Merge pull request #362 from gradha/pr_adds_fold_templates_to_sequtils

Adds fold templates to sequtils

lib/pure/collections/sequtils.nim

@@ -151,10 +151,81 @@ template toSeq*(iter: expr): expr {.immediate.} =
   for x in iter: add(result, x)
   result
 
+template foldl*(sequence, operation: expr): expr =
+  ## Template to fold a sequence from left to right, returning the accumulation.
+  ##
+  ## The sequence is required to have at least a single element. Debug versions
+  ## of your program will assert in this situation but release versions will
+  ## happily go ahead. If the sequence has a single element it will be returned
+  ## without applying ``operation``.
+  ##
+  ## The ``operation`` parameter should be an expression which uses the
+  ## variables ``a`` and ``b`` for each step of the fold. Since this is a left
+  ## fold, for non associative binary operations like substraction think that
+  ## the sequence of numbers 1, 2 and 3 will be parenthesized as (((1) - 2) -
+  ## 3).  Example:
+  ##
+  ## .. code-block:: nimrod
+  ##   let
+  ##     numbers = @[5, 9, 11]
+  ##     addition = foldl(numbers, a + b)
+  ##     substraction = foldl(numbers, a - b)
+  ##     multiplication = foldl(numbers, a * b)
+  ##     words = @["nim", "rod", "is", "cool"]
+  ##     concatenation = foldl(words, a & b)
+  ##   assert addition == 25, "Addition is (((5)+9)+11)"
+  ##   assert substraction == -15, "Substraction is (((5)-9)-11)"
+  ##   assert multiplication == 495, "Multiplication is (((5)*9)*11)"
+  ##   assert concatenation == "nimrodiscool"
+  assert sequence.len > 0, "Can't fold empty sequences"
+  var result {.gensym.}: type(sequence[0])
+  result = sequence[0]
+  for i in countup(1, sequence.len - 1):
+    let
+      a {.inject.} = result
+      b {.inject.} = sequence[i]
+    result = operation
+  result
+
+template foldr*(sequence, operation: expr): expr =
+  ## Template to fold a sequence from right to left, returning the accumulation.
+  ##
+  ## The sequence is required to have at least a single element. Debug versions
+  ## of your program will assert in this situation but release versions will
+  ## happily go ahead. If the sequence has a single element it will be returned
+  ## without applying ``operation``.
+  ##
+  ## The ``operation`` parameter should be an expression which uses the
+  ## variables ``a`` and ``b`` for each step of the fold. Since this is a right
+  ## fold, for non associative binary operations like substraction think that
+  ## the sequence of numbers 1, 2 and 3 will be parenthesized as (1 - (2 -
+  ## (3))). Example:
+  ##
+  ## .. code-block:: nimrod
+  ##   let
+  ##     numbers = @[5, 9, 11]
+  ##     addition = foldr(numbers, a + b)
+  ##     substraction = foldr(numbers, a - b)
+  ##     multiplication = foldr(numbers, a * b)
+  ##     words = @["nim", "rod", "is", "cool"]
+  ##     concatenation = foldr(words, a & b)
+  ##   assert addition == 25, "Addition is (5+(9+(11)))"
+  ##   assert substraction == 7, "Substraction is (5-(9-(11)))"
+  ##   assert multiplication == 495, "Multiplication is (5*(9*(11)))"
+  ##   assert concatenation == "nimrodiscool"
+  assert sequence.len > 0, "Can't fold empty sequences"
+  var result {.gensym.}: type(sequence[0])
+  result = sequence[sequence.len - 1]
+  for i in countdown(sequence.len - 2, 0):
+    let
+      a {.inject.} = sequence[i]
+      b {.inject.} = result
+    result = operation
+  result
+
 when isMainModule:
   import strutils
-  proc toStr(x: int): string {.procvar.} = $x
+  block: # concat test
-  # concat test
     let
       s1 = @[1, 2, 3]
       s2 = @[4, 5]
@@ -162,7 +233,7 @@ when isMainModule:
       total = concat(s1, s2, s3)
     assert total == @[1, 2, 3, 4, 5, 6, 7]
 
-  # duplicates test
+  block: # duplicates test
     let
       dup1 = @[1, 1, 3, 4, 2, 2, 8, 1, 4]
       dup2 = @["a", "a", "c", "d", "d"]
@@ -171,7 +242,7 @@ when isMainModule:
     assert unique1 == @[1, 3, 4, 2, 8]
     assert unique2 == @["a", "c", "d"]
 
-  # zip test
+  block: # zip test
     let
       short = @[1, 2, 3]
       long = @[6, 5, 4, 3, 2, 1]
@@ -183,7 +254,7 @@ when isMainModule:
     assert zip1[2].b == 4
     assert zip2[2].b == "three"
 
-  # filter proc test
+  block: # filter proc test
     let
       colors = @["red", "yellow", "black"]
       f1 = filter(colors, proc(x: string): bool = x.len < 6)
@@ -191,19 +262,19 @@ when isMainModule:
     assert f1 == @["red", "black"]
     assert f2 == @["yellow"]
 
-  # filter iterator test
+  block: # filter iterator test
     let numbers = @[1, 4, 5, 8, 9, 7, 4]
     for n in filter(numbers, proc (x: int): bool = x mod 2 == 0):
       echo($n)
     # echoes 4, 8, 4 in separate lines
 
-  # filterIt test
+  block: # filterIt test
     let
       temperatures = @[-272.15, -2.0, 24.5, 44.31, 99.9, -113.44]
       acceptable = filterIt(temperatures, it < 50 and it > -10)
     assert acceptable == @[-2.0, 24.5, 44.31]
 
-  # toSeq test
+  block: # toSeq test
     let
       numeric = @[1, 2, 3, 4, 5, 6, 7, 8, 9]
       odd_numbers = toSeq(filter(numeric) do (x: int) -> bool:
@@ -211,4 +282,30 @@ when isMainModule:
           result = true)
     assert odd_numbers == @[1, 3, 5, 7, 9]
 
+  block: # foldl tests
+    let
+      numbers = @[5, 9, 11]
+      addition = foldl(numbers, a + b)
+      substraction = foldl(numbers, a - b)
+      multiplication = foldl(numbers, a * b)
+      words = @["nim", "rod", "is", "cool"]
+      concatenation = foldl(words, a & b)
+    assert addition == 25, "Addition is (((5)+9)+11)"
+    assert substraction == -15, "Substraction is (((5)-9)-11)"
+    assert multiplication == 495, "Multiplication is (((5)*9)*11)"
+    assert concatenation == "nimrodiscool"
+
+  block: # foldr tests
+    let
+      numbers = @[5, 9, 11]
+      addition = foldr(numbers, a + b)
+      substraction = foldr(numbers, a - b)
+      multiplication = foldr(numbers, a * b)
+      words = @["nim", "rod", "is", "cool"]
+      concatenation = foldr(words, a & b)
+    assert addition == 25, "Addition is (5+(9+(11)))"
+    assert substraction == 7, "Substraction is (5-(9-(11)))"
+    assert multiplication == 495, "Multiplication is (5*(9*(11)))"
+    assert concatenation == "nimrodiscool"
+
   echo "Finished doc tests"