From 6ff5acae03650fd2dcf693ff953e74731cc3ab82 Mon Sep 17 00:00:00 2001
From: Mattias Wadman <mattias.wadman@gmail.com>
Date: Tue, 19 Oct 2021 01:53:37 +0200
Subject: [PATCH] gif: Support GIF87a

---
 format/gif/gif.go    |  3 ++-
 pkg/decode/decode.go | 22 ++++++++++++++--------
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/format/gif/gif.go b/format/gif/gif.go
index 689c0129..451652ea 100644
--- a/format/gif/gif.go
+++ b/format/gif/gif.go
@@ -53,7 +53,8 @@ func fieldColorMap(d *decode.D, name string, bitDepth int) {
 func gifDecode(d *decode.D, in interface{}) interface{} {
 	d.Endian = decode.LittleEndian
 
-	d.FieldValidateUTF8("header", "GIF89a")
+	d.FieldValidateUTF8Any("header", 6, []string{"GIF87a", "GIF89a"})
+
 	d.FieldU16("width")
 	d.FieldU16("height")
 	gcpFollows := d.FieldBool("gcp_follows")
diff --git a/pkg/decode/decode.go b/pkg/decode/decode.go
index 79e2f699..da836f04 100644
--- a/pkg/decode/decode.go
+++ b/pkg/decode/decode.go
@@ -727,25 +727,31 @@ func (d *D) FieldValidateUTF8Fn(name string, v string, fn func() string) {
 	}
 }
 
-func (d *D) FieldValidateUTF8(name string, v string) {
+func (d *D) FieldValidateUTF8Any(name string, nBytes int, vs []string) {
 	pos := d.Pos()
+	found := false
 	s := d.FieldStrFn(name, func() (string, string) {
-		nBytes := len(v)
 		str, err := d.TryUTF8(nBytes)
 		if err != nil {
 			panic(IOError{Err: err, Name: name, Op: "FieldValidateUTF8", Size: int64(nBytes) * 8, Pos: d.Pos()})
 		}
-		s := "Correct"
-		if str != v {
-			s = "Incorrect"
+		for _, v := range vs {
+			if v == str {
+				found = true
+				return str, "Correct"
+			}
 		}
-		return str, s
+		return str, "Incorrect"
 	})
-	if s != v {
-		panic(ValidateError{Reason: fmt.Sprintf("expected %s found %s", v, s), Pos: pos})
+	if !found {
+		panic(ValidateError{Reason: fmt.Sprintf("expected any of %v found %s", vs, s), Pos: pos})
 	}
 }
 
+func (d *D) FieldValidateUTF8(name string, v string) {
+	d.FieldValidateUTF8Any(name, len(v), []string{v})
+}
+
 func (d *D) ValidateAtLeastBitsLeft(nBits int64) {
 	bl := d.BitsLeft()
 	if bl < nBits {