feat: Add Decoder.NormalizeHeader

Closes: #45

decoder.go

@@ -1,6 +1,7 @@
 package csvutil
 
 import (
+	"errors"
 	"io"
 	"reflect"
 )
@@ -202,6 +203,27 @@ func (d *Decoder) Header() []string {
 	return header
 }
 
+// NormalizeHeader applies f to every column in the header. It returns error
+// if calling f results in conflicting header columns.
+//
+// NormalizeHeader must be called before Decode.
+func (d *Decoder) NormalizeHeader(f func(string) string) error {
+	set := make(map[string]int, len(d.header))
+	for i, s := range d.header {
+		set[f(s)] = i
+	}
+
+	if len(set) != len(d.header) {
+		return errors.New("csvutil: normalize header results in conflicting columns")
+	}
+
+	for s, i := range set {
+		d.header[i] = s
+	}
+	d.hmap = set
+	return nil
+}
+
 // Unused returns a list of column indexes that were not used during decoding
 // due to lack of matching struct field.
 func (d *Decoder) Unused() []int {

decoder_test.go

@@ -2405,6 +2405,45 @@ s,1,3.14,true
 			t.Log(e)
 		})
 	})
+
+	t.Run("normalize header", func(t *testing.T) {
+		csvr := csv.NewReader(strings.NewReader("STRING,INT\nfirst,1"))
+		dec, err := NewDecoder(csvr)
+		if err != nil {
+			t.Fatalf("want err == nil; got %v", err)
+		}
+
+		if err := dec.NormalizeHeader(strings.ToLower); err != nil {
+			t.Fatalf("want err=nil; got %v", err)
+		}
+
+		var data struct {
+			String string `csv:"string"`
+			Int    int    `csv:"int"`
+		}
+		if err := dec.Decode(&data); err != nil {
+			t.Fatalf("want err=nil; got %v", err)
+		}
+
+		if data.String != "first" {
+			t.Errorf("want String=first; got %s", data.String)
+		}
+		if data.Int != 1 {
+			t.Errorf("want Int=1; got %d", data.Int)
+		}
+	})
+
+	t.Run("normalize header - duplicate error", func(t *testing.T) {
+		csvr := csv.NewReader(strings.NewReader("STRING,string\nfirst,1"))
+		dec, err := NewDecoder(csvr)
+		if err != nil {
+			t.Fatalf("want err == nil; got %v", err)
+		}
+
+		if err := dec.NormalizeHeader(strings.ToLower); err == nil {
+			t.Fatal("want err not to be nil")
+		}
+	})
 }
 
 func BenchmarkDecode(b *testing.B) {