From c07ce50908403b5b15075e6f18871e1ff4c25a37 Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Wed, 11 Jul 2018 19:28:13 +1000
Subject: [PATCH] json/schema: modularise the validator objects

---
 CMakeLists.txt                |  15 ++++
 json/schema.cpp               |  28 ++++++++
 json/schema.hpp               |  15 ++++
 json/schema/base.cpp          |  39 +++++++++++
 json/schema/base.hpp          |  51 ++++++++++++++
 json/schema/combine.cpp       |  78 +++++++++++++++++++++
 json/schema/combine.hpp       |  52 ++++++++++++++
 json/schema/except.hpp        |  64 +++++++++++++++++
 json/schema/fwd.hpp           |  30 ++++++++
 json/schema/inequality.cpp    |  75 ++++++++++++++++++++
 json/schema/inequality.hpp    |  49 +++++++++++++
 json/schema/introspection.hpp |  35 ++++++++++
 json/schema/length.cpp        |  83 ++++++++++++++++++++++
 json/schema/length.hpp        |  42 +++++++++++
 json/schema/properties.cpp    |  81 +++++++++++++++++++++
 json/schema/properties.hpp    |  40 +++++++++++
 json/schema/type.cpp          | 128 ++++++++++++++++++++++++++++++++++
 json/schema/type.hpp          |  47 +++++++++++++
 tools/json-schema.cpp         |  15 ++--
 19 files changed, 963 insertions(+), 4 deletions(-)
 create mode 100644 json/schema/base.cpp
 create mode 100644 json/schema/base.hpp
 create mode 100644 json/schema/combine.cpp
 create mode 100644 json/schema/combine.hpp
 create mode 100644 json/schema/except.hpp
 create mode 100644 json/schema/fwd.hpp
 create mode 100644 json/schema/inequality.cpp
 create mode 100644 json/schema/inequality.hpp
 create mode 100644 json/schema/introspection.hpp
 create mode 100644 json/schema/length.cpp
 create mode 100644 json/schema/length.hpp
 create mode 100644 json/schema/properties.cpp
 create mode 100644 json/schema/properties.hpp
 create mode 100644 json/schema/type.cpp
 create mode 100644 json/schema/type.hpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index ec2685f3..6566c5af 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -298,6 +298,21 @@ list (
     json/flat.hpp
     json/schema.cpp
     json/schema.hpp
+    json/schema/fwd.hpp
+    json/schema/base.cpp
+    json/schema/base.hpp
+    json/schema/combine.cpp
+    json/schema/combine.hpp
+    json/schema/except.hpp
+    json/schema/introspection.hpp
+    json/schema/length.cpp
+    json/schema/length.hpp
+    json/schema/inequality.cpp
+    json/schema/inequality.hpp
+    json/schema/properties.cpp
+    json/schema/properties.hpp
+    json/schema/type.cpp
+    json/schema/type.hpp
     json/tree.cpp
     json/tree.hpp
     json2/fwd.hpp
diff --git a/json/schema.cpp b/json/schema.cpp
index 47e9eb8d..af4038be 100644
--- a/json/schema.cpp
+++ b/json/schema.cpp
@@ -538,3 +538,31 @@ json::schema::validate (json::tree::node &data,
     auto schema_object = json::tree::parse (util::view(schema_data).cast<const char*> ());
     validate (data, schema_object->as_object ());
 }
+
+
+///////////////////////////////////////////////////////////////////////////////
+#include "schema/except.hpp"
+#include "schema/base.hpp"
+
+#include "../io.hpp"
+#include "../view.hpp"
+
+#include <map>
+
+json::schema::validator::validator(std::experimental::filesystem::path const &path):
+    validator (*json::tree::parse (path))
+{ ; }
+
+
+json::schema::validator::validator (json::tree::node const &definition):
+    m_base (definition)
+{ ; }
+
+
+bool
+json::schema::validator::validate (json::tree::node &data) const
+{
+    std::vector<util::json::schema::constraint::failure> res;
+    m_base.validate (std::back_inserter(res), data);
+    return res.empty ();
+}
diff --git a/json/schema.hpp b/json/schema.hpp
index 4398bf1d..b0d524ac 100644
--- a/json/schema.hpp
+++ b/json/schema.hpp
@@ -18,12 +18,27 @@
 #define __UTIL_JSON_SCHEMA_HPP
 
 #include "fwd.hpp"
+#include "schema/combine.hpp"
 
 #include <experimental/filesystem>
 
 
 ///////////////////////////////////////////////////////////////////////////////
 namespace json::schema {
+    class validator {
+    public:
+        validator (json::tree::node const &definition);
+        validator (std::experimental::filesystem::path const&);
+
+        bool validate (json::tree::node &data) const;
+
+    private:
+        std::string m_version;
+
+        util::json::schema::constraint::combine m_base;
+    };
+
+
     // Validate the json tree using the provide schema object or path.
     //
     // Note that the data object being validated may be altered in the process
diff --git a/json/schema/base.cpp b/json/schema/base.cpp
new file mode 100644
index 00000000..41377731
--- /dev/null
+++ b/json/schema/base.cpp
@@ -0,0 +1,39 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "base.hpp"
+
+#include "type.hpp"
+#include "length.hpp"
+#include "inequality.hpp"
+#include "properties.hpp"
+
+using util::json::schema::constraint::base;
+
+
+///////////////////////////////////////////////////////////////////////////////
+std::unique_ptr<base>
+base::instantiate (std::string const &name, ::json::tree::node const &def)
+{
+    if (name == "type")       return std::make_unique<type> (def);
+    if (name == "minLength")  return std::make_unique<min_length> (def);
+    if (name == "maxLength")  return std::make_unique<max_length> (def);
+    if (name == "minimum")    return std::make_unique<minimum> (def);
+    if (name == "maximum")    return std::make_unique<maximum> (def);
+    if (name == "properties") return std::make_unique<properties> (def);
+
+    throw unknown_constraint (name);
+}
diff --git a/json/schema/base.hpp b/json/schema/base.hpp
new file mode 100644
index 00000000..44bffecd
--- /dev/null
+++ b/json/schema/base.hpp
@@ -0,0 +1,51 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "except.hpp"
+
+#include <string>
+#include <vector>
+#include <iosfwd>
+
+
+namespace util::json::schema::constraint {
+    struct failure {
+        constraint::base const &rule;
+        ::json::tree::node const &target;
+    };
+
+
+    class base {
+    public:
+        using output_container = std::vector<failure>;
+        using output_iterator = std::back_insert_iterator<output_container>;
+
+        virtual ~base () = default;
+        virtual output_iterator validate (output_iterator res, ::json::tree::node &) const noexcept = 0;
+        virtual std::ostream& describe (std::ostream&) const = 0;
+
+        static std::unique_ptr<base> instantiate (std::string const &name, ::json::tree::node const&);
+    };
+
+
+    inline std::ostream&
+    operator<< (std::ostream &os, base const &obj)
+    {
+        return obj.describe (os);
+    }
+}
diff --git a/json/schema/combine.cpp b/json/schema/combine.cpp
new file mode 100644
index 00000000..436af908
--- /dev/null
+++ b/json/schema/combine.cpp
@@ -0,0 +1,78 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "combine.hpp"
+
+#include "../tree.hpp"
+
+#include "../../iterator.hpp"
+
+using util::json::schema::constraint::combine;
+
+
+///////////////////////////////////////////////////////////////////////////////
+combine::combine (::json::tree::node const &def)
+{
+    for (auto const &[key,val]: def.as_object ()) {
+        if (key == "$schema") {
+            m_version = val->as_string ();
+            continue;
+        }
+
+        if (key == "default") {
+            m_default = val->clone ();
+            continue;
+        }
+
+        m_constraints.push_back (instantiate (key, *val));
+    }
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+combine::output_iterator
+combine::validate (output_iterator res, ::json::tree::node &data) const noexcept
+{
+    for (auto const &i: m_constraints)
+        res = i->validate (res, data);
+    return res;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+std::ostream&
+combine::describe (std::ostream &os) const
+{
+    return os << "{ combine: [ ";
+    for (auto const &i: m_constraints)
+        os << *i << ", ";
+    return os << " ] }";
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+bool
+combine::has_default (void) const
+{
+    return !!m_default;
+}
+
+//-----------------------------------------------------------------------------
+std::unique_ptr<::json::tree::node>
+combine::default_value (void) const
+{
+    return m_default->clone ();
+}
diff --git a/json/schema/combine.hpp b/json/schema/combine.hpp
new file mode 100644
index 00000000..5e6b714c
--- /dev/null
+++ b/json/schema/combine.hpp
@@ -0,0 +1,52 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "base.hpp"
+#include "../tree.hpp"
+
+#include <memory>
+#include <vector>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace util::json::schema::constraint {
+    class combine final : public base {
+    public:
+        combine (::json::tree::node const &def);
+
+        combine (combine const&) = delete;
+        combine& operator= (combine const&) = delete;
+
+        combine (combine &&) = default;
+        combine& operator= (combine &&) = default;
+
+        virtual ~combine () = default;
+
+        output_iterator validate (output_iterator res, ::json::tree::node &) const noexcept override;
+        std::ostream& describe (std::ostream&) const override;
+
+        bool has_default (void) const;
+        std::unique_ptr<::json::tree::node> default_value (void) const;
+
+    private:
+        std::string m_version;
+        std::unique_ptr<::json::tree::node> m_default;
+
+        std::vector<std::unique_ptr<base>> m_constraints;
+    };
+}
\ No newline at end of file
diff --git a/json/schema/except.hpp b/json/schema/except.hpp
new file mode 100644
index 00000000..8fa23c45
--- /dev/null
+++ b/json/schema/except.hpp
@@ -0,0 +1,64 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "fwd.hpp"
+#include "introspection.hpp"
+#include "../fwd.hpp"
+
+#include <exception>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace util::json::schema {
+    class error : public std::exception { };
+
+
+    template <typename T>
+    class constraint_error : public error {
+    public:
+        virtual ~constraint_error () = default;
+        constraint_error (::json::tree::node const &_def):
+            def (_def)
+        { ; }
+
+        char const* what (void) const noexcept override
+        {
+            return util::type_name_v<T>;
+        }
+
+        ::json::tree::node const &def;
+    };
+
+
+    class unknown_constraint : public error {
+    public:
+        unknown_constraint (std::string const &name):
+            m_name (name)
+        { ; }
+
+        virtual ~unknown_constraint () = default;
+
+        char const* what (void) const noexcept override
+        {
+            return m_name.c_str ();
+        }
+
+    private:
+        std::string m_name;
+    };
+}
diff --git a/json/schema/fwd.hpp b/json/schema/fwd.hpp
new file mode 100644
index 00000000..6922f953
--- /dev/null
+++ b/json/schema/fwd.hpp
@@ -0,0 +1,30 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+
+namespace util::json::schema::constraint {
+    struct failure;
+
+    class base;
+
+    class type;
+    class combine;
+    template <typename> class length;
+    template <template <typename> class> class inequality;
+    class properties;
+}
\ No newline at end of file
diff --git a/json/schema/inequality.cpp b/json/schema/inequality.cpp
new file mode 100644
index 00000000..c0087169
--- /dev/null
+++ b/json/schema/inequality.cpp
@@ -0,0 +1,75 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "inequality.hpp"
+
+#include "../tree.hpp"
+
+using util::json::schema::constraint::inequality;
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <template <typename> class ComparatorT>
+inequality<ComparatorT>::inequality (::json::tree::node const &def)
+{
+    if (!def.is_number ())
+        throw constraint_error<inequality> (def);
+
+    auto const &val = def.as_number ();
+    switch (val.repr ()) {
+    case ::json::tree::number::UINT: m_value = val.uint (); break;
+    case ::json::tree::number::SINT: m_value = val.sint (); break;
+    case ::json::tree::number::REAL: m_value = val.real (); break;
+    }
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <template <typename> class ComparatorT>
+typename inequality<ComparatorT>::output_iterator
+inequality<ComparatorT>::validate (util::json::schema::constraint::base::output_iterator res,
+                                  ::json::tree::node &target) const noexcept
+{
+    if (!target.is_number ()) {
+        return *res++ = failure { .rule = *this, .target = target };
+    }
+
+    // HACK: this shouldn't be casting to double for the comparison, but it
+    // greatly simplifies the logic.
+    auto const &value = target.as_number ();
+    std::visit([&,this] (auto const &inner) noexcept {
+        if (!ComparatorT<double>{} (value.as<double> (), inner))
+           *res++ = failure { .rule = *this, .target = target };
+    }, m_value);
+
+    return res;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <template <typename> class ComparatorT>
+std::ostream&
+inequality<ComparatorT>::describe (std::ostream &os) const
+{
+    os << "{ " << util::type_name_v<inequality> << ": ";
+    std::visit ([&os] (auto const &i) -> void { os << i; }, m_value);
+    return os << " }";
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template class util::json::schema::constraint::inequality<std::less_equal>;
+template class util::json::schema::constraint::inequality<std::greater_equal>;
diff --git a/json/schema/inequality.hpp b/json/schema/inequality.hpp
new file mode 100644
index 00000000..12994509
--- /dev/null
+++ b/json/schema/inequality.hpp
@@ -0,0 +1,49 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "base.hpp"
+
+#include "../tree.hpp"
+
+#include <functional>
+#include <variant>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace util::json::schema::constraint {
+    template <template <typename> class ComparatorT>
+    class inequality final : public base {
+    public:
+        inequality (::json::tree::node const&);
+        virtual ~inequality () = default;
+
+        output_iterator validate (output_iterator, ::json::tree::node &target) const noexcept override;
+        std::ostream& describe (std::ostream&) const override;
+
+    private:
+        using uint_t = ::json::tree::number::uint_t;
+        using sint_t = ::json::tree::number::sint_t;
+        using real_t = ::json::tree::number::real_t;
+
+        std::variant<uint_t,sint_t,real_t> m_value;
+    };
+
+
+    using minimum = inequality<std::greater_equal>;
+    using maximum = inequality<std::less_equal>;
+}
diff --git a/json/schema/introspection.hpp b/json/schema/introspection.hpp
new file mode 100644
index 00000000..44b0838e
--- /dev/null
+++ b/json/schema/introspection.hpp
@@ -0,0 +1,35 @@
+#pragma once
+
+#include "../../introspection.hpp"
+
+#include "fwd.hpp"
+
+namespace util {
+    #define SCHEMA_INTROSPECTION(K)                                 \
+    namespace detail {                                              \
+        struct type_name_cruft_json_schema_constraint_##K {         \
+            static constexpr const char value[] = #K;               \
+        };                                                          \
+    }                                                               \
+                                                                    \
+    template <>                                                     \
+    struct type_name<::util::json::schema::constraint::K> :         \
+        public detail::type_name_cruft_json_schema_constraint_##K   \
+    { };
+
+
+    template <template <typename> class OperatorT>
+    struct type_name<::util::json::schema::constraint::inequality<OperatorT>> {
+        static constexpr const char value[] = "inequality";
+    };
+
+    template <typename OperatorT>
+    struct type_name<::util::json::schema::constraint::length<OperatorT>> {
+        static constexpr const char value[] = "length";
+    };
+
+
+    MAP0(SCHEMA_INTROSPECTION, type, combine, properties)
+
+    #undef SCHEMA_INTROSPECTION
+}
diff --git a/json/schema/length.cpp b/json/schema/length.cpp
new file mode 100644
index 00000000..bd4d95ff
--- /dev/null
+++ b/json/schema/length.cpp
@@ -0,0 +1,83 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "length.hpp"
+
+#include "../tree.hpp"
+
+using util::json::schema::constraint::length;
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace {
+    template <typename ComparatorT>
+    struct field_name { };
+
+
+    template <>
+    struct field_name<std::greater_equal<std::size_t>> {
+        static constexpr char const *const value = "minLength";
+    };
+
+
+    template <>
+    struct field_name<std::less_equal<std::size_t>> {
+        static constexpr char const *const value = "maxLength";
+    };
+
+
+    template <typename ComparatorT>
+    static constexpr auto field_name_v = field_name<ComparatorT>::value;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename ComparatorT>
+length<ComparatorT>::length(::json::tree::node const &def):
+    m_length(def.as<std::size_t> ())
+{ ; }
+
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename ComparatorT>
+typename length<ComparatorT>::output_iterator
+length<ComparatorT>::validate (output_iterator res, ::json::tree::node &target) const noexcept
+{
+    if (!target.is_string ()) {
+        return *res++ = failure { .rule = *this, .target = target };
+    }
+
+    auto const &val = target.as_string ();
+    if (!ComparatorT{}(val.size (), m_length))
+        *res++ = failure { .rule = *this, .target = target };
+
+    return res;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <typename ComparatorT>
+std::ostream&
+length<ComparatorT>::describe (std::ostream &os) const
+{
+    return os << "{ " << ::field_name_v<ComparatorT> << ": " << m_length << " }";
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+template class util::json::schema::constraint::length<std::less_equal<std::size_t>>;
+template class util::json::schema::constraint::length<std::greater_equal<std::size_t>>;
diff --git a/json/schema/length.hpp b/json/schema/length.hpp
new file mode 100644
index 00000000..133ffcfb
--- /dev/null
+++ b/json/schema/length.hpp
@@ -0,0 +1,42 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "base.hpp"
+
+#include <functional>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace util::json::schema::constraint {
+    template <typename ComparatorT>
+    class length final : public base {
+    public:
+        length (::json::tree::node const&);
+        virtual ~length () = default;
+
+        output_iterator validate (output_iterator, ::json::tree::node &target) const noexcept override;
+        std::ostream& describe (std::ostream&) const override;
+
+    private:
+        std::size_t m_length;
+    };
+
+
+    using max_length = length<std::less_equal<std::size_t>>;
+    using min_length = length<std::greater_equal<std::size_t>>;
+}
diff --git a/json/schema/properties.cpp b/json/schema/properties.cpp
new file mode 100644
index 00000000..9958838b
--- /dev/null
+++ b/json/schema/properties.cpp
@@ -0,0 +1,81 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "properties.hpp"
+
+#include "../tree.hpp"
+
+using util::json::schema::constraint::properties;
+
+
+///////////////////////////////////////////////////////////////////////////////
+properties::properties (::json::tree::node const &def)
+{
+    if (!def.is_object ())
+        throw constraint_error<properties> (def);
+
+    for (auto const &[key,val]: def.as_object ()) {
+        m_properties.emplace (key, *val);
+    }
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+properties::output_iterator
+properties::validate (output_iterator res, ::json::tree::node &target) const noexcept
+{
+    if (!target.is_object ())
+        return *res++ = { .rule = *this, .target = target };
+
+    auto &obj = target.as_object ();
+
+    // validate the keys that are present against the property schemas
+    for (const auto &[key,val]: obj) {
+        auto const pos = m_properties.find (key);
+        if (pos == m_properties.end ())
+            continue;
+
+        res = pos->second.validate (res, *val);
+    }
+
+    // check if there's a key in the schema that isn't present but has a default
+    for (auto const &[key,doc]: m_properties) {
+        if (!doc.has_default ())
+            continue;
+
+        auto pos = obj.find (key);
+        if (pos == obj.end ())
+            obj.insert (key, doc.default_value ());
+    }
+
+    return res;
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+std::ostream&
+properties::describe (std::ostream &os) const
+{
+    os << "{ properties: [ ";
+
+    for (auto const &[key,val]: m_properties) {
+        os << key << ": ";
+        val.describe (os);
+        os << ", ";
+    }
+
+    return os << " ] }";
+}
diff --git a/json/schema/properties.hpp b/json/schema/properties.hpp
new file mode 100644
index 00000000..c5c632c6
--- /dev/null
+++ b/json/schema/properties.hpp
@@ -0,0 +1,40 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "base.hpp"
+#include "combine.hpp"
+
+#include <map>
+#include <string>
+#include <memory>
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace util::json::schema::constraint {
+    class properties : public base {
+    public:
+        properties (::json::tree::node const&);
+        virtual ~properties () = default;
+
+        output_iterator validate (output_iterator, ::json::tree::node &target) const noexcept override;
+        std::ostream& describe (std::ostream&) const override;
+
+    private:
+        std::map<std::string,combine> m_properties;
+    };
+}
diff --git a/json/schema/type.cpp b/json/schema/type.cpp
new file mode 100644
index 00000000..aae31297
--- /dev/null
+++ b/json/schema/type.cpp
@@ -0,0 +1,128 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "type.hpp"
+
+#include "except.hpp"
+
+#include "../../parse.hpp"
+
+using util::json::schema::constraint::type;
+
+
+///////////////////////////////////////////////////////////////////////////////
+template <>
+util::json::schema::constraint::type::type_t
+util::parse<util::json::schema::constraint::type::type_t> (util::view<const char*> str)
+{
+    if (str == "integer")  return ::util::json::schema::constraint::type::INTEGER;
+    if (str == "number")   return ::util::json::schema::constraint::type::NUMBER;
+    if (str == "string")   return ::util::json::schema::constraint::type::STRING;
+    if (str == "object")   return ::util::json::schema::constraint::type::OBJECT;
+    if (str == "array")    return ::util::json::schema::constraint::type::ARRAY;
+    if (str == "boolean")  return ::util::json::schema::constraint::type::BOOLEAN;
+    if (str == "null")     return ::util::json::schema::constraint::type::NONE;
+
+    throw std::invalid_argument (std::string (str.begin (), str.end ()));
+}
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace util::json::schema::constraint {
+    std::string const&
+    to_string (type::type_t t)
+    {
+        switch (t) {
+        case type::INTEGER: { static std::string s_name = "integer"; return s_name; }
+        case type::NUMBER:  { static std::string s_name = "number";  return s_name; }
+        case type::STRING:  { static std::string s_name = "string";  return s_name; }
+        case type::OBJECT:  { static std::string s_name = "object";  return s_name; }
+        case type::ARRAY:   { static std::string s_name = "array";   return s_name; }
+        case type::BOOLEAN: { static std::string s_name = "boolean"; return s_name; }
+        case type::NONE:    { static std::string s_name = "none";    return s_name; }
+        }
+
+        unreachable ();
+    }
+}
+
+
+static type::type_t
+valid_types (::json::tree::node const &def)
+{
+    if (def.is_string ()) {
+        return util::parse<type::type_t> (def.as_string ());
+    } else if (def.is_array ()) {
+        std::underlying_type_t<type::type_t> accum = 0;
+        for (auto const &i: def.as_array ())
+            accum |= util::parse<type::type_t> (i.as_string ());
+        return type::type_t (accum);
+    } else {
+        throw util::json::schema::constraint_error<type> (def);
+    }
+}
+
+
+
+///////////////////////////////////////////////////////////////////////////////
+type::type (::json::tree::node const &def):
+    m_type (valid_types (def))
+{ ; }
+
+
+///////////////////////////////////////////////////////////////////////////////
+static bool
+is_valid (type::type_t t, ::json::tree::node const &target) noexcept
+{
+    switch (target.type ()) {
+    case ::json::tree::STRING:  return t & type::STRING;
+    case ::json::tree::OBJECT:  return t & type::OBJECT;
+    case ::json::tree::ARRAY:   return t & type::ARRAY;
+    case ::json::tree::BOOLEAN: return t & type::BOOLEAN;
+    case ::json::tree::NONE:    return t & type::NONE;
+
+    case ::json::tree::NUMBER:
+        if (t & type::NUMBER)
+            return true;
+
+        switch (target.as_number ().repr ()) {
+        case ::json::tree::number::UINT:
+        case ::json::tree::number::SINT:
+            return t & type::INTEGER;
+
+        case ::json::tree::number::REAL:
+            return false;
+        }
+    }
+
+    unreachable ();
+}
+
+
+type::output_iterator
+type::validate (output_iterator res, ::json::tree::node &target) const noexcept
+{
+    if (!is_valid (m_type, target))
+        *res++ = failure { .rule = *this, .target = target };
+    return res;
+}
+
+
+std::ostream&
+type::describe (std::ostream &os) const
+{
+    return os << "{type: " << to_string (m_type) << " }";
+}
diff --git a/json/schema/type.hpp b/json/schema/type.hpp
new file mode 100644
index 00000000..e729aadb
--- /dev/null
+++ b/json/schema/type.hpp
@@ -0,0 +1,47 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#pragma once
+
+#include "base.hpp"
+
+#include "../tree.hpp"
+
+
+///////////////////////////////////////////////////////////////////////////////
+namespace util::json::schema::constraint {
+    class type final : public base {
+    public:
+        enum type_t {
+            INTEGER  = 1 << 0,
+            NUMBER   = 1 << 1,
+            STRING   = 1 << 2,
+            OBJECT   = 1 << 3,
+            ARRAY    = 1 << 4,
+            BOOLEAN  = 1 << 5,
+            NONE     = 1 << 6,
+        };
+
+        type (::json::tree::node const &definition);
+        virtual ~type () = default;
+
+        output_iterator validate (output_iterator, ::json::tree::node &target) const noexcept override;
+        std::ostream& describe (std::ostream&) const override;
+
+    private:
+        type_t const m_type;
+    };
+}
diff --git a/tools/json-schema.cpp b/tools/json-schema.cpp
index 4d06dddc..4db7e158 100644
--- a/tools/json-schema.cpp
+++ b/tools/json-schema.cpp
@@ -19,6 +19,8 @@
 #include "json/schema.hpp"
 #include "json/tree.hpp"
 
+#include "json/schema/except.hpp"
+
 #include "io.hpp"
 
 #include <experimental/filesystem>
@@ -47,15 +49,20 @@ main (int argc, char **argv) {
     }
 
     try {
-        const util::mapped_file schema_src (argv[ARG_SCHEMA]);
         const util::mapped_file input_src  (argv[ARG_INPUT]);
 
-        auto schema = json::tree::parse (util::view{schema_src}.cast<const char*>());
-        auto input  = json::tree::parse (util::view{input_src} .cast<const char*>());
+        json::schema::validator schema (argv[ARG_SCHEMA]);
+        auto input = json::tree::parse (util::view{input_src} .cast<const char*>());
 
-        json::schema::validate (*input, schema->as_object ());
+        if (!schema.validate (*input)) {
+            std::cerr << "fail\n";
+            return EXIT_FAILURE;
+        }
 
         std::cout << *input << '\n';
+    } catch (util::json::schema::error const &x) {
+        std::cerr << "schema: " << x.what () << '\n';
+        return EXIT_FAILURE;
     } catch (const json::error &e) {
         std::cerr << "error: " << e.what () << '\n';
         return EXIT_FAILURE;