Add a substring modification

modifiers/modifiers.go

@@ -26,5 +26,6 @@ func New() *mold.Transformer {
 	mod.Register("strip_punctuation", stripPunctuation)
 	mod.Register("camel", camelCase)
 	mod.Register("default", defaultValue)
+	mod.Register("substr", subStr)
 	return mod
 }

modifiers/string.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"reflect"
 	"regexp"
+	"strconv"
 	"strings"
 	"unicode"
 	"unicode/utf8"
@@ -210,3 +211,42 @@ func camelCase(ctx context.Context, fl mold.FieldLevel) error {
 	}
 	return nil
 }
+
+func subStr(ctx context.Context, fl mold.FieldLevel) error {
+	switch fl.Field().Kind() {
+	case reflect.String:
+		val := fl.Field().String()
+		params := strings.SplitN(fl.Param(), "-", 2)
+		if len(params) == 0 || len(params[0]) == 0 {
+			return nil
+		}
+
+		start, err := strconv.Atoi(params[0])
+		if err != nil {
+			return err
+		}
+
+		end := len(val)
+		if len(params) >= 2 {
+			end, err = strconv.Atoi(params[1])
+			if err != nil {
+				return err
+			}
+		}
+
+		if len(val) < start {
+			fl.Field().SetString("")
+			return nil
+		}
+		if len(val) < end {
+			end = len(val)
+		}
+		if start > end {
+			fl.Field().SetString("")
+			return nil
+		}
+
+		fl.Field().SetString(val[start:end])
+	}
+	return nil
+}

modifiers/string_test.go

@@ -938,3 +938,109 @@ var (
 		reflect.TypeOf(sql.NullString{}): true,
 	}
 )
+
+func TestSubStr(t *testing.T) {
+	conform := New()
+
+	s := "123"
+	expected := "123"
+
+	type Test struct {
+		String string `mod:"substr=0-3"`
+	}
+
+	tt := Test{String: s}
+	err := conform.Struct(context.Background(), &tt)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if tt.String != expected {
+		t.Fatalf("Unexpected value '%s'\n", tt.String)
+	}
+
+	tag := "substr=f-3"
+	err = conform.Field(context.Background(), &s, tag)
+	if err == nil {
+		t.Fatalf("Unexpected value '%s' instead of error for tag %s\n", s, tag)
+	}
+	tag = "substr=2-f"
+	err = conform.Field(context.Background(), &s, tag)
+	if err == nil {
+		t.Fatalf("Unexpected value '%s' instead of error for tag %s\n", s, tag)
+	}
+
+	tests := []struct {
+		tag      string
+		expected string
+	}{
+		{
+			tag:      "substr",
+			expected: "123",
+		},
+		{
+			tag:      "substr=0-1",
+			expected: "1",
+		},
+		{
+			tag:      "substr=0-3",
+			expected: "123",
+		},
+		{
+			tag:      "substr=0-2",
+			expected: "12",
+		},
+		{
+			tag:      "substr=1-2",
+			expected: "2",
+		},
+		{
+			tag:      "substr=3-3",
+			expected: "",
+		},
+		{
+			tag:      "substr=4-5",
+			expected: "",
+		},
+		{
+			tag:      "substr=2-1",
+			expected: "",
+		},
+		{
+			tag:      "substr=2-5",
+			expected: "3",
+		},
+		{
+			tag:      "substr=2",
+			expected: "3",
+		},
+	}
+	for _, test := range tests {
+		st := s
+
+		err = conform.Field(context.Background(), &st, test.tag)
+		if err != nil {
+			log.Fatal(err)
+		}
+		if st != test.expected {
+			t.Fatalf("Unexpected value '%s' for tag %s\n", st, test.tag)
+		}
+
+		var iface interface{}
+		err = conform.Field(context.Background(), &iface, test.tag)
+		if err != nil {
+			log.Fatal(err)
+		}
+		if iface != nil {
+			t.Fatalf("Unexpected value '%v'\n", nil)
+		}
+
+		iface = s
+		err = conform.Field(context.Background(), &iface, test.tag)
+		if err != nil {
+			log.Fatal(err)
+		}
+		if iface != test.expected {
+			t.Fatalf("Unexpected value '%v'\n", iface)
+		}
+	}
+}