Store fromPos/toPos as 16-bit ints, and reorder fields for better packing

Hopefully, graphs will never get wider than 32768 characters. (They would get
kind of hard to navigate if they did...)

This reduces the size of the Pipe struct from 48 to 32 bytes, which makes a
significant difference when there are many millions of instances.

pkg/gui/presentation/graph/graph.go

@@ -23,12 +23,12 @@ const (
 )
 
 type Pipe struct {
-	fromPos  int
-	toPos    int
 	fromHash *string
 	toHash   *string
-	kind     PipeKind
 	style    *style.TextStyle
+	fromPos  int16
+	toPos    int16
+	kind     PipeKind
 }
 
 var (
@@ -37,11 +37,11 @@ var (
 	StartCommitHash     = "START"
 )
 
-func (self Pipe) left() int {
+func (self Pipe) left() int16 {
 	return min(self.fromPos, self.toPos)
 }
 
-func (self Pipe) right() int {
+func (self Pipe) right() int16 {
 	return max(self.fromPos, self.toPos)
 }
 
@@ -107,7 +107,7 @@ func RenderAux(pipeSets [][]Pipe, commits []*models.Commit, selectedCommitHashPt
 }
 
 func getNextPipes(prevPipes []Pipe, commit *models.Commit, getStyle func(c *models.Commit) *style.TextStyle) []Pipe {
-	maxPos := 0
+	maxPos := int16(0)
 	for _, pipe := range prevPipes {
 		if pipe.toPos > maxPos {
 			maxPos = pipe.toPos
@@ -133,6 +133,9 @@ func getNextPipes(prevPipes []Pipe, commit *models.Commit, getStyle func(c *mode
 	}
 
 	// a taken spot is one where a current pipe is ending on
+	// Note: this set and similar ones below use int instead of int16 because
+	// that's much more efficient. We cast the int16 values we store in these
+	// sets to int on every access.
 	takenSpots := set.New[int]()
 	// a traversed spot is one where a current pipe is starting on, ending on, or passing through
 	traversedSpots := set.New[int]()
@@ -155,41 +158,41 @@ func getNextPipes(prevPipes []Pipe, commit *models.Commit, getStyle func(c *mode
 	traversedSpotsForContinuingPipes := set.New[int]()
 	for _, pipe := range currentPipes {
 		if !equalHashes(pipe.toHash, commit.HashPtr()) {
-			traversedSpotsForContinuingPipes.Add(pipe.toPos)
+			traversedSpotsForContinuingPipes.Add(int(pipe.toPos))
 		}
 	}
 
-	getNextAvailablePosForContinuingPipe := func() int {
+	getNextAvailablePosForContinuingPipe := func() int16 {
-		i := 0
+		i := int16(0)
 		for {
-			if !traversedSpots.Includes(i) {
+			if !traversedSpots.Includes(int(i)) {
 				return i
 			}
 			i++
 		}
 	}
 
-	getNextAvailablePosForNewPipe := func() int {
+	getNextAvailablePosForNewPipe := func() int16 {
-		i := 0
+		i := int16(0)
 		for {
 			// a newly created pipe is not allowed to end on a spot that's already taken,
 			// nor on a spot that's been traversed by a continuing pipe.
-			if !takenSpots.Includes(i) && !traversedSpotsForContinuingPipes.Includes(i) {
+			if !takenSpots.Includes(int(i)) && !traversedSpotsForContinuingPipes.Includes(int(i)) {
 				return i
 			}
 			i++
 		}
 	}
 
-	traverse := func(from, to int) {
+	traverse := func(from, to int16) {
 		left, right := from, to
 		if left > right {
 			left, right = right, left
 		}
 		for i := left; i <= right; i++ {
-			traversedSpots.Add(i)
+			traversedSpots.Add(int(i))
 		}
-		takenSpots.Add(to)
+		takenSpots.Add(int(to))
 	}
 
 	for _, pipe := range currentPipes {
@@ -232,7 +235,7 @@ func getNextPipes(prevPipes []Pipe, commit *models.Commit, getStyle func(c *mode
 				style:    getStyle(commit),
 			})
 
-			takenSpots.Add(availablePos)
+			takenSpots.Add(int(availablePos))
 		}
 	}
 
@@ -241,7 +244,7 @@ func getNextPipes(prevPipes []Pipe, commit *models.Commit, getStyle func(c *mode
 			// continuing on, potentially moving left to fill in a blank spot
 			last := pipe.toPos
 			for i := pipe.toPos; i > pos; i-- {
-				if takenSpots.Includes(i) || traversedSpots.Includes(i) {
+				if takenSpots.Includes(int(i)) || traversedSpots.Includes(int(i)) {
 					break
 				} else {
 					last = i
@@ -275,8 +278,8 @@ func renderPipeSet(
 	selectedCommitHashPtr *string,
 	prevCommit *models.Commit,
 ) string {
-	maxPos := 0
+	maxPos := int16(0)
-	commitPos := 0
+	commitPos := int16(0)
 	startCount := 0
 	for _, pipe := range pipes {
 		if pipe.kind == STARTS {
@@ -292,7 +295,7 @@ func renderPipeSet(
 	}
 	isMerge := startCount > 1
 
-	cells := lo.Map(lo.Range(maxPos+1), func(i int, _ int) *Cell {
+	cells := lo.Map(lo.Range(int(maxPos)+1), func(i int, _ int) *Cell {
 		return &Cell{cellType: CONNECTION, style: &style.FgDefault}
 	})