sort the objs in the output more consistently

When CatalogSort is enabled, the sort.Strings routine is used to order
the "keys" used for the various objects being put into the PDF. Use
sort.SliceStable instead, and specify a function Less function that, if
possible, uses an attribute of the objs being sorted. This is more
deterministic.

1 files changed, 5 insertions, 5 deletions

diff --git a/fpdf.go b/fpdf.go
index ad29d18..b8875a5 100644
--- a/fpdf.go
+++ b/fpdf.go
@@ -3199,7 +3199,7 @@ func (f *Fpdf) putfonts() {
 			fileList = append(fileList, file)
 		}
 		if f.catalogSort {
-			sort.Strings(fileList)
+			sort.SliceStable(fileList, func(i, j int) bool { return fileList[i] < fileList[j] })
 		}
 		for _, file = range fileList {
 			info = f.fontFiles[file]
@@ -3249,7 +3249,7 @@ func (f *Fpdf) putfonts() {
 			keyList = append(keyList, key)
 		}
 		if f.catalogSort {
-			sort.Strings(keyList)
+			sort.SliceStable(keyList, func(i, j int) bool { return keyList[i] < keyList[j] })
 		}
 		for _, key = range keyList {
 			font = f.fonts[key]
@@ -3350,7 +3350,7 @@ func (f *Fpdf) putimages() {
 		keyList = append(keyList, key)
 	}
 	if f.catalogSort {
-		sort.Strings(keyList)
+		sort.SliceStable(keyList, func(i, j int) bool { return f.images[keyList[i]].w < f.images[keyList[j]].w })
 	}
 	for _, key = range keyList {
 		f.putimage(f.images[key])
@@ -3430,7 +3430,7 @@ func (f *Fpdf) putxobjectdict() {
 			keyList = append(keyList, key)
 		}
 		if f.catalogSort {
-			sort.Strings(keyList)
+			sort.SliceStable(keyList, func(i, j int) bool { return f.images[keyList[i]].i < f.images[keyList[j]].i })
 		}
 		for _, key = range keyList {
 			image = f.images[key]
@@ -3464,7 +3464,7 @@ func (f *Fpdf) putresourcedict() {
 			keyList = append(keyList, key)
 		}
 		if f.catalogSort {
-			sort.Strings(keyList)
+			sort.SliceStable(keyList, func(i, j int) bool { return f.fonts[keyList[i]].I < f.fonts[keyList[j]].I })
 		}
 		for _, key = range keyList {
 			font = f.fonts[key]