reimplementation to include all the tests: use string addition

config.json

@@ -185,7 +185,12 @@
         "uuid": "99408193-d0ce-4c68-8cab-e680f3ed56a4",
         "practices": [],
         "prerequisites": [],
-        "difficulty": 2
+        "difficulty": 5,
+        "topics": [
+          "math",
+          "strings",
+          "loops"
+        ]
       },
       {
         "slug": "hello-world",

exercises/practice/grains/.docs/instructions.append.md

@@ -6,6 +6,7 @@ According to [the Vim docs][number]:
 
 That means that Vim cannot express any number `2^63` or greater as an integer.
 
-For the purposes of this exercise, we will declare that in Vimland, chess boards are 7x7 and have **49** squares.
+For this exercise, you will return the values as strings.
+That means you will have to implement a way to do addition on two _string_ operands.
 
 [number]: https://vimhelp.org/eval.txt.html#expr-number

exercises/practice/grains/.meta/config.json

@@ -1,6 +1,7 @@
 {
   "authors": [
-    "BNAndras"
+    "BNAndras",
+    "glennj"
   ],
   "files": {
     "solution": [

exercises/practice/grains/.meta/example.vim

@@ -1,11 +1,39 @@
+" add two strings as numbers from right to left, as if by hand
+function! StringAdd(a, b) abort
+  " zero-left-pad so the numbers are the same length
+  let len = [a:a->strlen(), a:b->strlen()]->max()
+  let a = printf('%0*s', len, a:a)
+  let b = printf('%0*s', len, a:b)
+  let result = ""
+  let carry = 0
+
+  for i in range(len - 1, 0, -1)
+    let c = carry + a->strpart(i, 1)->str2nr() + b->strpart(i, 1)->str2nr()
+    let result = $"{c % 10}{result}"
+    let carry = c / 10
+  endfor
+  return $"{carry}{result}"->trim('0', 1)
+endfunction
+
+" populate the squares, doubling each previous, cache in the script scope
+let s:grains = ['1']
+for i in range(1, 63)
+  eval s:grains->add(StringAdd(s:grains[i - 1], s:grains[i - 1]))
+endfor
+
+" return the grains on square `number`
 function! Square(number) abort
-  if a:number < 1 || a:number > 49
-    throw 'square must be between 1 and 49'
+  if a:number < 1 || a:number > 64
+    throw 'square must be between 1 and 64'
   endif
-
-  return float2nr(pow(2, (a:number-1)))
+  return s:grains[a:number - 1]
 endfunction
 
+" return the total number of grains
 function! Total() abort
-  return float2nr(pow(2, 49) - 1)
+  let total = '0'
+  for i in range(64)
+    let total = StringAdd(total, s:grains[i])
+  endfor
+  return total
 endfunction

exercises/practice/grains/.meta/tests.toml

@@ -29,7 +29,6 @@ description = "returns the number of grains on the square -> grains on square 32
 
 [c73b470a-5efb-4d53-9ac6-c5f6487f227b]
 description = "returns the number of grains on the square -> grains on square 64"
-include = false
 
 [1d47d832-3e85-4974-9466-5bd35af484e3]
 description = "returns the number of grains on the square -> square 0 is invalid"
@@ -39,8 +38,6 @@ description = "returns the number of grains on the square -> negative square is
 
 [a95e4374-f32c-45a7-a10d-ffec475c012f]
 description = "returns the number of grains on the square -> square greater than 64 is invalid"
-include = false
 
 [6eb07385-3659-4b45-a6be-9dc474222750]
 description = "returns the total number of grains on the board"
-include = false

exercises/practice/grains/grains.vader

@@ -1,57 +1,57 @@
 
 Execute (grains on square 1):
   let g:square = 1
-  let g:expected = 1
+  let g:expected = "1"
   AssertEqual g:expected, Square(g:square)
 
 Execute (grains on square 2):
   let g:square = 2
-  let g:expected = 2
+  let g:expected = "2"
   AssertEqual g:expected, Square(g:square)
 
 Execute (grains on square 3):
   let g:square = 3
-  let g:expected = 4
+  let g:expected = "4"
   AssertEqual g:expected, Square(g:square)
 
 Execute (grains on square 4):
   let g:square = 4
-  let g:expected = 8
+  let g:expected = "8"
   AssertEqual g:expected, Square(g:square)
 
 Execute (grains on square 16):
   let g:square = 16
-  let g:expected = 32768
+  let g:expected = "32768"
   AssertEqual g:expected, Square(g:square)
 
 Execute (grains on square 32):
   let g:square = 32
-  let g:expected = 2147483648
+  let g:expected = "2147483648"
   AssertEqual g:expected, Square(g:square)
 
-Execute (grains on square 49):
-  let g:square = 49
-  let g:expected = 281474976710656
+Execute (grains on square 64):
+  let g:square = 64
+  let g:expected = "9223372036854775808"
   AssertEqual g:expected, Square(g:square)
 
 Execute (square 0 is invalid):
   let g:square = 0
-  let g:expected = "square must be between 1 and 49"
+  let g:expected = "square must be between 1 and 64"
   AssertThrows call Square(g:square)
   AssertEqual g:expected, g:vader_exception
 
 Execute (negative square is invalid):
   let g:square = -1
-  let g:expected = "square must be between 1 and 49"
+  let g:expected = "square must be between 1 and 64"
   AssertThrows call Square(g:square)
   AssertEqual g:expected, g:vader_exception
 
-Execute (square greater than 49 is invalid):
-  let g:square = 50
-  let g:expected = "square must be between 1 and 49"
+Execute (square greater than 64 is invalid):
+  let g:square = 65
+  let g:expected = "square must be between 1 and 64"
   AssertThrows call Square(g:square)
   AssertEqual g:expected, g:vader_exception
 
-Execute (returns the total number of grains on the board for 49 squares):
-  let g:expected = 562949953421311
+Execute (returns the total number of grains on the board):
+  let g:expected = "18446744073709551615"
   AssertEqual g:expected, Total()

exercises/practice/grains/grains.vim

@@ -1,22 +1,22 @@
 "
-" Returns the number of grains on a chessboard square given the grains on each square
-" double from the previous square. 
-" Throws an error if the square is below 1 or above 49.
+" Returns the number of grains (as a string) on a chessboard square given the
+" grains on each square double from the previous square. 
+" Throws an error if the square is below 1 or above 64.
 "
 " Examples:
 "
 "   :echo Square(16)
 "   32768
 "
 "   :echo Square(-1)
-"   E605: Exception not caught: square must be between 1 and 49
+"   E605: Exception not caught: square must be between 1 and 64
 "
 function! Square(number) abort
   " your code goes here
 endfunction
 
 "
-" Returns the total number of grains for a filled chessboard
+" Returns the total number of grains (as a string) for a filled chessboard
 "
 function! Total() abort
   " your code goes here