<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:opf="http://www.idpf.org/2007/opf"
           xmlns:dc="http://purl.org/dc/elements/1.1/"
           xmlns:xml="http://www.w3.org/XML/1998/namespace"
           targetNamespace="http://www.idpf.org/2007/opf"
           elementFormDefault="qualified">

    <xs:annotation>
        <xs:documentation>Schema for OPF 2.0 (Open Packaging Format)</xs:documentation>
    </xs:annotation>

    <xs:import namespace="http://purl.org/dc/elements/1.1/" schemaLocation="dc.txt"/>
    <xs:import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="xml.txt"/>

    <!-- Define custom yes/no type -->
    <xs:simpleType name="yes_or_no">
        <xs:restriction base="xs:string">
            <xs:enumeration value="yes"/>
            <xs:enumeration value="no"/>
        </xs:restriction>
    </xs:simpleType>

    <!-- Package -->
    <xs:element name="package">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="opf:metadata"/>
                <xs:element ref="opf:manifest"/>
                <xs:element ref="opf:spine"/>
                <xs:element ref="opf:guide" minOccurs="0"/>
                <xs:element ref="opf:tours" minOccurs="0"/>
            </xs:sequence>
            <xs:attribute name="version" type="xs:string" use="required" fixed="2.0"/>
            <xs:attribute name="unique-identifier" type="xs:string" use="required"/>
            <xs:attribute ref="xml:lang"/>
        </xs:complexType>
    </xs:element>

    <!-- Metadata -->
    <xs:element name="metadata">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="dc:title" maxOccurs="unbounded"/>
                <xs:element ref="dc:language" maxOccurs="unbounded"/>
                <xs:element ref="dc:identifier" maxOccurs="unbounded"/>
                <xs:element ref="dc:creator" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:contributor" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:publisher" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:subject" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:description" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:date" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:type" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:format" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:source" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:relation" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:coverage" minOccurs="0" maxOccurs="unbounded"/>
                <xs:element ref="dc:rights" minOccurs="0" maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>

    <!-- Manifest -->
    <xs:element name="manifest">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="opf:item" maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>

    <xs:element name="item">
        <xs:complexType>
            <xs:attribute name="id" type="xs:ID" use="required"/>
            <xs:attribute name="href" type="xs:anyURI" use="required"/>
            <xs:attribute name="media-type" type="xs:string" use="required"/>
            <xs:attribute name="fallback" type="xs:IDREF"/>
        </xs:complexType>
    </xs:element>

    <!-- Spine -->
    <xs:element name="spine">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="opf:itemref" maxOccurs="unbounded"/>
            </xs:sequence>
            <xs:attribute name="toc" type="xs:IDREF"/>
        </xs:complexType>
    </xs:element>

    <xs:element name="itemref">
        <xs:complexType>
            <xs:attribute name="idref" type="xs:IDREF" use="required"/>
            <xs:attribute name="linear" type="yes_or_no" default="yes"/>
        </xs:complexType>
    </xs:element>

    <!-- Guide -->
    <xs:element name="guide">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="opf:reference" maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>

    <xs:element name="reference">
        <xs:complexType>
            <xs:attribute name="type" use="required"/>
            <xs:attribute name="title" type="xs:string"/>
            <xs:attribute name="href" type="xs:anyURI" use="required"/>
        </xs:complexType>
    </xs:element>

    <!-- Tours -->
    <xs:element name="tours">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="opf:tour" maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>

    <xs:element name="tour">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="opf:site" maxOccurs="unbounded"/>
            </xs:sequence>
            <xs:attribute name="id" type="xs:ID" use="required"/>
            <xs:attribute name="title" type="xs:string" use="required"/>
        </xs:complexType>
    </xs:element>

    <xs:element name="site">
        <xs:complexType>
            <xs:attribute name="title" type="xs:string" use="required"/>
            <xs:attribute name="href" type="xs:anyURI" use="required"/>
        </xs:complexType>
    </xs:element>

</xs:schema>