Added the ability to have multiple pages in a template (#216)

* Added the ability to have multiple pages in a template

Templates are now aware of the number of pages they have. The main
additions are the FromPage and FromPages methods which allow you to
extract a Template from a template if you pass in the page number. It
follows the same page mechanism as FPDF where pages start in the index
1.

* Update fpdf_test.go

3 files changed, 99 insertions, 17 deletions

diff --git a/fpdf_test.go b/fpdf_test.go
index 3e731fd..2fbca9c 100644
--- a/fpdf_test.go
+++ b/fpdf_test.go
@@ -64,6 +64,42 @@ func TestFpdfImplementPdf(t *testing.T) {
 	var _ gofpdf.Pdf = (*gofpdf.Tpl)(nil)
 }
 
+// TestPagedTemplate ensures new paged templates work
+func TestPagedTemplate(t *testing.T) {
+	pdf := gofpdf.New("P", "mm", "A4", "")
+	tpl := pdf.CreateTemplate(func(t *gofpdf.Tpl) {
+		// this will be the second page, as a page is already
+		// created by default
+		t.AddPage()
+		t.AddPage()
+		t.AddPage()
+	})
+
+	if tpl.NumPages() != 4 {
+		t.Fatalf("The template does not have the correct number of pages %d", tpl.NumPages())
+	}
+
+	tplPages := tpl.FromPages()
+	for x := 0; x < len(tplPages); x++ {
+		pdf.AddPage()
+		pdf.UseTemplate(tplPages[x])
+	}
+
+	// get the last template
+	tpl2, err := tpl.FromPage(tpl.NumPages())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// the objects should be the exact same, as the
+	// template will represent the last page by default
+	// therefore no new id should be set, and the object
+	// should be the same object
+	if fmt.Sprintf("%p", tpl2) != fmt.Sprintf("%p", tpl) {
+		t.Fatal("Template no longer respecting initial template object")
+	}
+}
+
 // TestIssue0116 addresses issue 116 in which library silently fails after
 // calling CellFormat when no font has been set.
 func TestIssue0116(t *testing.T) {
diff --git a/template.go b/template.go
index d27bc40..1bcd25d 100644
--- a/template.go
+++ b/template.go
@@ -124,6 +124,9 @@ type Template interface {
 	Bytes() []byte
 	Images() map[string]*ImageInfoType
 	Templates() []Template
+	NumPages() int
+	FromPage(int) (Template, error)
+	FromPages() []Template
 	Serialize() ([]byte, error)
 	gob.GobDecoder
 	gob.GobEncoder
diff --git a/template_impl.go b/template_impl.go
index 6a98e58..f7eeee1 100644
--- a/template_impl.go
+++ b/template_impl.go
@@ -3,6 +3,7 @@ package gofpdf
 import (
 	"bytes"
 	"encoding/gob"
+	"errors"
 	"fmt"
 )
 
@@ -32,10 +33,15 @@ func newTpl(corner PointType, size SizeType, orientationStr, unitStr, fontDirStr
 	if copyFrom != nil {
 		tpl.loadParamsFromFpdf(copyFrom)
 	}
-	tpl.Fpdf.SetAutoPageBreak(false, 0)
 	tpl.Fpdf.AddPage()
 	fn(&tpl)
-	bytes := tpl.Fpdf.pages[tpl.Fpdf.page].Bytes()
+
+	bytes := make([][]byte, len(tpl.Fpdf.pages))
+	// skip the first page as it will always be empty
+	for x := 1; x < len(bytes); x++ {
+		bytes[x] = tpl.Fpdf.pages[x].Bytes()
+	}
+
 	templates := make([]Template, 0, len(tpl.Fpdf.templates))
 	for _, key := range templateKeyList(tpl.Fpdf.templates, true) {
 		templates = append(templates, tpl.Fpdf.templates[key])
@@ -43,7 +49,7 @@ func newTpl(corner PointType, size SizeType, orientationStr, unitStr, fontDirStr
 	images := tpl.Fpdf.images
 
 	id := GenerateTemplateID()
-	template := FpdfTpl{id, corner, size, bytes, images, templates}
+	template := FpdfTpl{id, corner, size, bytes, images, templates, tpl.Fpdf.page}
 	return &template
 }
 
@@ -52,9 +58,10 @@ type FpdfTpl struct {
 	id        int64
 	corner    PointType
 	size      SizeType
-	bytes     []byte
+	bytes     [][]byte
 	images    map[string]*ImageInfoType
 	templates []Template
+	page      int
 }
 
 // ID returns the global template identifier
@@ -69,7 +76,42 @@ func (t *FpdfTpl) Size() (corner PointType, size SizeType) {
 
 // Bytes returns the actual template data, not including resources
 func (t *FpdfTpl) Bytes() []byte {
-	return t.bytes
+	return t.bytes[t.page]
+}
+
+// FromPage creates a new template from a specific Page
+func (t *FpdfTpl) FromPage(page int) (Template, error) {
+	// pages start at 1
+	if page == 0 {
+		return nil, errors.New("Pages start at 1 No template will have a page 0")
+	}
+
+	if page > t.NumPages() {
+		return nil, fmt.Errorf("The template does not have a page %d", page)
+	}
+	// if it is already pointing to the correct page
+	// there is no need to create a new template
+	if t.page == page {
+		return t, nil
+	}
+
+	t2 := *t
+	t2.id = GenerateTemplateID()
+	t2.page = page
+	return &t2, nil
+}
+
+// FromPages creates a template slice with all the pages within a template.
+func (t *FpdfTpl) FromPages() []Template {
+	p := make([]Template, t.NumPages())
+	for x := 1; x <= t.NumPages(); x++ {
+		// the only error is when accessing a
+		// non existing template... that can't happen
+		// here
+		p[x-1], _ = t.FromPage(x)
+	}
+
+	return p
 }
 
 // Images returns a list of the images used in this template
@@ -82,6 +124,13 @@ func (t *FpdfTpl) Templates() []Template {
 	return t.templates
 }
 
+// NumPages returns the number of available pages within the template. Look at FromPage and FromPages on access to that content.
+func (t *FpdfTpl) NumPages() int {
+	// the first page is empty to
+	// make the pages begin at one
+	return len(t.bytes) - 1
+}
+
 // Serialize turns a template into a byte string for later deserialization
 func (t *FpdfTpl) Serialize() ([]byte, error) {
 	b := new(bytes.Buffer)
@@ -160,6 +209,9 @@ func (t *FpdfTpl) GobEncode() ([]byte, error) {
 	if err == nil {
 		err = encoder.Encode(t.bytes)
 	}
+	if err == nil {
+		err = encoder.Encode(t.page)
+	}
 
 	return w.Bytes(), err
 }
@@ -227,6 +279,9 @@ func (t *FpdfTpl) GobDecode(buf []byte) error {
 	if err == nil {
 		err = decoder.Decode(&t.bytes)
 	}
+	if err == nil {
+		err = decoder.Decode(&t.page)
+	}
 
 	return err
 }
@@ -260,15 +315,3 @@ func (t *Tpl) loadParamsFromFpdf(f *Fpdf) {
 	t.Fpdf.fontStyle = f.fontStyle
 	t.Fpdf.ws = f.ws
 }
-
-// AddPage does nothing because you cannot add pages to a template
-func (t *Tpl) AddPage() {
-}
-
-// AddPageFormat does nothign because you cannot add pages to a template
-func (t *Tpl) AddPageFormat(orientationStr string, size SizeType) {
-}
-
-// SetAutoPageBreak does nothing because you cannot add pages to a template
-func (t *Tpl) SetAutoPageBreak(auto bool, margin float64) {
-}