move box layout stuff into its own package

pkg/gui/boxlayout/boxlayout.go

@@ -0,0 +1,145 @@
+package boxlayout
+
+import "math"
+
+type Dimensions struct {
+	X0 int
+	X1 int
+	Y0 int
+	Y1 int
+}
+
+const (
+	ROW = iota
+	COLUMN
+)
+
+// to give a high-level explanation of what's going on here. We layout our views by arranging a bunch of boxes in the window.
+// If a box has children, it needs to specify how it wants to arrange those children: ROW or COLUMN.
+// If a box represents a view, you can put the view name in the viewName field.
+// When determining how to divvy-up the available height (for row children) or width (for column children), we first
+// give the boxes with a static `size` the space that they want. Then we apportion
+// the remaining space based on the weights of the dynamic boxes (you can't define
+// both size and weight at the same time: you gotta pick one). If there are two
+// boxes, one with weight 1 and the other with weight 2, the first one gets 33%
+// of the available space and the second one gets the remaining 66%
+
+type Box struct {
+	// Direction decides how the children boxes are laid out. ROW means the children will each form a row i.e. that they will be stacked on top of eachother.
+	Direction int // ROW or COLUMN
+
+	// function which takes the width and height assigned to the box and decides which orientation it will have
+	ConditionalDirection func(width int, height int) int
+
+	Children []*Box
+
+	// function which takes the width and height assigned to the box and decides the layout of the children.
+	ConditionalChildren func(width int, height int) []*Box
+
+	// ViewName refers to the name of the view this box represents, if there is one
+	ViewName string
+
+	// static Size. If parent box's direction is ROW this refers to height, otherwise width
+	Size int
+
+	// dynamic size. Once all statically sized children have been considered, Weight decides how much of the remaining space will be taken up by the box
+	// TODO: consider making there be one int and a type enum so we can't have size and Weight simultaneously defined
+	Weight int
+}
+
+func (b *Box) isStatic() bool {
+	return b.Size > 0
+}
+
+func (b *Box) getDirection(width int, height int) int {
+	if b.ConditionalDirection != nil {
+		return b.ConditionalDirection(width, height)
+	}
+	return b.Direction
+}
+
+func (b *Box) getChildren(width int, height int) []*Box {
+	if b.ConditionalChildren != nil {
+		return b.ConditionalChildren(width, height)
+	}
+	return b.Children
+}
+
+func ArrangeViews(root *Box, x0, y0, width, height int) map[string]Dimensions {
+	children := root.getChildren(width, height)
+	if len(children) == 0 {
+		// leaf node
+		if root.ViewName != "" {
+			dimensionsForView := Dimensions{X0: x0, Y0: y0, X1: x0 + width - 1, Y1: y0 + height - 1}
+			return map[string]Dimensions{root.ViewName: dimensionsForView}
+		}
+		return map[string]Dimensions{}
+	}
+
+	direction := root.getDirection(width, height)
+
+	var availableSize int
+	if direction == COLUMN {
+		availableSize = width
+	} else {
+		availableSize = height
+	}
+
+	// work out size taken up by children
+	reservedSize := 0
+	totalWeight := 0
+	for _, child := range children {
+		// assuming either size or weight are non-zero
+		reservedSize += child.Size
+		totalWeight += child.Weight
+	}
+
+	remainingSize := availableSize - reservedSize
+	if remainingSize < 0 {
+		remainingSize = 0
+	}
+
+	unitSize := 0
+	extraSize := 0
+	if totalWeight > 0 {
+		unitSize = remainingSize / totalWeight
+		extraSize = remainingSize % totalWeight
+	}
+
+	result := map[string]Dimensions{}
+	offset := 0
+	for _, child := range children {
+		var boxSize int
+		if child.isStatic() {
+			boxSize = child.Size
+		} else {
+			// TODO: consider more evenly distributing the remainder
+			boxSize = unitSize * child.Weight
+			boxExtraSize := int(math.Min(float64(extraSize), float64(child.Weight)))
+			boxSize += boxExtraSize
+			extraSize -= boxExtraSize
+		}
+
+		var resultForChild map[string]Dimensions
+		if direction == COLUMN {
+			resultForChild = ArrangeViews(child, x0+offset, y0, boxSize, height)
+		} else {
+			resultForChild = ArrangeViews(child, x0, y0+offset, width, boxSize)
+		}
+
+		result = mergeDimensionMaps(result, resultForChild)
+		offset += boxSize
+	}
+
+	return result
+}
+
+func mergeDimensionMaps(a map[string]Dimensions, b map[string]Dimensions) map[string]Dimensions {
+	result := map[string]Dimensions{}
+	for _, dimensionMap := range []map[string]Dimensions{a, b} {
+		for k, v := range dimensionMap {
+			result[k] = v
+		}
+	}
+	return result
+}