Fix crash on paging down

Fixes #65.

m/screenLines.go

@@ -31,6 +31,15 @@ type RenderedLine struct {
 	cells []twin.Cell
 }
 
+func (sl *ScreenLines) lastInputLineOneBased() int {
+	if sl.inputLines.lines == nil {
+		panic("nil input lines, cannot make up a last input line number")
+	}
+
+	// Offsets figured out through trial-and-error...
+	return sl.inputLines.firstLineOneBased + len(sl.inputLines.lines) - 1
+}
+
 // Render screen lines into an array of lines consisting of Cells.
 //
 // The second return value is the same as firstInputLineOneBased, but decreased
@@ -45,6 +54,10 @@ func (sl *ScreenLines) renderScreenLines() ([][]twin.Cell, int) {
 		sl.firstInputLineOneBased = 1
 	}
 
+	if sl.firstInputLineOneBased > sl.lastInputLineOneBased() {
+		sl.firstInputLineOneBased = sl.lastInputLineOneBased()
+	}
+
 	allPossibleLines := sl.renderAllLines()
 
 	// Find which index in allPossibleLines the user wants to see at the top of
@@ -111,10 +124,7 @@ func (sl *ScreenLines) toScreenLinesArray(allPossibleLines []RenderedLine, first
 
 func (sl *ScreenLines) renderAllLines() []RenderedLine {
 	// Count the length of the last line number
-	//
-	// Offsets figured out through trial-and-error...
-	lastLineOneBased := sl.inputLines.firstLineOneBased + len(sl.inputLines.lines) - 1
-	numberPrefixLength := len(formatNumber(uint(lastLineOneBased))) + 1
+	numberPrefixLength := len(formatNumber(uint(sl.lastInputLineOneBased()))) + 1
 	if numberPrefixLength < 4 {
 		// 4 = space for 3 digits followed by one whitespace
 		//

m/screenLines_test.go

@@ -6,7 +6,7 @@ import (
 	"gotest.tools/assert"
 )
 
-func testCropping(t *testing.T, contents string, firstIndex int, lastIndex int, expected string) {
+func testHorizontalCropping(t *testing.T, contents string, firstIndex int, lastIndex int, expected string) {
 	screenLines := ScreenLines{width: 1 + lastIndex - firstIndex, leftColumnZeroBased: firstIndex}
 	lineContents := NewLine(contents).HighlightedTokens(nil)
 	screenLine := screenLines.createScreenLine(nil, 0, lineContents)
@@ -14,27 +14,27 @@ func testCropping(t *testing.T, contents string, firstIndex int, lastIndex int,
 }
 
 func TestCreateScreenLine(t *testing.T) {
-	testCropping(t, "abc", 0, 10, "abc")
+	testHorizontalCropping(t, "abc", 0, 10, "abc")
 }
 
 func TestCreateScreenLineCanScrollLeft(t *testing.T) {
-	testCropping(t, "abc", 1, 10, "<c")
+	testHorizontalCropping(t, "abc", 1, 10, "<c")
 }
 
 func TestCreateScreenLineCanScrollRight(t *testing.T) {
-	testCropping(t, "abc", 0, 1, "a>")
+	testHorizontalCropping(t, "abc", 0, 1, "a>")
 }
 
 func TestCreateScreenLineCanAlmostScrollRight(t *testing.T) {
-	testCropping(t, "abc", 0, 2, "abc")
+	testHorizontalCropping(t, "abc", 0, 2, "abc")
 }
 
 func TestCreateScreenLineCanScrollBoth(t *testing.T) {
-	testCropping(t, "abcde", 1, 3, "<c>")
+	testHorizontalCropping(t, "abcde", 1, 3, "<c>")
 }
 
 func TestCreateScreenLineCanAlmostScrollBoth(t *testing.T) {
-	testCropping(t, "abcd", 1, 3, "<cd")
+	testHorizontalCropping(t, "abcd", 1, 3, "<cd")
 }
 
 func TestEmpty(t *testing.T) {
@@ -53,3 +53,55 @@ func TestEmpty(t *testing.T) {
 	assert.Equal(t, len(rendered), 0)
 	assert.Equal(t, firstScreenLine, 0)
 }
+
+func TestOverflowDown(t *testing.T) {
+	// Set up a single line input
+	line := Line{
+		raw: "hej",
+	}
+	inputLines := InputLines{
+		lines:             []*Line{&line},
+		firstLineOneBased: 1,
+	}
+
+	// Set up a single line screen
+	screenLines := ScreenLines{
+		inputLines: &inputLines,
+		height:     1,
+		width:      10, // Longer than the raw line, we're testing vertical overflow, not horizontal
+
+		// This value can be anything and should be clipped, that's what we're testing
+		firstInputLineOneBased: 42,
+	}
+
+	rendered, firstScreenLine := screenLines.renderScreenLines()
+	assert.Equal(t, len(rendered), 1)
+	assert.Equal(t, "hej", rowToString(rendered[0]))
+	assert.Equal(t, firstScreenLine, 1)
+}
+
+func TestOverflowUp(t *testing.T) {
+	// Set up a single line input
+	line := Line{
+		raw: "hej",
+	}
+	inputLines := InputLines{
+		lines:             []*Line{&line},
+		firstLineOneBased: 1,
+	}
+
+	// Set up a single line screen
+	screenLines := ScreenLines{
+		inputLines: &inputLines,
+		height:     1,
+		width:      10, // Longer than the raw line, we're testing vertical overflow, not horizontal
+
+		// This value can be anything and should be clipped, that's what we're testing
+		firstInputLineOneBased: 0,
+	}
+
+	rendered, firstScreenLine := screenLines.renderScreenLines()
+	assert.Equal(t, len(rendered), 1)
+	assert.Equal(t, "hej", rowToString(rendered[0]))
+	assert.Equal(t, firstScreenLine, 1)
+}