caser 1.0.0

Change text between PascalCase, camelCase, and snake_case
Documentation 
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use std::iter::FromIterator;

pub enum Case {
    // snake_case
    SnakeCase,
    // PascalCase
    PascalCase,
    // camelCase
    CamelCase,
}

impl Case {
    pub fn transform(&self, string: &str) -> String {
        let mut chars: Vec<char> = vec![];
        return match self {
            Self::SnakeCase=> {
                for ch in string.chars() {
                    if ch.is_uppercase() {
                        chars.push('_');
                        chars.extend(ch.to_lowercase());
                    } else {
                        chars.push(ch);
                    }
                }

                let first_is_underscore = chars.first().map_or(false, |ch| *ch == '_');
                if first_is_underscore {
                    String::from_iter(chars.into_iter().skip(1))
                } else {
                    String::from_iter(chars)
                }
            }
            Self::PascalCase => {
                let mut uppercase_next = false;
                for (i, ch) in string.chars().enumerate() {
                        if i == 0 {
                            chars.extend(ch.to_uppercase())
                        } else if ch == '_' {
                            uppercase_next = true;
                        } else if uppercase_next {
                            chars.extend(ch.to_uppercase());
                            uppercase_next = false;
                        } else {
                            chars.push(ch);
                        }
                }

                String::from_iter(chars)
            }
            Self::CamelCase => {
                let mut uppercase_next = false;
                for (i, ch) in string.chars().enumerate() {
                        if i == 0 {
                            chars.extend(ch.to_lowercase())
                        } else if ch == '_' {
                            uppercase_next = true;
                        } else if uppercase_next {
                            chars.extend(ch.to_uppercase());
                            uppercase_next = false;
                        } else {
                            chars.push(ch);
                        }
                }

                String::from_iter(chars)
            }
        }
    }
}

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

    #[test]
    fn camel_to_snake() {
        let input = "camelToSnake";
        let expected = "camel_to_snake";
        assert_eq!(expected, SnakeCase.transform(input))
    }

    #[test]
    fn pascal_to_snake() {
        let input = "PascalToSnake";
        let expected = "pascal_to_snake";
        assert_eq!(expected, SnakeCase.transform(input))
    }

    #[test]
    fn snake_to_camel() {
        let input = "snake_to_camel";
        let expected = "snakeToCamel";
        assert_eq!(expected, CamelCase.transform(input))
    }

    #[test]
    fn pascal_to_camel() {
        let input = "PascalToCamel";
        let expected = "pascalToCamel";
        assert_eq!(expected, CamelCase.transform(input))
    }

    #[test]
    fn snake_to_pascal() {
        let input = "snake_to_pascal";
        let expected = "SnakeToPascal";
        assert_eq!(expected, PascalCase.transform(input))
    }

    #[test]
    fn camel_to_pascal() {
        let input = "camelToPascal";
        let expected = "CamelToPascal";
        assert_eq!(expected, PascalCase.transform(input))
    }
}