Expression hashing.

expreduce/builtin_system.go

@@ -6,6 +6,13 @@ import "fmt"
 import "os"
 import "io/ioutil"
 import "github.com/op/go-logging"
+import "hash/fnv"
+
+func hashEx(e Ex) uint64 {
+	h := fnv.New64a()
+	e.Hash(&h)
+	return h.Sum64()
+}
 
 func exprToN(es *EvalState, e Ex) Ex {
 	asInt, isInt := e.(*Integer)
@@ -634,5 +641,17 @@ func GetSystemDefinitions() (defs []Definition) {
 		Bootstrap: true,
 		Attributes: []string{"HoldAll"},
 	})
+	defs = append(defs, Definition{
+		Name:  "Hash",
+		Usage: "`Hash[expr]` returns an integer hash of `expr`.",
+		legacyEvalFn: func(this *Expression, es *EvalState) Ex {
+			if len(this.Parts) != 2 {
+				return this
+			}
+			i := big.NewInt(0)
+			i.SetUint64(hashEx(this.Parts[1]))
+			return &Integer{i}
+		},
+	})
 	return
 }

expreduce/cas.go

@@ -4,6 +4,8 @@
 
 package expreduce
 
+import "hash"
+
 type ToStringFnType (func(*Expression, string) (bool, string))
 
 // A nasty global to keep track of ToString functions. TODO: Fix this.
@@ -17,4 +19,5 @@ type Ex interface {
 	IsEqual(b Ex, cl *CASLogger) string
 	DeepCopy() Ex
 	NeedsEval() bool
+	Hash(h *hash.Hash64)
 }

expreduce/ex_expression.go

@@ -3,6 +3,7 @@ package expreduce
 import "bytes"
 import "math/big"
 import "sort"
+import "hash"
 
 type Expression struct {
 	Parts []Ex
@@ -388,6 +389,13 @@ func (this *Expression) NeedsEval() bool {
 	return false
 }
 
+func (this *Expression) Hash(h *hash.Hash64) {
+	(*h).Write([]byte{72, 5, 244, 86, 5, 210, 69, 30})
+	for _, part := range this.Parts {
+		part.Hash(h)
+	}
+}
+
 func NewExpression(parts []Ex) *Expression {
 	return &Expression{Parts: parts}
 }

expreduce/ex_integer.go

@@ -2,6 +2,7 @@ package expreduce
 
 import "fmt"
 import "math/big"
+import "hash"
 
 // Integer numbers represented by big.Int
 type Integer struct {
@@ -52,3 +53,9 @@ func (this *Integer) NeedsEval() bool {
 func NewInt(i int64) *Integer {
 	return &Integer{big.NewInt(i)}
 }
+
+func (this *Integer) Hash(h *hash.Hash64) {
+	(*h).Write([]byte{242, 99, 84, 113, 102, 46, 118, 94})
+	bytes, _ := this.Val.MarshalText()
+	(*h).Write(bytes)
+}

expreduce/ex_rational.go

@@ -2,6 +2,7 @@ package expreduce
 
 import "fmt"
 import "math/big"
+import "hash"
 
 type Rational struct {
 	Num *big.Int
@@ -86,3 +87,11 @@ func (this *Rational) AsBigRat() *big.Rat {
 func (this *Rational) NeedsEval() bool {
 	return false
 }
+
+func (this *Rational) Hash(h *hash.Hash64) {
+	(*h).Write([]byte{90, 82, 214, 51, 52, 7, 7, 33})
+	nBytes, _ := this.Num.MarshalText()
+	(*h).Write(nBytes)
+	dBytes, _ := this.Den.MarshalText()
+	(*h).Write(dBytes)
+}

expreduce/ex_real.go

@@ -3,6 +3,7 @@ package expreduce
 import "fmt"
 import "math/big"
 import "bytes"
+import "hash"
 
 // Floating point numbers represented by big.Float
 type Flt struct {
@@ -66,3 +67,9 @@ func RationalToFlt(r *Rational) (*Flt, bool) {
 func (this *Flt) NeedsEval() bool {
 	return false
 }
+
+func (this *Flt) Hash(h *hash.Hash64) {
+	(*h).Write([]byte{195, 244, 76, 249, 227, 115, 88, 251})
+	bytes, _ := this.Val.MarshalText()
+	(*h).Write(bytes)
+}

expreduce/ex_string.go

@@ -1,6 +1,7 @@
 package expreduce
 
 import "fmt"
+import "hash"
 
 type String struct {
 	Val string
@@ -36,3 +37,8 @@ func (this *String) DeepCopy() Ex {
 	thiscopy := *this
 	return &thiscopy
 }
+
+func (this *String) Hash(h *hash.Hash64) {
+	(*h).Write([]byte{102, 206, 57, 172, 207, 100, 198, 133})
+	(*h).Write([]byte(this.Val))
+}

expreduce/ex_symbol.go

@@ -2,6 +2,7 @@ package expreduce
 
 import "fmt"
 import "sort"
+import "hash"
 
 // Symbols are defined by a string-based name
 type Symbol struct {
@@ -225,3 +226,8 @@ func (this *Attributes) toStrings() []string {
 func (this *Symbol) NeedsEval() bool {
 	return false
 }
+
+func (this *Symbol) Hash(h *hash.Hash64) {
+	(*h).Write([]byte{107, 10, 247, 23, 33, 221, 163, 156})
+	(*h).Write([]byte(this.Name))
+}