Rope buffer: Fix bug in PosToLineCol and add test case

ui/buffer/rope.go

@@ -290,28 +290,33 @@ func (b *RopeBuffer) PosToLineCol(pos int) (int, int) {
 	var line, col int
 	var wasAtNewline bool
 
-	_rope := (*rope.Node)(b)
+	if pos <= 0 {
-	_rope.EachLeaf(func(n *rope.Node) bool {
+		return line, col
+	}
+
+	(*rope.Node)(b).EachLeaf(func(n *rope.Node) bool {
 		data := n.Value()
 		var i int
 		for i < len(data) {
-			if pos <= 0 {
+			if wasAtNewline { // Start of line
-				return true
+				if data[i] != '\n' { // If the start of this line does not happen to be a delim...
-			}
+					wasAtNewline = false // Say we weren't previously at a delimiter
-
+				}
-			if data[i] == '\n' { // End of line
+				line, col = line+1, 0
+			} else if data[i] == '\n' { // End of line
 				wasAtNewline = true
 				col++
-			} else if wasAtNewline { // Start of line
-				wasAtNewline = false
-				line, col = line+1, 0
 			} else {
-				col++ // Normal byte
+				col++
 			}
 
 			_, size := utf8.DecodeRune(data[i:])
 			i += size
 			pos -= size
+
+			if pos < 0 {
+				return true
+			}
 		}
 		return false
 	})

ui/buffer/rope_test.go

@@ -2,6 +2,49 @@ package buffer
 
 import "testing"
 
+func TestRopePosToLineCol(t *testing.T) {
+	var buf Buffer = NewRopeBuffer([]byte("line0\nline1\n\nline3\n"))
+	//line0
+	//line1
+	//
+	//line3
+	//
+
+	startLine, startCol := buf.PosToLineCol(0)
+	if startLine != 0 {
+		t.Errorf("Expected startLine == 0, got %v", startLine)
+		t.Fail()
+	}
+
+	if startCol != 0 {
+		t.Errorf("Expected startCol == 0, got %v", startCol)
+		t.Fail()
+	}
+
+	endPos := buf.Len()-1
+	endLine, endCol := buf.PosToLineCol(endPos)
+	t.Logf("endPos = %v", endPos)
+	if endLine != 3 {
+		t.Errorf("Expected endLine == 3, got %v", endLine)
+	}
+
+	if endCol != 5 {
+		t.Errorf("Expected endCol == 5, got %v", endCol)
+	}
+
+	line1Pos := 11 // Byte index of the delim separating line1 and line 2
+	line1Line, line1Col := buf.PosToLineCol(line1Pos)
+	if line1Line != 1 {
+		t.Errorf("Expected line1Line == 1, got %v", line1Line)
+		t.Fail()
+	}
+
+	if line1Col != 5 {
+		t.Errorf("Expected line1Col == 5, got %v", line1Col)
+		t.Fail()
+	}
+}
+
 func TestRopeInserting(t *testing.T) {
 	var buf Buffer = NewRopeBuffer([]byte("some"))
 	buf.Insert(0, 4, []byte(" text\n")) // Insert " text" after "some"