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    use crate::Error;
    use alloc::string::String;
    use parity_wasm::elements::{Local, ValueType};

    #[cfg(test)]
    use assert_matches::assert_matches;

    /// Locals are the concatenation of a slice of function parameters
    /// with function declared local variables.
    ///
    /// Local variables are given in the form of groups represented by pairs
    /// of a value_type and a count.
    #[derive(Debug)]
    pub struct Locals<'a> {
        params: &'a [ValueType],
        local_groups: &'a [Local],
        count: u32,
    }

    impl<'a> Locals<'a> {
        /// Create a new wrapper around declared variables and parameters.
        pub fn new(params: &'a [ValueType], local_groups: &'a [Local]) -> Result<Locals<'a>, Error> {
            let mut acc = params.len() as u32;
            for locals_group in local_groups {
                acc = acc
                    .checked_add(locals_group.count())
                    .ok_or_else(|| Error(String::from("Locals range not in 32-bit range")))?;
            }

            Ok(Locals {
                params,
                local_groups,
                count: acc,
            })
        }

        /// Returns parameter count.
        pub fn param_count(&self) -> u32 {
            self.params.len() as u32
        }

        /// Returns total count of all declared locals and paramaterers.
        pub fn count(&self) -> u32 {
            self.count
        }

        /// Returns the type of a local variable (either a declared local or a param).
        ///
        /// Returns `Err` in the case of overflow or when idx falls out of range.
        pub fn type_of_local(&self, idx: u32) -> Result<ValueType, Error> {
            if let Some(param) = self.params.get(idx as usize) {
                return Ok(*param);
            }

            // If an index doesn't point to a param, then we have to look into local declarations.
            let mut start_idx = self.param_count();
            for locals_group in self.local_groups {
                let end_idx = start_idx
                    .checked_add(locals_group.count())
                    .ok_or_else(|| Error(String::from("Locals range not in 32-bit range")))?;

                if idx >= start_idx && idx < end_idx {
                    return Ok(locals_group.value_type());
                }

                start_idx = end_idx;
            }

            // We didn't find anything, that's an error.
            // At this moment `start_idx` should hold the count of all locals
            // (since it's either set to the `end_idx` or equal to `params.len()`)
            let total_count = start_idx;

            Err(Error(format!(
                "Trying to access local with index {} when there are only {} locals",
                idx, total_count
            )))
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn locals_it_works() {
            let params = vec![ValueType::I32, ValueType::I64];
            let local_groups = vec![Local::new(2, ValueType::F32), Local::new(2, ValueType::F64)];
            let locals = Locals::new(&params, &local_groups).unwrap();

            assert_matches!(locals.type_of_local(0), Ok(ValueType::I32));
            assert_matches!(locals.type_of_local(1), Ok(ValueType::I64));
            assert_matches!(locals.type_of_local(2), Ok(ValueType::F32));
            assert_matches!(locals.type_of_local(3), Ok(ValueType::F32));
            assert_matches!(locals.type_of_local(4), Ok(ValueType::F64));
            assert_matches!(locals.type_of_local(5), Ok(ValueType::F64));
            assert_matches!(locals.type_of_local(6), Err(_));
        }

        #[test]
        fn locals_no_declared_locals() {
            let params = vec![ValueType::I32];
            let locals = Locals::new(&params, &[]).unwrap();

            assert_matches!(locals.type_of_local(0), Ok(ValueType::I32));
            assert_matches!(locals.type_of_local(1), Err(_));
        }

        #[test]
        fn locals_no_params() {
            let local_groups = vec![Local::new(2, ValueType::I32), Local::new(3, ValueType::I64)];
            let locals = Locals::new(&[], &local_groups).unwrap();

            assert_matches!(locals.type_of_local(0), Ok(ValueType::I32));
            assert_matches!(locals.type_of_local(1), Ok(ValueType::I32));
            assert_matches!(locals.type_of_local(2), Ok(ValueType::I64));
            assert_matches!(locals.type_of_local(3), Ok(ValueType::I64));
            assert_matches!(locals.type_of_local(4), Ok(ValueType::I64));
            assert_matches!(locals.type_of_local(5), Err(_));
        }

        #[test]
        fn locals_u32_overflow() {
            let local_groups = vec![
                Local::new(u32::max_value(), ValueType::I32),
                Local::new(1, ValueType::I64),
            ];
            assert_matches!(Locals::new(&[], &local_groups), Err(_));
        }
    }

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