin [`in` checks](https://kotlinlang.org/docs/operator-overloading.html#in-operator)
 are translated to the corresponding `contains` calls
-and [`..`](https://kotlinlang.org/docs/reference/operator-overloading.html#arithmetic)
+and [`..`](https://kotlinlang.org/docs/operator-overloading.html#arithmetic-operators)
 to `rangeTo` calls:
 
 ```kotlin

Generics/Generic functions/task.md

@@ -1,6 +1,6 @@
 ## Generic functions
 
-Learn about [generic functions](https://kotlinlang.org/docs/reference/generics.html#generic-functions).
+Learn about [generic functions](https://kotlinlang.org/docs/generics.html#generic-functions).
 Make the code compile by implementing a `partitionTo` function that splits
 a collection into two collections according to the predicate.
 

Introduction/Default arguments/src/Task.kt

@@ -1,9 +1,9 @@
-fun foo(name: String, number: Int = 42, toUpperCase: Boolean = false) =
-        (if (toUpperCase) name.toUpperCase() else name) + number
+fun foo(name: String, number: Int = 42, upperCase: Boolean = false) =
+        (if (upperCase) name.uppercase() else name) + number
 
 fun useFoo() = listOf(
         foo("a"),
         foo("b", number = 1),
-        foo("c", toUpperCase = true),
-        foo(name = "d", number = 2, toUpperCase = true)
+        foo("c", upperCase = true),
+        foo(name = "d", number = 2, upperCase = true)
 )

Introduction/Default arguments/task-info.yaml

@@ -4,7 +4,7 @@ files:
   visible: true
   placeholders:
   - offset: 0
-    length: 69
+    length: 67
     placeholder_text: 'fun foo(name: String, number: Int, toUpperCase: Boolean)'
 - name: test/tests.kt
   visible: false

Introduction/Default arguments/task.md

@@ -19,4 +19,4 @@ public String foo(String name) {
 
 You can replace all these Java overloads with one function in Kotlin.
 Change the declaration of the `foo` function in a way that makes the code using `foo` compile.
-Use [default and named](https://kotlinlang.org/docs/reference/functions.html#default-arguments) arguments.
+Use [default and named](https://kotlinlang.org/docs/functions.html#default-arguments) arguments.

Introduction/Hello, world!/task.md

@@ -1,6 +1,6 @@
 ## Simple Functions
 
-Check out the [function syntax](https://kotlinlang.org/docs/reference/basic-syntax.html#defining-functions)
+Check out the [function syntax](https://kotlinlang.org/docs/basic-syntax.html#defining-functions)
 and change the code to make the function `start` return the string `"OK"`.
 
 In the Kotlin Koans tasks the function `TODO()` will throw an exception.

Introduction/Lambdas/task.md

@@ -1,7 +1,7 @@
 ## Lambdas
 
 Kotlin supports functional programming.
-Learn about [lambdas](https://kotlinlang.org/docs/reference/lambdas.html#lambda-expressions-and-anonymous-functions) in Kotlin.
+Learn about [lambdas](https://kotlinlang.org/docs/lambdas.html#lambda-expressions-and-anonymous-functions) in Kotlin.
 
 Pass a lambda to the [`any`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/any.html)
 function to check if the collection contains an even number.

Introduction/Named arguments/task.md

@@ -3,7 +3,7 @@
 Make the function `joinOptions()` return the list in a JSON format (e.g., `[a, b, c]`)
 by specifying only two arguments. 
 
-[Default and named](https://kotlinlang.org/docs/reference/functions.html#default-arguments)
+[Default and named](https://kotlinlang.org/docs/functions.html#default-arguments)
 arguments help to minimize the number of overloads and improve the readability of the function invocation.
 The library function [`joinToString`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/join-to-string.html)
 is declared with default values for parameters:

Introduction/Nothing type/task.md

@@ -1,8 +1,8 @@
 ## Nothing type
 
-[Nothing type](https://kotlinlang.org/docs/reference/exceptions.html#the-nothing-type)
+[Nothing type](https://kotlinlang.org/docs/exceptions.html#the-nothing-type)
 can be used as a return type for a function that always throws an exception.
-When you call such a function, the compiler uses the information that it throws an exception.
+When you call such a function, the compiler uses the information that the execution doesn't continue beyond the function.
 
 Specify `Nothing` return type for the `failWithWrongAge` function.
 Note that without the `Nothing` type the `checkAge` function doesn't compile

Introduction/Nullable types/task.md

@@ -1,7 +1,7 @@
 ## Nullable types
 
 Learn about
-[null safety and safe calls](https://kotlinlang.org/docs/reference/null-safety.html)
+[null safety and safe calls](https://kotlinlang.org/docs/null-safety.html)
 in Kotlin and rewrite the following Java code so that it only has one `if` expression:
 
 ```java

Introduction/Triple-quoted strings/task.md

@@ -1,6 +1,6 @@
 ## Triple-quoted strings
 
-Learn about the [different string literals and string templates](https://kotlinlang.org/docs/reference/basic-types.html#string-literals)
+Learn about the [different string literals and string templates](https://kotlinlang.org/docs/basic-types.html#string-literals)
 in Kotlin.
 
 You can use the handy library functions 

Properties/Delegates examples/task.md

@@ -1,4 +1,4 @@
 ## Delegates example
 
-Learn about [delegated properties](https://kotlinlang.org/docs/reference/delegated-properties.html)
+Learn about [delegated properties](https://kotlinlang.org/docs/delegated-properties.html)
 and make the property lazy using delegates.

Properties/Delegates how it works/task.md

@@ -1,6 +1,6 @@
 ## Delegates
 
-You can declare your own [delegates](https://kotlinlang.org/docs/reference/delegated-properties.html#property-delegate-requirements).
+You can declare your own [delegates](https://kotlinlang.org/docs/delegated-properties.html#property-delegate-requirements).
 Implement the methods of the class `EffectiveDate` so you can delegate to it.
 Store only the time in milliseconds in the `timeInMillis` property.
 

Properties/Properties/task.md

@@ -1,6 +1,6 @@
 ## Properties
 
-Learn about the [properties](https://kotlinlang.org/docs/reference/properties.html) in Kotlin.
+Learn about the [properties](https://kotlinlang.org/docs/properties.html) in Kotlin.
 
 Add a custom setter to `PropertyExample.propertyWithCounter` so that
 the `counter` property is incremented every time the `propertyWithCounter` is assigned. 

build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-    ext.kotlin_version = '1.4.10'
+    ext.kotlin_version = '1.5.10'
 
     repositories {
         mavenCentral()

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 #Wed Jun 26 18:26:37 CEST 2019
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.3-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.1.1-all.zip
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStorePath=wrapper/dists