Add support for expression evaluation

go.mod

@@ -8,7 +8,7 @@ require (
 	github.com/766b/chi-prometheus v0.0.0-20180509160047-46ac2b31aa30
 	github.com/99designs/basicauth-go v0.0.0-20160802081356-2a93ba0f464d
 	github.com/Masterminds/squirrel v1.1.1-0.20191017225151-12f2162c8d8d
-	github.com/PaesslerAG/gval v1.0.1 // indirect
+	github.com/PaesslerAG/gval v1.0.1
 	github.com/PaesslerAG/jsonpath v0.1.1 // indirect
 	github.com/SentimensRG/ctx v0.0.0-20180729130232-0bfd988c655d
 	github.com/crusttech/go-oidc v0.0.0-20180918092017-982855dad3e1

pkg/migrate/README.adoc

@@ -171,3 +171,20 @@ The following value mapping maps `sys_status` field's values; the left one into
   }
 }
 ----
+
+The system also provides support for arbitrary mathematical expressions.
+If you wish to perform an expression, prefix the mapped value with `=EVL=`; for example `=EVL=numFmt(cell, \"%.0f\")`.
+
+Variables:
+* current cell -- `cell`.
+
+The following example will remove the decimal point from every `sys_rating` in the given source.
+
+[source,json]
+----
+{
+  "sys_rating": {
+    "*": "=EVL=numFmt(cell, \"%.0f\")"
+  }
+}
+----

pkg/migrate/types/eval.go

@@ -0,0 +1,25 @@
+package types
+
+import (
+	"fmt"
+	"strconv"
+
+	"github.com/PaesslerAG/gval"
+)
+
+// generates a simple gval language to be used within the migration
+func exprs() gval.Language {
+	return gval.NewLanguage(
+		gval.JSON(),
+		gval.Arithmetic(),
+
+		gval.Function("numFmt", func(number, format string) (string, error) {
+			nn, err := strconv.ParseFloat(number, 64)
+			if err != nil {
+				return "", err
+			}
+
+			return fmt.Sprintf(format, nn), nil
+		}),
+	)
+}

pkg/migrate/types/node.go

@@ -1,6 +1,7 @@
 package types
 
 import (
+	"context"
 	"encoding/csv"
 	"errors"
 	"fmt"
@@ -72,6 +73,10 @@ type (
 	}
 )
 
+const (
+	evalPrefix = "=EVL="
+)
+
 // helper, to determine if the two nodes are equal
 func (n *Node) Compare(to *Node) bool {
 	return n.Name == to.Name && n.spliced == to.spliced
@@ -392,6 +397,8 @@ func importNodeSource(n *Node, users map[string]uint64, repo repository.RecordRe
 		return r
 	}
 
+	lng := exprs()
+
 	for {
 	looper:
 		record, err := n.Reader.Read()
@@ -526,10 +533,26 @@ func importNodeSource(n *Node, users map[string]uint64, repo repository.RecordRe
 
 				for i, v := range values {
 					if fmp, ok := n.ValueMap[h]; ok {
+						nvl := ""
 						if mpv, ok := fmp[v]; ok {
-							v = mpv
+							nvl = mpv
 						} else if mpv, ok := fmp["*"]; ok {
-							v = mpv
+							nvl = mpv
+						}
+
+						if nvl != "" && strings.HasPrefix(nvl, evalPrefix) {
+							opp := nvl[len(evalPrefix):len(nvl)]
+							ev, err := lng.NewEvaluable(opp)
+							if err != nil {
+								return nil, err
+							}
+
+							v, err = ev.EvalString(context.Background(), map[string]interface{}{"cell": v})
+							if err != nil {
+								return nil, err
+							}
+						} else if nvl != "" {
+							v = nvl
 						}
 					}