Initial version

Author: NoHomey

Package.swift

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+import PackageDescription
+
+let package = Package(
+    name: "swift-array-variations"
+)

Sources/swift_array_variations.swift

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+internal func strike(_ array: [Int], from: [Int]) -> [Int] {
+    var result: Array<Int> = []
+
+    for element in from {
+        if array.index(of: element) == nil {
+            result.append(element)
+        }
+    }
+
+    return result
+}
+
+extension Array where Element: Equatable {
+    func variations(class count: Int) -> [[Element]] {
+        let length = ((self.count - count + 1)...self.count).reduce(1, { a, b in a * b })
+        let indexes = Array<Int>(0..<self.count)
+        var repeats = Array<Int>(repeating: length / self.count, count: count)
+        var divisor = self.count
+        for i in 1..<count {
+            divisor -= 1
+            repeats[i] = repeats[i - 1] / divisor
+        }
+        var result = Array<Array<Int>>(repeating: [], count: length)
+        var k = 0
+        for i in 0..<count {
+            k = 0
+            while k < length {
+                for number in strike(result[k], from: indexes) {
+                    for _ in 0..<repeats[i] {
+                        result[k].append(number)
+                        k += 1
+                    }
+                }
+            }
+        }
+
+        return result.map { variation in variation.map { element in self[element] } }
+    }
+}

Tests/LinuxMain.swift

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+import XCTest
+@testable import swift_array_variationsTests
+
+XCTMain([
+     testCase(swift_array_variationsTests.allTests),
+])

Tests/swift-array-variationsTests/swift_array_variationsTests.swift

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+import XCTest
+@testable import swift_array_variations
+
+class swift_array_variationsTests: XCTestCase {
+    func test_3_2_Ints() {
+        let expect = [[0, 1], [0, 2], [1, 0], [1, 2], [2, 0], [2, 1]]
+        let result = Array<Int>(0...2).variations(class: 2)
+
+        XCTAssertEqual(result.count, 6)
+        XCTAssertEqual(result.count, expect.count)
+
+        for i in 0..<6 {
+            XCTAssertEqual(result[i].count, 2)
+            for j in 0...1 {
+                XCTAssertEqual(result[i][j], expect[i][j])
+            }
+        }
+    }
+
+    func test_3_3_String() {
+        let expect = [
+            ["test", "unit-test", "XCTest"],
+            ["test", "XCTest", "unit-test"],
+            ["unit-test", "test", "XCTest"],
+            ["unit-test", "XCTest", "test"],
+            ["XCTest", "test", "unit-test"],
+            ["XCTest", "unit-test", "test"]
+        ]
+        let result = ["test", "unit-test", "XCTest"].variations(class: 3)
+
+        XCTAssertEqual(result.count, 6)
+        XCTAssertEqual(result.count, expect.count)
+
+        for i in 0..<6 {
+            XCTAssertEqual(result[i].count, 3)
+            for j in 0...1 {
+                XCTAssertEqual(result[i][j], expect[i][j])
+            }
+        }
+    }
+
+    func test_10_6_Ints() {
+        let result = Array<Int>(1...10).variations(class: 6)
+
+        XCTAssertEqual(result.count, 151200)
+
+        for i in 0..<151200 {
+            XCTAssertEqual(result[i].count, 6)
+        }
+    }
+
+    static var allTests : [(String, (swift_array_variationsTests) -> () throws -> Void)] {
+        return [
+            ("test 2-permutation of [0, 1, 2]", test_3_2_Ints),
+            ("test 3-permutation of [\"test\", \"unit-test\", \"XCTest\"]", test_3_3_String),
+            ("test 6-permutation of 10", test_10_6_Ints),
+        ]
+    }
+}