Make some more tests pass

m/linenumbers/linenumber.go

@@ -96,7 +96,7 @@ func (l LineNumber) Format() string {
 // If both lines are the same this method will return 1.
 func (l LineNumber) CountLinesTo(next LineNumber) int {
 	if l.number > next.number {
-		panic(fmt.Errorf("line numbers must be ordered, got %d-%d", l.number, next.number))
+		panic(fmt.Errorf("line numbers must be ordered, got %s-%s", l.Format(), next.Format()))
 	}
 
 	return 1 + next.AsZeroBased() - l.AsZeroBased()

m/linenumbers/linenumber_test.go

@@ -45,3 +45,15 @@ func TestLineNumberEquality(t *testing.T) {
 	assert.Equal(t, LineNumberFromZeroBased(1), LineNumberFromOneBased(2),
 		"Two different ways of representing the same line number should be equal")
 }
+
+func TestLineNumberCountLinesTo(t *testing.T) {
+	assert.Equal(t,
+		LineNumberFromZeroBased(0).CountLinesTo(LineNumberFromZeroBased(0)),
+		1, // Count is inclusive, so countint from 0 to 0 is 1
+	)
+
+	assert.Equal(t,
+		LineNumberFromZeroBased(0).CountLinesTo(LineNumberFromZeroBased(1)),
+		2, // Count is inclusive, so countint from 0 to 1 is 2
+	)
+}

m/reader.go

@@ -638,17 +638,16 @@ func (reader *Reader) getLinesUnlocked(firstLine linenumbers.LineNumber, wantedL
 
 	lastLine := firstLine.NonWrappingAdd(wantedLineCount - 1)
 
-	if lastLine.AsZeroBased() >= len(reader.lines) {
-		lastLine = *linenumbers.LineNumberFromLength(len(reader.lines))
-	}
-
 	// Prevent reading past the end of the available lines
-	if firstLine.CountLinesTo(lastLine) < wantedLineCount && !firstLine.IsZero() {
-		// 5 would mean we've gone 5 lines too far
-		overshoot := wantedLineCount - firstLine.CountLinesTo(lastLine)
-		firstLine = firstLine.NonWrappingAdd(-overshoot)
+	maxLineNumber := *linenumbers.LineNumberFromLength(len(reader.lines))
+	if lastLine.IsAfter(maxLineNumber) {
+		lastLine = maxLineNumber
 
-		return reader.getLinesUnlocked(firstLine, wantedLineCount)
+		// If one line was requested, then first and last should be exactly the
+		// same, and we would get there by adding zero.
+		firstLine = lastLine.NonWrappingAdd(1 - wantedLineCount)
+
+		return reader.getLinesUnlocked(firstLine, firstLine.CountLinesTo(lastLine))
 	}
 
 	returnLines := reader.lines[firstLine.AsZeroBased() : lastLine.AsZeroBased()+1]

m/scrollPosition_test.go

@@ -36,7 +36,7 @@ func testCanonicalize1000(t *testing.T, withStatusBar bool, currentStartLine lin
 		},
 	}
 
-	assert.Equal(t, lastVisiblePosition.lineNumber(&pager), lastVisibleLine)
+	assert.Equal(t, *lastVisiblePosition.lineNumber(&pager), lastVisibleLine)
 }
 
 func TestCanonicalize1000WithStatusBar(t *testing.T) {