From fc51fe732bd438f480a582e45f8fb5cc57c29b31 Mon Sep 17 00:00:00 2001
From: mappu <mappu04@gmail.com>
Date: Tue, 11 Jul 2017 17:14:00 +1200
Subject: [PATCH] diff: initial implementation

---
 diff/diff.go | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)
 create mode 100644 diff/diff.go

diff --git a/diff/diff.go b/diff/diff.go
new file mode 100644
index 0000000..e6c3213
--- /dev/null
+++ b/diff/diff.go
@@ -0,0 +1,96 @@
+package diff
+
+import (
+	"html"
+	"io"
+	"strings"
+)
+
+/*
+Paul's Simple Diff Algorithm v 0.1
+(C) Paul Butler 2007 <http://www.paulbutler.org/>
+May be used and distributed under the zlib/libpng license.
+https://github.com/paulgb/simplediff/
+*/
+
+var Separator string = "\n"
+
+type Operation int8
+
+const (
+	OP_INSERTED  Operation = 1
+	OP_UNCHANGED           = 2
+	OP_REMOVED             = 3
+)
+
+type Instruction struct {
+	Op      Operation
+	Content []string
+}
+
+func (this *Instruction) RenderHTML() string {
+	switch this.Op {
+	case OP_INSERTED:
+		return `<ins>` + strings.Join(this.Content, Separator) + `</ins>`
+	case OP_REMOVED:
+		return `<del>` + strings.Join(this.Content, Separator) + `</del>`
+	default: //OP_UNCHANGED:
+		return strings.Join(this.Content, Separator)
+	}
+}
+
+func Diff(o, n []string) []Instruction {
+	maxl := 0
+	omax := 0
+	nmax := 0
+	matrix := make(map[int]map[int]int, len(o)+1)
+
+	for oindex, ovalue := range o {
+
+		// Find keys in new, where value matchines ovalue
+		nkeys := make([]int, 0)
+		for nindex, nvalue := range n {
+			if nvalue == ovalue {
+				nkeys = append(nkeys, nindex)
+			}
+		}
+
+		// Build diff matrix
+		for nindex, _ := range n {
+			if _, ok := matrix[oindex]; !ok {
+				matrix[oindex] = make(map[int]int, len(n)+1)
+			}
+			matrix[oindex][nindex] = matrix[oindex-1][nindex-1] + 1
+
+			if matrix[oindex][nindex] > maxl {
+				maxl = matrix[oindex][nindex]
+				omax = oindex + 1 - maxl
+				nmax = nindex + 1 - maxl
+			}
+		}
+	}
+
+	if maxl == 0 {
+		return []Instruction{
+			Instruction{OP_REMOVED, o},
+			Instruction{OP_INSERTED, n},
+		}
+	}
+
+	ret := Diff(o[0:omax], n[0:nmax])
+	if maxl != nmax {
+		ret = append(ret, Instruction{OP_UNCHANGED, n[nmax:maxl]})
+	}
+	ret = append(ret, Diff(o[0:omax+maxl], n[0:nmax+maxl])...)
+
+	return ret
+}
+
+// HtmlDiff splits o and n by newlines, and writes an HTML-safe version to the dest writer.
+func HtmlDiff(o, n string, dest io.Writer) {
+	ops := Diff(strings.Split(html.EscapeString(o), Separator), strings.Split(html.EscapeString(n), Separator))
+
+	for _, op := range ops {
+		dest.Write([]byte(op.RenderHTML()))
+	}
+}