model: add a reference to width table format and simplify the code

pdf/model/font_composite.go

@@ -439,25 +439,37 @@ func NewCompositePdfFontFromTTFFile(filePath string) (*PdfFont, error) {
 	// Construct W array.  Stores character code to width mappings.
 	wArr := &core.PdfObjectArray{}
 
-	for i := uint16(0); int(i) < len(runes); {
+	// 9.7.4.3 Glyph metrics in CIDFonts
 
-		j := i + 1
+	// In the first format, c shall be an integer specifying a starting CID value; it shall be followed by an array of
-		for int(j) < len(runes) {
+	// n numbers that shall specify the widths for n consecutive CIDs, starting with c.
-			if runeToWidthMap[runes[i]] != runeToWidthMap[runes[j]] {
+	// The second format shall define the same width, w, as a number, for all CIDs in the range c_first to c_last.
+
+	// We always use the second format.
+
+	// TODO(dennwc): this can be done on GIDs instead of runes
+	for i := 0; i < len(runes); {
+		w := runeToWidthMap[runes[i]]
+
+		li := i
+		for j := i + 1; j < len(runes); j++ {
+			w2 := runeToWidthMap[runes[j]]
+			if w == w2 {
+				li = j
+			} else {
 				break
 			}
-			j++
 		}
 
 		// The W maps from CID to width, here CID = GID.
 		gid1 := ttf.Chars[runes[i]]
-		gid2 := ttf.Chars[runes[j-1]]
+		gid2 := ttf.Chars[runes[li]]
 
 		wArr.Append(core.MakeInteger(int64(gid1)))
 		wArr.Append(core.MakeInteger(int64(gid2)))
-		wArr.Append(core.MakeInteger(int64(runeToWidthMap[runes[i]])))
+		wArr.Append(core.MakeInteger(int64(w)))
 
-		i = j
+		i = li + 1
 	}
 	cidfont.W = core.MakeIndirectObject(wArr)