Implement Grains exercise (JuliaLang#89)

* Implement Grains exercise

Contributes to exercism/julia#87. Loosely based on Python
implementation.

* Revert to pre-0.7 feature set

In v0.6-, `DomainError` takes no arguments.

* Add exercise to config.json

Update README according to specification.

* Add exercise dependency and fix README

* Fix bug and add backwards compatibility

config.json

@@ -2,8 +2,7 @@
   "language": "Julia",
   "active": true,
   "blurb": "Julia is an open-source high-level, dynamic programming language whose sweet spot is technical and scientific computing. It is convenient for day-to-day work and fast enough for high performance computing.",
-  "exercises": [
-    {
+  "exercises": [{
       "uuid": "a668410d-41aa-4710-a68f-54521da6486d",
       "slug": "hello-world",
       "core": true,
@@ -358,6 +357,16 @@
         "mathematics",
         "iterators"
       ]
+    },
+    {
+      "uuid": "2ed6e171-04e1-b080-f383-a801b2564b3e0f4f1ee",
+      "slug": "grains",
+      "core": false,
+      "unlocked_by": "difference-of-squares",
+      "difficulty": 1,
+      "topics": [
+        "exceptions"
+      ]
     }
   ],
   "foregone": [

exercises/grains/README.md

@@ -0,0 +1,33 @@
+# Grains
+
+Calculate the number of grains of wheat on a chessboard given that the number
+on each square doubles.
+
+There once was a wise servant who saved the life of a prince. The king
+promised to pay whatever the servant could dream up. Knowing that the
+king loved chess, the servant told the king he would like to have grains
+of wheat. One grain on the first square of a chess board. Two grains on
+the next. Four on the third, and so on.
+
+There are 64 squares on a chessboard.
+
+Write code that shows:
+- how many grains were on each square (`on_square`)
+- the total number of grains (`total_after`)
+
+## For bonus points
+
+Did you get the tests passing and the code clean? If you want to, these
+are some additional things you could try:
+
+- Optimize for speed.
+- Optimize for readability.
+
+Then please share your thoughts in a comment on the submission. Did this
+experiment make the code better? Worse? Did you learn anything from it?
+
+## Source
+JavaRanch Cattle Drive, exercise 6 [http://www.javaranch.com/grains.jsp](http://www.javaranch.com/grains.jsp)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/grains/example.jl

@@ -0,0 +1,23 @@
+"""Calculate the number of grains on square `square`."""
+function on_square(square)
+    check_square_input(square)
+    2 ^ (square - 1)
+end
+
+"""Calculate the total number of grains after square `square`."""
+function total_after(square)
+    check_square_input(square)
+    sum(map(on_square, 1:square))
+end
+
+"""Validate an arbitrary square."""
+function check_square_input(square)
+    square == 0 && throw(DomainError(square, "Square input of zero is invalid."))
+    square <  0 && throw(DomainError(square, "Negative square input is invalid."))
+    square > 64 && throw(DomainError(square, "Square input greater than 64 is invalid."))
+end
+
+if VERSION < v"0.7"  # backwards compatibility
+    Base.DomainError(val) = DomainError()
+    Base.DomainError(val, msg) = DomainError()
+end

exercises/grains/grains.jl

@@ -0,0 +1,9 @@
+"""Calculate the number of grains on square `square`."""
+function on_square(square)
+
+end
+
+"""Calculate the total number of grains after square `square`."""
+function total_after(square)
+
+end

exercises/grains/runtests.jl

@@ -0,0 +1,27 @@
+using Base.Test
+
+include("grains.jl")
+
+@testset "On squares" begin
+    @testset "On square $s" for s = UInt64(1):64
+        @test on_square(s) == 2^(s-1)
+        @test total_after(s) == 2^s - 1
+    end
+end
+
+@testset "Invalid values" begin
+    @testset "Zero" begin
+        @test_throws DomainError on_square(0)
+        @test_throws DomainError total_after(0)
+    end
+
+    @testset "Negative" begin
+        @test_throws DomainError on_square(-1)
+        @test_throws DomainError total_after(-1)
+    end
+
+    @testset "Greater than 64" begin
+        @test_throws DomainError on_square(65)
+        @test_throws DomainError total_after(65)
+    end
+end