Rusthero solution

encoder_.gitignore

/target
**/*.rs.bk

encoder_.idea_markdown-navigator_profiles_settings.xml

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encoder_.idea_misc.xml

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encoder_.idea_modules.xml

<?xml version="1.0" encoding="UTF-8"?>
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      <module fileurl="file://$PROJECT_DIR$/encoder.iml" filepath="$PROJECT_DIR$/encoder.iml" />
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encoder_.idea_vcs.xml

<?xml version="1.0" encoding="UTF-8"?>
<project version="4">
  <component name="VcsDirectoryMappings">
    <mapping directory="" vcs="Git" />
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encoder_Cargo.lock

# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
[[package]]
name = "encoder"
version = "0.1.0"
dependencies = [
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dependencies = [
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[[package]]
name = "quote"
version = "0.6.12"
source = "registry+https://github.com/rust-lang/crates.io-index"
dependencies = [
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encoder_Cargo.toml

[package]
name = "encoder"
version = "0.1.0"
authors = ["michele <[email protected]>"]
edition = "2018"

[dependencies]

[dev-dependencies]
rstest = "0.2.2"

encoder_encoder.iml

<?xml version="1.0" encoding="UTF-8"?>
<module type="RUST_MODULE" version="4">
  <component name="NewModuleRootManager" inherit-compiler-output="true">
    <exclude-output />
    <content url="file://$MODULE_DIR$">
      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
      <sourceFolder url="file://$MODULE_DIR$/examples" isTestSource="false" />
      <sourceFolder url="file://$MODULE_DIR$/tests" isTestSource="true" />
      <sourceFolder url="file://$MODULE_DIR$/benches" isTestSource="true" />
      <excludeFolder url="file://$MODULE_DIR$/target" />
    </content>
    <orderEntry type="inheritedJdk" />
    <orderEntry type="sourceFolder" forTests="false" />
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</module>

encoder_src_main.rs

fn main() {
    let mut data = "2216224441".to_owned();
    let steps = 40;

    for _ in 0..steps {
        data = encode(&data);
    }
    println!("New data after '{}' steps = {}", steps, data);
    println!("Len = {}", data.len());
}

struct CounterState {
    char: char,
    occurrences: usize
}

impl CounterState {
    pub fn new(c: char) -> Self {
        Self { char: c, occurrences: 1 }
    }

    fn increment(&mut self) {
        self.occurrences += 1;
    }

    fn same(&self, other: char) -> bool {
        self.char == other
    }
}

struct Encoder<I: Iterator<Item=char>> {
    stream: I,
    state: Option<CounterState>
}

impl<I: Iterator<Item=char>> Encoder<I> {
    pub fn new(stream: I) -> Self {
        Self {stream, state: None }
    }

    fn replace_state(&mut self, c: char) -> Option<CounterState> {
        std::mem::replace(&mut self.state, Some(CounterState::new(c)))
    }
}

impl<I: Iterator<Item=char>> Iterator for Encoder<I> {
    type Item = CounterState;

    fn next(&mut self) -> Option<Self::Item> {
        while let Some(c) = self.stream.next() {
            match self.state {
                None => self.state = Some(CounterState::new(c)),
                Some(ref mut cs) if cs.same(c) => { cs.increment(); },
                _ => return self.replace_state(c)
            }
        };
        std::mem::replace(&mut self.state, None)
    }
}

pub fn encode(input: &str) -> String {
    Encoder::new(input.chars()).
        map(|cs| format!("{}{}", cs.occurrences, cs.char))
        .collect()
}

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

    #[rstest_parametrize(input, expected,
        case("1", "11"),
        case("2", "12"),
        case("11", "21"),
        case("31", "1311"),
        case("3211", "131221"),
        case("111223", "312213"),
        case("2216224441", "221116223411"),
    )]
    fn standard_cases(input: &str, expected: &str) {
        assert_eq!(&encode(input), expected)
    }
}