grains exercise: test against integer values as strings. (#326)

* To work around Vimscripts 2^63 limit for integers, the tests
has been created to expect the integral values as strings.

config.json

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

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

@@ -0,0 +1,13 @@
+## Maximum integer value
+
+According to [the Vim docs][number]:
+
+> Assuming 64 bit numbers are used (see v:numbersize) an unsigned number is truncated to 0x7fffffffffffffff or 9223372036854775807.
+
+In other words, Vimscript cannot express any number `2^63` or greater as an integer.
+
+Some of the tests for this exercise require 64 bit integers which is beyond the integer size limitation ov Vimscript.
+Because of this limitation, the results of the calculations are tested against a string which expresses the integer value, rather than expressing the answer as Integer.
+Can you solve this by avoiding numbers that are larger than the language will allow directly?
+
+[number]: https://vimhelp.org/eval.txt.html#expr-number

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

@@ -1,6 +1,8 @@
 {
   "authors": [
-    "BNAndras"
+    "BNAndras",
+    "KOTP",
+    "glennj"
   ],
   "files": {
     "solution": [

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

@@ -1,11 +1,59 @@
-function! Square(number) abort
-  if a:number < 1 || a:number > 64
-    throw 'square must be between 1 and 64'
-  endif
+" Helper function to add two numbers represented as strings
+function! StringAdd(num1, num2)
+    let carry = 0
+    let result = ''
 
-  return float2nr(pow(2, (a:number-1)))
+    " Pad the shorter number with leading zeros
+    let len1 = strlen(a:num1)
+    let len2 = strlen(a:num2)
+    if len1 < len2
+        let a:num1 = repeat('0', len2 - len1) . a:num1
+    elseif len2 < len1
+        let a:num2 = repeat('0', len1 - len2) . a:num2
+    endif
+
+    " Add digits from right to left
+    for i in range(strlen(a:num1) - 1, 0, -1)
+        let sum = str2nr(a:num1[i]) + str2nr(a:num2[i]) + carry
+        let carry = sum >= 10 ? 1 : 0
+        let result = string(sum % 10) . result
+    endfor
+
+    " Add the last carry if it exists
+    if carry > 0
+        let result = '1' . result
+    endif
+
+    return result
 endfunction
 
-function! Total() abort
-  return float2nr(pow(2, 64) - 1)
+" Function to calculate grains on a specific square using string manipulation
+function! Square(n)
+    if a:n < 1 || a:n > 64
+        throw 'square must be between 1 and 64'
+    endif
+
+    " Start with 1 grain on the first square
+    let grains = '1'
+    for i in range(2, a:n)
+        " Double the grains by adding it to itself
+        let grains = StringAdd(grains, grains)
+    endfor
+
+    return grains
+endfunction
+
+" Function to calculate the total grains on the chessboard using string manipulation
+function! Total()
+    let total = '0'
+
+    " Accumulate grains for each square from 1 to 64
+    let grains = '1'
+    for i in range(1, 64)
+        let total = StringAdd(total, grains)
+        " Double grains for the next square
+        let grains = StringAdd(grains, grains)
+    endfor
+
+    return total
 endfunction

exercises/practice/grains/grains.vader

@@ -1,37 +1,37 @@
 
 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 64):
   let g:square = 64
-  let g:expected = 9223372036854775807
+  let g:expected = "9223372036854775808"
   AssertEqual g:expected, Square(g:square)
 
 Execute (square 0 is invalid):
@@ -53,5 +53,5 @@ Execute (square greater than 64 is invalid):
   AssertEqual g:expected, g:vader_exception
 
 Execute (returns the total number of grains on the board):
-  let g:expected = 9223372036854775807
+  let g:expected = "18446744073709551615"
   AssertEqual g:expected, Total()

exercises/practice/grains/grains.vim

@@ -1,6 +1,6 @@
 "
-" Returns the number of grains on a chessboard square given the grains on each square
-" double from the previous square. 
+" 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:
@@ -16,7 +16,7 @@ function! Square(number) abort
 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