Add a hashing system to Entity

entity/toplevel.go

@@ -1,10 +1,32 @@
 package entity
 
 import "fmt"
+import "crypto/md5"
 import "encoding/base64"
 import "github.com/google/uuid"
 import "git.tebibyte.media/fspl/fspl/errors"
 
+// A hash represents a hash sum that fits within a uint64.
+type Hash [8]byte
+
+// NewHash creates a new truncated md5 hash using the specified data.
+func NewHash (data []byte) Hash {
+	sum := md5.Sum(data)
+	hash := Hash { }
+	copy(hash[:], sum[:8])
+	return hash
+}
+
+// Number converts the hash into a uint64.
+func (hash Hash) Number () uint64 {
+	var number uint64
+	for _, part := range hash {
+		number <<= 8
+		number |=  uint64(part)
+	}
+	return number
+}
+
 // Key globally indexes top level entities in contexts where modules matter.
 type Key struct {
 	Unit   uuid.UUID
@@ -20,6 +42,12 @@ func (key Key) String () string {
 	return out
 }
 
+// Hash returns a representation of the hash of this key that fits within a
+// uint64.
+func (key Key) Hash () Hash {
+	return NewHash([]byte("Key:" + key.String()))
+}
+
 // LinkName returns the name that the entity it refers to will be given when
 // compiled.
 func (key Key) LinkName () string {