Record how lines run through the cells of the commit graph

A cell of the commit graph only records which of its edges are touched
by lines. That's all the box drawing characters can show, but it
doesn't say how the lines connect. A vertical line with a horizontal
line passing behind it touches all four edges. So does a cell in which
the line from above bends to the left, the line below comes in from the
left, and a horizontal line passes through.

For the lines at the top and bottom edges, record whether they run
straight on or bend to the left or right, and record whether a line
passes through horizontally. Nothing reads this yet. It prepares for
drawing the graph with symbols that can show these differences.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

pkg/gui/presentation/graph/cell.go

@@ -21,17 +21,33 @@ const (
 	MERGE
 )
 
+// How a line that touches the top or bottom edge of a cell runs within it
+type verticalLine uint8
+
+const (
+	noLine verticalLine = iota
+	// On to the opposite edge, or into the commit symbol
+	straightLine
+	// Bends towards the left edge
+	lineToLeft
+	// Bends towards the right edge
+	lineToRight
+)
+
 type Cell struct {
-	up, down, left, right bool
-	cellType              cellType
-	rightStyle            *style.TextStyle
-	style                 *style.TextStyle
+	// The lines that touch the top and bottom edges
+	up, down verticalLine
+	// Whether lines touch the left and right edges
+	left, right bool
+	// Whether a line passes through from the left edge to the right edge
+	horizontal bool
+	cellType   cellType
+	rightStyle *style.TextStyle
+	style      *style.TextStyle
 }
 
 func (cell *Cell) render(writer io.StringWriter) {
-	up, down, left, right := cell.up, cell.down, cell.left, cell.right
-
-	first, second := getBoxDrawingChars(up, down, left, right)
+	first, second := getBoxDrawingChars(cell.up != noLine, cell.down != noLine, cell.left, cell.right)
 	var adjustedFirst string
 	switch cell.cellType {
 	case CONNECTION:
@@ -98,34 +114,34 @@ func cachedSprint(style style.TextStyle, str string) string {
 }
 
 func (cell *Cell) reset() {
-	cell.up = false
-	cell.down = false
+	cell.up = noLine
+	cell.down = noLine
 	cell.left = false
 	cell.right = false
+	cell.horizontal = false
 }
 
-func (cell *Cell) setUp(style *style.TextStyle) *Cell {
-	cell.up = true
+func (cell *Cell) setUp(style *style.TextStyle, line verticalLine) *Cell {
+	cell.up = line
 	cell.style = style
 	return cell
 }
 
-func (cell *Cell) setDown(style *style.TextStyle) *Cell {
-	cell.down = true
+func (cell *Cell) setDown(style *style.TextStyle, line verticalLine) *Cell {
+	cell.down = line
 	cell.style = style
 	return cell
 }
 
 func (cell *Cell) setLeft(style *style.TextStyle) *Cell {
 	cell.left = true
-	if !cell.up && !cell.down {
+	if cell.up == noLine && cell.down == noLine {
 		// vertical trumps left
 		cell.style = style
 	}
 	return cell
 }
 
-//nolint:unparam
 func (cell *Cell) setRight(style *style.TextStyle, override bool) *Cell {
 	cell.right = true
 	if cell.rightStyle == nil || override {
@@ -134,6 +150,11 @@ func (cell *Cell) setRight(style *style.TextStyle, override bool) *Cell {
 	return cell
 }
 
+func (cell *Cell) setHorizontal(style *style.TextStyle, overrideRightStyle bool) *Cell {
+	cell.horizontal = true
+	return cell.setLeft(style).setRight(style, overrideRightStyle)
+}
+
 func (cell *Cell) setStyle(style *style.TextStyle) *Cell {
 	cell.style = style
 	return cell

pkg/gui/presentation/graph/graph.go

@@ -303,17 +303,17 @@ func renderPipeSet(
 
 		if left != right {
 			for i := left + 1; i < right; i++ {
-				cells[i].setLeft(style).setRight(style, overrideRightStyle)
+				cells[i].setHorizontal(style, overrideRightStyle)
 			}
 			cells[left].setRight(style, overrideRightStyle)
 			cells[right].setLeft(style)
 		}
 
 		if pipe.kind == STARTS || pipe.kind == CONTINUES {
-			cells[pipe.toPos].setDown(style)
+			cells[pipe.toPos].setDown(style, lineTowards(pipe.toPos, pipe.fromPos))
 		}
 		if pipe.kind == TERMINATES || pipe.kind == CONTINUES {
-			cells[pipe.fromPos].setUp(style)
+			cells[pipe.fromPos].setUp(style, lineTowards(pipe.fromPos, pipe.toPos))
 		}
 	}
 
@@ -375,6 +375,19 @@ func renderPipeSet(
 	return writer.String()
 }
 
+// The course of a line through the top or bottom edge of the cell at pos,
+// given the column of the pipe's other end
+func lineTowards(pos, otherPos int16) verticalLine {
+	switch {
+	case otherPos < pos:
+		return lineToLeft
+	case otherPos > pos:
+		return lineToRight
+	default:
+		return straightLine
+	}
+}
+
 func equalHashes(a, b *string) bool {
 	// if our selectedCommitHashPtr is nil, there is no selected commit
 	if a == nil || b == nil {