nanotemplate 0.1.0

/*!
A minimal templating engine that renders a string from the template, replacing all instances of `{placeholder}` with given values.

The engine is strict:

* all placeholders must have values provided,
* all provided values must have matching placeholder,
* a single placeholder can be used multiple times and will be expanded in all places.

Values are provided as an iterable object that provides placeholder name and value pairs.

Placeholder name may only contain letters (`a` to `z`, `A` to `Z`), numbers, dash, underscore and a dot to limit the risk of accidental placeholders in templates.

Hash maps are not used; all data is kept in vectors making it `O(N + N * M)` where `N` is number of placeholders and `M` is number of values. This should still be very fast for templates with small number of placeholders as no data is copied (works with slices).

```rust
use nanotemplate::template;

assert_eq!(
    template("Hello, my name is {name}!", &[("name", "nanotemplate")]).unwrap(),
    "Hello, my name is nanotemplate!".to_owned());
```

Also comes with simple CLI utility:

```sh
echo "Hello, my name is {name}!" | nanotemplate name=nanotemplate
```

*/

use std::error::Error;
use std::fmt::{self, Display};

#[derive(Debug)]
pub enum TemplateError {
    MissingValueForPlaceholder(String),
    NoPlaceholder(String),
}

impl Display for TemplateError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            TemplateError::MissingValueForPlaceholder(placeh) => write!(f, "template is missing value for placeholder {:?}", placeh),
            TemplateError::NoPlaceholder(placeh) => write!(f, "template has no placeholder {:?}", placeh),
        }
    }
}

impl Error for TemplateError {}

/// Provides a way to get placeholder name and value for template expansion.
pub trait PlaceholderValuePair<'k, 'v> {
    fn get_placeholder(&self) -> &'k str;
    fn get_value(&self) -> &'v str;
}

impl<'k, 'v> PlaceholderValuePair<'k, 'v> for (&'k str, &'v str) {
    fn get_placeholder(&self) -> &'k str {
        self.0
    }

    fn get_value(&self) -> &'v str {
        self.1
    }
}

impl<'k, 'v> PlaceholderValuePair<'k, 'v> for &(&'k str, &'v str) {
    fn get_placeholder(&self) -> &'k str {
        self.0
    }

    fn get_value(&self) -> &'v str {
        self.1
    }
}

impl<'t> PlaceholderValuePair<'t, 't> for [&'t str; 2] {
    fn get_placeholder(&self) -> &'t str {
        self[0]
    }

    fn get_value(&self) -> &'t str {
        self[1]
    }
}

impl<'t> PlaceholderValuePair<'t, 't> for &[&'t str; 2] {
    fn get_placeholder(&self) -> &'t str {
        self[0]
    }

    fn get_value(&self) -> &'t str {
        self[1]
    }
}

/// Renders a string from the tempalte, replacing all instances of {placeholder} with given values.
///
/// Placeholder name may only contain letters, numbers, dash, underscore and a dot.
pub fn template<'k, 'v, T: PlaceholderValuePair<'k, 'v>, I: IntoIterator<Item = T>>(template: &str, values: I) -> Result<String, TemplateError> {
    let mut vars = Vec::new();
    let mut slices = Vec::new();

    let mut from = 0;
    let mut candidate = None;

    for (i, c) in template.char_indices() {
        match c {
            '{' => candidate = Some(i),
            '}' => if let Some(placeh) = candidate.take() {
                slices.push(&template[from .. placeh]);
                vars.push((&template[placeh + 1 .. i], None));
                from = i + 1;
            }
            c => match c {
                'a'..='z' | 'A'..='Z' | '0'..='9' | '-' | '_' | '.' => (),
                _ => candidate = None,
            }
        }
    }

    slices.push(&template[from .. template.len()]);

    for t in values {
        let (tplaceh, tvalue) = (t.get_placeholder(), t.get_value());
        let mut found = false;

        for &mut (placeh, ref mut value) in vars.iter_mut() {
            if placeh == tplaceh {
                *value = Some(tvalue);
                found = true;
            }
        }

        if !found {
            return Err(TemplateError::NoPlaceholder(tplaceh.to_owned()))
        }
    }

    let mut out = String::new();
    let len = slices.len();

    for (i, s) in slices.into_iter().enumerate() {
        out.push_str(s);

        if i < len - 1 {
            let slot = &vars[i];
            let value = slot.1
                .as_ref()
                .ok_or_else(|| TemplateError::MissingValueForPlaceholder(slot.0.to_owned()))?;
            out.push_str(value);
        }
    }

    Ok(out)
}

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

    #[test]
    fn test_template_empty() {
        assert_eq!(&template::<&(&str, &str), _>("", None).unwrap(), "");
    }

    #[test]
    fn test_template_none() {
        assert_eq!(&template::<&(&str, &str), _>("hello world", None).unwrap(), "hello world");
    }

    #[test]
    fn test_template_single() {
        assert_eq!(&template("hello {foo} world", Some(("foo", "bar"))).unwrap(), "hello bar world");
    }

    #[test]
    fn test_template_single_variable_multi() {
        assert_eq!(&template("{foo} hello {foo} world {foo}", Some(("foo", "bar"))).unwrap(), "bar hello bar world bar");
    }

    #[test]
    fn test_template_many_vars_slice() {
        assert_eq!(&template("{foo} hello {bar} world {baz}", &[("foo", "1"), ("bar", "2"), ("baz" ,"3")]).unwrap(), "1 hello 2 world 3");
    }

    #[test]
    fn test_template_many_vars_slice_array() {
        assert_eq!(&template("{foo} hello {bar} world {baz}", &[["foo", "1"], ["bar", "2"], ["baz" ,"3"]]).unwrap(), "1 hello 2 world 3");
    }

    #[test]
    fn test_template_many_vars() {
        assert_eq!(&template("{foo} hello {bar} world {baz}", vec![("foo", "1"), ("bar", "2"), ("baz" ,"3")]).unwrap(), "1 hello 2 world 3");
    }

    #[test]
    fn test_template_nested_reverse() {
        assert_eq!(&template("}}}{foo}{{ hello }}}{bar}{{{ world }}{baz}{{{", vec![("foo", "1"), ("bar", "2"), ("baz" ,"3")]).unwrap(), "}}}1{{ hello }}}2{{{ world }}3{{{");
    }

    #[test]
    fn test_template_nested() {
        assert_eq!(&template("{{{{foo}}} hello {{{{bar}}}} world {{{baz}}}}", vec![("foo", "1"), ("bar", "2"), ("baz" ,"3")]).unwrap(), "{{{1}} hello {{{2}}} world {{3}}}");
    }

    #[test]
    fn test_template_json() {
        let tpl = r#"{"menu": {
  "id": "file",
  "value": {File},
  "{Popup}": {
    "menuitem": [
      {"value": "{New}", "onclick": "CreateNewDoc()"},
      {"value": "{Open}", "onclick": "OpenDoc()"},
      {"value": "{Close}", "onclick": "CloseDoc()"}
    ]
  }
}}"#;

        let out = r#"{"menu": {
  "id": "file",
  "value": "File",
  "popup": {
    "menuitem": [
      {"value": "New", "onclick": "CreateNewDoc()"},
      {"value": "Open", "onclick": "OpenDoc()"},
      {"value": "Close", "onclick": "CloseDoc()"}
    ]
  }
}}"#;

        assert_eq!(&template(tpl, vec![("File", "\"File\""), ("Popup", "popup"), ("New", "New"), ("Open" ,"Open"), ("Close", "Close")]).unwrap(), out);
    }

    #[test]
    fn test_template_many_vars_space() {
        assert_eq!(&template("{foo} hello {foo bar} world {baz}", vec![("foo", "1"), ("baz" ,"3")]).unwrap(), "1 hello {foo bar} world 3");
        assert_eq!(&template("{foo.bar} hello {foo+bar} world {foo_baz}", vec![("foo.bar", "1"), ("foo_baz" ,"3")]).unwrap(), "1 hello {foo+bar} world 3");
    }

    #[test]
    #[should_panic(expected = "template is missing value for placeholder \\\"foo\\\"")]
    fn test_template_missing_placeholder() {
        template::<&(&str, &str), _>("{foo} hello {foo} world {foo}", &[]).map_err(|e| e.to_string()).unwrap();
    }

    #[test]
    #[should_panic(expected = "template has no placeholder \\\"bar\\\"")]
    fn test_template_no_placeholder() {
        template("{foo} hello {foo} world {foo}", vec![("foo", "1"), ("bar", "2")]).map_err(|e| e.to_string()).unwrap();
    }
}