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    // TODO: Uncomment
    // #![warn(missing_docs)]

    #![cfg_attr(not(feature = "std"), no_std)]

    #[cfg(not(feature = "std"))]
    #[macro_use]
    extern crate alloc;
    #[cfg(feature = "std")]
    extern crate std as alloc;

    pub mod stack;

    /// Index of default linear memory.
    pub const DEFAULT_MEMORY_INDEX: u32 = 0;
    /// Index of default table.
    pub const DEFAULT_TABLE_INDEX: u32 = 0;

    /// Maximal number of pages that a wasm instance supports.
    pub const LINEAR_MEMORY_MAX_PAGES: u32 = 65536;

    use alloc::{string::String, vec::Vec};
    use core::fmt;
    #[cfg(feature = "std")]
    use std::error;

    use self::context::ModuleContextBuilder;
    use parity_wasm::elements::{
        BlockType, ExportEntry, External, FuncBody, GlobalEntry, GlobalType, InitExpr, Instruction,
        Internal, MemoryType, Module, ResizableLimits, TableType, Type, ValueType,
    };

    pub mod context;
    pub mod func;
    pub mod util;

    #[cfg(test)]
    mod tests;

    // TODO: Consider using a type other than String, because
    // of formatting machinary is not welcomed in substrate runtimes.
    #[derive(Debug)]
    pub struct Error(pub String);

    impl fmt::Display for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "{}", self.0)
        }
    }

    #[cfg(feature = "std")]
    impl error::Error for Error {
        fn description(&self) -> &str {
            &self.0
        }
    }

    impl From<stack::Error> for Error {
        fn from(e: stack::Error) -> Error {
            Error(format!("Stack: {}", e))
        }
    }

    pub trait Validator {
        type Output;
        type FuncValidator: FuncValidator;
        fn new(module: &Module) -> Self;
        fn on_function_validated(
            &mut self,
            index: u32,
            output: <<Self as Validator>::FuncValidator as FuncValidator>::Output,
        );
        fn finish(self) -> Self::Output;
    }

    pub trait FuncValidator {
        type Output;
        fn new(ctx: &func::FunctionValidationContext, body: &FuncBody) -> Self;
        fn next_instruction(
            &mut self,
            ctx: &mut func::FunctionValidationContext,
            instruction: &Instruction,
        ) -> Result<(), Error>;
        fn finish(self) -> Self::Output;
    }

    /// A module validator that just validates modules and produces no result.
    pub struct PlainValidator;

    impl Validator for PlainValidator {
        type Output = ();
        type FuncValidator = PlainFuncValidator;
        fn new(_module: &Module) -> PlainValidator {
            PlainValidator
        }
        fn on_function_validated(
            &mut self,
            _index: u32,
            _output: <<Self as Validator>::FuncValidator as FuncValidator>::Output,
        ) -> () {
            ()
        }
        fn finish(self) -> () {
            ()
        }
    }

    /// A function validator that just validates modules and produces no result.
    pub struct PlainFuncValidator;

    impl FuncValidator for PlainFuncValidator {
        type Output = ();

        fn new(_ctx: &func::FunctionValidationContext, _body: &FuncBody) -> PlainFuncValidator {
            PlainFuncValidator
        }

        fn next_instruction(
            &mut self,
            ctx: &mut func::FunctionValidationContext,
            instruction: &Instruction,
        ) -> Result<(), Error> {
            ctx.step(instruction)
        }

        fn finish(self) -> () {
            ()
        }
    }

    pub fn validate_module<V: Validator>(module: &Module) -> Result<V::Output, Error> {
        let mut context_builder = ModuleContextBuilder::new();
        let mut imported_globals = Vec::new();
        let mut validation = V::new(&module);

        // Copy types from module as is.
        context_builder.set_types(
            module
                .type_section()
                .map(|ts| {
                    ts.types()
                        .into_iter()
                        .map(|&Type::Function(ref ty)| ty)
                        .cloned()
                        .collect()
                })
                .unwrap_or_default(),
        );

        // Fill elements with imported values.
        for import_entry in module
            .import_section()
            .map(|i| i.entries())
            .unwrap_or_default()
        {
            match *import_entry.external() {
                External::Function(idx) => context_builder.push_func_type_index(idx),
                External::Table(ref table) => context_builder.push_table(table.clone()),
                External::Memory(ref memory) => context_builder.push_memory(memory.clone()),
                External::Global(ref global) => {
                    context_builder.push_global(global.clone());
                    imported_globals.push(global.clone());
                }
            }
        }

        // Concatenate elements with defined in the module.
        if let Some(function_section) = module.function_section() {
            for func_entry in function_section.entries() {
                context_builder.push_func_type_index(func_entry.type_ref())
            }
        }
        if let Some(table_section) = module.table_section() {
            for table_entry in table_section.entries() {
                validate_table_type(table_entry)?;
                context_builder.push_table(table_entry.clone());
            }
        }
        if let Some(mem_section) = module.memory_section() {
            for mem_entry in mem_section.entries() {
                validate_memory_type(mem_entry)?;
                context_builder.push_memory(mem_entry.clone());
            }
        }
        if let Some(global_section) = module.global_section() {
            for global_entry in global_section.entries() {
                validate_global_entry(global_entry, &imported_globals)?;
                context_builder.push_global(global_entry.global_type().clone());
            }
        }

        let context = context_builder.build();

        let function_section_len = module
            .function_section()
            .map(|s| s.entries().len())
            .unwrap_or(0);
        let code_section_len = module.code_section().map(|s| s.bodies().len()).unwrap_or(0);
        if function_section_len != code_section_len {
            return Err(Error(format!(
                "length of function section is {}, while len of code section is {}",
                function_section_len, code_section_len
            )));
        }

        // validate every function body in user modules
        if function_section_len != 0 {
            // tests use invalid code
            let function_section = module
                .function_section()
                .expect("function_section_len != 0; qed");
            let code_section = module
                .code_section()
                .expect("function_section_len != 0; function_section_len == code_section_len; qed");
            // check every function body
            for (index, function) in function_section.entries().iter().enumerate() {
                let function_body = code_section
                    .bodies()
                    .get(index as usize)
                    .ok_or(Error(format!("Missing body for function {}", index)))?;

                let output = func::drive::<V::FuncValidator>(&context, function, function_body)
                    .map_err(|Error(ref msg)| {
                        Error(format!(
                            "Function #{} reading/validation error: {}",
                            index, msg
                        ))
                    })?;
                validation.on_function_validated(index as u32, output);
            }
        }

        // validate start section
        if let Some(start_fn_idx) = module.start_section() {
            let (params, return_ty) = context.require_function(start_fn_idx)?;
            if return_ty != BlockType::NoResult || params.len() != 0 {
                return Err(Error(
                    "start function expected to have type [] -> []".into(),
                ));
            }
        }

        // validate export section
        if let Some(export_section) = module.export_section() {
            let mut export_names = export_section
                .entries()
                .iter()
                .map(ExportEntry::field)
                .collect::<Vec<_>>();

            export_names.sort_unstable();

            for (fst, snd) in export_names.iter().zip(export_names.iter().skip(1)) {
                if fst == snd {
                    return Err(Error(format!("duplicate export {}", fst)));
                }
            }

            for export in export_section.entries() {
                match *export.internal() {
                    Internal::Function(function_index) => {
                        context.require_function(function_index)?;
                    }
                    Internal::Global(global_index) => {
                        context.require_global(global_index, Some(false))?;
                    }
                    Internal::Memory(memory_index) => {
                        context.require_memory(memory_index)?;
                    }
                    Internal::Table(table_index) => {
                        context.require_table(table_index)?;
                    }
                }
            }
        }

        // validate import section
        if let Some(import_section) = module.import_section() {
            for import in import_section.entries() {
                match *import.external() {
                    External::Function(function_type_index) => {
                        context.require_function_type(function_type_index)?;
                    }
                    External::Global(ref global_type) => {
                        if global_type.is_mutable() {
                            return Err(Error(format!(
                                "trying to import mutable global {}",
                                import.field()
                            )));
                        }
                    }
                    External::Memory(ref memory_type) => {
                        validate_memory_type(memory_type)?;
                    }
                    External::Table(ref table_type) => {
                        validate_table_type(table_type)?;
                    }
                }
            }
        }

        // there must be no greater than 1 table in tables index space
        if context.tables().len() > 1 {
            return Err(Error(format!(
                "too many tables in index space: {}",
                context.tables().len()
            )));
        }

        // there must be no greater than 1 linear memory in memory index space
        if context.memories().len() > 1 {
            return Err(Error(format!(
                "too many memory regions in index space: {}",
                context.memories().len()
            )));
        }

        // use data section to initialize linear memory regions
        if let Some(data_section) = module.data_section() {
            for data_segment in data_section.entries() {
                context.require_memory(data_segment.index())?;
                let offset = data_segment
                    .offset()
                    .as_ref()
                    .ok_or_else(|| Error("passive memory segments are not supported".into()))?;
                let init_ty = expr_const_type(&offset, context.globals())?;
                if init_ty != ValueType::I32 {
                    return Err(Error("segment offset should return I32".into()));
                }
            }
        }

        // use element section to fill tables
        if let Some(element_section) = module.elements_section() {
            for element_segment in element_section.entries() {
                context.require_table(element_segment.index())?;
                let offset = element_segment
                    .offset()
                    .as_ref()
                    .ok_or_else(|| Error("passive element segments are not supported".into()))?;
                let init_ty = expr_const_type(&offset, context.globals())?;
                if init_ty != ValueType::I32 {
                    return Err(Error("segment offset should return I32".into()));
                }

                for function_index in element_segment.members() {
                    context.require_function(*function_index)?;
                }
            }
        }

        Ok(validation.finish())
    }

    fn validate_limits(limits: &ResizableLimits) -> Result<(), Error> {
        if let Some(maximum) = limits.maximum() {
            if limits.initial() > maximum {
                return Err(Error(format!(
                    "maximum limit {} is less than minimum {}",
                    maximum,
                    limits.initial()
                )));
            }
        }
        Ok(())
    }

    fn validate_memory_type(memory_type: &MemoryType) -> Result<(), Error> {
        let initial = memory_type.limits().initial();
        let maximum: Option<u32> = memory_type.limits().maximum();
        validate_memory(initial, maximum).map_err(Error)
    }

    pub fn validate_memory(initial: u32, maximum: Option<u32>) -> Result<(), String> {
        if initial > LINEAR_MEMORY_MAX_PAGES {
            return Err(format!(
                "initial memory size must be at most {} pages",
                LINEAR_MEMORY_MAX_PAGES
            ));
        }
        if let Some(maximum) = maximum {
            if initial > maximum {
                return Err(format!(
                    "maximum limit {} is less than minimum {}",
                    maximum, initial,
                ));
            }

            if maximum > LINEAR_MEMORY_MAX_PAGES {
                return Err(format!(
                    "maximum memory size must be at most {} pages",
                    LINEAR_MEMORY_MAX_PAGES
                ));
            }
        }
        Ok(())
    }

    fn validate_table_type(table_type: &TableType) -> Result<(), Error> {
        validate_limits(table_type.limits())
    }

    fn validate_global_entry(global_entry: &GlobalEntry, globals: &[GlobalType]) -> Result<(), Error> {
        let init = global_entry.init_expr();
        let init_expr_ty = expr_const_type(init, globals)?;
        if init_expr_ty != global_entry.global_type().content_type() {
            return Err(Error(format!(
                "Trying to initialize variable of type {:?} with value of type {:?}",
                global_entry.global_type().content_type(),
                init_expr_ty
            )));
        }
        Ok(())
    }

    /// Returns type of this constant expression.
    fn expr_const_type(init_expr: &InitExpr, globals: &[GlobalType]) -> Result<ValueType, Error> {
        let code = init_expr.code();
        if code.len() != 2 {
            return Err(Error(
                "Init expression should always be with length 2".into(),
            ));
        }
        let expr_ty: ValueType = match code[0] {
            Instruction::I32Const(_) => ValueType::I32,
            Instruction::I64Const(_) => ValueType::I64,
            Instruction::F32Const(_) => ValueType::F32,
            Instruction::F64Const(_) => ValueType::F64,
            Instruction::GetGlobal(idx) => match globals.get(idx as usize) {
                Some(target_global) => {
                    if target_global.is_mutable() {
                        return Err(Error(format!("Global {} is mutable", idx)));
                    }
                    target_global.content_type()
                }
                None => {
                    return Err(Error(format!(
                        "Global {} doesn't exists or not yet defined",
                        idx
                    )));
                }
            },
            _ => return Err(Error("Non constant opcode in init expr".into())),
        };
        if code[1] != Instruction::End {
            return Err(Error("Expression doesn't ends with `end` opcode".into()));
        }
        Ok(expr_ty)
    }

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