pmap/pmap.go

package pmap

import (
	"fmt"
	"math/bits"
)

const (
	nodeDegree = 16 // branch factor of nodes
	nodeShift  = 4
	nodeMask   = 0b1111
)

type Key = int
type Value = int

type Map interface {
	Get(Key) (Value, bool)
	Set(Key, Value) Map
	Del(Key) Map
	Len() int
}

type pmap struct {
	root interface{}
	len  int
	hash func(Key) uint32
}

// A Map implemented as a hashed trie
type node struct {
	child  []interface{}
	bitmap uint32
}

func bitmask(shiftedHash uint32) uint32 {
	return uint32(1) << (shiftedHash & nodeMask)
}

func (n *node) index(mask uint32) int {
	return bits.OnesCount32(n.bitmap & (mask - 1))
}

type collision struct {
	hash uint32
	leaf []leaf
}

type leaf struct {
	k Key
	v Value
}

type HashFunc = func(Key) uint32

func New(hash HashFunc) Map {
	if hash == nil {
		panic("pmap.New: nil hash")
	}
	return pmap{hash: hash}
}

func (p pmap) Len() int {
	return p.len
}

func (p pmap) Get(k Key) (Value, bool) {
	var zero Value
	h := p.hash(k)
	return lookup(p.root, h, k, zero)
}

func (p pmap) Set(k Key, v Value) Map {
	h := p.hash(k)
	root, added := insert(p.root, h, k, v, p.hash)
	p.root = root
	if added {
		p.len++
	}
	//pretty.Println(p)
	return p
}

func (p pmap) Del(k Key) Map {
	return p
}

func (n *node) check() {
	if bits.OnesCount32(n.bitmap) != len(n.child) {
		panic(fmt.Sprintf("pmap: corrupt bitmap b=%#b len=%d", n.bitmap, len(n.child)))
	}
}

func (n *node) getNode(shift, hash uint32, key Key) interface{} {
	n.check()
	m := bitmask(hash >> shift)
	if n.bitmap&m == 0 {
		return nil
	}
	return n.child[n.index(m)]
}

func (n collision) getNode(hash uint32, key Key) interface{} {
	if hash != n.hash {
		return nil
	}
	for i := range n.leaf {
		if key == n.leaf[i].k {
			return n.leaf[i]
		}
	}
	return nil
}

func lookup(root interface{}, hash uint32, key Key, zero Value) (Value, bool) {
	shift := 0
	cur := root
	for {
		switch n := cur.(type) {
		case nil:
			return zero, false
		case leaf:
			if n.k == key {
				return n.v, true
			} else {
				return zero, false
			}
		case *node:
			cur = n.getNode(shift, hash, key)
			shift += nodeShift
		case collision:
			cur = n.getNode(hash, key)
		default:
			panic("pmap: unhandled case in lookup")
		}
	}
}

func singleton(key Key, val Value, hash, shift uint32) *node {
	return newnode(leaf{key, val}, hash, shift)
}

func newnode(child interface{}, hash, shift uint32) *node {
	n := &node{}
	a := [1]interface{}{child}
	n.child = a[:]
	n.bitmap = bitmask(hash >> shift)
	n.check()
	return n
}

func insert(n interface{}, hash uint32, key Key, val Value, hashFn HashFunc) (newNode interface{}, added bool) {
	if n == nil {
		return leaf{key, val}, true
	}
	var _insert func(n interface{}, shift uint32) interface{}
	_insert = func(n interface{}, shift uint32) interface{} {
		//fmt.Printf("insert %v %x %#v\n", shift, hash, n)
		switch n := n.(type) {
		//case nil:
		//	added = true
		//	return leaf{key, val}
		case leaf:
			if n.k == key {
				// replace existing entry
				added = false
				return leaf{key, val}
			} else if h := hashFn(n.k); h == hash {
				// collision
				added = true
				return collision{hash, []leaf{{key, val}, n}}
			} else {
				if h>>shift == hash>>shift {
					panic("pmap: infinite loop in insert")
				}
				// not a collision, so we must still have some hash bits left
				// split the trie
				x := newnode(n, h, shift)
				return _insert(x, shift)
			}
		case *node:
			c := n.getNode(shift, hash, key)
			if c == nil {
				// new node
				c = leaf{key, val}
				added = true

				m := bitmask(hash >> shift)
				x := &node{bitmap: n.bitmap | m}
				i := x.index(m)
				x.child = make([]interface{}, len(n.child)+1)
				copy(x.child[:i], n.child[:i])
				x.child[i] = c
				copy(x.child[i+1:], n.child[i:])
				x.check()
				return x
			} else {
				c = _insert(c, shift+nodeShift)
				// TODO: short circuit if c unchanged?
				m := bitmask(hash >> shift)
				x := &node{bitmap: n.bitmap}
				i := x.index(m)
				x.child = make([]interface{}, len(n.child))
				copy(x.child, n.child)
				x.child[i] = c
				x.check()
				return x
			}
		case collision:
			if n.hash != hash {
				// not a collision, so we must still have some hash bits left
				// split the trie
				x := newnode(n, n.hash, shift)
				return _insert(x, shift)
			}
			for i := range n.leaf {
				if key == n.leaf[i].k {
					// replace existing entry
					l := make([]leaf, len(n.leaf))
					l[0] = leaf{key, val}
					copy(l[1:], n.leaf[:i])
					copy(l[1+i:], n.leaf[i+1:])
					added = false
					return collision{hash, l}
				}
			}
			// new collision
			added = true
			return collision{hash, append([]leaf{{key, val}}, n.leaf...)}
		default:
			panic("pmap: unhandled case in insert")
		}
	}
	newNode = _insert(n, 0)
	return
}

type stats struct {
	count          int
	maxHeight      int
	avgHeight      float64
	avgDegree      float64
	nodeCount      int
	leafCount      int
	collisionCount int
	collidedKeys   int
	emptySlots     int
}

func (p pmap) stats() stats {
	var s stats
	var th float64
	var td float64
	var visit func(n interface{}, h int)
	visit = func(n interface{}, h int) {
		switch n := n.(type) {
		case leaf:
			s.count++
			s.leafCount++
			th += float64(h)
			if s.maxHeight < h {
				s.maxHeight = h
			}
		case *node:
			s.count++
			s.nodeCount++
			for i := range n.child {
				if n.child[i] != nil {
					td += 1.0
					visit(n.child[i], h+1)
				} else {
					s.emptySlots++
				}
			}
		case collision:
			s.count++
			s.collisionCount++
			s.collidedKeys += len(n.leaf)
		default:
			panic("pmap: unhandled case in stats")
		}
	}
	visit(p.root, 1)
	if s.leafCount > 0 {
		s.avgHeight = th / float64(s.leafCount)
	}
	if s.nodeCount > 0 {
		s.avgDegree = td / float64(s.nodeCount)
	}
	return s
}

func (s stats) String() string {
	return fmt.Sprintf(
		"count          = %d\n"+
			"maxHeight      = %d\n"+
			"avgHeight      = %g\n"+
			"avgDegree      = %g\n"+
			"nodeCount      = %d\n"+
			"leafCount      = %d\n"+
			"collisionCount = %d\n"+
			"collidedKeys   = %d\n"+
			"emptySlots     = %d\n",
		s.count,
		s.maxHeight,
		s.avgHeight,
		s.avgDegree,
		s.nodeCount,
		s.leafCount,
		s.collisionCount,
		s.collidedKeys,
		s.emptySlots,
	)
}