Fix incorrect error for very long lines (#48)

Fix a bug where an io.EOF error would incorrectly be returned when a
line of data contains bufio.MaxScanTokenSize characters or greater.
The decoder will now return ErrTooLong when a line is too long to
decode.

Fixes #47

decode.go

@@ -11,6 +11,12 @@ import (
 	"strings"
 )
 
+var (
+	// ErrTooLong indicates a line was too long to decode. Currently, the maximum
+	// decodable line length is bufio.MaxScanTokenSize-1.
+	ErrTooLong = bufio.ErrTooLong
+)
+
 // Unmarshal parses fixed width encoded data and stores the
 // result in the value pointed to by v. If v is nil or not a
 // pointer, Unmarshal returns an InvalidUnmarshalError.
@@ -95,6 +101,9 @@ func (d *Decoder) SetUseCodepointIndices(use bool) {
 //
 // In the case that v points to a slice value, Decode will read until
 // the end of its input.
+//
+// Currently, the maximum decodable line length is bufio.MaxScanTokenSize-1. ErrTooLong
+// is returned if a line is encountered that too long to decode.
 func (d *Decoder) Decode(v interface{}) error {
 	rv := reflect.ValueOf(v)
 	if rv.Kind() != reflect.Ptr || rv.IsNil() {
@@ -158,9 +167,15 @@ func (d *Decoder) scan(data []byte, atEOF bool) (advance int, token []byte, err
 	return 0, nil, nil
 }
 
+// readLine reads the next line of data. False is returned if there is no remaining data
+// to read.
 func (d *Decoder) readLine(v reflect.Value) (err error, ok bool) {
 	ok = d.scanner.Scan()
 	if !ok {
+		if d.scanner.Err() != nil {
+			return d.scanner.Err(), false
+		}
+
 		d.done = true
 		return nil, false
 	}

decode_test.go

@@ -1,6 +1,7 @@
 package fixedwidth
 
 import (
+	"bufio"
 	"bytes"
 	"encoding"
 	"fmt"
@@ -396,6 +397,22 @@ func TestDecode_EOF(t *testing.T) {
 	}
 }
 
+// Verify that lines longer than the bufio.Scanner buffer decode correctly. See
+// https://github.com/ianlopshire/go-fixedwidth/issues/47.
+func TestDecode_VeryLongLines(t *testing.T) {
+	var s struct {
+		Field1 string `fixed:"1,10"`
+	}
+
+	// Fill a line with bufio.MaxScanTokenSize charters.
+	data := bytes.Repeat([]byte("a"), bufio.MaxScanTokenSize)
+
+	d := NewDecoder(bytes.NewReader(data))
+	if err := d.Decode(&s); err != ErrTooLong {
+		t.Errorf("Decode should have returned ErrTooLong. Returned: %v", err)
+	}
+}
+
 func TestNewRawValue(t *testing.T) {
 	for _, tt := range []struct {
 		name     string