stat/card: allow unmarshaling into the zero value

stat/card/card.go

@@ -7,8 +7,11 @@
 package card
 
 import (
+	"fmt"
+	"hash"
 	"math"
 	"reflect"
+	"sync"
 )
 
 const (
@@ -59,3 +62,48 @@ func typeNameOf(v interface{}) string {
 	}
 	return prefix + t.PkgPath() + "." + t.Name()
 }
+
+// hashes holds registered hashes.
+var hashes sync.Map // map[string]userType
+
+type userType struct {
+	fn  reflect.Value // Holds a func() hash.Hash{32,64}.
+	typ reflect.Type  // Type of the returned hash implementation.
+}
+
+// RegisterHash registers a function that returns a new hash.Hash32 or hash.Hash64
+// to the name of the type implementing the interface. The value of fn must be a
+// func() hash.Hash32 or func() hash.Hash64, otherwise RegisterHash will panic.
+// RegisterHash will panic if there is not a unique mapping from the name to the
+// returned type.
+func RegisterHash(fn interface{}) {
+	const invalidType = "card: must register func() hash.Hash32 or func() hash.Hash64"
+
+	rf := reflect.ValueOf(fn)
+	rt := rf.Type()
+	if rf.Kind() != reflect.Func {
+		panic(invalidType)
+	}
+	if rt.NumIn() != 0 {
+		panic(invalidType)
+	}
+	if rt.NumOut() != 1 {
+		panic(invalidType)
+	}
+	h := rf.Call(nil)[0].Interface()
+	var name string
+	var h32 hash.Hash32
+	var h64 hash.Hash64
+	switch rf.Type().Out(0) {
+	case reflect.TypeOf(&h32).Elem(), reflect.TypeOf(&h64).Elem():
+		name = typeNameOf(h)
+	default:
+		panic(invalidType)
+	}
+	user := userType{fn: rf, typ: reflect.TypeOf(h)}
+	ut, dup := hashes.LoadOrStore(name, user)
+	stored := ut.(userType)
+	if dup && stored.typ != user.typ {
+		panic(fmt.Sprintf("card: registering duplicate types for %q: %s != %s", name, stored.typ, user.typ))
+	}
+}

stat/card/card_test.go

@@ -7,10 +7,12 @@ package card
 import (
 	"encoding"
 	"fmt"
+	"hash"
 	"hash/fnv"
 	"io"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 
 	"golang.org/x/exp/rand"
@@ -203,39 +205,45 @@ type counterEncoder interface {
 }
 
 var counterEncoderTests = []struct {
-	name     string
+	name           string
-	count    int
+	count          int
-	src, dst func() counterEncoder
+	src, dst, zdst func() counterEncoder
 }{
 	{
 		name: "HyperLogLog32-4-4-FNV-1a", count: 1e3,
-		src: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
+		src:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
-		dst: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
+		dst:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
+		zdst: func() counterEncoder { return &HyperLogLog32{} },
 	},
 	{
 		name: "HyperLogLog32-4-8-FNV-1a", count: 1e3,
-		src: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
+		src:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
-		dst: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(8, fnv.New32a())) },
+		dst:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(8, fnv.New32a())) },
+		zdst: func() counterEncoder { return &HyperLogLog32{} },
 	},
 	{
 		name: "HyperLogLog32-8-4-FNV-1a", count: 1e3,
-		src: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(8, fnv.New32a())) },
+		src:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(8, fnv.New32a())) },
-		dst: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
+		dst:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog32(4, fnv.New32a())) },
+		zdst: func() counterEncoder { return &HyperLogLog32{} },
 	},
 	{
 		name: "HyperLogLog64-4-4-FNV-1a", count: 1e3,
-		src: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
+		src:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
-		dst: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
+		dst:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
+		zdst: func() counterEncoder { return &HyperLogLog64{} },
 	},
 	{
 		name: "HyperLogLog64-4-8-FNV-1a", count: 1e3,
-		src: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
+		src:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
-		dst: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(8, fnv.New64a())) },
+		dst:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(8, fnv.New64a())) },
+		zdst: func() counterEncoder { return &HyperLogLog64{} },
 	},
 	{
 		name: "HyperLogLog64-8-4-FNV-1a", count: 1e3,
-		src: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(8, fnv.New64a())) },
+		src:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(8, fnv.New64a())) },
-		dst: func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
+		dst:  func() counterEncoder { return mustCounterEncoder(NewHyperLogLog64(4, fnv.New64a())) },
+		zdst: func() counterEncoder { return &HyperLogLog64{} },
 	},
 }
 
@@ -247,6 +255,11 @@ func mustCounterEncoder(c counterEncoder, err error) counterEncoder {
 }
 
 func TestBinaryEncoding(t *testing.T) {
+	RegisterHash(fnv.New32a)
+	RegisterHash(fnv.New64a)
+	defer func() {
+		hashes = sync.Map{}
+	}()
 	for _, test := range counterEncoderTests {
 		rnd := rand.New(rand.NewSource(1))
 		src := test.src()
@@ -277,12 +290,56 @@ func TestBinaryEncoding(t *testing.T) {
 			t.Errorf("unexpected error unmarshaling binary for %s: %v", test.name, err)
 			continue
 		}
+		zdst := test.zdst()
+		err = zdst.UnmarshalBinary(buf)
+		if err != nil {
+			t.Errorf("unexpected error unmarshaling binary into zero receiver for %s: %v", test.name, err)
+			continue
+		}
 		gotSrc := src.Count()
 		gotDst := dst.Count()
+		gotZdst := zdst.Count()
 
 		if gotSrc != gotDst {
 			t.Errorf("unexpected count for %s: got:%.0f want:%.0f", test.name, gotDst, gotSrc)
 		}
+		if gotSrc != gotZdst {
+			t.Errorf("unexpected count for %s into zero receiver: got:%.0f want:%.0f", test.name, gotZdst, gotSrc)
+		}
+	}
+}
+
+var invalidRegisterTests = []struct {
+	fn     interface{}
+	panics bool
+}{
+	{fn: int(0), panics: true},
+	{fn: func() {}, panics: true},
+	{fn: func(int) {}, panics: true},
+	{fn: func() int { return 0 }, panics: true},
+	{fn: func() hash.Hash { return fnv.New32a() }, panics: true},
+	{fn: func() hash.Hash32 { return fnv.New32a() }, panics: false},
+	{fn: func() hash.Hash { return fnv.New64a() }, panics: true},
+	{fn: func() hash.Hash64 { return fnv.New64a() }, panics: false},
+}
+
+func TestRegisterInvalid(t *testing.T) {
+	for _, test := range invalidRegisterTests {
+		var r interface{}
+		func() {
+			defer func() {
+				r = recover()
+			}()
+			RegisterHash(test.fn)
+		}()
+		panicked := r != nil
+		if panicked != test.panics {
+			if panicked {
+				t.Errorf("unexpected panic for %T", test.fn)
+			} else {
+				t.Errorf("expected panic for %T", test.fn)
+			}
+		}
 	}
 }
 

stat/card/generate_64bit.sh

@@ -14,5 +14,6 @@ echo -e '// Code generated by "go generate gonum.org/v1/gonum/stat/card"; DO NOT
 	-e 's/rho32/rho64/' \
 	-e 's/HyperLogLog32/HyperLogLog64/g' \
 	-e 's/Hash32/Hash64/' \
+	-e 's/hash32/hash64/' \
 	-e 's/w32/w64/g' \
 >> hll64.go

stat/card/hll32.go

@@ -179,9 +179,6 @@ func (h *HyperLogLog32) MarshalBinary() ([]byte, error) {
 // return. The receiver must have a non-nil hash function value that is
 // the same type as the one that was stored in the binary data.
 func (h *HyperLogLog32) UnmarshalBinary(b []byte) error {
-	if h.hash == nil {
-		return errors.New("card: hash function not set")
-	}
 	dec := gob.NewDecoder(bytes.NewReader(b))
 	var size uint8
 	err := dec.Decode(&size)
@@ -196,9 +193,16 @@ func (h *HyperLogLog32) UnmarshalBinary(b []byte) error {
 	if err != nil {
 		return err
 	}
-	dstHash := typeNameOf(h.hash)
+	if h.hash == nil {
-	if dstHash != srcHash {
+		h.hash = hash32For(srcHash)
-		return fmt.Errorf("card: mismatched hash function: dst=%s src=%s", dstHash, srcHash)
+		if h.hash == nil {
+			return fmt.Errorf("card: hash function not set and no hash registered for %q", srcHash)
+		}
+	} else {
+		dstHash := typeNameOf(h.hash)
+		if dstHash != srcHash {
+			return fmt.Errorf("card: mismatched hash function: dst=%s src=%s", dstHash, srcHash)
+		}
 	}
 	err = dec.Decode(&h.p)
 	if err != nil {
@@ -212,3 +216,12 @@ func (h *HyperLogLog32) UnmarshalBinary(b []byte) error {
 	}
 	return nil
 }
+
+func hash32For(name string) hash.Hash32 {
+	fn, ok := hashes.Load(name)
+	if !ok {
+		return nil
+	}
+	h, _ := fn.(userType).fn.Call(nil)[0].Interface().(hash.Hash32)
+	return h
+}

stat/card/hll64.go

@@ -181,9 +181,6 @@ func (h *HyperLogLog64) MarshalBinary() ([]byte, error) {
 // return. The receiver must have a non-nil hash function value that is
 // the same type as the one that was stored in the binary data.
 func (h *HyperLogLog64) UnmarshalBinary(b []byte) error {
-	if h.hash == nil {
-		return errors.New("card: hash function not set")
-	}
 	dec := gob.NewDecoder(bytes.NewReader(b))
 	var size uint8
 	err := dec.Decode(&size)
@@ -198,9 +195,16 @@ func (h *HyperLogLog64) UnmarshalBinary(b []byte) error {
 	if err != nil {
 		return err
 	}
-	dstHash := typeNameOf(h.hash)
+	if h.hash == nil {
-	if dstHash != srcHash {
+		h.hash = hash64For(srcHash)
-		return fmt.Errorf("card: mismatched hash function: dst=%s src=%s", dstHash, srcHash)
+		if h.hash == nil {
+			return fmt.Errorf("card: hash function not set and no hash registered for %q", srcHash)
+		}
+	} else {
+		dstHash := typeNameOf(h.hash)
+		if dstHash != srcHash {
+			return fmt.Errorf("card: mismatched hash function: dst=%s src=%s", dstHash, srcHash)
+		}
 	}
 	err = dec.Decode(&h.p)
 	if err != nil {
@@ -214,3 +218,12 @@ func (h *HyperLogLog64) UnmarshalBinary(b []byte) error {
 	}
 	return nil
 }
+
+func hash64For(name string) hash.Hash64 {
+	fn, ok := hashes.Load(name)
+	if !ok {
+		return nil
+	}
+	h, _ := fn.(userType).fn.Call(nil)[0].Interface().(hash.Hash64)
+	return h
+}