Fetch page from outline

lex.go

@@ -404,6 +404,11 @@ type objptr struct {
 	gen uint16
 }
 
+// Equal returns true if two object references point to the same object.
+func (a objptr) Equal(b objptr) bool {
+	return a.id == b.id
+}
+
 type objdef struct {
 	ptr objptr
 	obj object

page.go

@@ -50,6 +50,44 @@ Search:
 	return Page{}
 }
 
+func (p Page) findAdjacent(direction int) Page {
+	lookingFor := p.V.ptr
+	found := false
+	pages := p.V.Key("Parent")
+FindNext:
+	for pages.Key("Type").Name() == "Pages" {
+		var kid Value
+		kids := pages.Key("Kids")
+		for i := 0; i < kids.Len(); i++ {
+			if direction == 1 {
+				kid = kids.Index(i)
+			} else {
+				kid = kids.Index(kids.Len() - i - 1)
+			}
+			if found {
+				if kid.Key("Type").Name() == "Page" {
+					return Page{kid}
+				} else if kid.Key("Type").Name() == "Pages" {
+					pages = kid
+					continue FindNext
+				}
+			} else if kid.ptr.Equal(lookingFor) {
+				found = true
+			}
+		}
+		lookingFor = pages.ptr
+		found = false
+		pages = pages.Key("Parent")
+	}
+	return Page{}
+}
+
+// Next returns the next page in the document.
+func (p Page) Next() Page { return p.findAdjacent(1) }
+
+// Prev returns the previous page in the document.
+func (p Page) Prev() Page { return p.findAdjacent(-1) }
+
 // NumPage returns the number of pages in the PDF file.
 func (r *Reader) NumPage() int {
 	return int(r.Trailer().Key("Root").Key("Pages").Key("Count").Int64())
@@ -647,6 +685,7 @@ func (x TextHorizontal) Less(i, j int) bool {
 type Outline struct {
 	Title string    // title for this element
 	Child []Outline // child elements
+	dest  Value     // destination, which will need to be resolved
 }
 
 // Outline returns the document outline.
@@ -659,8 +698,27 @@ func (r *Reader) Outline() Outline {
 func buildOutline(entry Value) Outline {
 	var x Outline
 	x.Title = entry.Key("Title").Text()
+	x.dest = entry.Key("Dest")
 	for child := entry.Key("First"); child.Kind() == Dict; child = child.Key("Next") {
 		x.Child = append(x.Child, buildOutline(child))
 	}
 	return x
 }
+
+// Page returns the page of the document corresponding to an outline (aka
+// table of contents) entry.
+func (o Outline) Page() Page {
+	root := o.dest.r.Trailer().Key("Root")
+	dests := root.Key("Names").Key("Dests")
+	if dests.IsNull() {
+		dests = root.Key("Dests") // PDF 1.1
+	}
+
+	if o.dest.Kind() == String {
+		dest := dests.nameTreeLookup(o.dest.String()).Key("D").Index(0)
+		if dest.Key("Type").Name() == "Page" {
+			return Page{dest}
+		}
+	} // non-named destinations are not supported
+	return Page{}
+}

read.go

@@ -73,6 +73,7 @@ import (
 	"os"
 	"sort"
 	"strconv"
+	"strings"
 )
 
 // A Reader is a single PDF file open for reading.
@@ -664,6 +665,41 @@ func (v Value) Key(key string) Value {
 	return v.r.resolve(v.ptr, x[name(key)])
 }
 
+// nameTreeLookup looks up a value from a name tree.  Name trees are similar
+// to dictionaries, so this is analogous to Key().  See 7.9.6 of the PDF seec.
+func (v Value) nameTreeLookup(key string) Value {
+NTSearch:
+	for {
+		limits := v.Key("Limits")
+		if !limits.IsNull() && (strings.Compare(key, limits.Index(0).String()) == -1 || strings.Compare(key, limits.Index(1).String()) == 1) {
+			break // key is outside stated limits
+		}
+
+		names := v.Key("Names")
+		if !names.IsNull() {
+			for i := 0; i < names.Len(); i += 2 {
+				if names.Index(i).String() == key {
+					return names.Index(i + 1)
+				}
+			}
+			break // name not found
+		}
+
+		kids := v.Key("Kids")
+		for i := 0; i < kids.Len(); i++ {
+			kid := kids.Index(i)
+			limits := kid.Key("Limits")
+			if !limits.IsNull() && strings.Compare(key, limits.Index(0).String()) >= 0 && strings.Compare(key, limits.Index(1).String()) <= 0 {
+				v = kid
+				continue NTSearch
+			}
+		}
+		break // key is not within any kid's limits
+	}
+
+	return Value{}
+}
+
 // Keys returns a sorted list of the keys in the dictionary v.
 // If v is a stream, Keys applies to the stream's header dictionary.
 // If v.Kind() != Dict and v.Kind() != Stream, Keys returns nil.