added new editdistance stdlib module, deprecated strutils.editDistance, refs #6301

2 files changed, 310 insertions, 1 deletions

diff --git a/lib/pure/editdistance.nim b/lib/pure/editdistance.nim
new file mode 100644
index 000000000..63fa4cf72
--- /dev/null
+++ b/lib/pure/editdistance.nim
@@ -0,0 +1,306 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2018 Nim contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## This module implements an algorithm to compute the
+## `edit distance`:idx: between two Unicode strings.
+
+import unicode
+
+proc len(r: Rune): int {.noSideEffect.} =
+  ## Returns the number of bytes the rune ``r`` takes
+  let v = r.uint32
+  if v <= 0x007F: result = 1
+  elif v <= 0x07FF: result = 2
+  elif v <= 0xFFFF: result = 3
+  elif v <= 0x1FFFFF: result = 4
+  elif v <= 0x3FFFFFF: result = 5
+  elif v <= 0x7FFFFFFF: result = 6
+  else: result = 1
+
+proc editDistance*(a, b: string): int {.noSideEffect.} =
+  ## Returns the unicode-rune edit distance between ``a`` and ``b``.
+  ##
+  ## This uses the `Levenshtein`:idx: distance algorithm with only a linear
+  ## memory overhead.
+  if len(a) > len(b):
+    # make ``b`` the longer string
+    return editDistance(b, a)
+  # strip common prefix
+  var
+    i_start = 0 ## The character starting index of the first rune in both strings ``a`` and ``b``
+    i_next_a = 0
+    i_next_b = 0
+    rune_a, rune_b: Rune
+    len_runes_a = 0 ## The number of relevant runes in string ``a``.
+    len_runes_b = 0 ## The number of relevant runes in string ``b``.
+  block commonPrefix:
+    # ``a`` is the shorter string
+    while i_start < len(a):
+      i_next_a = i_start
+      a.fastRuneAt(i_next_a, rune_a, doInc = true)
+      i_next_b = i_start
+      b.fastRuneAt(i_next_b, rune_b, doInc = true)
+      if rune_a != rune_b:
+        inc(len_runes_a)
+        inc(len_runes_b)
+        break
+      i_start = i_next_a
+  var
+    # we know that we are either at the start of the strings
+    # or that the current value of rune_a is not equal to rune_b
+    # => start search for common suffix after the current rune (``i_next_*``)
+    i_end_a = i_next_a ## The exclusive upper index bound of string ``a``.
+    i_end_b = i_next_b ## The exclusive upper index bound of string ``b``.
+    i_current_a = i_next_a
+    i_current_b = i_next_b
+  block commonSuffix:
+    var
+      add_runes_a = 0
+      add_runes_b = 0
+    while i_current_a < len(a) and i_current_b < len(b):
+      i_next_a = i_current_a
+      a.fastRuneAt(i_next_a, rune_a)
+      i_next_b = i_current_b
+      b.fastRuneAt(i_next_b, rune_b)
+      inc(add_runes_a)
+      inc(add_runes_b)
+      if rune_a != rune_b:
+        i_end_a = i_next_a
+        i_end_b = i_next_b
+        inc(len_runes_a, add_runes_a)
+        inc(len_runes_b, add_runes_b)
+        add_runes_a = 0
+        add_runes_b = 0
+      i_current_a = i_next_a
+      i_current_b = i_next_b
+    if i_current_a >= len(a): # ``a`` exhausted
+      if i_current_b < len(b): # ``b`` not exhausted
+        i_end_a = i_current_a
+        i_end_b = i_current_b
+        inc(len_runes_a, add_runes_a)
+        inc(len_runes_b, add_runes_b)
+        while true:
+          b.fastRuneAt(i_end_b, rune_b)
+          inc(len_runes_b)
+          if i_end_b >= len(b): break
+    elif i_current_b >= len(b): # ``b`` exhausted and ``a`` not exhausted
+      i_end_a = i_current_a
+      i_end_b = i_current_b
+      inc(len_runes_a, add_runes_a)
+      inc(len_runes_b, add_runes_b)
+      while true:
+        a.fastRuneAt(i_end_a, rune_a)
+        inc(len_runes_a)
+        if i_end_a >= len(a): break
+  block specialCases:
+    # trivial cases:
+    if len_runes_a == 0: return len_runes_b
+    if len_runes_b == 0: return len_runes_a
+    # another special case:
+    if len_runes_a == 1:
+      a.fastRuneAt(i_start, rune_a, doInc = false)
+      var i_current_b = i_start
+      while i_current_b < i_end_b:
+        b.fastRuneAt(i_current_b, rune_b, doInc = true)
+        if rune_a == rune_b: return len_runes_b - 1
+      return len_runes_b
+  # common case:
+  var
+    len1 = len_runes_a + 1
+    len2 = len_runes_b + 1
+    row: seq[int]
+  let half = len_runes_a div 2
+  newSeq(row, len2)
+  var e = i_start + len2 - 1 # end marker
+  # initialize first row:
+  for i in 1 .. (len2 - half - 1): row[i] = i
+  row[0] = len1 - half - 1
+  i_current_a = i_start
+  var
+    char2p_i = -1
+    char2p_prev: int
+  for i in 1 .. (len1 - 1):
+    i_next_a = i_current_a
+    a.fastRuneAt(i_next_a, rune_a)
+    var
+      char2p: int
+      D, x: int
+      p: int
+    if i >= (len1 - half):
+      # skip the upper triangle:
+      let offset = i + half - len1
+      if char2p_i == i:
+        b.fastRuneAt(char2p_prev, rune_b)
+        char2p = char2p_prev
+        char2p_i = i + 1
+      else:
+        char2p = i_start
+        for j in 0 ..< offset:
+          rune_b = b.runeAt(char2p)
+          inc(char2p, rune_b.len)
+        char2p_i = i + 1
+        char2p_prev = char2p
+      p = offset
+      rune_b = b.runeAt(char2p)
+      var c3 = row[p] + (if rune_a != rune_b: 1 else: 0)
+      inc(char2p, rune_b.len)
+      inc(p)
+      x = row[p] + 1
+      D = x
+      if x > c3: x = c3
+      row[p] = x
+      inc(p)
+    else:
+      p = 1
+      char2p = i_start
+      D = i
+      x = i
+    if i <= (half + 1):
+      # skip the lower triangle:
+      e = len2 + i - half - 2
+    # main:
+    while p <= e:
+      dec(D)
+      rune_b = b.runeAt(char2p)
+      var c3 = D + (if rune_a != rune_b: 1 else: 0)
+      inc(char2p, rune_b.len)
+      inc(x)
+      if x > c3: x = c3
+      D = row[p] + 1
+      if x > D: x = D
+      row[p] = x
+      inc(p)
+    # lower triangle sentinel:
+    if i <= half:
+      dec(D)
+      rune_b = b.runeAt(char2p)
+      var c3 = D + (if rune_a != rune_b: 1 else: 0)
+      inc(x)
+      if x > c3: x = c3
+      row[p] = x
+    i_current_a = i_next_a
+  result = row[e]
+
+proc editDistanceAscii*(a, b: string): int {.noSideEffect.} =
+  ## Returns the edit distance between `a` and `b`.
+  ##
+  ## This uses the `Levenshtein`:idx: distance algorithm with only a linear
+  ## memory overhead.
+  var len1 = a.len
+  var len2 = b.len
+  if len1 > len2:
+    # make `b` the longer string
+    return editDistanceAscii(b, a)
+
+  # strip common prefix:
+  var s = 0
+  while s < len1 and a[s] == b[s]:
+    inc(s)
+    dec(len1)
+    dec(len2)
+  # strip common suffix:
+  while len1 > 0 and len2 > 0 and a[s+len1-1] == b[s+len2-1]:
+    dec(len1)
+    dec(len2)
+  # trivial cases:
+  if len1 == 0: return len2
+  if len2 == 0: return len1
+
+  # another special case:
+  if len1 == 1:
+    for j in s..s+len2-1:
+      if a[s] == b[j]: return len2 - 1
+    return len2
+
+  inc(len1)
+  inc(len2)
+  var half = len1 shr 1
+  # initalize first row:
+  #var row = cast[ptr array[0..high(int) div 8, int]](alloc(len2*sizeof(int)))
+  var row: seq[int]
+  newSeq(row, len2)
+  var e = s + len2 - 1 # end marker
+  for i in 1..len2 - half - 1: row[i] = i
+  row[0] = len1 - half - 1
+  for i in 1 .. len1 - 1:
+    var char1 = a[i + s - 1]
+    var char2p: int
+    var D, x: int
+    var p: int
+    if i >= len1 - half:
+      # skip the upper triangle:
+      var offset = i - len1 + half
+      char2p = offset
+      p = offset
+      var c3 = row[p] + ord(char1 != b[s + char2p])
+      inc(p)
+      inc(char2p)
+      x = row[p] + 1
+      D = x
+      if x > c3: x = c3
+      row[p] = x
+      inc(p)
+    else:
+      p = 1
+      char2p = 0
+      D = i
+      x = i
+    if i <= half + 1:
+      # skip the lower triangle:
+      e = len2 + i - half - 2
+    # main:
+    while p <= e:
+      dec(D)
+      var c3 = D + ord(char1 != b[char2p + s])
+      inc(char2p)
+      inc(x)
+      if x > c3: x = c3
+      D = row[p] + 1
+      if x > D: x = D
+      row[p] = x
+      inc(p)
+    # lower triangle sentinel:
+    if i <= half:
+      dec(D)
+      var c3 = D + ord(char1 != b[char2p + s])
+      inc(x)
+      if x > c3: x = c3
+      row[p] = x
+  result = row[e]
+
+
+when isMainModule:
+  doAssert editDistance("", "") == 0
+  doAssert editDistance("kitten", "sitting") == 3 # from Wikipedia
+  doAssert editDistance("flaw", "lawn") == 2 # from Wikipedia
+
+  doAssert editDistance("привет", "превет") == 1
+  doAssert editDistance("Åge", "Age") == 1
+  # editDistance, one string is longer in bytes, but shorter in rune length
+  # first string: 4 bytes, second: 6 bytes, but only 3 runes
+  doAssert editDistance("aaaa", "×××") == 4
+
+  block veryLongStringEditDistanceTest:
+    const cap = 256
+    var
+      s1 = newStringOfCap(cap)
+      s2 = newStringOfCap(cap)
+    while len(s1) < cap:
+      s1.add 'a'
+    while len(s2) < cap:
+      s2.add 'b'
+    doAssert editDistance(s1, s2) == cap
+
+  block combiningCodePointsEditDistanceTest:
+    const s = "A\xCC\x8Age"
+    doAssert editDistance(s, "Age") == 1
+
+  doAssert editDistanceAscii("", "") == 0
+  doAssert editDistanceAscii("kitten", "sitting") == 3 # from Wikipedia
+  doAssert editDistanceAscii("flaw", "lawn") == 2 # from Wikipedia
diff --git a/lib/pure/strutils.nim b/lib/pure/strutils.nim
index 296901558..a130c5a65 100644
--- a/lib/pure/strutils.nim
+++ b/lib/pure/strutils.nim
@@ -1775,8 +1775,10 @@ proc validIdentifier*(s: string): bool {.noSideEffect,
       if s[i] notin IdentChars: return false
     return true
 
+{.push warning[Deprecated]: off.}
 proc editDistance*(a, b: string): int {.noSideEffect,
-  rtl, extern: "nsuEditDistance".} =
+  rtl, extern: "nsuEditDistance",
+  deprecated: "use editdistance.editDistanceAscii instead".} =
   ## Returns the edit distance between `a` and `b`.
   ##
   ## This uses the `Levenshtein`:idx: distance algorithm with only a linear
@@ -1862,6 +1864,7 @@ proc editDistance*(a, b: string): int {.noSideEffect,
       if x > c3: x = c3
       row[p] = x
   result = row[e]
+{.pop.}
 
 # floating point formating:
 when not defined(js):