#pragma once

#include <functional>
#include <map>
#include <memory>
#include <set>
#include <string_view>
#include <unordered_map>
#include <unordered_set>

#include <jvalidate/constraint/array_constraint.h>
#include <jvalidate/constraint/general_constraint.h>
#include <jvalidate/constraint/number_constraint.h>
#include <jvalidate/constraint/object_constraint.h>
#include <jvalidate/constraint/string_constraint.h>

#include <jvalidate/compat/enumerate.h>
#include <jvalidate/detail/expect.h>
#include <jvalidate/detail/parser_context.h>
#include <jvalidate/detail/vocabulary.h>
#include <jvalidate/enum.h>
#include <jvalidate/forward.h>
#include <jvalidate/vocabulary.h>

namespace jvalidate {
/**
 * @brief A factory object for the generation of constraints in JSON Schema
 * Parsing.
 *
 * Unless specified, the reference numbers in function documentation
 * refer to the Draft2020_12 specification, located on the following webpage(s):
 * https://json-schema.org/draft/2020-12/json-schema-validation OR
 * https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-01
 *
 * The ConstraintFactory supports, by default, all of the vocabulary of every
 * JSON Schema Draft covered by the Version enum.
 *
 * @tparam A The concrete Adapter type being used with this factory.
 * By providing this as a template parameter to ConstraintFactory - it allows us
 * to write code that does not need to operate on an abstract interface type
 * with a lot of annoying pointers and indirections.
 */
template <Adapter A> class ConstraintFactory {
public:
  using pConstraint = std::unique_ptr<constraint::Constraint>;
  using DependentKeyword = vocabulary::DependentKeyword;
  static constexpr auto Removed = vocabulary::Removed;
  static constexpr auto Literal = vocabulary::Literal;
  static constexpr auto Keyword = vocabulary::Keyword;
  static constexpr auto KeywordMap = vocabulary::KeywordMap;
  static constexpr auto PostConstraint = vocabulary::PostConstraint;

  /**
   * @brief In order to support multiple schema versions in a single instance of
   * a ConstraintFactory, we need to be able to describe which version a keyword
   * becomes part of the language vocabulary, and what (if any) version it
   * leaves the vocabulary after.
   *
   * To do this, we store an ordered map of Version enum onto Make (see above),
   * and then use {@see std::map::lower_bound} to determine which Make object is
   * the most approriate for the schema version being evaluated.
   *
   * For example:
   * The "additionalProperties" constraint is the same across all versions, and
   * so can be represented using only a function pointer.
   *   {"additionalProperties", &Self::additionalProperties}
   *
   * The "const" constraint was not added until Draft06, so we include the
   * version when constructing its constraint bindings like so:
   *   {"const", {schema::Version::Draft06, &Self::isConstant}}
   *
   * The "divisibleBy" constraint was removed in favor of "multipleOf" in
   * Draft04, and therefore is represented as:
   *   {"divisibleBy", {{schema::Version::Earliest, &Self::multipleOf},
   *                    {schema::Version::Draft04, Removed}}},
   *   {"multipleOf",  {schema::Version::Draft04, &Self::multipleOf}}
   *
   * A small number of rare constraints change their meaning when moving from
   * one draft version to another in such a significant way that it makes more
   * sense to use different MakeConstraint functions for them.
   *   {"items", {{schema::Version::Earliest, &Self::itemsTupleOrVector},
   *              {schema::Version::Draft2020_12, &Self::additionalItems}}}
   */
  struct Versioned {
    template <typename M = vocabulary::Metadata<A>>
    Versioned(M make) : data{{schema::Version::Earliest, make}} {}
    Versioned(schema::Version version, vocabulary::Metadata<A> make) : data{{version, make}} {}
    Versioned(std::initializer_list<std::pair<schema::Version const, vocabulary::Metadata<A>>> init)
        : data(init) {}

    std::map<schema::Version, vocabulary::Metadata<A>, std::greater<>> data;
  };
  using Store = std::unordered_map<std::string_view, Versioned>;

private:
  using Self = ConstraintFactory<A>;

private:
  std::unordered_map<std::string_view, Versioned> constraints_{
      {"$defs", {schema::Version::Draft2019_09, KeywordMap}},
      {"additionalItems",
       {{schema::Version::Earliest, {&Self::additionalItems, Keyword}},
        {schema::Version::Draft2020_12, Removed}}},
      {"additionalProperties", {{&Self::additionalProperties, Keyword}}},
      {"allOf", {schema::Version::Draft04, {&Self::allOf, Keyword}}},
      {"anyOf", {schema::Version::Draft04, {&Self::anyOf, Keyword}}},
      {"const", {schema::Version::Draft06, &Self::isConstant}},
      {"contains", {schema::Version::Draft06, &Self::contains}},
      {"definitions", KeywordMap},
      {"dependencies", {{&Self::dependencies, KeywordMap}}},
      {"dependentRequired", {schema::Version::Draft2019_09, &Self::dependentRequired}},
      {"dependentSchemas", {schema::Version::Draft2019_09, {&Self::dependentSchemas, KeywordMap}}},
      {"disallow",
       {{schema::Version::Earliest, &Self::disallowDraft3}, {schema::Version::Draft04, Removed}}},
      {"divisibleBy",
       {{schema::Version::Earliest, &Self::multipleOf}, {schema::Version::Draft04, Removed}}},
      {"else", {{schema::Version::Draft07, DependentKeyword{"if"}}}},
      {"enum", &Self::isInEnumuration},
      {"exclusiveMaximum", {schema::Version::Draft06, &Self::exclusiveMaximum}},
      {"exclusiveMinimum", {schema::Version::Draft06, &Self::exclusiveMinimum}},
      {"extends",
       {{schema::Version::Earliest, &Self::extendsDraft3}, {schema::Version::Draft04, Removed}}},
      {"format", &Self::format},
      {"if", {schema::Version::Draft07, {&Self::ifThenElse, Keyword}}},
      {"items",
       {{schema::Version::Earliest, {&Self::itemsTupleOrVector, Keyword}},
        {schema::Version::Draft2020_12, {&Self::additionalItems, Keyword}}}},
      {"maxItems", &Self::maxItems},
      {"maxLength", &Self::maxLength},
      {"maxProperties", {schema::Version::Draft04, &Self::maxProperties}},
      {"maximum", &Self::maximum},
      {"minItems", &Self::minItems},
      {"minLength", &Self::minLength},
      {"minProperties", {schema::Version::Draft04, &Self::minProperties}},
      {"minimum", &Self::minimum},
      {"multipleOf", {schema::Version::Draft04, &Self::multipleOf}},
      {"not", {schema::Version::Draft04, {&Self::isNot, Keyword}}},
      {"oneOf", {schema::Version::Draft04, {&Self::oneOf, Keyword}}},
      {"pattern", &Self::pattern},
      {"patternProperties", {{&Self::patternProperties, KeywordMap}}},
      {"prefixItems", {schema::Version::Draft2020_12, {&Self::prefixItems, Keyword}}},
      {"properties",
       {{schema::Version::Earliest, {&Self::propertiesDraft3, KeywordMap}},
        {schema::Version::Draft04, {&Self::properties, KeywordMap}}}},
      {"propertyNames", {schema::Version::Draft06, &Self::propertyNames}},
      {"required", {schema::Version::Draft04, &Self::required}},
      {"then", {schema::Version::Draft07, DependentKeyword{"if"}}},
      {"type",
       {{schema::Version::Earliest, &Self::typeDraft3}, {schema::Version::Draft04, &Self::type}}},
      {"unevaluatedItems",
       {schema::Version::Draft2019_09, {&Self::unevaluatedItems, PostConstraint}}},
      {"unevaluatedProperties",
       {schema::Version::Draft2019_09, {&Self::unevaluatedProperties, PostConstraint}}},
      {"uniqueItems", &Self::uniqueItems},
  };

public:
  ConstraintFactory() = default;

  static pConstraint ptr(auto && in) {
    return std::make_unique<constraint::Constraint>(std::move(in));
  }

  /**
   * @brief Construct a new ConstraintFactory, with a pre-defined list of user
   * keywords to be injected into the vocabulary. Does not override any keywords
   * that currently exist. Operates equivalently to calling with_user_keyword
   * for each element of init.
   *
   * @param init A list of keyword => Versioned constraint generators
   */
  ConstraintFactory(std::initializer_list<std::pair<std::string_view, Versioned>> init) {
    constraints_.insert(init.begin(), init.end());
  }

  /**
   * @brief A "with-er" function that adds a user-defined keyword to the
   * vocabulary. This keyword cannot already exist in the schema (although it is
   * not asserted).
   *
   * Only usable on rval references to prevent injections.
   *
   * @param word The keyword being added
   * @param make The Versioned constraint generators
   *
   * @returns This factory
   */
  ConstraintFactory<A> && with_user_keyword(std::string_view word, Versioned make) && {
    constraints_.insert(word, std::move(make));
    return *this;
  }

  /**
   * @brief A "with-er" function that overrides a draft-defined keyword to the
   * vocabulary. This keyword is expected to already exist (although it is not
   * asserted).
   *
   * Only usable on rval references to prevent injections.
   *
   * @param word The keyword being overwritten
   * @param make The Versioned constraint generators
   *
   * @returns This factory
   */
  ConstraintFactory<A> && override_keyword(std::string_view word, Versioned make) && {
    constraints_[word] = std::move(make);
    return *this;
  }

  detail::Vocabulary<A> keywords(schema::Version version) const {
    std::unordered_map<std::string_view, vocabulary::Metadata<A>> rval;
    for (auto const & [key, versions] : constraints_) {
      if (auto it = versions.data.lower_bound(version); it != versions.data.end() && it->second) {
        rval.emplace(key, it->second);
      }
    }

    return detail::Vocabulary<A>(version, std::move(rval));
  }

  // SECTION: Untyped Constraints

  /**
   * @brief Parser for the "type" constraint (6.1.1) for JSON Schema Draft04
   * and up. This validates that a JSON document instance is of a specified
   * type or list of types.
   *
   * @pre context.schema MUST be either a string, or an array of strings.
   * @pre each string must be one of the six JSON primitives, or "integer"
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns If the value at "type" is a string, then we return a constraint
   * that is true if the type of the instance is of the type represented by the
   * string. If the value is an array, then the constraint will validate if the
   * instance is any of the listed types.
   *
   * @throws std::runtime_error if precondition #1 is broken
   * @throws std::out_of_range  if precondition #2 is broken
   */
  static auto type(detail::ParserContext<A> const & context) {
    static std::unordered_map<std::string_view, adapter::Type> const s_type_names{
        {"null", adapter::Type::Null},       {"boolean", adapter::Type::Boolean},
        {"integer", adapter::Type::Integer}, {"number", adapter::Type::Number},
        {"string", adapter::Type::String},   {"array", adapter::Type::Array},
        {"object", adapter::Type::Object},
    };

    auto to_type = [](std::string_view type) {
      EXPECT_M(s_type_names.contains(type), "Unknown type " << type);
      return s_type_names.at(type);
    };

    adapter::Type const type = context.schema.type();
    if (type == adapter::Type::String) {
      return ptr(constraint::TypeConstraint{{to_type(context.schema.as_string())}});
    }

    EXPECT(type == adapter::Type::Array);
    std::set<adapter::Type> types;
    for (auto subschema : context.schema.as_array()) {
      types.insert(to_type(subschema.as_string()));
    }
    return ptr(constraint::TypeConstraint{types});
  }

  /**
   * @brief Parser for the "type" constraint (5.1) for JSON Schema Draft03
   * (https://json-schema.org/draft-03/draft-zyp-json-schema-03.pdf). This
   * validates that a JSON document instance is of a specified type, or list of
   * types/subschemas.
   *
   * Despite the draft document not indicating so, it is considered legal for a
   * type constraint to allow subschemas when in array form.
   *
   * Additionally, it supports the additional type enumeration "any", which is
   * equivalent to not having a type constraint at all.
   *
   * @pre context.schema MUST be either a string, or an array of
   * strings/subschemas.
   * @pre each string MUST be one of the six JSON primitives, "integer",
   * or "any"
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns If the value at "type" is a string, then we return a constraint
   * that is true if the type of the instance is of the type represented by the
   * string. If the value is an array, then the constraint will validate if the
   * instance is any of the listed types or validated by the subschema.
   *
   * @throws std::runtime_error if precondition #1 is broken
   * @throws std::out_of_range  if precondition #2 is broken
   */
  static pConstraint typeDraft3(detail::ParserContext<A> const & context) {
    static std::unordered_map<std::string_view, adapter::Type> const s_type_names{
        {"null", adapter::Type::Null},       {"boolean", adapter::Type::Boolean},
        {"integer", adapter::Type::Integer}, {"number", adapter::Type::Number},
        {"string", adapter::Type::String},   {"array", adapter::Type::Array},
        {"object", adapter::Type::Object},
    };

    auto to_type = [](std::string_view type) {
      EXPECT_M(s_type_names.contains(type), "Unknown type " << type);
      return s_type_names.at(type);
    };

    adapter::Type const type = context.schema.type();
    if (type == adapter::Type::String) {
      if (context.schema.as_string() == "any") {
        return nullptr; // nullptr is a synonym for "always accept"
      }
      return ptr(constraint::TypeConstraint{{to_type(context.schema.as_string())}});
    }

    EXPECT(type == adapter::Type::Array);
    std::vector<constraint::SubConstraint> children;

    std::set<adapter::Type> types;
    for (auto const & [index, subschema] : detail::enumerate(context.schema.as_array())) {
      if (subschema.type() != adapter::Type::String) {
        children.push_back(context.child(subschema, index).node());
      } else if (subschema.as_string() == "any") {
        return nullptr; // nullptr is a synonym for "always accept"
      } else {
        types.insert(to_type(subschema.as_string()));
      }
    }

    children.push_back(ptr(constraint::TypeConstraint{types}));
    return ptr(constraint::AnyOfConstraint{std::move(children)});
  }

  /**
   * @brief Parser for the "disallow" constraint (5.25) for JSON Schema Draft03.
   * This constraint has the same preconditions and parsing rules as "type"
   * (Draft03) does, but inverts it.
   *
   * @pre context.schema MUST be either a string, or an array of
   * strings/subschemas.
   * @pre each string must be one of the six JSON primitives, "integer",
   * or "any"
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns not(typeDraft3())
   *
   * @throws {@see ConstraintFactory::typeDraft3}
   */
  static pConstraint disallowDraft3(detail::ParserContext<A> const & context) {
    return ptr(constraint::NotConstraint{typeDraft3(context)});
  }

  /**
   * @brief Parser for the "extends" constraint (5.26) for JSON Schema Draft03.
   * This validates that a JSON document instance meets both the parent schema,
   * and the child schema(s).
   *
   * The draft document shows examples that make sense as "$ref" constraints,
   * but the actual form of the "extends" is another schema, or an array of
   * schemas.
   * In Draft04 - the array/object form is replaced by including an "allOf"
   * constraint will all of the subschemas.
   * In Draft2019_09 - the single-object form can be implemented using "$ref",
   * since the parsing rules of reference handling have been relaxed.
   *
   * @pre context.schema MUST be either an object, or an array
   * @pre each object MUST be valid as a top-level schema
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns An AllOf constraint matching the one/many requested subschema(s).
   *
   * @throws std::runtime_error if precondition #1 is broken
   * @throws any std::exception if precondition #2 is broken
   */
  static pConstraint extendsDraft3(detail::ParserContext<A> const & context) {
    std::vector<constraint::SubConstraint> children;
    switch (context.schema.type()) {
    case adapter::Type::Object:
      children.push_back(context.node());
      break;
    case adapter::Type::Array: {
      for (auto const & [index, subschema] : detail::enumerate(context.schema.as_array())) {
        children.push_back(context.child(subschema, index).node());
      }
      break;
    }
    default:
      JVALIDATE_THROW(std::runtime_error, "extends must be a schema of array-of-schemas");
    }

    return ptr(constraint::AllOfConstraint{std::move(children)});
  }

  /**
   * @brief Parser for the "if" constraint (10.2.2.1, 10.2.2.2, 10.2.2.3). This
   * constraint is divided between three keywords.
   * If the "if" keyword is present, then we will attempt to validate the "then"
   * and "else keywords as well. All three are implemented as subschemas.
   * The evaluation forms an if-then-else, if-then, or if-else block, depending
   * on which of "then" and "else" are present.
   * There is no rule preventing an "if" keyword from existing without either
   * of the "then" or "else" keywords, but doing so only serves the purpose of
   * annotation gathering.
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A ContainsConstraint, with optional minimum and maximum matching
   */
  static pConstraint ifThenElse(detail::ParserContext<A> const & context) {
    schema::Node const * then_ = context.fixed_schema(true);
    if (context.parent->contains("then")) {
      then_ = context.neighbor("then").node();
    }

    schema::Node const * else_ = context.fixed_schema(true);
    if (context.parent->contains("else")) {
      else_ = context.neighbor("else").node();
    }

    return ptr(constraint::ConditionalConstraint{context.node(), then_, else_});
  }

  /**
   * @brief Parser for the "enum" constraint (6.1.2) for JSON Schema Draft04
   * and up. This validates that the JSON document instance is equal to one of
   * the given JSON document samples.
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A constraint that checks equality against a set of values.
   */
  static auto isInEnumuration(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Array);

    std::vector<std::unique_ptr<adapter::Const const>> rval;
    for (auto subschema : context.schema.as_array()) {
      rval.push_back(subschema.freeze());
    }

    return ptr(constraint::EnumConstraint{std::move(rval)});
  }

  /**
   * @brief Parser for the "const" constraint (6.1.3) for JSON Schema Draft04
   * and up. This validates that the JSON document instance is equal to the
   * given JSON document samples.
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A constraint that checks equality against a single value.
   */
  static auto isConstant(detail::ParserContext<A> const & context) {
    constraint::EnumConstraint rval;
    rval.enumeration.push_back(context.schema.freeze());
    return ptr(rval);
  }

  /**
   * @brief Parser for a "allOf" constraint (10.2.1.1). This constraint
   * validates that all of the underlying schemas validate the instance.
   *
   * @pre context.schema is an array
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A AllOfConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static auto allOf(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Array);

    std::vector<constraint::SubConstraint> rval;
    for (auto const & [index, subschema] : detail::enumerate(context.schema.as_array())) {
      rval.push_back(context.child(subschema, index).node());
    }

    return ptr(constraint::AllOfConstraint{std::move(rval)});
  }

  /**
   * @brief Parser for a "anyOf" constraint (10.2.1.2). This constraint
   * validates that any of the underlying schemas validate the instance.
   *
   * @pre context.schema is an array
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A AnyOfConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static auto anyOf(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Array);

    std::vector<constraint::SubConstraint> rval;
    for (auto const & [index, subschema] : detail::enumerate(context.schema.as_array())) {
      rval.push_back(context.child(subschema, index).node());
    }

    return ptr(constraint::AnyOfConstraint{std::move(rval)});
  }

  /**
   * @brief Parser for a "oneOf" constraint (10.2.1.3). This constraint
   * validates that exactly one of the underlying schemas validate the instance.
   *
   * @pre context.schema is an array
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A OneOfConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static auto oneOf(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Array);

    std::vector<schema::Node const *> rval;
    for (auto const & [index, subschema] : detail::enumerate(context.schema.as_array())) {
      rval.push_back(context.child(subschema, index).node());
    }

    return ptr(constraint::OneOfConstraint{rval});
  }

  /**
   * @brief Parser for a "not" constraint (10.2.1.4). This constraint inverts
   * the acceptance of the underlying schema.
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A NotConstraint
   */
  static auto isNot(detail::ParserContext<A> const & context) {
    return ptr(constraint::NotConstraint{context.node()});
  }

  // SECTION: Numeric Constraints

  /**
   * @brief Parser for the "minimum" constraint (6.2.4). This constraint
   * validates numberic JSON instances where `instance >= context.schema`
   * Before Draft06, this constraint must test for/evaluate the neighbor keyword
   * "exclusiveMinimum", which is a boolean.
   * Starting in Draft06, the "exclusiveMinimum" constraint exists separately,
   * and this constraint represents "inclusive minimum".
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MinimumConstraint
   *
   * @throws If the contained value is not interpretable as a number
   * @throws std::runtime_error if version < Draft06 AND exclusiveMinimum exists
   * and is not a boolean.
   */
  static auto minimum(detail::ParserContext<A> const & context) {
    double value = context.schema.as_number();
    if (context.vocab->version() < schema::Version::Draft06 &&
        context.parent->contains("exclusiveMinimum")) {
      auto exclusive = (*context.parent)["exclusiveMinimum"];
      EXPECT(exclusive.type() == adapter::Type::Boolean);
      return ptr(constraint::MinimumConstraint{value, exclusive.as_boolean()});
    }
    return ptr(constraint::MinimumConstraint{value, false});
  }

  /**
   * @brief Parser for the "exclusiveMinimum" constraint (6.2.5) for JSON Schema
   * Draft06 and up. This constraint validates numberic JSON instances where
   * `instance > context.schema`
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MinimumConstraint
   *
   * @throws If the contained value is not interpretable as a number
   */
  static pConstraint exclusiveMinimum(detail::ParserContext<A> const & context) {
    double value = context.schema.as_number();
    return ptr(constraint::MinimumConstraint{value, true});
  }

  /**
   * @brief Parser for the "maximum" constraint (6.2.2). This constraint
   * validates numberic JSON instances where `instance <= context.schema`
   * Before Draft06, this constraint must test for/evaluate the neighbor keyword
   * "exclusiveMaximum", which is a boolean.
   * Starting in Draft06, the "exclusiveMaximum" constraint exists separately,
   * and this constraint represents "inclusive maximum".
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MaximumConstraint
   *
   * @throws If the contained value is not interpretable as a number
   * @throws std::runtime_error if version < Draft06 AND exclusiveMaximum exists
   * and is not a boolean.
   */
  static auto maximum(detail::ParserContext<A> const & context) {
    double value = context.schema.as_number();
    if (context.vocab->version() < schema::Version::Draft06 &&
        context.parent->contains("exclusiveMaximum")) {
      auto exclusive = (*context.parent)["exclusiveMaximum"];
      EXPECT(exclusive.type() == adapter::Type::Boolean);
      return ptr(constraint::MaximumConstraint{value, exclusive.as_boolean()});
    }
    return ptr(constraint::MaximumConstraint{value, false});
  }

  /**
   * @brief Parser for the "exclusiveMaximum" constraint (6.2.3) for JSON Schema
   * Draft06 and up. This constraint validates numberic JSON instances where
   * `instance < context.schema`
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MaximumConstraint
   *
   * @throws If the contained value is not interpretable as a number
   */
  static pConstraint exclusiveMaximum(detail::ParserContext<A> const & context) {
    double value = context.schema.as_number();
    return ptr(constraint::MaximumConstraint{value, true});
  }

  /**
   * @brief Parser for the "multipleOf" constraint (6.2.1) for JSON Schema
   * Draft04 and up. In Draft03 this covers the "divisibleBy" constraint (5.24).
   * This constraint validates numeric JSON instances where
   * `instance / context.schema` is a whole number. Because of differences in
   * handling of numbers between C++, Python, JavaScript, etc. there are some
   * edge cases which cannot be properly handled.
   *
   * @pre context.schema matches { "type": "number" }
   * @pre context.schema.as_number() > 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MultipleOfConstraint
   *
   * @throws If the contained value is not interpretable as a number
   */
  static auto multipleOf(detail::ParserContext<A> const & context) {
    double value = context.schema.as_number();
    return ptr(constraint::MultipleOfConstraint{value});
  }

  // SECTION: String Constraints

  /**
   * @brief Parser for the "minLength" constraint (6.3.2). This constraint
   * validates string JSON instances has a length >= context.schema, as per
   * RFC 8259.
   *
   * @pre context.schema MUST be an integer
   * @pre context.schema >= 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MinLengthConstraint
   *
   * @throws If the contained value is not interpretable as an integer
   */
  static auto minLength(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Integer ||
           context.schema.type() == adapter::Type::Number);
    return ptr(constraint::MinLengthConstraint{context.schema.as_integer()});
  }

  /**
   * @brief Parser for the "maxLength" constraint (6.3.1). This constraint
   * validates string JSON instances have a length <= context.schema, as per
   * RFC 8259.
   *
   * @pre context.schema MUST be an integer
   * @pre context.schema >= 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MaxLengthConstraint
   *
   * @throws If the contained value is not interpretable as an integer
   */
  static auto maxLength(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Integer ||
           context.schema.type() == adapter::Type::Number);
    return ptr(constraint::MaxLengthConstraint{context.schema.as_integer()});
  }

  /**
   * @brief Parser for the "pattern" constraint (6.3.3). This constraint
   * validates string JSON instances match an ECMA-262 compatible regular
   * expression.
   *
   * This function does not attempt to compile the regular expression,
   * meaning that if the pattern is not a valid regex, it will fail at a later
   * point.
   *
   * @pre context.schema MUST be a string
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A PatternConstraint
   *
   * @throws If the contained value is not interpretable as a string
   */
  static auto pattern(detail::ParserContext<A> const & context) {
    return ptr(constraint::PatternConstraint{context.schema.as_string()});
  }

  /**
   * @brief Parser for the "format" constraint, which validates string JSON
   * instances against one of several pre-defined formats that either would
   * be unnecessarily complicated to represent as PatternConstraint, prone to
   * user-error when done so, or cannot be represented as regular expressions
   * at all.
   *
   * @pre context.schema MUST be a string
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A FormatConstraint, if the vocabulary enabled "format assertions",
   * then this constraint will actually validate the JSON instance. Otherwise,
   * it simply annotates that the field is expected to match the format.
   *
   * @throws If the contained value is not interpretable as a string
   */
  static auto format(detail::ParserContext<A> const & context) {
    return ptr(constraint::FormatConstraint{context.schema.as_string(),
                                            context.vocab->is_format_assertion()});
  }

  // SECTION: Array Constraints

  /**
   * @brief Parser for the "contains" constraint (10.3.1.3, 6.4.4, 6.4.5). This
   * constraint is divided between three different keywords.
   * The "contains" keyword acts as a subschema, and is required for parsing.
   * "minContains" and "maxContains" act as boundaries on the number of matches,
   * such that the number of array elements matching the "contains" schema is at
   * least "minContains" and at most "maxContains".
   *
   * If "minContains" is null, then it is the equivalent of 1.
   *   A "minContains" value of zero is only meaningful in the context of
   *   setting an upper-bound.
   * If "maxContains" is null, then it is the equivalent of INFINITY
   *
   * @pre context.schema MUST be a valid JSON schema
   * @pre context.parent["maxContains"] is null OR an integer >= 0
   * @pre context.parent["minContains"] is null OR an integer >= 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A ContainsConstraint, with optional minimum and maximum matching
   */
  static auto contains(detail::ParserContext<A> const & context) {
    if (context.vocab->version() < schema::Version::Draft2019_09) {
      return ptr(constraint::ContainsConstraint{context.node()});
    }

    std::optional<size_t> maximum;
    std::optional<size_t> minimum;
    if (context.parent->contains("maxContains")) {
      maximum = (*context.parent)["maxContains"].as_integer();
    }
    if (context.parent->contains("minContains")) {
      minimum = (*context.parent)["minContains"].as_integer();
    }

    return ptr(constraint::ContainsConstraint{context.node(), minimum, maximum});
  }

  /**
   * @brief Parser for the "minItems" constraint (6.4.2). This constraint
   * validates array JSON instances have a length <= context.schema.
   *
   * @pre context.schema MUST be an integer
   * @pre context.schema >= 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MinItemsConstraint
   *
   * @throws If the contained value is not interpretable as an integer
   */
  static auto minItems(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Integer ||
           context.schema.type() == adapter::Type::Number);
    return ptr(constraint::MinItemsConstraint{context.schema.as_integer()});
  }

  /**
   * @brief Parser for the "maxItems" constraint (6.4.1). This constraint
   * validates array JSON instances have a length <= context.schema.
   *
   * @pre context.schema MUST be an integer
   * @pre context.schema >= 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MaxItemsConstraint
   *
   * @throws If the contained value is not interpretable as an integer
   */
  static auto maxItems(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Integer ||
           context.schema.type() == adapter::Type::Number);
    return ptr(constraint::MaxItemsConstraint{context.schema.as_integer()});
  }

  /**
   * @brief Parser for the "prefixItems" constraint (10.3.1.1) for JSON Schema
   * Draft2020_12 and up. This constraint validates the first N elements of an
   * array JSON instance with its own subschemas.
   *
   * @pre context.schema MUST be an array
   * @pre context.schema MUST have at least 1 element
   * @pre each element of context.schema MUST be a valid JSON Schema
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A TupleConstraint
   *
   * @throws std::runtime_error if preconditions #1 or #2 are broken
   * @throws if precondition #3 is broken
   */
  static auto prefixItems(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Array);

    std::vector<schema::Node const *> rval;
    for (auto const & [index, subschema] : detail::enumerate(context.schema.as_array())) {
      rval.push_back(context.child(subschema, index).node());
    }

    return ptr(constraint::TupleConstraint{rval});
  }

  /**
   * @brief Parser for the "additionalItems" constraint (9.3.1.1) for JSON
   * Schema Draft2019_09 and prior, and the "items" constraint (10.3.1.2) for
   * JSON Schema Draft2020_12 and up.
   * This constraint validates an array JSON instance starting from the N-th
   * element, as determined by the "items" schema (<= Draft2019_09), or the
   * "prefixItems" schema (>= Draft2020_12).
   *
   * @pre context.schema MUST be a valid JSON Schema (including boolean schema)
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns An AdditionalItemsConstraint, unless the special condition
   * described below is met.
   *
   * @throws if the precondition is broken
   */
  static pConstraint additionalItems(detail::ParserContext<A> const & context) {
    std::string const prefix =
        context.vocab->version() >= schema::Version::Draft2020_12 ? "prefixItems" : "items";

    auto const & parent = *context.parent;
    // Before Draft 2020-12, the "items" could be either a subschema or a tuple.
    // When not provided, we therefore treat it as an "accept-all" schema, and
    // thus will never have additionalItems to process. Similarly - if it is an
    // Object, then it must act on all items.
    if (context.vocab->version() < schema::Version::Draft2020_12 &&
        (not parent.contains(prefix) || parent[prefix].type() == adapter::Type::Object)) {
      return nullptr;
    }

    size_t const n = parent[prefix].array_size();
    // Prior to Draft06, boolean schemas were not allowed in a general context,
    // they were instead reserved for the "additionalItems" and
    // "additionalProperties" keywords.
    if (context.vocab->version() < schema::Version::Draft06 &&
        context.schema.type() == adapter::Type::Boolean) {
      return ptr(constraint::AdditionalItemsConstraint{context.always(), n});
    }

    return ptr(constraint::AdditionalItemsConstraint{context.node(), n});
  }

  /**
   * @brief Parser for the "items" constraint (9.3.1.1) for JSON Schema
   * Draft2019_09 and prior.
   * https://json-schema.org/draft/2019-09/draft-handrews-json-schema-02
   * This constraint validates either the first N elements of an array JSON
   * instance with its own subschemas, or all elements of an array JSON instance
   * with its single subschema.
   *
   * @pre context.schema MUST satisfy the preconditions of either
   * {@see ConstraintFactory::prefixItems} or
   * {@see ConstraintFactory::additionalItems}.
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns If the schema is an array, a TupleConstraint. If the schema is an
   * object, an AdditionalItemsConstraint.
   *
   * @throws {@see ConstraintFactory::prefixItems}
   * @throws {@see ConstraintFactory::additionalItems}
   */
  static pConstraint itemsTupleOrVector(detail::ParserContext<A> const & context) {
    if (context.schema.type() == adapter::Type::Array) {
      return prefixItems(context);
    }

    return ptr(constraint::AdditionalItemsConstraint{context.node(), 0});
  }

  /**
   * @brief Parser for the "unevaluatedItems" constraint (11.2). This constraint
   * validates every element of an array JSON instance that is not handled by
   * any other subschema of this schema's parent.
   * In terms of annotation, we flag instance paths as "Accept", "Reject", or
   * "Noop" - "unevaluatedItems" constraints will be run after all other
   * constraints, applying to every item that is labeled "Noop" (or was never
   * visited to get even that tag).
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns An AdditionalPropertiesConstraint
   */
  static auto unevaluatedItems(detail::ParserContext<A> const & context) {
    return ptr(constraint::UnevaluatedItemsConstraint{context.node()});
  }

  /**
   * @brief Parser for the "uniqueItems" constraint (6.4.3). This constraint
   * validates array JSON instances where no member of the array is repeated.
   * In other words: `std::set{instance}.size() == instance.size()`
   *
   * @pre context.schema MUST be a boolean
   *
   * @returns An "accept-all" constraint if the schema is "false", else a
   * UniqueItemsConstraint.
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static pConstraint uniqueItems(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Boolean);
    if (not context.schema.as_boolean()) {
      return nullptr;
    }

    return ptr(constraint::UniqueItemsConstraint{});
  }

  // SECTION: Object Constraints

  /**
   * @brief Parser for the "required" constraint (6.5.3) starting in Draft04.
   * In Draft03, the required keyword is a boolean property of subschemas, and
   * so must be evaluated by {@see ConstraintFactory::propertiesDraft3}.
   * This constraint validates object JSON instances MUST contain every property
   * in the schema array provided.
   *
   * @pre context.schema MUST be an array of strings
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A RequiredConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static auto required(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Array);

    std::unordered_set<std::string> rval;
    for (auto subschema : context.schema.as_array()) {
      EXPECT(subschema.type() == adapter::Type::String);
      rval.insert(subschema.as_string());
    }

    return ptr(constraint::RequiredConstraint{rval});
  }

  /**
   * @brief Parser for the "minProperties" constraint (6.5.2). This constraint
   * validates object JSON instances have a length <= context.schema.
   *
   * @pre context.schema MUST be an integer
   * @pre context.schema >= 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MinPropertiesConstraint
   *
   * @throws If the contained value is not interpretable as an integer
   */
  static auto minProperties(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Integer ||
           context.schema.type() == adapter::Type::Number);
    return ptr(constraint::MinPropertiesConstraint{context.schema.as_integer()});
  }

  /**
   * @brief Parser for the "maxProperties" constraint (6.5.1). This constraint
   * validates object JSON instances have a length <= context.schema.
   *
   * @pre context.schema MUST be an integer
   * @pre context.schema >= 0
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A MaxPropertiesConstraint
   *
   * @throws If the contained value is not interpretable as an integer
   */
  static auto maxProperties(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Integer ||
           context.schema.type() == adapter::Type::Number);
    return ptr(constraint::MaxPropertiesConstraint{context.schema.as_integer()});
  }

  /**
   * @brief Parser for the "patternProperties" constraint (10.3.2.2). This
   * constraint validates an object JSON instance where each key in the instance
   * is checked against each of the ECMA-262 compatible regular expression keys
   * in this constraint, and validated against the subschema of that pattern if
   * matched. EVERY subschema whose pattern matches is validated against, so the
   * order does not strictly matter.
   *
   * This function does not attempt to compile the regular expression(s),
   * meaning that if the pattern is not a valid regex, it will fail at a later
   * point.
   *
   * @pre context.schema is an object
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A PatternPropertiesConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static auto patternProperties(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Object);

    std::vector<std::pair<std::string, schema::Node const *>> rval;
    for (auto [prop, subschema] : context.schema.as_object()) {
      rval.emplace_back(prop, context.child(subschema, prop).node());
    }

    return ptr(constraint::PatternPropertiesConstraint{rval});
  }

  /**
   * @brief Parser for the "properties" constraint (10.3.2.1) for JSON Schema
   * Draft04 and up. This constraint validates an object JSON instance where if
   * a key in the instance is in this constraint, then the value is validated
   * against the subschema.
   *
   * @pre context.schema is an object
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A PropertiesConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static auto properties(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Object);

    std::map<std::string, schema::Node const *> rval;
    for (auto [prop, subschema] : context.schema.as_object()) {
      rval.emplace(prop, context.child(subschema, prop).node());
    }

    return ptr(constraint::PropertiesConstraint{rval});
  }

  /**
   * @brief Parser for the "properties" constraint (5.2) for JSON Schema
   * Draft03. This constraint validates an object JSON instance where if a key
   * in the instance is in this constraint, then the value is validated against
   * the subschema.
   *
   * The Draft03 version of this method differs from the general version because
   * of the way that the "required" keyword is handled in Draft03, vs others.
   * In Draft03, "required" is a boolean field, that may be placed in each
   * subschema of the properties constraint.
   * There would be two possible ways to handle this:
   * 1) Implement a constraint like "MustBeVisitedConstraint", and make it so
   *    the ValidationVisitor will look-forward into properties constraints to
   *    check for its presence.
   * 2) During Schema parsing, scan the children of the properties constraint
   *    for the required keyword. If present, add a RequiredConstraint in our
   *    output.
   *
   * @pre context.schema is an object
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A PropertiesConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   */
  static pConstraint propertiesDraft3(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Object);

    std::unordered_set<std::string> required;
    for (auto [prop, subschema] : context.schema.as_object()) {
      EXPECT(subschema.type() == adapter::Type::Object);
      if (auto sub = subschema.as_object();
          sub.contains("required") && sub["required"].as_boolean()) {
        required.insert(prop);
      }
    }

    if (required.empty()) {
      return properties(context);
    }

    std::vector<constraint::SubConstraint> rval;
    rval.push_back(properties(context));
    rval.push_back(ptr(constraint::RequiredConstraint{std::move(required)}));
    return ptr(constraint::AllOfConstraint{std::move(rval)});
  }

  /**
   * @brief Parser for the "propertyNames" constraint (10.3.2.4). This
   * constraint validates the keys of an object JSON instance against a
   * subschema, the values of the object are ignored.
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A PropertyNamesConstraint
   */
  static auto propertyNames(detail::ParserContext<A> const & context) {
    return ptr(constraint::PropertyNamesConstraint{context.node()});
  }

  /**
   * @brief Parser for the "unevaluatedProperties" constraint (11.3). This
   * constraint validates every element of an object JSON instance that is not
   * handled by any other subschema of this schema's parent.
   * In terms of annotation, we flag instance paths as "Accept", "Reject", or
   * "Noop" - "unevaluatedProperties" constraints will be run after all other
   * constraints, applying to every property that is labeled "Noop" (or was
   * never visited to get even that tag).
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns An AdditionalPropertiesConstraint
   */
  static auto unevaluatedProperties(detail::ParserContext<A> const & context) {
    return ptr(constraint::UnevaluatedPropertiesConstraint{context.node()});
  }

  /**
   * @brief Parser for the "additionalProperties" constraint (10.3.2.3). This
   * constraint validates every element of an object JSON instance that is not
   * handled by a "properties" or "patternProperties" constraint in the same
   * parent schema.
   *
   * Constrast this with the "unevaluatedProperties" of Draft2019_09, which is
   * able to investigate various complex interactions and nested schemas.
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns An AdditionalPropertiesConstraint
   */
  static auto additionalProperties(detail::ParserContext<A> const & context) {
    std::unordered_set<std::string> properties;
    std::vector<std::string> patterns;

    auto const & parent = *context.parent;
    if (parent.contains("properties")) {
      for (auto [key, _] : parent["properties"].as_object()) {
        properties.insert(key);
      }
    }
    if (parent.contains("patternProperties")) {
      for (auto [key, _] : parent["patternProperties"].as_object()) {
        patterns.push_back(key);
      }
    }

    using C = constraint::AdditionalPropertiesConstraint; // Otherwise - the formatting is ugly
    // Prior to Draft06, boolean schemas were not allowed in a general context,
    // they were instead reserved for the "additionalItems" and
    // "additionalProperties" keywords.
    if (context.vocab->version() < schema::Version::Draft06 &&
        context.schema.type() == adapter::Type::Boolean) {
      return ptr(C{context.always(), properties, patterns});
    }

    return ptr(C{context.node(), properties, patterns});
  }

  /**
   * @brief Parser for the "dependencies" constraint (6.5.7) until JSON Schema
   * Draft2019_09
   * (https://json-schema.org/draft-07/draft-handrews-json-schema-validation-01)
   * This constraint creates an if-then relationship where "if property X exists
   * in the instance, then we must validate schema Y (object) or properties Y...
   * must also exist (array)".
   * It is not required for every key in this schema to be contained within the
   * instance being validated.
   *
   * In Draft03, we additionally permit
   * In Draft2019_09, this was split into the "dependentSchemas" and
   * "dependentRequired" keyword.
   *
   * @pre context.schema MUST be an object
   * @pre all object values in context.schema are valid JSON Schemas, or a
   * lists of strings
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A DependenciesConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   * @throws if precondition #2 is broken
   */
  static auto dependencies(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Object);

    std::map<std::string, schema::Node const *> schemas;
    std::map<std::string, std::unordered_set<std::string>> required;
    for (auto [prop, subschema] : context.schema.as_object()) {
      if (subschema.type() == adapter::Type::Array) {
        // Option 1) dependentRequired
        for (auto key : subschema.as_array()) {
          EXPECT(key.type() == adapter::Type::String);
          required[prop].insert(key.as_string());
        }
      } else if (context.vocab->version() <= schema::Version::Draft03 &&
                 subschema.type() == adapter::Type::String) {
        // Option 2) Special single-element dependentRequired in Draft03
        required[prop].insert(subschema.as_string());
      } else {
        // Option 3) dependentSchemas
        schemas.emplace(prop, context.child(subschema, prop).node());
      }
    }

    return ptr(constraint::DependenciesConstraint{schemas, required});
  }

  /**
   * @brief Parser for the "dependentSchemas" constraint (10.2.2.4) for
   * JSON Schema Draft2019_09 and up. This constraint creates an if-then
   * relationship where "if property X exists in the instance, then we must
   * validate schema Y". It is not required for every key in this schema to be
   * contained within the instance being validated.
   *
   * Before Draft2019_09, this was part of the "dependencies" keyword, along
   * with "dependentRequired".
   *
   * @pre context.schema MUST be an object
   * @pre all object values in context.schema are lists of strings
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A DependenciesConstraint
   *
   * @throws std::runtime_error if precondition #1 is broken
   * @throws if precondition #2 is broken
   */
  static auto dependentSchemas(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Object);

    std::map<std::string, schema::Node const *> rval;
    for (auto [prop, subschema] : context.schema.as_object()) {
      rval.emplace(prop, context.child(subschema, prop).node());
    }

    return ptr(constraint::DependenciesConstraint{rval});
  }

  /**
   * @brief Parser for the "dependentRequired" constraint (6.5.4) for
   * JSON Schema Draft2019_09 and up. This constraint creates an if-then
   * relationship where "if property X exists in the instance, properties Y...
   * must also exist". It is not required for every key in this schema to be
   * contained within the instance being validated.
   *
   * Before Draft2019_09, this was part of the "dependencies" keyword, along
   * with "dependentSchemas".
   *
   * @pre context.schema MUST be an object
   * @pre all object values in context.schema are valid JSON Schemas
   *
   * @param context The operating context of the schema parsing.
   *
   * @returns A DependenciesConstraint
   *
   * @throws std::runtime_error if any preconditions are broken
   */
  static auto dependentRequired(detail::ParserContext<A> const & context) {
    EXPECT(context.schema.type() == adapter::Type::Object);

    std::map<std::string, std::unordered_set<std::string>> rval;
    for (auto [prop, subschema] : context.schema.as_object()) {
      EXPECT(subschema.type() == adapter::Type::Array);
      for (auto key : subschema.as_array()) {
        EXPECT(key.type() == adapter::Type::String);
        rval[prop].insert(key.as_string());
      }
    }

    return ptr(constraint::DependenciesConstraint{{}, rval});
  }
};
}