Persistent vector and list hash to the same value

Signed-off-by: James Hamlin <jfhamlin@gmail.com>

Author: jfhamlin

pkg/lang/apersistentvector.go

@@ -34,6 +34,8 @@ type (
 )
 
 var (
+	_ Hasher = (*PersistentVector)(nil)
+
 	_ ASeq       = (*apvSeq)(nil)
 	_ IndexedSeq = (*apvSeq)(nil)
 	_ IReduce    = (*apvSeq)(nil)
@@ -203,6 +205,19 @@ func apersistentVectorHashEq(hc *uint32, a APersistentVector) uint32 {
 	return hash
 }
 
+func apersistentVectorHash(hc *uint32, a APersistentVector) uint32 {
+	if *hc != 0 {
+		return *hc
+	}
+	var n int
+	var hash uint32 = 1
+	for ; n < a.Count(); n++ {
+		hash = 31*hash + Hash(a.Nth(n))
+	}
+	*hc = hash
+	return hash
+}
+
 func apersistentVectorInvoke(a APersistentVector, args ...any) any {
 	if len(args) != 1 {
 		panic(NewIllegalArgumentError(fmt.Sprintf("vector apply expects one argument, got %d", len(args))))

pkg/lang/aseq.go

@@ -92,8 +92,9 @@ func aseqHash(hc *uint32, a ASeq) uint32 {
 	hash := uint32(1)
 	for s := a.Seq(); s != nil; s = s.Next() {
 		var h uint32
-		if s.First() != nil {
-			h = Hash(s.First())
+		first := s.First()
+		if first != nil {
+			h = Hash(first)
 		}
 		hash = 31*hash + h
 	}

pkg/lang/hashes_test.go

@@ -0,0 +1,30 @@
+package lang
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestHashEquivalences(t *testing.T) {
+	// test cases are sets of values that should hash to the same value
+	testCases := [][]any{
+		{nil, uint32(0)},
+		{NewList(NewKeyword("a"), NewKeyword("b")), NewVector(NewKeyword("a"), NewKeyword("b"))},
+	}
+
+	for i, group := range testCases {
+		group := group // capture range variable
+		t.Run(fmt.Sprintf("group_%d", i), func(t *testing.T) {
+			if len(group) < 2 {
+				t.Fatalf("test case must have at least two elements")
+			}
+			expectedHash := Hash(group[0])
+			for _, v := range group[1:] {
+				h := Hash(v)
+				if h != expectedHash {
+					t.Errorf("Hash(%v [%T]) = %d; want %d", v, v, h, expectedHash)
+				}
+			}
+		})
+	}
+}

pkg/lang/list.go

@@ -41,6 +41,7 @@ var (
 	_ IReduce         = (*EmptyList)(nil)
 	_ IReduceInit     = (*EmptyList)(nil)
 	_ IHashEq         = (*EmptyList)(nil)
+	_ Hasher          = (*EmptyList)(nil)
 )
 
 func (e *EmptyList) xxx_sequential() {}
@@ -109,10 +110,6 @@ func (e *EmptyList) Meta() IPersistentMap {
 	return e.meta
 }
 
-func (e *EmptyList) Hash() uint32 {
-	return 1
-}
-
 var (
 	emptyHashOrdered = murmur3.HashOrdered(nil, HashEq)
 )
@@ -121,6 +118,10 @@ func (e *EmptyList) HashEq() uint32 {
 	return emptyHashOrdered
 }
 
+func (e *EmptyList) Hash() uint32 {
+	return 1
+}
+
 func (e *EmptyList) WithMeta(meta IPersistentMap) any {
 	if e.meta == meta {
 		return e

pkg/lang/vector.go

@@ -227,6 +227,10 @@ func (v *Vector) HashEq() uint32 {
 	return apersistentVectorHashEq(&v.hasheq, v)
 }
 
+func (v *Vector) Hash() uint32 {
+	return apersistentVectorHash(&v.hash, v)
+}
+
 func (v *Vector) ReduceInit(f IFn, init any) any {
 	res := init
 	for i := 0; i < v.Count(); i++ {