diff --git a/acedoc.go b/acedoc.go
index b0f0802..1a48df2 100644
--- a/acedoc.go
+++ b/acedoc.go
@@ -1,46 +1,10 @@
 package acedoc
 
 import (
-	"fmt"
 	"strings"
 	"sync"
 )
 
-type DeltaAction bool
-
-const (
-	DeltaInsert DeltaAction = true
-	DeltaRemove DeltaAction = false
-)
-
-type Delta struct {
-	Action     DeltaAction
-	Lines      []string
-	Start, End Position
-}
-
-func Remove(row, col uint, str string) Delta {
-	lines := strings.Split(str, "\n")
-	return Delta{
-		Action: DeltaRemove,
-		Lines:  lines,
-		Start:  Position{row, col},
-		End:    Position{row + uint(len(lines)-1), uint(len(lines[len(lines)-1]))},
-	}
-
-}
-
-func Insert(row, col uint, str string) Delta {
-	lines := strings.Split(str, "\n")
-	return Delta{
-		Action: DeltaInsert,
-		Lines:  lines,
-		Start:  Position{row, col},
-		End:    Position{row + uint(len(lines)-1), uint(len(lines[len(lines)-1]))},
-	}
-
-}
-
 type Document struct {
 	mtx   sync.RWMutex
 	Lines []string
@@ -69,78 +33,6 @@ func (d Delta) NLines() uint {
 	return n
 }
 
-func (d *Document) Apply(dl Delta) error {
-	d.mtx.Lock()
-	defer d.mtx.Unlock()
-
-	err := d.validate(dl)
-	if err != nil {
-		return fmt.Errorf("delta does not apply to document: %v", err)
-	}
-
-	row := dl.Start.Row
-	col := dl.Start.Column
-	line := d.Lines[row]
-
-	switch dl.Action {
-	case DeltaInsert:
-
-		if dl.NLines() == 1 {
-			d.Lines[row] = line[:col] + dl.Line(0) + line[col:]
-		} else {
-			newlines := []string{}
-			if row != 0 {
-				newlines = append(newlines, d.Lines[:row-1]...) // old content
-			}
-			newlines = append(newlines, d.Lines[row][:col]+dl.Lines[0])
-			newlines = append(newlines, dl.Lines[1:]...) // new content
-			newlines[len(newlines)-1] += d.Lines[row][col:]
-			if uint(len(d.Lines)) > row {
-				newlines = append(newlines, d.Lines[row+1:]...) // old content
-			}
-			d.Lines = newlines
-		}
-
-	case DeltaRemove:
-		endCol := dl.End.Column
-		endRow := dl.End.Row
-
-		if row == endRow {
-			d.Lines[row] = line[:col] + line[endCol:]
-		} else {
-			newlines := []string{}
-			newlines = append(newlines, d.Lines[:row]...)
-			newlines = append(newlines, d.Lines[endRow:]...)
-			newlines[endRow] = newlines[endRow][endCol:]
-			d.Lines = newlines
-		}
-	}
-
-	return nil
-}
-
-func (d *Document) validate(dl Delta) error {
-	if !d.InDocument(dl.Start) {
-		return fmt.Errorf("start is not in document")
-	}
-
-	if dl.Action == DeltaRemove && !d.InDocument(dl.End) {
-		return fmt.Errorf("end is not in document for remove")
-	}
-
-	if dl.NLines() != dl.NRows() {
-		return fmt.Errorf("delta has %d lines, but positions show range of %d lines", dl.NLines(), dl.NRows())
-	}
-
-	lastDlLine := dl.Line(dl.NRows() - 1)
-
-	if uint(len(lastDlLine)) != dl.Cols() {
-		return fmt.Errorf("delta has %d chars on the final line, but positions show range of %d chars", len(lastDlLine), dl.Cols())
-	}
-
-	return nil
-}
-
 func (d Document) Line(i uint) string {
 	if i >= uint(len(d.Lines)) {
 		return ""
@@ -156,6 +48,7 @@ func (d Delta) Line(i uint) string {
 
 func (d Delta) NRows() uint {
 	r := d.End.Row - d.Start.Row
+	r += 1
 	if r == 0 {
 		return 1
 	}
diff --git a/acedoc_test.go b/acedoc_test.go
index 15f8f8f..7de21fd 100644
--- a/acedoc_test.go
+++ b/acedoc_test.go
@@ -11,18 +11,47 @@ func TestDelta(t *testing.T) {
 	}
 
 	cases := []c{
-		{"empty", "", "", Delta{}},
-		{"empty with content", "a", "a", Delta{}},
-		{"insert with no content", "a", "a", Insert(0, 1, "")},
-		{"insert", "a", "ab", Insert(0, 1, "b")},
-		{"insert across line", "a", "a\nb", Insert(0, 1, "\nb")},
-		{"insert new line", "ab", "a\nb", Insert(0, 1, "\n")},
-		{"insert lines at beginning", "ab", "\n\nab", Insert(0, 0, "\n\n")},
-		{"insert lines at end", "ab", "ab\n\n", Insert(0, 2, "\n\n")},
-		{"insert more lines", "a\nb", "a\n\n\nb", Insert(0, 1, "\n\n")},
-
-		{"remove", "a", "", Remove(0, 0, "a")},
-		{"remove across line", "a\nb", "a", Remove(0, 1, "\nb")},
+
+		// empty tests
+
+		{"empty",
+			"", "", Delta{}},
+
+		{"empty with content",
+			"a", "a", Delta{}},
+
+		// insert tests
+
+		{"insert with no content",
+			"a", "a", Insert(0, 1, "")},
+
+		{"insert",
+			"a", "ab", Insert(0, 1, "b")},
+
+		{"insert across line",
+			"a", "a\nb", Insert(0, 1, "\nb")},
+
+		{"insert new line",
+			"ab", "a\nb", Insert(0, 1, "\n")},
+
+		{"insert lines at beginning",
+			"ab", "\n\nab", Insert(0, 0, "\n\n")},
+
+		{"insert lines at end",
+			"ab", "ab\n\n", Insert(0, 2, "\n\n")},
+
+		{"insert more lines",
+			"a\nb", "a\n\n\nb", Insert(0, 1, "\n\n")},
+
+		// remove tests
+		{"remove",
+			"a", "", Remove(0, 0, "a")},
+
+		{"remove part",
+			"aa", "a", Remove(0, 0, "a")},
+
+		{"remove across line",
+			"a\nb", "a", Remove(0, 1, "\nb")},
 	}
 
 	for _, c := range cases {
diff --git a/delta.go b/delta.go
index b74f9a0..2a983bc 100644
--- a/delta.go
+++ b/delta.go
@@ -1,6 +1,14 @@
 package acedoc
 
+import (
+	"fmt"
+	"strings"
+)
+
 /* ***** BEGIN LICENSE BLOCK *****
+*
+* Original license for JS algorithm, which has been rewritten in Go:
+*
 * Distributed under the BSD license:
 *
 * Copyright (c) 2010, Ajax.org B.V.
@@ -29,3 +37,124 @@ package acedoc
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * ***** END LICENSE BLOCK ***** */
+
+type DeltaAction bool
+
+const (
+	DeltaInsert DeltaAction = true
+	DeltaRemove DeltaAction = false
+)
+
+type Delta struct {
+	Action     DeltaAction
+	Lines      []string
+	Start, End Position
+}
+
+func Remove(row, col uint, str string) Delta {
+	lines := strings.Split(str, "\n")
+	nlines := len(lines) - 1
+	return Delta{
+		Action: DeltaRemove,
+		Lines:  lines,
+		Start:  Position{row, col},
+		End:    Position{row + uint(nlines), uint(len(lines[nlines]))},
+	}
+}
+
+func Insert(row, col uint, str string) Delta {
+	lines := strings.Split(str, "\n")
+	nlines := uint(len(lines) - 1)
+	return Delta{
+		Action: DeltaInsert,
+		Lines:  lines,
+		Start:  Position{row, col},
+		End:    Position{row + nlines, uint(len(lines[nlines]))},
+	}
+}
+
+func (d *Document) applyInsert(dl Delta) error {
+
+	row := dl.Start.Row
+	col := dl.Start.Column
+	line := d.Lines[row]
+
+	if dl.NLines() == 1 {
+		d.Lines[row] = line[:col] + dl.Line(0) + line[col:]
+	} else {
+		newlines := []string{}
+		if row != 0 {
+			newlines = append(newlines, d.Lines[:row-1]...) // old content
+		}
+		newlines = append(newlines, d.Lines[row][:col]+dl.Lines[0])
+		newlines = append(newlines, dl.Lines[1:]...) // new content
+		newlines[len(newlines)-1] += d.Lines[row][col:]
+		if uint(len(d.Lines)) > row {
+			newlines = append(newlines, d.Lines[row+1:]...) // old content
+		}
+		d.Lines = newlines
+	}
+
+	return nil
+}
+
+func (d *Document) applyRemove(dl Delta) error {
+	row := dl.Start.Row
+	col := dl.Start.Column
+	line := d.Lines[row]
+	endCol := dl.End.Column
+	endRow := dl.End.Row
+
+	if row == endRow {
+		d.Lines[row] = line[:col] + line[endCol:]
+	} else {
+		d.Lines[endRow] = d.Lines[row][:col] + d.Lines[endRow][endCol:]
+		newlines := []string{}
+		newlines = append(newlines, d.Lines[:row]...)
+		newlines = append(newlines, d.Lines[endRow:]...)
+		d.Lines = newlines
+	}
+
+	return nil
+}
+
+func (d *Document) Apply(dl Delta) error {
+	d.mtx.Lock()
+	defer d.mtx.Unlock()
+
+	err := d.validate(dl)
+	if err != nil {
+		return fmt.Errorf("delta does not apply to document: %v", err)
+	}
+
+	switch dl.Action {
+	case DeltaInsert:
+		return d.applyInsert(dl)
+	case DeltaRemove:
+		return d.applyRemove(dl)
+	}
+
+	return nil
+}
+
+func (d *Document) validate(dl Delta) error {
+	if !d.InDocument(dl.Start) {
+		return fmt.Errorf("start is not in document")
+	}
+
+	if dl.Action == DeltaRemove && !d.InDocument(dl.End) {
+		return fmt.Errorf("end is not in document for remove")
+	}
+
+	if dl.NLines() != dl.NRows() {
+		return fmt.Errorf("delta has %d lines, but positions show range of %d lines", dl.NLines(), dl.NRows())
+	}
+
+	lastDlLine := dl.Line(dl.NRows() - 1)
+
+	if uint(len(lastDlLine)) != dl.Cols() {
+		return fmt.Errorf("delta has %d chars on the final line, but positions show range of %d chars", len(lastDlLine), dl.Cols())
+	}
+
+	return nil
+}