repo_description: An embedded key/value database for Go.
repo_topics: database go key-value
code: package bolt
  
  import (
  	"bytes"
  	"fmt"
  	"sort"
  )
  
  // Cursor represents an iterator that can traverse over all key/value pairs in a bucket in sorted order.
  // Cursors see nested buckets with value == nil.
  // Cursors can be obtained from a transaction and are valid as long as the transaction is open.
  //
  // Keys and values returned from the cursor are only valid for the life of the transaction.
  //
  // Changing data while traversing with a cursor may cause it to be invalidated
  // and return unexpected keys and/or values. You must reposition your cursor
  // after mutating data.
  type Cursor struct {
  	bucket *Bucket
  	stack  []elemRef
  }
  
  // Bucket returns the bucket that this cursor was created from.
  func (c *Cursor) Bucket() *Bucket {
  	return c.bucket
  }
  
  // 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.
  // The returned key and value are only valid for the life of the transaction.
  func (c *Cursor) First() (key []byte, value []byte) {
  	_assert(c.bucket.tx.db != nil, "tx closed")
  	c.stack = c.stack[:0]
  	p, n := c.bucket.pageNode(c.bucket.root)
  	c.stack = append(c.stack, elemRef{page: p, node: n, index: 0})
  	c.first()
  
  	// If we land on an empty page then move to the next value.
  	// https://github.com/boltdb/bolt/issues/450
  	if c.stack[len(c.stack)-1].count() == 0 {
  		c.next()
  	}
  
  	k, v, flags := c.keyValue()
  	if (flags & uint32(bucketLeafFlag)) != 0 {
  		return k, nil
  	}
  	return k, v
  
  }
  
  // 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.
  // The returned key and value are only valid for the life of the transaction.
  func (c *Cursor) Last() (key []byte, value []byte) {
  	_assert(c.bucket.tx.db != nil, "tx closed")
  	c.stack = c.stack[:0]
  	p, n := c.bucket.pageNode(c.bucket.root)
  	ref := elemRef{page: p, node: n}
  	ref.index = ref.count() - 1
  	c.stack = append(c.stack, ref)
  	c.last()
  	k, v, flags := c.keyValue()
  	if (flags & uint32(bucketLeafFlag)) != 0 {
  		return k, nil
  	}
  	return k, v
  }
  
  // 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.
  // The returned key and value are only valid for the life of the transaction.
  func (c *Cursor) Next() (key []byte, value []byte) {
  	_assert(c.bucket.tx.db != nil, "tx closed")
  	k, v, flags := c.next()
  	if (flags & uint32(bucketLeafFlag)) != 0 {
  		return k, nil
  	}
  	return k, v
  }
  
  // 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.
  // The returned key and value are only valid for the life of the transaction.
  func (c *Cursor) Prev() (key []byte, value []byte) {
  	_assert(c.bucket.tx.db != nil, "tx closed")
  
  	// 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()
  	k, v, flags := c.keyValue()
  	if (flags & uint32(bucketLeafFlag)) != 0 {
  		return k, nil
  	}
  	return k, v
  }
  
  // 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 key is returned.
  // The returned key and value are only valid for the life of the transaction.
  func (c *Cursor) Seek(seek []byte) (key []byte, value []byte) {
  	k, v, flags := c.seek(seek)
  
  	// If we ended up after the last element of a page then move to the next one.
  	if ref := &c.stack[len(c.stack)-1]; ref.index >= ref.count() {
  		k, v, flags = c.next()
  	}
  
  	if k == nil {
  		return nil, nil
  	} else if (flags & uint32(bucketLeafFlag)) != 0 {
  		return k, nil
  	}
  	return k, v
  }
  
  // Delete removes the current key/value under the cursor from the bucket.
  // Delete fails if current key/value is a bucket or if the transaction is not writable.
  func (c *Cursor) Delete() error {
  	if c.bucket.tx.db == nil {
  		return ErrTxClosed
  	} else if !c.bucket.Writable() {
  		return ErrTxNotWritable
  	}
  
  	key, _, flags := c.keyValue()
  	// Return an error if current value is a bucket.
  	if (flags & bucketLeafFlag) != 0 {
  		return ErrIncompatibleValue
  	}
  	c.node().del(key)
  
  	return nil
  }
  
  // seek moves the cursor to a given key and returns it.
  // If the key does not exist then the next key is used.
  func (c *Cursor) seek(seek []byte) (key []byte, value []byte, flags uint32) {
  	_assert(c.bucket.tx.db != nil, "tx closed")
  
  	// Start from root page/node and traverse to correct page.
  	c.stack = c.stack[:0]
  	c.search(seek, c.bucket.root)
  	ref := &c.stack[len(c.stack)-1]
  
  	// If the cursor is pointing to the end of page/node then return nil.
  	if ref.index >= ref.count() {
  		return nil, nil, 0
  	}
  
  	// If this is a bucket then return a nil value.
  	return c.keyValue()
  }
  
  // first moves the cursor to the first leaf element under the last page in the stack.
  func (c *Cursor) first() {
  	for {
  		// Exit when we hit a leaf page.
  		var ref = &c.stack[len(c.stack)-1]
  		if ref.isLeaf() {
  			break
  		}
  
  		// Keep adding pages pointing to the first element to the stack.
  		var pgid pgid
  		if ref.node != nil {
  			pgid = ref.node.inodes[ref.index].pgid
  		} else {
  			pgid = ref.page.branchPageElement(uint16(ref.index)).pgid
  		}
  		p, n := c.bucket.pageNode(pgid)
  		c.stack = append(c.stack, elemRef{page: p, node: n, index: 0})
  	}
  }
  
  // last moves the cursor to the last leaf element under the last page in the stack.
  func (c *Cursor) last() {
  	for {
  		// Exit when we hit a leaf page.
  		ref := &c.stack[len(c.stack)-1]
  		if ref.isLeaf() {
  			break
  		}
  
  		// Keep adding pages pointing to the last element in the stack.
  		var pgid pgid
  		if ref.node != nil {
  			pgid = ref.node.inodes[ref.index].pgid
  		} else {
  			pgid = ref.page.branchPageElement(uint16(ref.index)).pgid
  		}
  		p, n := c.bucket.pageNode(pgid)
  
  		var nextRef = elemRef{page: p, node: n}
  		nextRef.index = nextRef.count() - 1
  		c.stack = append(c.stack, nextRef)
  	}
  }
  
  // next moves to the next leaf element and returns the key and value.
  // If the cursor is at the last leaf element then it stays there and returns nil.
  func (c *Cursor) next() (key []byte, value []byte, flags uint32) {
  	for {
  		// 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.
  		var i int
  		for i = len(c.stack) - 1; i >= 0; i-- {
  			elem := &c.stack[i]
  			if elem.index < elem.count()-1 {
  				elem.index++
  				break
  			}
  		}
  
  		// If we've hit the root page then stop and return. This will leave the
  		// cursor on the last element of the last page.
  		if i == -1 {
  			return nil, nil, 0
  		}
  
  		// Otherwise start from where we left off in the stack and find the
  		// first element of the first leaf page.
  		c.stack = c.stack[:i+1]
  		c.first()
  
  		// If this is an empty page then restart and move back up the stack.
  		// https://github.com/boltdb/bolt/issues/450
  		if c.stack[len(c.stack)-1].count() == 0 {
  			continue
  		}
  
  		return c.keyValue()
  	}
  }
  
  // search recursively performs a binary search against a given page/node until it finds a given key.
  func (c *Cursor) search(key []byte, pgid pgid) {
  	p, n := c.bucket.pageNode(pgid)
  	if p != nil && (p.flags&(branchPageFlag|leafPageFlag)) == 0 {
  		panic(fmt.Sprintf("invalid page type: %d: %x", p.id, p.flags))
  	}
  	e := elemRef{page: p, node: n}
  	c.stack = append(c.stack, e)
  
  	// If we're on a leaf page/node then find the specific node.
  	if e.isLeaf() {
  		c.nsearch(key)
  		return
  	}
  
  	if n != nil {
  		c.searchNode(key, n)
  		return
  	}
  	c.searchPage(key, p)
  }
  
  func (c *Cursor) searchNode(key []byte, n *node) {
  	var exact bool
  	index := sort.Search(len(n.inodes), 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(n.inodes[i].key, key)
  		if ret == 0 {
  			exact = true
  		}
  		return ret != -1
  	})
  	if !exact && index > 0 {
  		index--
  	}
  	c.stack[len(c.stack)-1].index = index
  
  	// Recursively search to the next page.
  	c.search(key, n.inodes[index].pgid)
  }
  
  func (c *Cursor) searchPage(key []byte, p *page) {
  	// 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 = index
  
  	// Recursively search to the next page.
  	c.search(key, inodes[index].pgid)
  }
  
  // nsearch searches the leaf node on the top of the stack for a key.
  func (c *Cursor) nsearch(key []byte) {
  	e := &c.stack[len(c.stack)-1]
  	p, n := e.page, e.node
  
  	// If we have a node then search its inodes.
  	if n != nil {
  		index := sort.Search(len(n.inodes), func(i int) bool {
  			return bytes.Compare(n.inodes[i].key, key) != -1
  		})
  		e.index = index
  		return
  	}
  
  	// If we have a page then search its leaf elements.
  	inodes := p.leafPageElements()
  	index := sort.Search(int(p.count), func(i int) bool {
  		return bytes.Compare(inodes[i].key(), key) != -1
  	})
  	e.index = index
  }
  
  // keyValue returns the key and value of the current leaf element.
  func (c *Cursor) keyValue() ([]byte, []byte, uint32) {
  	ref := &c.stack[len(c.stack)-1]
  	if ref.count() == 0 || ref.index >= ref.count() {
  		return nil, nil, 0
  	}
  
  	// Retrieve value from node.
  	if ref.node != nil {
  		inode := &ref.node.inodes[ref.index]
  		return inode.key, inode.value, inode.flags
  	}
  
  	// Or retrieve value from page.
  	elem := ref.page.leafPageElement(uint16(ref.index))
  	return elem.key(), elem.value(), elem.flags
  }
  
  // node returns the node that the cursor is currently positioned on.
  func (c *Cursor) node() *node {
  	_assert(len(c.stack) > 0, "accessing a node with a zero-length cursor stack")
  
  	// If the top of the stack is a leaf node then just return it.
  	if ref := &c.stack[len(c.stack)-1]; ref.node != nil && ref.isLeaf() {
  		return ref.node
  	}
  
  	// Start from root and traverse down the hierarchy.
  	var n = c.stack[0].node
  	if n == nil {
  		n = c.bucket.node(c.stack[0].page.id, nil)
  	}
  	for _, ref := range c.stack[:len(c.stack)-1] {
  		_assert(!n.isLeaf, "expected branch node")
  		n = n.childAt(int(ref.index))
  	}
  	_assert(n.isLeaf, "expected leaf node")
  	return n
  }
  
  // elemRef represents a reference to an element on a given page/node.
  type elemRef struct {
  	page  *page
  	node  *node
  	index int
  }
  
  // isLeaf returns whether the ref is pointing at a leaf page/node.
  func (r *elemRef) isLeaf() bool {
  	if r.node != nil {
  		return r.node.isLeaf
  	}
  	return (r.page.flags & leafPageFlag) != 0
  }
  
  // count returns the number of inodes or page elements.
  func (r *elemRef) count() int {
  	if r.node != nil {
  		return len(r.node.inodes)
  	}
  	return int(r.page.count)
  }
  