package bolt

import (
	"bytes"
	"sort"
)

// Cursor represents an iterator that can traverse over all key/value pairs in a bucket in sorted order.
// Cursors can be obtained from a Transaction and are valid as long as the Transaction is open.
type Cursor struct {
	transaction *Transaction
	root        pgid
	stack       []pageElementRef
}

// First moves the cursor to the first item in the bucket and returns its key and value.
// If the bucket is empty then a nil key and value are returned.
func (c *Cursor) First() (key []byte, value []byte) {
	if len(c.stack) > 0 {
		c.stack = c.stack[:0]
	}
	p := c.transaction.page(c.root)
	c.stack = append(c.stack, pageElementRef{page: p, index: 0})
	c.first()
	return c.keyValue()
}

// Last moves the cursor to the last item in the bucket and returns its key and value.
// If the bucket is empty then a nil key and value are returned.
func (c *Cursor) Last() (key []byte, value []byte) {
	if len(c.stack) > 0 {
		c.stack = c.stack[:0]
	}
	p := c.transaction.page(c.root)
	c.stack = append(c.stack, pageElementRef{page: p, index: p.count - 1})
	c.last()
	return c.keyValue()
}

// Next moves the cursor to the next item in the bucket and returns its key and value.
// If the cursor is at the end of the bucket then a nil key and value are returned.
func (c *Cursor) Next() (key []byte, value []byte) {
	// Attempt to move over one element until we're successful.
	// Move up the stack as we hit the end of each page in our stack.
	for i := len(c.stack) - 1; i >= 0; i-- {
		elem := &c.stack[i]
		if elem.index < elem.page.count-1 {
			elem.index++
			break
		}
		c.stack = c.stack[:i]
	}

	// If we've hit the end then return nil.
	if len(c.stack) == 0 {
		return nil, nil
	}

	// Move down the stack to find the first element of the first leaf under this branch.
	c.first()
	return c.keyValue()
}

// Prev moves the cursor to the previous item in the bucket and returns its key and value.
// If the cursor is at the beginning of the bucket then a nil key and value are returned.
func (c *Cursor) Prev() (key []byte, value []byte) {
	// Attempt to move back one element until we're successful.
	// Move up the stack as we hit the beginning of each page in our stack.
	for i := len(c.stack) - 1; i >= 0; i-- {
		elem := &c.stack[i]
		if elem.index > 0 {
			elem.index--
			break
		}
		c.stack = c.stack[:i]
	}

	// If we've hit the end then return nil.
	if len(c.stack) == 0 {
		return nil, nil
	}

	// Move down the stack to find the last element of the last leaf under this branch.
	c.last()
	return c.keyValue()
}

// Seek moves the cursor to a given key and returns it.
// If the key does not exist then the next key is used. If no keys
// follow, a nil value is returned.
func (c *Cursor) Seek(seek []byte) (key []byte, value []byte) {
	// Start from root page and traverse to correct page.
	c.stack = c.stack[:0]
	c.search(seek, c.transaction.page(c.root))
	p, index := c.top()

	// If the cursor is pointing to the end of page then return nil.
	if index == p.count {
		return nil, nil
	}

	return c.element().key(), c.element().value()
}

// first moves the cursor to the first leaf element under the last page in the stack.
func (c *Cursor) first() {
	p := c.stack[len(c.stack)-1].page
	for {
		// Exit when we hit a leaf page.
		if (p.flags & leafPageFlag) != 0 {
			break
		}

		// Keep adding pages pointing to the first element to the stack.
		p = c.transaction.page(p.branchPageElement(c.stack[len(c.stack)-1].index).pgid)
		c.stack = append(c.stack, pageElementRef{page: p, index: 0})
	}
}

// last moves the cursor to the last leaf element under the last page in the stack.
func (c *Cursor) last() {
	p := c.stack[len(c.stack)-1].page
	for {
		// Exit when we hit a leaf page.
		if (p.flags & leafPageFlag) != 0 {
			break
		}

		// Keep adding pages pointing to the last element in the stack.
		p = c.transaction.page(p.branchPageElement(c.stack[len(c.stack)-1].index).pgid)
		c.stack = append(c.stack, pageElementRef{page: p, index: p.count - 1})
	}
}

// search recursively performs a binary search against a given page until it finds a given key.
func (c *Cursor) search(key []byte, p *page) {
	_assert((p.flags&(branchPageFlag|leafPageFlag)) != 0, "invalid page type: "+p.typ())
	e := pageElementRef{page: p}
	c.stack = append(c.stack, e)

	// If we're on a leaf page then find the specific node.
	if (p.flags & leafPageFlag) != 0 {
		c.nsearch(key, p)
		return
	}

	// Binary search for the correct range.
	inodes := p.branchPageElements()

	var exact bool
	index := sort.Search(int(p.count), func(i int) bool {
		// TODO(benbjohnson): Optimize this range search. It's a bit hacky right now.
		// sort.Search() finds the lowest index where f() != -1 but we need the highest index.
		ret := bytes.Compare(inodes[i].key(), key)
		if ret == 0 {
			exact = true
		}
		return ret != -1
	})
	if !exact && index > 0 {
		index--
	}
	c.stack[len(c.stack)-1].index = uint16(index)

	// Recursively search to the next page.
	c.search(key, c.transaction.page(inodes[index].pgid))
}

// nsearch searches a leaf node for the index of the node that matches key.
func (c *Cursor) nsearch(key []byte, p *page) {
	e := &c.stack[len(c.stack)-1]

	// Binary search for the correct leaf node index.
	inodes := p.leafPageElements()
	index := sort.Search(int(p.count), func(i int) bool {
		return bytes.Compare(inodes[i].key(), key) != -1
	})
	e.index = uint16(index)
}

// top returns the page and leaf node that the cursor is currently pointing at.
func (c *Cursor) top() (*page, uint16) {
	ptr := c.stack[len(c.stack)-1]
	return ptr.page, ptr.index
}

// element returns the leaf element that the cursor is currently positioned on.
func (c *Cursor) element() *leafPageElement {
	ref := c.stack[len(c.stack)-1]
	return ref.page.leafPageElement(ref.index)
}

// keyValue returns the key and value of the current leaf element.
func (c *Cursor) keyValue() ([]byte, []byte) {
	ref := &c.stack[len(c.stack)-1]
	if ref.index >= ref.page.count {
		return nil, nil
	}
	e := ref.page.leafPageElement(ref.index)
	return e.key(), e.value()
}

// node returns the node that the cursor is currently positioned on.
func (c *Cursor) node(t *RWTransaction) *node {
	_assert(len(c.stack) > 0, "accessing a node with a zero-length cursor stack")

	// Start from root and traverse down the hierarchy.
	n := t.node(c.stack[0].page.id, nil)
	for _, ref := range c.stack[:len(c.stack)-1] {
		_assert(!n.isLeaf, "expected branch node")
		_assert(ref.page.id == n.pgid, "node/page mismatch a: %d != %d", ref.page.id, n.childAt(int(ref.index)).pgid)
		n = n.childAt(int(ref.index))
	}
	_assert(n.isLeaf, "expected leaf node")
	_assert(n.pgid == c.stack[len(c.stack)-1].page.id, "node/page mismatch b: %d != %d", n.pgid, c.stack[len(c.stack)-1].page.id)
	return n
}