3 files changed, 75 insertions(+), 2 deletions(-)

M README.md
M src/simplediff.nim
M tests/testDiff.nim

@@ 31,7 31,9 @@ for diff in stringDiff("the word is blue", "the word is red", seps={' '}):
 
 Other convenience wrappers may be added in the future! Feel free to request one or submit a patch.
 
-See the `when isMainModule` block within `simplediff.nim` for an example of using these functions to build a simple diff application.
+`simplediff` also provides `prettyDiff`, which writes a formatted version of the diff to an output stream. It can be called directly or, if you need more flexibility than it offers, used as a starting point for building your own output.
+
+See the `when isMainModule` block within `simplediff.nim` for an example of a simple diff application.
 
 ## Contributing
 

@@ 1,3 1,5 @@
+import math
+import streams
 import strutils
 import tables
 


@@ 39,7 41,7 @@ proc diff*[T](itemsOld, itemsNew: openArray[T]): seq[Diff[T]] =
       var newSuffixLen = 1
       if ixOld > 0 and overlap.getOrDefault(ixOld - 1, 0) > 0:
         newSuffixLen = overlap.getOrDefault(ixOld - 1, 0) + 1
-      overlaptemp[ixOld] = newSuffixLen
+      overlapTemp[ixOld] = newSuffixLen
       if overlapTemp[ixOld] > subLength:
         subLength = overlapTemp[ixOld]
         subStartOld = ixOld - subLength + 1


@@ 73,6 75,42 @@ proc stringDiff*(s1, s2: string, seps: set[char] = Newlines): seq[Diff[string]] 
   return diff(split(s1, seps = seps), split(s2, seps = seps))
 
 
+proc prettyDiff*[T](itemsOld, itemsNew: openArray[T],
+    outStream: Stream = newFileStream(stdout), insertionPrefix = "++++",
+        deletionPrefix = "----") =
+  ## Calculate the diff of `itemsOld` and `itemsNew`
+  ## and write it to `outStream`.
+  ## `insertionPrefix` is prepended to lines that were inserted,
+  ## and `deletionPrefix` is prepended to lines that were deleted.
+  let diffed = diff(itemsOld, itemsNew)
+  var lineCounter = 1
+
+  let linePadding = int(floor(log10(float(itemsOld.len)))) + 2
+  proc formatLineCount(count: int): string =
+    align($lineCounter, linePadding) & " "
+
+  for ix, entry in diffed:
+    case entry.kind
+    of NoChange:
+      for line in entry.tokens:
+        lineCounter += 1
+    of Insertion:
+      if lineCounter != 1:
+        lineCounter -= 1
+      for line in entry.tokens:
+        outStream.writeLine(formatLineCount(lineCounter) &
+            insertionPrefix & " " & $line)
+      # Don't increment the lines when inserting at the beginning
+      # because we were adding at the zeroth line
+      if lineCounter != 1:
+        lineCounter += 1
+    of Deletion:
+      for line in entry.tokens:
+        outStream.writeLine(formatLineCount(lineCounter) &
+            deletionPrefix & " " & $line)
+        lineCounter += 1
+
+
 when isMainModule:
   import parseopt
   import terminal

@@ 1,3 1,4 @@
+import streams
 import strutils
 import unittest
 


@@ 71,3 72,35 @@ suite "test stringDiff":
       Diff[string](kind: Deletion, tokens: @["def", "123"]),
       Diff[string](kind: Insertion, tokens: @["fed", "abc"])
       ]
+
+
+suite "test prettyDiff":
+  var output: Stream
+  setup:
+    output = newStringStream()
+
+  test "correct output on unchanged strings":
+    prettyDiff(["hello"], ["hello"], outStream = output)
+    output.setPosition(0)
+    check output.readAll() == ""
+
+  test "correct output on deletion":
+    prettyDiff(["hello", "world"], ["hello"], outStream = output)
+    output.setPosition(0)
+    check output.readAll() == " 2 ---- world\n"
+
+  test "correct output on insertion":
+    prettyDiff(["hello"], ["hello", "world"], outStream = output)
+    output.setPosition(0)
+    check output.readAll() == " 1 ++++ world\n"
+
+  test "correct output on insertion, deletion, and unchanged":
+    prettyDiff(["same", "deletion"], ["insertion", "same", ],
+        outStream = output)
+    output.setPosition(0)
+    check output.readAll() == " 1 ++++ insertion\n 2 ---- deletion\n"
+
+  test "correct output on non-string tokens":
+    prettyDiff([1, 2], [3, 1], outStream = output)
+    output.setPosition(0)
+    check output.readAll() == " 1 ++++ 3\n 2 ---- 2\n"