add algorithm.merge (#16182)

* add merge to algorithm
* Apply suggestions from code review
* Update lib/pure/algorithm.nim
* Apply suggestions from code review
* Update changelog.md

Co-authored-by: ee7 <[email protected]>

Author: ringabout

changelog.md

@@ -212,6 +212,9 @@
 
 - `std/options` changed `$some(3)` to `"some(3)"` instead of `"Some(3)"`
   and `$none(int)` to `"none(int)"` instead of `"None[int]"`.
+  
+- Added `algorithm.merge`.
+
 
 - Added `std/jsfetch` module [Fetch](https://developer.mozilla.org/docs/Web/API/Fetch_API) wrapper for JavaScript target.
 

lib/pure/algorithm.nim

@@ -42,6 +42,8 @@ runnableExamples:
 ## * `sequtils module<sequtils.html>`_ for working with the built-in seq type
 ## * `tables module<tables.html>`_ for sorting tables
 
+import std/private/since
+
 type
   SortOrder* = enum
     Descending, Ascending
@@ -559,6 +561,98 @@ proc isSorted*[T](a: openArray[T], order = SortOrder.Ascending): bool =
     assert isSorted(e) == false
   isSorted(a, system.cmp[T], order)
 
+proc merge*[T](
+  result: var seq[T],
+  x, y: openArray[T], cmp: proc(x, y: T): int {.closure.}
+) {.since: (1, 5, 1).} =
+  ## Merges two sorted `openArray`. `x` and `y` are assumed to be sorted.
+  ## If you do not wish to provide your own `cmp`,
+  ## you may use `system.cmp` or instead call the overloaded
+  ## version of `merge`, which uses `system.cmp`.
+  ##
+  ## .. note:: The original data of `result` is not cleared,
+  ##    new data is appended to `result`.
+  ##
+  ## **See also:**
+  ## * `merge proc<#merge,seq[T],openArray[T],openArray[T]>`_
+  runnableExamples:
+    let x = @[1, 3, 6]
+    let y = @[2, 3, 4]
+
+    block:
+      var merged = @[7] # new data is appended to merged sequence
+      merged.merge(x, y, system.cmp[int])
+      assert merged == @[7, 1, 2, 3, 3, 4, 6]
+
+    block:
+      var merged = @[7] # if you only want new data, clear merged sequence first
+      merged.setLen(0)
+      merged.merge(x, y, system.cmp[int])
+      assert merged.isSorted
+      assert merged == @[1, 2, 3, 3, 4, 6]
+
+    import std/sugar
+
+    var res: seq[(int, int)]
+    res.merge([(1, 1)], [(1, 2)], (a, b) => a[0] - b[0])
+    assert res == @[(1, 1), (1, 2)]
+
+    assert seq[int].default.dup(merge([1, 3], [2, 4])) == @[1, 2, 3, 4]
+
+  let
+    sizeX = x.len
+    sizeY = y.len
+    oldLen = result.len
+
+  result.setLen(oldLen + sizeX + sizeY)
+
+  var
+    ix = 0
+    iy = 0
+    i = oldLen
+
+  while true:
+    if ix == sizeX:
+      while iy < sizeY:
+        result[i] = y[iy]
+        inc i
+        inc iy
+      return
+
+    if iy == sizeY:
+      while ix < sizeX:
+        result[i] = x[ix]
+        inc i
+        inc ix
+      return
+
+    let itemX = x[ix]
+    let itemY = y[iy]
+
+    if cmp(itemX, itemY) > 0: # to have a stable sort
+      result[i] = itemY
+      inc iy
+    else:
+      result[i] = itemX
+      inc ix
+
+    inc i
+
+proc merge*[T](result: var seq[T], x, y: openArray[T]) {.inline, since: (1, 5, 1).} =
+  ## Shortcut version of `merge` that uses `system.cmp[T]` as the comparison function.
+  ##
+  ## **See also:**
+  ## * `merge proc<#merge,seq[T],openArray[T],openArray[T],proc(T,T)>`_
+  runnableExamples:
+    let x = [5, 10, 15, 20, 25]
+    let y = [50, 40, 30, 20, 10].sorted
+
+    var merged: seq[int]
+    merged.merge(x, y)
+    assert merged.isSorted
+    assert merged == @[5, 10, 10, 15, 20, 20, 25, 30, 40, 50]
+  merge(result, x, y, system.cmp)
+
 proc product*[T](x: openArray[seq[T]]): seq[seq[T]] =
   ## Produces the Cartesian product of the array.
   ## Every element of the result is a combination of one element from each seq in `x`,

tests/stdlib/talgorithm.nim

@@ -4,7 +4,8 @@ discard """
 '''
 """
 #12928,10456
-import std/[sequtils, algorithm, json]
+
+import std/[sequtils, algorithm, json, sugar]
 
 proc test() = 
   try: 
@@ -114,3 +115,159 @@ block:
     doAssert binarySearch(moreData, 6) == -1
     doAssert binarySearch(moreData, 4711) == 4
     doAssert binarySearch(moreData, 4712) == -1
+
+# merge
+proc main() =
+  block:
+    var x = @[1, 7, 8, 11, 21, 33, 45, 99]
+    var y = @[6, 7, 9, 12, 57, 66]
+
+    var merged: seq[int]
+    merged.merge(x, y)
+    doAssert merged.isSorted
+    doAssert merged == sorted(x & y)
+
+  block:
+    var x = @[111, 88, 76, 56, 45, 31, 22, 19, 11, 3]
+    var y = @[99, 85, 83, 82, 69, 64, 48, 42, 33, 31, 26, 13]
+
+    var merged: seq[int]
+    merged.merge(x, y, (x, y) => -system.cmp(x, y))
+    doAssert merged.isSorted((x, y) => -system.cmp(x, y))
+    doAssert merged == sorted(x & y, SortOrder.Descending)
+
+  block:
+    var x: seq[int] = @[]
+    var y = @[1]
+
+    var merged: seq[int]
+    merged.merge(x, y)
+    doAssert merged.isSorted
+    doAssert merged.isSorted(SortOrder.Descending)
+    doAssert merged == @[1]
+
+  block:
+    var x = [1, 3, 5, 5, 7]
+    var y: seq[int] = @[]
+
+    var merged: seq[int]
+    merged.merge(x, y)
+    doAssert merged.isSorted
+    doAssert merged == @x
+
+  block:
+    var x = [1, 3, 5, 5, 7]
+    var y: seq[int] = @[]
+
+    var merged: seq[int] = @[1, 2, 3, 5, 6, 56, 99, 2, 34]
+    merged.merge(x, y)
+    doAssert merged == @[1, 2, 3, 5, 6, 56, 99, 2, 34, 1, 3, 5, 5, 7]
+
+
+  block:
+    var x: array[0, int]
+    var y = [1, 4, 6, 7, 9]
+
+    var merged: seq[int]
+    merged.merge(x, y)
+    doAssert merged.isSorted
+    doAssert merged == @y
+
+  block:
+    var x: array[0, int]
+    var y: array[0, int]
+
+    var merged: seq[int]
+    merged.merge(x, y)
+    doAssert merged.isSorted
+    doAssert merged.len == 0
+
+  block:
+    var x: array[0, int]
+    var y: array[0, int]
+
+    var merged: seq[int] = @[99, 99, 99]
+    merged.setLen(0)
+    merged.merge(x, y)
+    doAssert merged.isSorted
+    doAssert merged.len == 0
+
+  block:
+    var x: seq[int]
+    var y: seq[int]
+
+    var merged: seq[int]
+    merged.merge(x, y)
+    doAssert merged.isSorted
+    doAssert merged.len == 0
+
+  block:
+    type
+      Record = object
+        id: int
+    
+    proc r(id: int): Record =
+      Record(id: id)
+
+    proc cmp(x, y: Record): int =
+      if x.id == y.id: return 0
+      if x.id < y.id: return -1
+      result = 1
+
+    var x = @[r(-12), r(1), r(3), r(8), r(13), r(88)]
+    var y = @[r(4), r(7), r(12), r(13), r(77), r(99)]
+
+    var merged: seq[Record] = @[]
+    merged.merge(x, y, cmp)
+    doAssert merged.isSorted(cmp)
+    doAssert merged.len == 12
+
+  block:
+    type
+      Record = object
+        id: int
+    
+    proc r(id: int): Record =
+      Record(id: id)
+
+    proc ascendingCmp(x, y: Record): int =
+      if x.id == y.id: return 0
+      if x.id < y.id: return -1
+      result = 1
+
+    proc descendingCmp(x, y: Record): int =
+      if x.id == y.id: return 0
+      if x.id < y.id: return 1
+      result = -1
+
+    var x = @[r(-12), r(1), r(3), r(8), r(13), r(88)]
+    var y = @[r(4), r(7), r(12), r(13), r(77), r(99)]
+
+    var merged: seq[Record]
+    merged.setLen(0)
+    merged.merge(x, y, ascendingCmp)
+    doAssert merged.isSorted(ascendingCmp)
+    doAssert merged == sorted(x & y, ascendingCmp)
+
+    reverse(x)
+    reverse(y)
+
+    merged.setLen(0)
+    merged.merge(x, y, descendingCmp)
+    doAssert merged.isSorted(descendingCmp)
+    doAssert merged == sorted(x & y, ascendingCmp, SortOrder.Descending)
+
+    reverse(x)
+    reverse(y)
+    merged.setLen(0)
+    merged.merge(x, y, proc (x, y: Record): int = -descendingCmp(x, y))
+    doAssert merged.isSorted(proc (x, y: Record): int = -descendingCmp(x, y))
+    doAssert merged == sorted(x & y, ascendingCmp)
+
+
+  var x: seq[(int, int)]
+  x.merge([(1,1)], [(1,2)], (a,b) => a[0] - b[0])
+  doAssert x == @[(1, 1), (1, 2)]
+
+static: main()
+main()