fix: empty file previews as binary

[horriblename]

Fixes a bug where empty files are mislabeled as 'binary' in preview due to extraneous bytes being passed to the scanner

How to reproduce bug:

1. select an empty file for preview
2. preview shows up as "binary"

[joelim-work]

I haven't figured out if there's a way to just simply peek at the first two bytes, instead of actually reading them and then joining it back using io.MultiReader, but anyway the current fix looks fine for now.

[joelim-work]

How to reproduce bug:

1. select an empty file for preview
2. preview shows up as "binary"

Also add step 0. Ensure sixel option is enabled

[joelim-work]

Hi @horriblename

I merged this patch quickly because it was a bug and may confuse users who are testing the new sixel feature.

Do you think a better way to fix this might be to wrap the output in bufio.Reader and use that to Peek at the bytes?

Something like this:

diff --git a/nav.go b/nav.go
index 56dab74..ef98966 100644
--- a/nav.go
+++ b/nav.go
@@ -2,7 +2,6 @@ package main
 
 import (
 	"bufio"
-	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -802,7 +801,7 @@ func (nav *nav) preview(path string, win *win) {
 	reg := &reg{loadTime: time.Now(), path: path}
 	defer func() { nav.regChan <- reg }()
 
-	var reader io.Reader
+	var reader *bufio.Reader
 
 	if len(gOpts.previewer) != 0 {
 		cmd := exec.Command(gOpts.previewer, path,
@@ -836,7 +835,7 @@ func (nav *nav) preview(path string, win *win) {
 			}
 		}()
 		defer out.Close()
-		reader = out
+		reader = bufio.NewReader(out)
 	} else {
 		f, err := os.Open(path)
 		if err != nil {
@@ -845,14 +844,11 @@ func (nav *nav) preview(path string, win *win) {
 		}
 
 		defer f.Close()
-		reader = f
+		reader = bufio.NewReader(f)
 	}
 
-	prefix := make([]byte, 2)
 	if gOpts.sixel {
-		_, err := reader.Read(prefix)
-		reader = io.MultiReader(bytes.NewReader(prefix), reader)
-
+		prefix, err := reader.Peek(2)
 		if err == nil && string(prefix) == gSixelBegin {
 			b, err := io.ReadAll(reader)
 			if err != nil {

I haven't tested this very much though.

[horriblename]

I've thought of that but was worried that it would cause a 'double buffered' situation, i.e. reader has its buffer but then the buf := bufio.NewScanner(reader) (a bit further down) would keep another buffer. I didn't measure anything but this feels worse.

Another solution is to replace the scanner using bufio.Reader.ReadString('\n') instead but idk if we'd need to take care of DOS-format endlines \r\n

Hope this makes sense, if it doesn't, give me a few days until my exams are over, and I'll get back to you :)

[joelim-work]

That's OK, it was just an idea. The extra buffer in bufio.Reader probably does make it less efficient than io.MultiReader, but I haven't bothered to test to see if it makes any noticeable impact.