Added all source files

operator-fact.go

@@ -0,0 +1,46 @@
+// operator-fact.go
+package expr
+
+import "fmt"
+
+//-------- fact term
+
+func newFactTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		class:    classOperator,
+		kind:     kindInteger,
+		children: make([]*term, 0, 1),
+		position: posPostfix,
+		priority: priFact,
+		evalFunc: evalFact,
+	}
+}
+
+func evalFact(ctx exprContext, self *term) (v any, err error) {
+	var leftValue any
+
+	if leftValue, err = self.evalPrefix(ctx); err != nil {
+		return
+	}
+
+	if isInteger(leftValue) {
+		if i, _ := leftValue.(int64); i >= 0 {
+			f := int64(1)
+			for k := int64(1); k <= i; k++ {
+				f *= k
+			}
+			v = f
+		} else {
+			err = fmt.Errorf("factorial of a negative integer (%d) is not allowed", i)
+		}
+	} else {
+		err = self.errIncompatibleType(leftValue)
+	}
+	return
+}
+
+// init
+func init() {
+	registerTermConstructor(SymExclamation, newFactTerm)
+}