use std::borrow::Cow;

/// A variable name is one or more ASCII alphanumerical characters, including underscore.

/// Note that numerical variable names like `$45` are allowed too.

///

/// Try to parse a variable name from the beginning of `s`.

/// Return the index of the character following the varname.

/// This returns 0 if `s` doesn't have a prefix that is a variable name.

pub fn varname_prefix(s: &str) -> usize {

    for (idx, ch) in s.char_indices() {

        match ch {

            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' => {}

            _ => return idx,

        }

    }

    s.len()

}

/// A variable can contain either a regular expression or plain text.

#[derive(Debug, Clone, PartialEq, Eq)]

pub enum Value<'a> {

    /// Verbatim text.

    Text(Cow<'a, str>),

    /// Regular expression.

    Regex(Cow<'a, str>),

}

/// Resolve variables by name.

pub trait VariableMap {

    /// Get the value of the variable `varname`, or return `None` for an unknown variable name.

    fn lookup(&self, varname: &str) -> Option<Value>;

}

impl VariableMap for () {

    fn lookup(&self, _: &str) -> Option<Value> {

        None

    }

}

/// An empty variable map.

pub const NO_VARIABLES: &'static dyn VariableMap = &();

#[cfg(test)]

mod tests {

    #[test]

    fn varname() {

        use super::varname_prefix;

        assert_eq!(varname_prefix(""), 0);

        assert_eq!(varname_prefix("\0"), 0);

        assert_eq!(varname_prefix("_"), 1);

        assert_eq!(varname_prefix("0"), 1);

        assert_eq!(varname_prefix("01"), 2);

        assert_eq!(varname_prefix("b"), 1);

        assert_eq!(varname_prefix("C"), 1);

        assert_eq!(varname_prefix("."), 0);

        assert_eq!(varname_prefix(".s"), 0);

        assert_eq!(varname_prefix("0."), 1);

        assert_eq!(varname_prefix("01="), 2);

        assert_eq!(varname_prefix("0a)"), 2);

    }

}