//! Provides functions for getting instances of [`Schema`] using [`SchemaSystem`].
//!
//! ## Example:
//! In general, users will create one or more authorities and use them to build the [`SchemaSystem`].
//! Then this [`SchemaSystem`] is used to create instances of [`Schema`].
//!
//! ```
//! use ion_schema::authority::{MapDocumentAuthority, FileSystemDocumentAuthority, DocumentAuthority};
//! use ion_schema::system::SchemaSystem;
//! use std::path::Path;
//!
//! // create a vector of authorities and construct schema system
//! let authorities: Vec<Box<dyn DocumentAuthority>> = vec![Box::new(
//!             FileSystemDocumentAuthority::new(Path::new("schemas")),
//!         )];
//! let mut schema_system = SchemaSystem::new(authorities);
//!
//! // use this schema_system to load a schema as following
//! let schema = schema_system.load_schema("sample.isl");
//! ```

use crate::authority::DocumentAuthority;
use crate::isl::isl_import::{IslImport, IslImportType};
use crate::isl::isl_type::{IslType, IslTypeImpl};
use crate::isl::IslSchema;
use crate::result::{
    invalid_schema_error, unresolvable_schema_error, unresolvable_schema_error_raw, IonSchemaError,
    IonSchemaResult,
};
use crate::schema::Schema;
use crate::types::{BuiltInTypeDefinition, Nullability, TypeDefinition, TypeDefinitionImpl};
use ion_rs::value::owned::{text_token, OwnedElement, OwnedSymbolToken};
use ion_rs::value::reader::{element_reader, ElementReader};
use ion_rs::value::{Element, Sequence, Struct};
use ion_rs::IonType;
use std::collections::HashMap;
use std::io::ErrorKind;
use std::rc::Rc;

// TODO: Shift PendingTypes and TypeStore implementations to a separate module
/// Stores information about types that are in the process of being defined.
///
/// An ISL type definition can include types that are not yet fully defined.
/// For example, an ISL type definition might include:
/// * A reference to itself. This could happen in a recursive structure like a
///   linked list or binary tree.
/// * A nested anonymous type definition.
/// Because the [`SchemaSystem`] does not yet know the complete definition
/// of these types, it cannot find them in the [`TypeStore`].
/// An instance of [`PendingTypes`] is used to track information about types
/// that we do not have a complete definition for yet. When the
/// [`SchemaSystem`] finishes loading these types, the type definitions in
/// [`PendingTypes`] can be promoted the [`TypeStore`].
#[derive(Debug, Clone, Default)]
pub struct PendingTypes {
    builtin_type_ids_by_name: HashMap<String, TypeId>,
    ids_by_name: HashMap<String, TypeId>,
    parent: Option<(String, TypeId)>,
    types_by_id: Vec<Option<TypeDefinition>>, // a None in this vector represents a not-yet-resolved type
}

impl PendingTypes {
    /// Adds all the types from PendingTypes into given [`TypeStore`] including adding all the imported types into imports of [`TypeStore`].
    /// It also clears [`PendingTypes`] types for loading next set of types.
    /// This method is used after a schema named type/root type is loaded entirely into [`PendingTypes`]
    /// * `type_store` - The TypeStore which will be updated with the types within this PendingType
    /// * `load_isl_import` - If this argument is Some(isl_import), then we are not within an import process of schema.
    ///                       Based on given enum variant isl_import we will add the types to type_store.
    ///                       Otherwise we will add all the types from this PendingTypes to TypeStore.
    /// Returns true, if this update is not for an isl import type or it is for an isl import type but it is added to the type_store
    /// Otherwise, returns false if this update is for an isl import type and it is not yet added to the type_store.
    pub fn update_type_store(
        &mut self,
        type_store: &mut TypeStore,
        load_isl_import: Option<&IslImport>,
    ) -> IonSchemaResult<bool> {
        // if load_isl_import is not None, then match the enum variant and update type store with import types accordingly
        if let Some(import) = load_isl_import {
            match import {
                IslImport::Schema(_) => {
                    self.update_type_store_with_all_isl_imported_types(None, type_store)?;
                }
                IslImport::Type(isl_import) => {
                    // if import has a specified type to import then only add that type
                    if let Some(named_type_def) =
                        self.get_type_by_name_for_import(isl_import.type_name(), type_store)
                    {
                        type_store
                            .add_isl_imported_type(isl_import.alias().as_ref(), named_type_def?);
                        self.update_type_store_with_all_isl_imported_types(
                            Some(isl_import.type_name()),
                            type_store,
                        )?;
                    } else {
                        // if the named_type_def appears as None then it means we haven't reached
                        // the named type to be imported yet hence we return with false pointing
                        // we haven't yet resolved this import.
                        return Ok(false);
                    }
                }
                IslImport::TypeAlias(isl_import) => {
                    // if import has a specified type with an alias then renamed that type to the given alias and add it
                    if type_store
                        .imported_type_ids_by_name
                        .contains_key(isl_import.alias().as_ref().unwrap())
                    {
                        // if the type_store already has the import in it then return true (i.e. TypeAlias has already been imported)
                        return Ok(true);
                    } else if let Some(named_type_def) =
                        self.get_type_by_name_for_import(isl_import.type_name(), type_store)
                    {
                        let aliased_type_def = named_type_def?
                            .with_name(isl_import.alias().as_ref().unwrap().to_owned());
                        type_store
                            .add_isl_imported_type(isl_import.alias().as_ref(), aliased_type_def);
                        self.update_type_store_with_all_isl_imported_types(
                            Some(isl_import.type_name()),
                            type_store,
                        )?;
                    } else {
                        // if the named_type_def appears as None then it means we haven't reached
                        // the named type to be imported yet hence we return with false pointing
                        // we haven't yet resolved this import.
                        return Ok(false);
                    }
                }
            }
        } else {
            // if load_isl_import is None i.e. it is the root schema, then update type_store with all the types inside this PendingTypes
            self.update_type_store_with_all_types(type_store)?;
        }
        self.types_by_id.clear();
        self.ids_by_name.clear();
        Ok(true)
    }

    // helper method get named type for given import_type_name from this PendingTypes
    // this return type will be used by update_type_store method to then update type_store with this named type as import
    fn get_type_by_name_for_import(
        &self,
        import_type_name: &str,
        type_store: &mut TypeStore,
    ) -> Option<IonSchemaResult<TypeDefinitionImpl>> {
        return match self.ids_by_name.get(import_type_name) {
            Some(id) => self.types_by_id[*id]
                .to_owned()
                .and_then(|type_def| match type_def {
                    TypeDefinition::Named(named_type_def) => Some(Ok(named_type_def)),
                    TypeDefinition::Anonymous(_) => {
                        unreachable!(
                            "The TypeDefinition for the imported type '{}' was Anonymous.",
                            import_type_name
                        )
                    }
                    TypeDefinition::BuiltIn(_) => {
                        unreachable!(
                            "The TypeDefinition for the imported type '{}' was a builtin type.",
                            import_type_name
                        )
                    }
                }),
            None => match type_store.get_type_id_by_name(import_type_name) {
                Some(id) => match type_store.types_by_id[*id].to_owned() {
                    TypeDefinition::Named(named_type_def) => Some(Ok(named_type_def)),
                    TypeDefinition::Anonymous(_) => {
                        unreachable!(
                            "The TypeDefinition for the imported type '{}' was Anonymous.",
                            import_type_name
                        )
                    }
                    TypeDefinition::BuiltIn(_) => {
                        unreachable!(
                            "The TypeDefinition for the imported type '{}' was a builtin type.",
                            import_type_name
                        )
                    }
                },
                None => None,
            },
        };
    }

    // helper method to update type store with all the types from this PendingTypes
    fn update_type_store_with_all_types(&self, type_store: &mut TypeStore) -> IonSchemaResult<()> {
        for optional_type in &self.types_by_id {
            // return an error if any of the type in types_by_id vector is None/Unresolved
            let type_def = optional_type.to_owned().ok_or_else(|| {
                unresolvable_schema_error_raw("Unable to load schema due to unresolvable type")
            })?;

            match type_def {
                TypeDefinition::Named(named_type_def) => type_store.add_named_type(named_type_def),
                TypeDefinition::Anonymous(anonymous_type_def) => {
                    type_store.add_anonymous_type(anonymous_type_def)
                }
                TypeDefinition::BuiltIn(builtin_type) => type_store.add_builtin_type(&builtin_type),
            };
        }
        Ok(())
    }

    // helper method to update type store with all the types from this PendingTypes
    // import_type_name: this argument represents whether the import type is SchemaImport or a TypeImport (includes both TypeImport and TypeAliasImport)
    //                   None - represents its a schema import which imports all types into imported_type_ids_by_name section of type_store
    //                   Some(_) - represents a type import which import all types into types_by_id of type_store,
    //                             except specified import type as it will be already loaded by parent method that uses this helper method
    fn update_type_store_with_all_isl_imported_types(
        &self,
        isl_imported_type_name: Option<&str>,
        type_store: &mut TypeStore,
    ) -> IonSchemaResult<()> {
        for optional_type in &self.types_by_id {
            // return an error if any of the type in types_by_id vector is None/Unresolved
            let type_def = optional_type.to_owned().ok_or_else(|| {
                unresolvable_schema_error_raw("Unable to load schema due to unresolvable type")
            })?;

            match type_def.to_owned() {
                TypeDefinition::Named(named_type_def) => {
                    match isl_imported_type_name {
                        None => {
                            // imports all types into imported_type_ids_by_name section of type_store
                            type_store.add_isl_imported_type(None, named_type_def);
                        }
                        Some(import_type_name) => {
                            // skip the specified import type as it will be already loaded by parent method that uses this helper method
                            if named_type_def.name().as_ref().unwrap().eq(import_type_name) {
                                continue;
                            }
                            // import all types into types_by_id of type_store which will help resolving the given import type
                            type_store
                                .types_by_id
                                .push(TypeDefinition::Named(named_type_def));
                        }
                    }
                }
                TypeDefinition::Anonymous(anonymous_type_def) => {
                    type_store.add_anonymous_type(anonymous_type_def);
                }
                TypeDefinition::BuiltIn(builtin_type) => {
                    type_store.add_builtin_type(&builtin_type);
                }
            };
        }
        Ok(())
    }

    /// Returns total number of types stored in the [`TypeStore`]
    pub(crate) fn get_total_types(&self, type_store: &mut TypeStore) -> usize {
        self.types_by_id.len() + type_store.types_by_id.len()
    }

    /// Provides the [`TypeId`] associated with given name if it exists in the [`TypeStore`] or [`PendingTypes`]  
    /// Otherwise returns None
    pub(crate) fn get_type_id_by_name(
        &self,
        name: &str,
        type_store: &mut TypeStore,
    ) -> Option<TypeId> {
        match self.ids_by_name.get(name) {
            Some(id) => Some(*id + type_store.types_by_id.len()),
            None => type_store.get_type_id_by_name(name).copied(),
        }
    }

    /// Adds the [`NamedTypeDefinition`] and the associated name in the [`PendingTypes`] and returns the [`TypeId`] for it
    /// If the given name already exists in the [`TypeStore`] or [`PendingTypes`] it returns the associated [`TypeId`]
    pub(crate) fn add_named_type(
        &mut self,
        name: &str,
        type_def: TypeDefinitionImpl,
        type_store: &mut TypeStore,
    ) -> TypeId {
        if let Some(exists) = self.ids_by_name.get(name) {
            return exists.to_owned();
        }
        if let Some(exists) = type_store.get_type_id_by_name(name) {
            return exists.to_owned();
        }
        let type_id = self.types_by_id.len();
        self.ids_by_name.insert(name.to_owned(), type_id);
        self.types_by_id.push(Some(TypeDefinition::Named(type_def)));
        type_id + type_store.types_by_id.len()
    }

    /// Adds the [`BuiltInTypeDefinition`] in the [`PendingTypes`] and returns the [`TypeId`] for it
    /// If the given name already exists in the [`TypeStore`] or [`PendingTypes`] it returns the associated [`TypeId`]
    pub(crate) fn add_builtin_type(
        &mut self,
        builtin_type_definition: &BuiltInTypeDefinition,
        type_store: &mut TypeStore,
    ) -> TypeId {
        let builtin_type_name = match builtin_type_definition {
            BuiltInTypeDefinition::Atomic(ion_type, is_nullable) => match is_nullable {
                Nullability::Nullable => format!("${}", ion_type),
                Nullability::NotNullable => format!("{}", ion_type),
            },
            BuiltInTypeDefinition::Derived(other_type) => other_type.name().to_owned().unwrap(),
        };

        if let Some(exists) = self.builtin_type_ids_by_name.get(&builtin_type_name) {
            return exists.to_owned();
        }
        if let Some(exists) = type_store.get_builtin_type_id(&builtin_type_name) {
            return exists.to_owned();
        }
        let type_id = self.types_by_id.len();
        self.builtin_type_ids_by_name
            .insert(builtin_type_name, type_id);
        self.types_by_id.push(Some(TypeDefinition::BuiltIn(
            builtin_type_definition.to_owned(),
        )));
        type_id + type_store.types_by_id.len()
    }

    /// Updates the unresolved named type that was added as None while loading types in a schema
    /// with a resolved [TypeDefinition]
    pub(crate) fn update_named_type(
        &mut self,
        type_id: TypeId,
        name: &str,
        type_def: TypeDefinitionImpl,
        type_store: &mut TypeStore,
    ) -> TypeId {
        let type_id = type_id - type_store.types_by_id.len();
        if let Some(exists) = self.ids_by_name.get(name) {
            return exists.to_owned();
        }
        if let Some(exists) = type_store.get_type_id_by_name(name) {
            return exists.to_owned();
        }
        self.ids_by_name.insert(name.to_owned(), type_id);
        self.types_by_id[type_id] = Some(TypeDefinition::Named(type_def));
        type_id + type_store.types_by_id.len()
    }

    /// Updates the unresolved anonymous type that was added as None while loading types in a schema
    /// with a resolved [`TypeDefinition`]
    pub(crate) fn update_anonymous_type(
        &mut self,
        type_id: TypeId,
        type_def: TypeDefinitionImpl,
        type_store: &mut TypeStore,
    ) -> TypeId {
        self.types_by_id[type_id - type_store.types_by_id.len()] =
            Some(TypeDefinition::Anonymous(type_def));
        type_id
    }

    /// Adds parent information storing the name and possible TypeId of the parent
    pub(crate) fn add_parent(&mut self, name: String, type_store: &mut TypeStore) {
        // Parent information is used when resolving a self referencing type
        // while we resolve a type using the PendingTypes (a temporary type store used while we resolve a type definition)
        // the type id for any type definition should be the PendingType's types_by_id length in  + TypeStore's types_by_id length
        // This gives a correct type id when all the types within PendingTypes are shifted to TypeStore
        self.parent = Some((name, self.types_by_id.len() + type_store.types_by_id.len()))
    }

    /// Provides parent information: (parent name, type id)
    pub(crate) fn get_parent(&self) -> &Option<(String, TypeId)> {
        &self.parent
    }

    /// Clears parent information once that tree of types is traversed
    pub(crate) fn clear_parent(&mut self) {
        self.parent = None
    }

    /// Adds the unresolved type as None before it gets resolved and gets the associated [`TypeId`]
    pub(crate) fn add_type(&mut self, type_store: &mut TypeStore) -> TypeId {
        let type_id = self.types_by_id.len();
        self.types_by_id.push(None);
        type_id + type_store.types_by_id.len()
    }
}

/// Represents an array of BuiltIn derived ISL types
/// for more information: https://amzn.github.io/ion-schema/docs/spec.html#type-system
static DERIVED_ISL_TYPES: [&str; 10] = [
    "type::{ name: lob, one_of: [ blob, clob ] }",
    "type::{ name: number, one_of: [ decimal, float, int ] }",
    "type::{ name: text, one_of: [ string, symbol ] }",
    "type::{ name: $lob, one_of: [ $blob, $clob ] }",
    "type::{ name: $number, one_of: [ $decimal, $float, $int ] }",
    "type::{ name: $text, one_of: [ $string, $symbol ] }",
    "type::{ name: $any, one_of: [ $blob, $bool, $clob, $decimal,
                                    $float, $int, $string, $symbol, $timestamp,
                                    $list, $sexp, $struct, $null ] }",
    // this is just a place holder for document type,
    // IonSchemaElement::Document(_) type is used to verify the correctness on the validation side
    "type::{ name: document }",
    "type::{ name: nothing, not: $any }",
    "type::{ name: any, one_of: [ blob, bool, clob, decimal,
                                    float, int, string, symbol, timestamp,
                                    list, sexp, struct, document ] }",
];

pub type TypeId = usize;

/// Defines a cache that can be used to store resolved [`TypeDefinition`]s of a [`Schema`]
#[derive(Debug, Clone)]
pub struct TypeStore {
    builtin_type_ids_by_name: HashMap<String, TypeId>, // stores all the builtin types used within this schema
    imported_type_ids_by_name: HashMap<String, TypeId>, // stores all the imported types of a schema
    ids_by_name: HashMap<String, TypeId>, // stores named types defined within the schema
    types_by_id: Vec<TypeDefinition>,
}

impl Default for TypeStore {
    fn default() -> Self {
        let mut type_store = Self {
            builtin_type_ids_by_name: HashMap::new(),
            imported_type_ids_by_name: HashMap::new(),
            ids_by_name: HashMap::new(),
            types_by_id: Vec::new(),
        };
        type_store
            .preload()
            .expect("The type store didn't preload with built-in types correctly");
        type_store
    }
}

impl TypeStore {
    /// Preloads all [builtin isl types] into the TypeStore
    /// [builtin isl types]: https://amzn.github.io/ion-schema/docs/spec.html#type-system
    /// TODO: add document builtin type
    pub(crate) fn preload(&mut self) -> IonSchemaResult<()> {
        // add all ion types to the type store
        // TODO: this array can be turned into an iterator implementation in ion-rust for IonType
        use IonType::*;
        let built_in_atomic_types: [IonType; 12] = [
            Integer,
            Float,
            Decimal,
            Timestamp,
            String,
            Symbol,
            Boolean,
            Blob,
            Clob,
            SExpression,
            List,
            Struct,
        ];
        // add all the atomic ion types that doesn't allow nulls [type_ids: 0 - 11]
        for atomic_type in built_in_atomic_types {
            self.add_builtin_type(&BuiltInTypeDefinition::Atomic(
                atomic_type.to_owned(),
                Nullability::NotNullable,
            ));
        }

        // add all the atomic ion types that allows nulls [type_ids: 12 - 23]
        for atomic_type in built_in_atomic_types {
            self.add_builtin_type(&BuiltInTypeDefinition::Atomic(
                atomic_type.to_owned(),
                Nullability::Nullable,
            ));
        }

        // add $null to the built-in types [type_id: 24]
        self.add_builtin_type(&BuiltInTypeDefinition::Atomic(Null, Nullability::Nullable));

        // get the derived built in types map and related text value for given type_name [type_ids: 25 - 33]
        let pending_types = &mut PendingTypes::default();
        for text in DERIVED_ISL_TYPES {
            let isl_type = IslTypeImpl::from_owned_element(
                &element_reader()
                    .read_one(text.as_bytes())
                    .expect("parsing failed unexpectedly"),
                &mut vec![],
            )
            .unwrap();
            let type_def =
                BuiltInTypeDefinition::parse_from_isl_type(&isl_type, self, pending_types)?;
            self.add_builtin_type(&type_def);
        }
        Ok(())
    }

    /// Returns [`TypeId`]s stored in the [`TypeStore`] to be used by [`SchemaTypeIterator`]
    pub(crate) fn get_types(&self) -> Vec<TypeId> {
        self.ids_by_name.values().cloned().collect()
    }

    /// Returns import [`TypeId`]s stored in the [`TypeStore`] to be used by [`SchemaTypeIterator`]
    pub(crate) fn get_imports(&self) -> Vec<TypeId> {
        self.imported_type_ids_by_name.values().cloned().collect()
    }

    /// Provides the [`Type`] associated with given name if it exists in the [`TypeStore`]  
    /// Otherwise returns None
    pub(crate) fn get_type_by_name(&self, name: &str) -> Option<&TypeDefinition> {
        self.ids_by_name
            .get(name)
            .and_then(|id| self.types_by_id.get(*id))
            .or_else(|| {
                self.imported_type_ids_by_name
                    .get(name)
                    .and_then(|id| self.types_by_id.get(*id))
            })
    }

    /// Provides the [`TypeId`] associated with given name if it exists in the [`TypeStore`]  
    /// Otherwise returns None
    pub(crate) fn get_type_id_by_name(&self, name: &str) -> Option<&TypeId> {
        self.ids_by_name
            .get(name)
            .or_else(|| self.imported_type_ids_by_name.get(name))
    }

    /// Provides the [`TypeId`] associated with given type name if it exists in the [`TypeStore`]  
    /// Otherwise returns None
    pub(crate) fn get_builtin_type_id(&self, type_name: &str) -> Option<TypeId> {
        let type_name = match type_name {
            "int" => "integer",
            "bool" => "boolean",
            "$int" => "$integer",
            "$bool" => "$boolean",
            _ => type_name,
        };
        self.builtin_type_ids_by_name
            .get(type_name)
            .map(|t| t.to_owned())
    }

    /// Provides the [`Type`] associated with given [`TypeId`] if it exists in the [`TypeStore`]  
    /// Otherwise returns None
    pub(crate) fn get_type_by_id(&self, id: TypeId) -> Option<&TypeDefinition> {
        self.types_by_id.get(id)
    }

    /// Adds the [`NamedTypeDefinition`] and the associated name in the [`TypeStore`] and returns the [`TypeId`] for it
    /// If the name already exists in the [`TypeStore`] it returns the associated [`TypeId`]
    pub(crate) fn add_named_type(&mut self, type_def: TypeDefinitionImpl) -> TypeId {
        let name = type_def.name().as_ref().unwrap();
        if let Some(exists) = self.ids_by_name.get(name) {
            return exists.to_owned();
        }
        let type_id = self.types_by_id.len();
        self.ids_by_name.insert(name.to_owned(), type_id);
        self.types_by_id.push(TypeDefinition::Named(type_def));
        type_id
    }

    /// Adds the [BuiltInTypeDefinition] in the [TypeStore] and returns the [TypeId] for it
    /// If the name already exists in the [TypeStore] it returns the associated [TypeId]
    pub(crate) fn add_builtin_type(
        &mut self,
        builtin_type_definition: &BuiltInTypeDefinition,
    ) -> TypeId {
        let builtin_type_name = match builtin_type_definition {
            BuiltInTypeDefinition::Atomic(ion_type, is_nullable) => match is_nullable {
                Nullability::Nullable => format!("${}", ion_type),
                Nullability::NotNullable => format!("{}", ion_type),
            },
            BuiltInTypeDefinition::Derived(other_type) => other_type.name().to_owned().unwrap(),
        };

        if let Some(exists) = self.builtin_type_ids_by_name.get(&builtin_type_name) {
            return exists.to_owned();
        }
        let type_id = self.types_by_id.len();
        self.builtin_type_ids_by_name
            .insert(builtin_type_name, type_id);
        self.types_by_id
            .push(TypeDefinition::BuiltIn(builtin_type_definition.to_owned()));
        type_id
    }

    /// Adds the [`NamedTypeDefinition`] and the associated name as the imports of [`TypeStore`]
    ///  and returns the [`TypeId`] for it. If the name already exists in the [`TypeStore`] it returns the associated [`TypeId`]
    pub(crate) fn add_isl_imported_type(
        &mut self,
        alias: Option<&String>,
        type_def: TypeDefinitionImpl,
    ) -> TypeId {
        let name = match alias {
            None => type_def.name().as_ref().unwrap(),
            Some(name) => name,
        };

        if let Some(exists) = self.imported_type_ids_by_name.get(name) {
            return exists.to_owned();
        }
        let type_id = self.types_by_id.len();
        self.imported_type_ids_by_name
            .insert(name.to_owned(), type_id);
        self.types_by_id.push(TypeDefinition::Named(type_def));
        type_id
    }

    /// Adds the [`Type`] in the [`TypeStore`] and returns the [`TypeId`] for it
    pub(crate) fn add_anonymous_type(&mut self, type_def: TypeDefinitionImpl) -> TypeId {
        let type_id = self.types_by_id.len();
        self.types_by_id.push(TypeDefinition::Anonymous(type_def));
        type_id
    }
}

/// Provides functions to load [`Schema`] with type definitions using authorities for [`SchemaSystem`]
pub struct Resolver {
    authorities: Vec<Box<dyn DocumentAuthority>>,
    resolved_schema_cache: HashMap<String, Rc<Schema>>,
}

impl Resolver {
    pub fn new(authorities: Vec<Box<dyn DocumentAuthority>>) -> Self {
        Self {
            authorities,
            resolved_schema_cache: HashMap::new(),
        }
    }

    pub fn schema_from_isl_types<A: AsRef<str>, B: Into<Vec<IslType>>>(
        &self,
        id: A,
        isl_types: B,
    ) -> IonSchemaResult<Schema> {
        // create type_store and pending types which will be used to create type definition
        let type_store = &mut TypeStore::default();
        let pending_types = &mut PendingTypes::default();
        for isl_type in isl_types.into() {
            // convert [IslType] into [TypeDefinition]
            match isl_type {
                IslType::Named(named_isl_type) => {
                    TypeDefinitionImpl::parse_from_isl_type_and_update_pending_types(
                        &named_isl_type,
                        type_store,
                        pending_types,
                    )
                    .unwrap()
                }
                IslType::Anonymous(anonymous_isl_type) => {
                    TypeDefinitionImpl::parse_from_isl_type_and_update_pending_types(
                        &anonymous_isl_type,
                        type_store,
                        pending_types,
                    )
                    .unwrap()
                }
            };
        }
        // add all types from pending_types to type_store
        pending_types.update_type_store(type_store, None)?;
        Ok(Schema::new(id, Rc::new(type_store.to_owned())))
    }

    /// Converts given owned elements into ISL representation
    pub fn isl_schema_from_elements<I: Iterator<Item = OwnedElement>>(
        &mut self,
        elements: I,
        id: &str,
    ) -> IonSchemaResult<IslSchema> {
        // properties that will be stored in the ISL representation
        let mut isl_imports: Vec<IslImport> = vec![];
        let mut isl_types: Vec<IslTypeImpl> = vec![];
        let mut isl_inline_imports: Vec<IslImportType> = vec![];

        let mut found_header = false;
        let mut found_footer = false;

        for value in elements {
            let annotations: Vec<&OwnedSymbolToken> = value.annotations().collect();
            // load header for schema
            if annotations.contains(&&text_token("schema_header")) {
                found_header = true;
                if let Some(imports) = try_to!(value.as_struct())
                    .get("imports")
                    .and_then(|it| it.as_sequence())
                {
                    for import in imports.iter() {
                        let isl_import = IslImport::from_ion_element(import)?;
                        isl_imports.push(isl_import);
                    }
                }
            }
            // load types for schema
            else if annotations.contains(&&text_token("type")) {
                // convert OwnedElement to IslType
                let isl_type: IslTypeImpl =
                    IslTypeImpl::from_owned_element(&value, &mut isl_inline_imports)?;
                isl_types.push(isl_type);
            }
            // load footer for schema
            else if annotations.contains(&&text_token("schema_footer")) {
                found_footer = true;
            } else {
                continue;
            }
        }

        if found_footer ^ found_header {
            return invalid_schema_error("For any schema while a header and footer are both optional, a footer is required if a header is present (and vice-versa).");
        }

        Ok(IslSchema::new(isl_imports, isl_types, isl_inline_imports))
    }

    /// Converts given ISL representation into a [`Schema`]
    pub fn schema_from_isl_schema(
        &mut self,
        isl: IslSchema,
        id: &str,
        type_store: &mut TypeStore,
        load_isl_import: Option<&IslImport>,
    ) -> IonSchemaResult<Rc<Schema>> {
        // This is used while resolving an import, it is initialized as `false` to indicate that
        // the type to be imported is not yet added to the type_store.
        // This will be changed to `true` as soon as the type to be imported is resolved and is added to the type_store
        let mut added_imported_type_to_type_store = false;

        // Resolve all inline import types if there are any
        // this will help resolve all inline imports before they are used as a reference to another type
        for isl_inline_imported_type in isl.inline_imported_types() {
            let import_id = isl_inline_imported_type.id();
            let inline_imported_type = self.load_schema(
                import_id,
                type_store,
                Some(&IslImport::Type(isl_inline_imported_type.to_owned())),
            )?;
        }

        // Resolve all ISL imports
        for isl_import in isl.imports() {
            let import_id = isl_import.id();
            let imported_schema = self.load_schema(import_id, type_store, Some(isl_import))?;
        }

        // Resolve all ISL types and constraints
        for isl_type in isl.types() {
            let pending_types = &mut PendingTypes::default();

            // convert IslType to TypeDefinition
            let type_id: TypeId = TypeDefinitionImpl::parse_from_isl_type_and_update_pending_types(
                isl_type,
                type_store,
                pending_types,
            )?;

            // add all types from context to type_store
            added_imported_type_to_type_store =
                pending_types.update_type_store(type_store, load_isl_import)?;
        }

        // if currently loading an ISL import (i.e. load_isl_import != None)
        // then check if the type to be imported is added to the type_store or not
        if load_isl_import.is_some() && !added_imported_type_to_type_store {
            unreachable!(
                "Unable to load import: {} as the type/types were not added to the type_store correctly",
                id
            );
        }

        Ok(Rc::new(Schema::new(id, Rc::new(type_store.to_owned()))))
    }

    /// Loads a [`Schema`] with resolved [`Type`]s using authorities and type_store
    // If we are loading the root schema then this will be set to `None` ( i.e. in the beginning when
    // this method is called from the load_schema method of schema_system it is set to `None`)
    // Otherwise if we are loading an import of the schema then this will be set to `Some(isl_import)`
    // to be loaded (i.e. Inside schema_from_elements while loading imports this will be set to
    // `Some(isl_import)`)
    fn load_schema<A: AsRef<str>>(
        &mut self,
        id: A,
        type_store: &mut TypeStore,
        load_isl_import: Option<&IslImport>,
    ) -> IonSchemaResult<Rc<Schema>> {
        let id: &str = id.as_ref();
        if let Some(schema) = self.resolved_schema_cache.get(id) {
            return Ok(Rc::clone(schema));
        }

        for authority in &self.authorities {
            return match authority.elements(id) {
                Err(error) => match error {
                    IonSchemaError::IoError { source } => match source.kind() {
                        ErrorKind::NotFound => continue,
                        _ => Err(IonSchemaError::IoError { source }),
                    },
                    _ => Err(error),
                },
                Ok(schema_content) => {
                    let isl = self.isl_schema_from_elements(schema_content.into_iter(), id)?;
                    self.schema_from_isl_schema(isl, id, type_store, load_isl_import)
                }
            };
        }
        unresolvable_schema_error("Unable to load schema: ".to_owned() + id)
    }
}

/// Provides functions for instantiating instances of [`Schema`].
pub struct SchemaSystem {
    resolver: Resolver,
}

// TODO: make methods public based on the requirements
impl SchemaSystem {
    pub fn new(authorities: Vec<Box<dyn DocumentAuthority>>) -> Self {
        Self {
            resolver: Resolver::new(authorities),
        }
    }

    /// Requests each of the provided [`DocumentAuthority`]s, in order, to resolve the requested schema id
    /// until one successfully resolves it.
    /// If an authority throws an exception, resolution silently proceeds to the next authority.
    pub fn load_schema<A: AsRef<str>>(&mut self, id: A) -> IonSchemaResult<Rc<Schema>> {
        self.resolver
            .load_schema(id, &mut TypeStore::default(), None)
    }

    /// Returns authorities associated with this [`SchemaSystem`]
    fn authorities(&mut self) -> &[Box<dyn DocumentAuthority>] {
        &self.resolver.authorities
    }

    /// Adds the provided authority to the list of [`DocumentAuthority`]s.
    fn add_authority(&mut self, authority: Box<dyn DocumentAuthority>) {
        self.resolver.authorities.push(authority);
    }

    /// Replaces the list of [`DocumentAuthority`]s with a list containing only the specified authority.
    fn with_authority(&mut self, authority: Box<dyn DocumentAuthority>) {
        let authorities: Vec<Box<dyn DocumentAuthority>> = vec![authority];
        self.resolver.authorities = authorities;
    }

    // TODO: Use IntoIterator here instead of a Vec
    /// Replaces the list of [`DocumentAuthority`]s with the specified list of [`DocumentAuthority`]s.
    fn with_authorities(&mut self, authorities: Vec<Box<dyn DocumentAuthority>>) {
        self.resolver.authorities = authorities;
    }

    /// Creates a schema from given [`IslType`]s
    /// Note: This method assumes that there are no imported type definitions used for these [`IslType`]s
    pub fn schema_from_isl_types<A: AsRef<str>, B: Into<Vec<IslType>>>(
        &self,
        id: A,
        isl_types: B,
    ) -> IonSchemaResult<Schema> {
        self.resolver.schema_from_isl_types(id, isl_types)
    }

    /// Creates a type from given OwnedElement using [`TypeStore`]
    pub fn schema_type_from_element(
        &mut self,
        type_content: &OwnedElement,
        type_store: &mut TypeStore,
    ) -> IonSchemaResult<TypeId> {
        // convert to isl_type
        let mut isl_inline_imported_types = vec![];
        let isl_type =
            IslTypeImpl::from_owned_element(type_content, &mut isl_inline_imported_types)?;

        // Resolve all inline import types if there are any
        // this will help resolve all inline imports before they are used as a reference to another type
        for isl_inline_imported_type in isl_inline_imported_types {
            let import_id = isl_inline_imported_type.id();
            let inline_imported_type = self.resolver.load_schema(
                import_id,
                type_store,
                Some(&IslImport::Type(isl_inline_imported_type.to_owned())),
            )?;
        }

        // convert to type_def
        let pending_types = &mut PendingTypes::default();

        // convert IslType to TypeDefinition
        let type_id: TypeId = TypeDefinitionImpl::parse_from_isl_type_and_update_pending_types(
            &isl_type,
            type_store,
            pending_types,
        )?;

        // add all types from context to type_store
        pending_types.update_type_store(type_store, None)?;

        Ok(type_id)
    }
}

#[cfg(test)]
mod schema_system_tests {
    use super::*;
    use crate::authority::{FileSystemDocumentAuthority, MapDocumentAuthority};
    use crate::system::IonSchemaError::InvalidSchemaError;
    use std::path::Path;

    #[test]
    fn schema_system_add_authorities_test() {
        let mut schema_system = SchemaSystem::new(vec![Box::new(
            FileSystemDocumentAuthority::new(Path::new("src")),
        )]);
        schema_system.add_authority(Box::new(FileSystemDocumentAuthority::new(Path::new(
            "test",
        ))));
        let schema_system_authorities = schema_system.authorities();
        assert_eq!(2, schema_system_authorities.len());
    }

    #[test]
    fn schema_system_with_authority_test() {
        let mut schema_system = SchemaSystem::new(vec![Box::new(
            FileSystemDocumentAuthority::new(Path::new("src")),
        )]);
        schema_system.with_authority(Box::new(FileSystemDocumentAuthority::new(Path::new(
            "test",
        ))));
        let schema_system_authorities = schema_system.authorities();
        assert_eq!(1, schema_system_authorities.len());
    }

    #[test]
    fn schema_system_with_authorities_test() {
        let mut schema_system = SchemaSystem::new(vec![Box::new(
            FileSystemDocumentAuthority::new(Path::new("src")),
        )]);
        schema_system.with_authorities(vec![
            Box::new(FileSystemDocumentAuthority::new(Path::new("test"))),
            Box::new(FileSystemDocumentAuthority::new(Path::new("ion"))),
        ]);
        let schema_system_authorities = schema_system.authorities();
        assert_eq!(2, schema_system_authorities.len());
    }

    #[test]
    fn schema_system_map_authority_with_type_alias_import_test() {
        // map with (id, ion content)
        let map_authority = [
            (
                "sample_number.isl",
                r#"
                    schema_header::{
                      imports: [{ id: "sample_decimal.isl", type: my_decimal, as: other_decimal }],
                    }
                    
                    type::{
                      name: my_int,
                      type: int,
                    }
                    
                    type::{
                      name: my_number,
                      all_of: [
                        my_int,
                        other_decimal,
                      ],
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
            (
                "sample_decimal.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_decimal,
                      type: decimal,
                    }
                    
                    type::{
                      name: my_string,
                      type: string,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
        ];
        let mut schema_system =
            SchemaSystem::new(vec![Box::new(MapDocumentAuthority::new(map_authority))]);
        // verify if the schema loads without any errors
        let schema = schema_system.load_schema("sample_number.isl");
        assert!(schema.is_ok());
    }

    #[test]
    fn schema_system_map_authority_with_type_import_test() {
        // map with (id, ion content)
        let map_authority = [
            (
                "sample_number.isl",
                r#"
                    schema_header::{
                      imports: [{ id: "sample_decimal.isl", type: my_decimal }],
                    }
                    
                    type::{
                      name: my_int,
                      type: int,
                    }
                    
                    type::{
                      name: my_number,
                      all_of: [
                        my_int,
                        my_decimal,
                      ],
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
            (
                "sample_decimal.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_decimal,
                      type: decimal,
                    }
                    
                    type::{
                      name: my_string,
                      type: string,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
        ];
        let mut schema_system =
            SchemaSystem::new(vec![Box::new(MapDocumentAuthority::new(map_authority))]);
        // verify if the schema loads without any errors
        let schema = schema_system.load_schema("sample_number.isl");
        assert!(schema.is_ok());
    }

    #[test]
    fn schema_system_map_authority_with_schema_import_test() {
        // map with (id, ion content)
        let map_authority = [
            (
                "sample_import_string.isl",
                r#"
                    schema_header::{
                      imports: [{ id: "sample_string.isl" }],
                    }
                    
                    type::{
                      name: import_string,
                      type: my_string,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
            (
                "sample_string.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_string,
                      type: string,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
        ];
        let mut schema_system =
            SchemaSystem::new(vec![Box::new(MapDocumentAuthority::new(map_authority))]);
        // verify if the schema loads without any errors
        let schema = schema_system.load_schema("sample_import_string.isl");
        assert!(schema.is_ok());
    }

    #[test]
    fn schema_system_map_authority_with_inline_import_type_test() {
        // map with (id, ion content)
        let map_authority = [
            (
                "sample_number.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_int,
                      type: int,
                    }
                    
                    type::{
                      name: my_number,
                      all_of: [
                        my_int,
                        { id: "sample_decimal.isl", type: my_decimal },
                      ],
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
            (
                "sample_decimal.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_decimal,
                      type: decimal,
                    }
                    
                    type::{
                      name: my_string,
                      type: string,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
        ];
        let mut schema_system =
            SchemaSystem::new(vec![Box::new(MapDocumentAuthority::new(map_authority))]);
        // verify if the schema loads without any errors
        let schema = schema_system.load_schema("sample_number.isl");
        assert!(schema.is_ok());
    }

    #[test]
    fn schema_system_map_authority_with_incorrect_inline_import_type_test() {
        // map with (id, ion content)
        let map_authority = [
            (
                "sample_number.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_int,
                      type: int,
                    }
                    
                    type::{
                      name: my_number,
                      all_of: [
                        my_int,
                        { id: "sample_decimal.isl", type: my_decimal, as: other_decimal},
                      ],
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
            (
                "sample_decimal.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_decimal,
                      type: decimal,
                    }
                    
                    type::{
                      name: my_string,
                      type: string,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
        ];
        let mut schema_system =
            SchemaSystem::new(vec![Box::new(MapDocumentAuthority::new(map_authority))]);
        // verify if the schema loads with specific errors
        let schema = schema_system.load_schema("sample_number.isl");
        assert!(schema.is_err());
        assert!(matches!(schema.unwrap_err(), InvalidSchemaError { .. }));
    }

    #[test]
    fn schema_system_map_authority_with_isl_builtin_derived_types() {
        // map with (id, ion content)
        let map_authority = [
            (
                "sample.isl",
                r#"
                    schema_header::{
                      imports: [ { id: "sample_builtin_types.isl" } ],
                    }
                    
                    type::{
                        name: my_number,
                        type: number,
                    }
                    
                    type::{
                      name: my_type,
                      one_of: [
                        my_number,
                        my_text,
                        my_lob
                      ],
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
            (
                "sample_builtin_types.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_text,
                      type: text,
                    }
                    
                    type::{
                        name: my_lob,
                        type: lob,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
        ];
        let mut schema_system =
            SchemaSystem::new(vec![Box::new(MapDocumentAuthority::new(map_authority))]);
        // verify if the schema loads without any errors
        let schema = schema_system.load_schema("sample.isl");
        assert!(schema.is_ok());
    }

    #[test]
    fn schema_system_map_authority_with_isl_builtin_derived_nullable_types() {
        // map with (id, ion content)
        let map_authority = [
            (
                "sample.isl",
                r#"
                    schema_header::{
                      imports: [ { id: "sample_builtin_nullable_types.isl" } ],
                    }
                    
                    type::{
                        name: my_number,
                        type: $number,
                    }
                    
                    type::{
                        name: my_any,
                        type: $any,
                    }
                    
                    type::{
                      name: my_type,
                      one_of: [
                        my_number,
                        my_text,
                        my_lob
                      ],
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
            (
                "sample_builtin_nullable_types.isl",
                r#"
                    schema_header::{
                      imports: [],
                    }
                    
                    type::{
                      name: my_text,
                      type: $text,
                    }
                    
                    type::{
                        name: my_lob,
                        type: $lob,
                    }
                    
                    schema_footer::{
                    }
                "#,
            ),
        ];
        let mut schema_system =
            SchemaSystem::new(vec![Box::new(MapDocumentAuthority::new(map_authority))]);
        // verify if the schema loads without any errors
        let schema = schema_system.load_schema("sample.isl");
        assert!(schema.is_ok());
    }
}