diff --git a/pkg/utilfn/linediff.go b/pkg/utilfn/linediff.go
new file mode 100644
index 00000000..5d4a2e9e
--- /dev/null
+++ b/pkg/utilfn/linediff.go
@@ -0,0 +1,132 @@
+package utilfn
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"strings"
+)
+
+const LineDiffVersion = 0
+
+type LineDiffType struct {
+	Lines   []int
+	NewData []string
+}
+
+// simple encoding
+// a 0 means read a line from NewData
+// a non-zero number means read the 1-indexed line from OldData
+func applyDiff(oldData []string, diff LineDiffType) ([]string, error) {
+	rtn := make([]string, 0, len(diff.Lines))
+	newDataPos := 0
+	for i := 0; i < len(diff.Lines); i++ {
+		if diff.Lines[i] == 0 {
+			if newDataPos >= len(diff.NewData) {
+				return nil, fmt.Errorf("not enough newdata for diff")
+			}
+			rtn = append(rtn, diff.NewData[newDataPos])
+			newDataPos++
+		} else {
+			idx := diff.Lines[i] - 1 // 1-indexed
+			if idx < 0 || idx >= len(oldData) {
+				return nil, fmt.Errorf("diff index out of bounds %d old-data-len:%d", idx, len(oldData))
+			}
+			rtn = append(rtn, oldData[idx])
+		}
+	}
+	return rtn, nil
+}
+
+func putUVarint(buf *bytes.Buffer, viBuf []byte, ival int) {
+	l := binary.PutUvarint(viBuf, uint64(ival))
+	buf.Write(viBuf[0:l])
+}
+
+// simple encoding
+// write varints.  first version, then len, then len-number-of-varints, then fill the rest with newdata
+// [version] [len-varint] [varint]xlen... newdata (bytes)
+func encodeDiff(diff LineDiffType) []byte {
+	var buf bytes.Buffer
+	viBuf := make([]byte, binary.MaxVarintLen64)
+	putUVarint(&buf, viBuf, 0)
+	putUVarint(&buf, viBuf, len(diff.Lines))
+	for _, val := range diff.Lines {
+		putUVarint(&buf, viBuf, val)
+	}
+	for _, str := range diff.NewData {
+		buf.WriteString(str)
+		buf.WriteByte('\n')
+	}
+	return buf.Bytes()
+}
+
+func decodeDiff(diffBytes []byte) (LineDiffType, error) {
+	var rtn LineDiffType
+	r := bytes.NewBuffer(diffBytes)
+	version, err := binary.ReadUvarint(r)
+	if err != nil {
+		return rtn, fmt.Errorf("invalid diff, cannot read version: %v", err)
+	}
+	if version != LineDiffVersion {
+		return rtn, fmt.Errorf("invalid diff, bad version: %d", version)
+	}
+	linesLen64, err := binary.ReadUvarint(r)
+	if err != nil {
+		return rtn, fmt.Errorf("invalid diff, cannot read lines length: %v", err)
+	}
+	linesLen := int(linesLen64)
+	rtn.Lines = make([]int, linesLen)
+	for idx := 0; idx < linesLen; idx++ {
+		vi, err := binary.ReadUvarint(r)
+		if err != nil {
+			return rtn, fmt.Errorf("invalid diff, cannot read line %d: %v", idx, err)
+		}
+		rtn.Lines[idx] = int(vi)
+	}
+	restOfInput := string(r.Bytes())
+	rtn.NewData = strings.Split(restOfInput, "\n")
+	return rtn, nil
+}
+
+func makeDiff(oldData []string, newData []string) LineDiffType {
+	var rtn LineDiffType
+	oldDataMap := make(map[string]int) // 1-indexed
+	for idx, str := range oldData {
+		if _, found := oldDataMap[str]; found {
+			continue
+		}
+		oldDataMap[str] = idx + 1
+	}
+	rtn.Lines = make([]int, len(newData))
+	for idx, str := range newData {
+		oldIdx, found := oldDataMap[str]
+		if found {
+			rtn.Lines[idx] = oldIdx
+		} else {
+			rtn.Lines[idx] = 0
+			rtn.NewData = append(rtn.NewData, str)
+		}
+	}
+	return rtn
+}
+
+func MakeDiff(str1 string, str2 string) []byte {
+	str1Arr := strings.Split(str1, "\n")
+	str2Arr := strings.Split(str2, "\n")
+	diff := makeDiff(str1Arr, str2Arr)
+	return encodeDiff(diff)
+}
+
+func ApplyDiff(str1 string, diffBytes []byte) (string, error) {
+	diff, err := decodeDiff(diffBytes)
+	if err != nil {
+		return "", err
+	}
+	str1Arr := strings.Split(str1, "\n")
+	str2Arr, err := applyDiff(str1Arr, diff)
+	if err != nil {
+		return "", err
+	}
+	return strings.Join(str2Arr, "\n"), nil
+}
diff --git a/pkg/utilfn/utilfn_test.go b/pkg/utilfn/utilfn_test.go
new file mode 100644
index 00000000..d5d7c81b
--- /dev/null
+++ b/pkg/utilfn/utilfn_test.go
@@ -0,0 +1,48 @@
+package utilfn
+
+import (
+	"fmt"
+	"testing"
+)
+
+const Str1 = `
+hello
+line #2
+more
+stuff
+apple
+`
+
+const Str2 = `
+line #2
+apple
+grapes
+banana
+`
+
+const Str3 = `
+more
+stuff
+banana
+coconut
+`
+
+func testDiff(t *testing.T, str1 string, str2 string) {
+	diffBytes := MakeDiff(str1, str2)
+	fmt.Printf("diff-len: %d\n", len(diffBytes))
+	out, err := ApplyDiff(str1, diffBytes)
+	if err != nil {
+		t.Errorf("error in diff: %v", err)
+		return
+	}
+	if out != str2 {
+		t.Errorf("bad diff output")
+	}
+}
+
+func TestDiff(t *testing.T) {
+	testDiff(t, Str1, Str2)
+	testDiff(t, Str2, Str3)
+	testDiff(t, Str1, Str3)
+	testDiff(t, Str3, Str1)
+}