Add exercise: rotational-cipher (JuliaLang#44)

* dibs: I will implement exercise rotational-cipher

* Add exercise: rotational-cipher

* Added rotational-cipher to config

* rotational-cipher: test_skip instead of comment

* rotational-cipher: revert to comment

* Update HINTS-formatting

Author: SaschaMann

Diff for: config.json

@@ -215,6 +215,15 @@
         "mathematics"
       ]
     },
+    {
+      "slug": "rotational-cipher",
+      "difficulty": 2,
+      "topics": [
+        "string literals",
+        "metaprogramming",
+        "strings"
+      ]
+    },
     {
       "slug": "custom-set",
       "difficulty": 5,

Diff for: exercises/rotational-cipher/HINTS.md

@@ -0,0 +1,21 @@
+This is a good exercise to experiment with non-standard string literals and metaprogramming.
+
+A short introduction to non-standard string literals can be found in this [blog post](http://iaindunning.com/blog/julia-unicode.html). A detailed metaprogramming guide can be found in the [manual](http://docs.julialang.org/en/stable/manual/metaprogramming/).
+
+You can extend your solution by adding the functionality described below. To test your solution, you have to remove the comments at the end of `runtests.jl` before running the tests as usual.
+
+Bonus A only requires basics as outlined in the blog post. Bonus B requires significantly more knowledge of metaprogramming in Julia.
+
+## Bonus A
+Implement a string literal that acts as `ROT13` on the string:
+```julia
+R13"abcdefghijklmnopqrstuvwxyz" == "nopqrstuvwxyzabcdefghijklm"
+```
+
+## Bonus B
+Implement string literals `R<i>`, `i = 0, ..., 26`, that shift the string for `i` values:
+```julia
+R0"Hello, World!" == "Hello, World!"
+R4"Testing 1 2 3 testing" == "Xiwxmrk 1 2 3 xiwxmrk"
+R13"abcdefghijklmnopqrstuvwxyz" == "nopqrstuvwxyzabcdefghijklm"
+```

Diff for: exercises/rotational-cipher/example.jl

@@ -0,0 +1,16 @@
+function rotate(n::Int, c::Char)
+    if c in 'a':'z'
+        c = 'a' + (c - 'a' + n) % 26
+    elseif c in 'A':'Z'
+        c = 'A' + (c - 'A' + n) % 26
+    end
+    return c
+end
+
+rotate(n::Int, s::String) = join(rotate(n, c) for c in s)
+
+for n in 0:26
+    eval( :(macro $(Symbol(:R, n, :_str))(s::String)
+        :(rotate($$n, $s))
+    end))
+end

Diff for: exercises/rotational-cipher/rotational-cipher.jl

@@ -0,0 +1,51 @@
+using Base.Test
+
+include("rotational-cipher.jl")
+
+@testset "rotate function" begin
+    @testset "rotate by n" begin
+        @testset "no wrap" begin
+            @test rotate(1, "a") == "b"
+            @test rotate(1, 'a') == 'b'
+            @test rotate(13, "m") == "z"
+            @test rotate(13, 'm') == 'z'
+        end
+        @testset "wrap around" begin
+            @test rotate(13, "n") == "a"
+            @test rotate(13, 'n') == 'a'
+        end
+    end
+
+    @testset "full rotation" begin
+        @test rotate(26, "a") == "a"
+        @test rotate(26, 'a') == 'a'
+        @test rotate(0, "a") == "a"
+        @test rotate(0, 'a') == 'a'
+    end
+
+    @testset "full strings" begin
+        @test rotate(5, "OMG") == "TRL"
+        @test rotate(5, "O M G") == "T R L"
+        @test rotate(4, "Testing 1 2 3 testing") == "Xiwxmrk 1 2 3 xiwxmrk"
+        @test rotate(21, "Let's eat, Grandma!") == "Gzo'n zvo, Bmviyhv!"
+        @test rotate(13, "The quick brown fox jumps over the lazy dog.") == "Gur dhvpx oebja sbk whzcf bire gur ynml qbt."
+    end
+end
+
+# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
+# Additional exercises                                                #
+# Remove the comments for the optional bonus exercises from HINTS.md  #
+# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
+
+# Bonus A
+# @testset "string literal R13" begin
+#     @test R13"The quick brown fox jumps over the lazy dog." == "Gur dhvpx oebja sbk whzcf bire gur ynml qbt."
+# end
+
+# Bonus B
+# @testset "string literals" begin
+#     @test R5"OMG" == "TRL"
+#     @test R4"Testing 1 2 3 testing" == "Xiwxmrk 1 2 3 xiwxmrk"
+#     @test R21"Let's eat, Grandma!" == "Gzo'n zvo, Bmviyhv!"
+#     @test R13"The quick brown fox jumps over the lazy dog." == "Gur dhvpx oebja sbk whzcf bire gur ynml qbt."
+# end