feat: add AlignRecord flag to Decoder

Closes: #63

Author: jszwec

decoder.go

@@ -25,6 +25,21 @@ type Decoder struct {
 	// provided struct.
 	DisallowMissingColumns bool
 
+	// AlignRecord will cause Decoder to align returned record slice to the
+	// header in case Reader returns records of different lengths.
+	//
+	// This flag is supposed to work with csv.Reader.FieldsPerRecord set to -1
+	// which may cause this behavior.
+	//
+	// When header is longer than the record, it will populate the missing
+	// records with an empty string.
+	//
+	// When header is shorter than the record, it will slice the record to match
+	// header's length.
+	//
+	// When this flag is used, Decoder will not ever return ErrFieldCount.
+	AlignRecord bool
+
 	// If not nil, Map is a function that is called for each field in the csv
 	// record before decoding the data. It allows mapping certain string values
 	// for specific columns or types to a known format. Decoder calls Map with
@@ -394,7 +409,15 @@ func (d *Decoder) decodeStruct(v reflect.Value) (err error) {
 	}
 
 	if len(d.record) != len(d.header) {
-		return ErrFieldCount
+		if !d.AlignRecord {
+			return ErrFieldCount
+		}
+
+		if len(d.record) > len(d.header) {
+			d.record = d.record[:len(d.header)]
+		} else {
+			d.record = append(d.record, make([]string, len(d.header)-len(d.record))...)
+		}
 	}
 
 	return d.unmarshal(d.record, v)

decoder_test.go

@@ -2483,6 +2483,56 @@ s,1,3.14,true
 			t.Fatal("want err not to be nil")
 		}
 	})
+
+	t.Run("align record to header - header longer", func(t *testing.T) {
+		csvr := csv.NewReader(strings.NewReader("A,B,C\na,b"))
+		csvr.FieldsPerRecord = -1
+		dec, err := NewDecoder(csvr)
+		if err != nil {
+			t.Fatalf("want err == nil; got %v", err)
+		}
+		dec.AlignRecord = true
+
+		var data []struct {
+			A, B, C string
+		}
+		if err := dec.Decode(&data); err != nil {
+			t.Fatal("did not expect decode fail with:", err)
+		}
+
+		if len(data) != 1 {
+			t.Fatalf("expected data to be of length 1 got: %d", len(data))
+		}
+
+		if data[0].A != "a" || data[0].B != "b" || data[0].C != "" {
+			t.Errorf("expected \"a\", \"b\" and \"\"; got: %q, %q and %q", data[0].A, data[0].B, data[0].C)
+		}
+	})
+
+	t.Run("align record to header - header shorter", func(t *testing.T) {
+		csvr := csv.NewReader(strings.NewReader("A,B\na,b,c"))
+		csvr.FieldsPerRecord = -1
+		dec, err := NewDecoder(csvr)
+		if err != nil {
+			t.Fatalf("want err == nil; got %v", err)
+		}
+		dec.AlignRecord = true
+
+		var data []struct {
+			A, B string
+		}
+		if err := dec.Decode(&data); err != nil {
+			t.Fatal("did not expect decode fail with:", err)
+		}
+
+		if len(data) != 1 {
+			t.Fatalf("expected data to be of length 1 got: %d", len(data))
+		}
+
+		if data[0].A != "a" || data[0].B != "b" {
+			t.Errorf("expected \"a\" and \"b\"; got: %q and %q", data[0].A, data[0].B)
+		}
+	})
 }
 
 func BenchmarkDecode(b *testing.B) {

error.go

@@ -10,6 +10,8 @@ import (
 
 // ErrFieldCount is returned when header's length doesn't match the length of
 // the read record.
+//
+// This Error can be disabled with Decoder.AlignRecord = true.
 var ErrFieldCount = errors.New("wrong number of fields in record")
 
 // An UnmarshalTypeError describes a string value that was not appropriate for