// Package navigation implements the menu navigation stack for Nebara OS TUI.
package navigation

import "github.com/nebara/nebara-os/internal/model"

// Level represents one level in the navigation stack.
type Level struct {
	Items       []model.MenuItem
	SelectedIdx int
}

// Navigator maintains a stack of menu levels and tracks the current position.
type Navigator struct {
	stack []Level
}

// NewNavigator creates a Navigator rooted at the main menu.
func NewNavigator(items []model.MenuItem, initialIdx int) *Navigator {
	if initialIdx < 0 || initialIdx >= len(items) {
		initialIdx = 0
	}
	return &Navigator{
		stack: []Level{
			{Items: items, SelectedIdx: initialIdx},
		},
	}
}

// Current returns the active menu level.
func (n *Navigator) Current() *Level {
	if len(n.stack) == 0 {
		return nil
	}
	return &n.stack[len(n.stack)-1]
}

// Depth returns the current nesting depth (0 = main menu).
func (n *Navigator) Depth() int {
	return len(n.stack) - 1
}

// IsAtRoot reports whether the navigator is at the main menu level.
func (n *Navigator) IsAtRoot() bool {
	return len(n.stack) == 1
}

// SelectedItem returns the currently highlighted menu item.
func (n *Navigator) SelectedItem() *model.MenuItem {
	cur := n.Current()
	if cur == nil {
		return nil
	}
	return &cur.Items[cur.SelectedIdx]
}

// MoveUp moves the selection one row up, clamping at the first item.
func (n *Navigator) MoveUp() {
	cur := n.Current()
	if cur == nil {
		return
	}
	if cur.SelectedIdx > 0 {
		cur.SelectedIdx--
	}
}

// MoveDown moves the selection one row down, clamping at the last item.
func (n *Navigator) MoveDown() {
	cur := n.Current()
	if cur == nil {
		return
	}
	if cur.SelectedIdx < len(cur.Items)-1 {
		cur.SelectedIdx++
	}
}

// MoveFirst jumps to the first item.
func (n *Navigator) MoveFirst() {
	cur := n.Current()
	if cur != nil {
		cur.SelectedIdx = 0
	}
}

// MoveLast jumps to the last item.
func (n *Navigator) MoveLast() {
	cur := n.Current()
	if cur != nil {
		cur.SelectedIdx = len(cur.Items) - 1
	}
}

// Enter descends into the submenu of the currently selected item.
// Returns false when the item has no children (leaf node).
func (n *Navigator) Enter() bool {
	item := n.SelectedItem()
	if item == nil || len(item.Children) == 0 {
		return false
	}
	n.stack = append(n.stack, Level{
		Items:       item.Children,
		SelectedIdx: 0,
	})
	return true
}

// Back pops the current level from the stack, returning to the parent.
// Returns false when already at the root.
func (n *Navigator) Back() bool {
	if len(n.stack) <= 1 {
		return false
	}
	n.stack = n.stack[:len(n.stack)-1]
	return true
}

// BreadcrumbTitle builds a display title for the current navigation depth.
// It returns empty string when at the root.
func (n *Navigator) BreadcrumbTitle() string {
	if len(n.stack) <= 1 {
		return ""
	}
	// Return the label of the parent item whose children we are browsing.
	parent := n.stack[len(n.stack)-2]
	return parent.Items[parent.SelectedIdx].Label
}