[UP-453] Example remove unused resources (unidoc#227)

* added example code to solve issue UP-453

* uncomment init function

* added some annotation

* renamed file name and added some comments

* added more explanation

pages/pdf_split_and_remove_unused_resources.go

@@ -0,0 +1,130 @@
+/*
+ * This example shows how to split pdf pages and removes unused resources.
+ * When a big pdf file is split into small parts, each page gets its own copy of the `XObject` dictionary. This causes each part to have unnecessarily big size
+ * This example shows how to remove the unused resources from the XObject dictionary.
+ *
+ * Run as: go run pdf_split_and_remove_unused_resources.go <input.pdf> <output-dir>
+ * In this example the document is split in to 1 page small documents, the idea can be easily extended into any kind of page splitting.
+ */
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/unidoc/unipdf/v3/common"
+	"github.com/unidoc/unipdf/v3/common/license"
+	"github.com/unidoc/unipdf/v3/contentstream"
+	"github.com/unidoc/unipdf/v3/core"
+	"github.com/unidoc/unipdf/v3/model"
+)
+
+func init() {
+	// Make sure to load your metered License API key prior to using the library.
+	// If you need a key, you can sign up and create a free one at https://cloud.unidoc.io
+	err := license.SetMeteredKey(os.Getenv(`UNIDOC_LICENSE_API_KEY`))
+	if err != nil {
+		panic(err)
+	}
+}
+
+func main() {
+	if len(os.Args) < 3 {
+		fmt.Printf("Usage: go run pdf_split_and_remove_unused_resources.go <input.pdf> <output-dir>\n")
+		os.Exit(1)
+	}
+
+	inputPath := os.Args[1]
+
+	outputDir := os.Args[2]
+	splitPdfFile(inputPath, outputDir)
+}
+
+// splitPdfFile splits inputFile and saves the output files in `outputDir`.
+func splitPdfFile(inputFile string, outputDir string) {
+	pdfBytes, err := os.ReadFile(inputFile)
+	if err != nil {
+		log.Fatalf("Failed to read file: %v", err)
+	}
+
+	r := bytes.NewReader(pdfBytes)
+	pdfReader, err := model.NewPdfReader(r)
+	if err != nil {
+		log.Fatalf("Failed to create reader: %v", err)
+	}
+
+	numPages, err := pdfReader.GetNumPages()
+	if err != nil {
+		log.Fatalf("Failed to get number of pages: %v", err)
+	}
+	for pageIdx := 0; pageIdx < numPages; pageIdx++ {
+		pdfPage, _ := pdfReader.GetPage(pageIdx + 1)
+		cleanUnusedXobjects(pdfPage)
+		w := model.NewPdfWriter()
+		if err := w.AddPage(pdfPage); err != nil {
+			log.Fatalf("Failed to convert pageIdx = %d, %v", pageIdx, err)
+		}
+		if err := w.WriteToFile(fmt.Sprintf("./%s/output/doc_page_%d_new.pdf", outputDir, pageIdx)); err != nil {
+			log.Fatalf("Failed to write to file %d: %v", pageIdx, err)
+		}
+	}
+}
+
+// cleanUnusedXobjects removes entries of unused XObjects from teh Resource's XObjects dictionary.
+func cleanUnusedXobjects(page *model.PdfPage) {
+	contents, err := page.GetAllContentStreams()
+	if err != nil {
+		common.Log.Debug("failed to get page content stream")
+	}
+	parser := contentstream.NewContentStreamParser(contents)
+	operations, err := parser.Parse()
+	if err != nil {
+		common.Log.Debug("failed to parse content stream")
+	}
+	usedObjectsNames := []string{}
+	for _, op := range *operations {
+		operand := op.Operand
+		// Check for `Do` (Draw XObject) operator.
+		if operand == "Do" {
+			params := op.Params
+			imageName := params[0].String()
+			usedObjectsNames = append(usedObjectsNames, imageName)
+		}
+	}
+
+	xObject := page.Resources.XObject
+	dict, ok := xObject.(*core.PdfObjectDictionary)
+	if ok {
+		keys := getKeys(dict)
+		for _, k := range keys {
+			if exists(k, usedObjectsNames) {
+				continue
+			}
+			name := *core.MakeName(k)
+			dict.Remove(name)
+		}
+	}
+
+}
+
+// getKeys gets the keys of the dictionary `dict`.
+func getKeys(dict *core.PdfObjectDictionary) []string {
+	keys := []string{}
+	for _, k := range dict.Keys() {
+		keys = append(keys, k.String())
+	}
+	return keys
+}
+
+// exists checks if `element` exists in `elements`.
+func exists(element string, elements []string) bool {
+	for _, el := range elements {
+		if element == el {
+			return true
+		}
+	}
+	return false
+}