//! Minimal strict templating for prompt and text assets.//!//! Supported syntax://! - `{{ name }}` placeholder interpolation//! - `{{{{` for a literal `{{`//! - `}}}}` for a literal `}}`use std::collections::BTreeMap;use std::collections::BTreeSet;use std::error::Error;use std::fmt;#[derive(Debug, Clone, PartialEq, Eq)]pub enum TemplateParseError {    EmptyPlaceholder { start: usize },    NestedPlaceholder { start: usize },    UnmatchedClosingDelimiter { start: usize },    UnterminatedPlaceholder { start: usize },}impl fmt::Display for TemplateParseError {    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {        match self {            Self::EmptyPlaceholder { start } => {                write!(f, "template placeholder at byte {start} is empty")            }            Self::NestedPlaceholder { start } => {                write!(                    f,                    "template placeholder starting at byte {start} contains a nested `{{`"                )            }            Self::UnmatchedClosingDelimiter { start } => {                write!(f, "template contains an unmatched `}}` at byte {start}")            }            Self::UnterminatedPlaceholder { start } => {                write!(                    f,                    "template placeholder starting at byte {start} is missing `}}`"                )            }        }    }}impl Error for TemplateParseError {}#[derive(Debug, Clone, PartialEq, Eq)]pub enum TemplateRenderError {    DuplicateValue { name: String },    ExtraValue { name: String },    MissingValue { name: String },}impl fmt::Display for TemplateRenderError {    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {        match self {            Self::DuplicateValue { name } => {                write!(f, "template value `{name}` was provided more than once")            }            Self::ExtraValue { name } => {                write!(f, "template value `{name}` is not used by this template")            }            Self::MissingValue { name } => {                write!(f, "template placeholder `{name}` is missing a value")            }        }    }}impl Error for TemplateRenderError {}#[derive(Debug, Clone, PartialEq, Eq)]pub enum TemplateError {    Parse(TemplateParseError),    Render(TemplateRenderError),}impl fmt::Display for TemplateError {    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {        match self {            Self::Parse(err) => err.fmt(f),            Self::Render(err) => err.fmt(f),        }    }}impl Error for TemplateError {    fn source(&self) -> Option<&(dyn Error + 'static)> {        match self {            Self::Parse(err) => Some(err),            Self::Render(err) => Some(err),        }    }}impl From<TemplateParseError> for TemplateError {    fn from(value: TemplateParseError) -> Self {        Self::Parse(value)    }}impl From<TemplateRenderError> for TemplateError {    fn from(value: TemplateRenderError) -> Self {        Self::Render(value)    }}#[derive(Debug, Clone, PartialEq, Eq)]enum Segment {    Literal(String),    Placeholder(String),}#[derive(Debug, Clone, PartialEq, Eq)]pub struct Template {    placeholders: BTreeSet<String>,    segments: Vec<Segment>,}impl Template {    pub fn parse(source: &str) -> Result<Self, TemplateParseError> {        let mut placeholders = BTreeSet::new();        let mut segments = Vec::new();        let mut literal_start = 0usize;        let mut cursor = 0usize;        while cursor < source.len() {            let rest = &source[cursor..];            if rest.starts_with("{{{{") {                push_literal(&mut segments, &source[literal_start..cursor]);                push_literal(&mut segments, "{{");                cursor += "{{{{".len();                literal_start = cursor;                continue;            }            if rest.starts_with("}}}}") {                push_literal(&mut segments, &source[literal_start..cursor]);                push_literal(&mut segments, "}}");                cursor += "}}}}".len();                literal_start = cursor;                continue;            }            if rest.starts_with("{{") {                push_literal(&mut segments, &source[literal_start..cursor]);                let (placeholder, next_cursor) = parse_placeholder(source, cursor)?;                placeholders.insert(placeholder.clone());                segments.push(Segment::Placeholder(placeholder));                cursor = next_cursor;                literal_start = cursor;                continue;            }            if rest.starts_with("}}") {                return Err(TemplateParseError::UnmatchedClosingDelimiter { start: cursor });            }            let Some(ch) = rest.chars().next() else {                break;            };            cursor += ch.len_utf8();        }        push_literal(&mut segments, &source[literal_start..]);        Ok(Self {            placeholders,            segments,        })    }    pub fn placeholders(&self) -> impl ExactSizeIterator<Item = &str> {        self.placeholders.iter().map(String::as_str)    }    pub fn render<I, K, V>(&self, variables: I) -> Result<String, TemplateRenderError>    where        I: IntoIterator<Item = (K, V)>,        K: AsRef<str>,        V: AsRef<str>,    {        let variables = build_variable_map(variables)?;        for placeholder in &self.placeholders {            if !variables.contains_key(placeholder.as_str()) {                return Err(TemplateRenderError::MissingValue {                    name: placeholder.clone(),                });            }        }        for name in variables.keys() {            if !self.placeholders.contains(name.as_str()) {                return Err(TemplateRenderError::ExtraValue { name: name.clone() });            }        }        let mut rendered = String::new();        for segment in &self.segments {            match segment {                Segment::Literal(literal) => rendered.push_str(literal),                Segment::Placeholder(name) => {                    let Some(value) = variables.get(name.as_str()) else {                        return Err(TemplateRenderError::MissingValue { name: name.clone() });                    };                    rendered.push_str(value);                }            }        }        Ok(rendered)    }}pub fn render<I, K, V>(template: &str, variables: I) -> Result<String, TemplateError>where    I: IntoIterator<Item = (K, V)>,    K: AsRef<str>,    V: AsRef<str>,{    Template::parse(template)?        .render(variables)        .map_err(Into::into)}fn push_literal(segments: &mut Vec<Segment>, literal: &str) {    if literal.is_empty() {        return;    }    if let Some(Segment::Literal(existing)) = segments.last_mut() {        existing.push_str(literal);    } else {        segments.push(Segment::Literal(literal.to_string()));    }}fn parse_placeholder(source: &str, start: usize) -> Result<(String, usize), TemplateParseError> {    let placeholder_start = start + "{{".len();    let mut cursor = placeholder_start;    while cursor < source.len() {        let rest = &source[cursor..];        if rest.starts_with("{{") {            return Err(TemplateParseError::NestedPlaceholder { start });        }        if rest.starts_with("}}") {            let placeholder = source[placeholder_start..cursor].trim();            if placeholder.is_empty() {                return Err(TemplateParseError::EmptyPlaceholder { start });            }            return Ok((placeholder.to_string(), cursor + "}}".len()));        }        let Some(ch) = rest.chars().next() else {            break;        };        cursor += ch.len_utf8();    }    Err(TemplateParseError::UnterminatedPlaceholder { start })}fn build_variable_map<I, K, V>(    variables: I,) -> Result<BTreeMap<String, String>, TemplateRenderError>where    I: IntoIterator<Item = (K, V)>,    K: AsRef<str>,    V: AsRef<str>,{    let mut map = BTreeMap::new();    for (name, value) in variables {        let name = name.as_ref().to_string();        if map            .insert(name.clone(), value.as_ref().to_string())            .is_some()        {            return Err(TemplateRenderError::DuplicateValue { name });        }    }    Ok(map)}#[cfg(test)]mod tests {    use super::Template;    use super::TemplateError;    use super::TemplateParseError;    use super::TemplateRenderError;    use super::render;    use pretty_assertions::assert_eq;    #[test]    fn render_replaces_placeholders_with_and_without_whitespace() {        let rendered = render(            "Hello, {{ name }}. You are in {{place}}. {{ name }} is repeated.",            [("name", "Codex"), ("place", "codex-rs")],        )        .unwrap();        assert_eq!(            rendered,            "Hello, Codex. You are in codex-rs. Codex is repeated."        );    }    #[test]    fn parsed_templates_can_be_reused() {        let template = Template::parse("{{greeting}}, {{ name }}!").unwrap();        assert_eq!(            template.render([("greeting", "Hello"), ("name", "Codex")]),            Ok("Hello, Codex!".to_string())        );        assert_eq!(            template.render([("greeting", "Hi"), ("name", "builder")]),            Ok("Hi, builder!".to_string())        );    }    #[test]    fn placeholders_are_sorted_and_unique() {        let template = Template::parse("{{ b }} {{ a }} {{ b }}").unwrap();        assert_eq!(template.placeholders().collect::<Vec<_>>(), vec!["a", "b"]);    }    #[test]    fn render_supports_multiline_templates_and_adjacent_placeholders() {        let rendered = render(            "Line 1: {{first}}{{second}}\nLine 2: {{ third }}",            [("first", "A"), ("second", "B"), ("third", "C")],        )        .unwrap();        assert_eq!(rendered, "Line 1: AB\nLine 2: C");    }    #[test]    fn render_supports_literal_delimiter_escapes() {        let rendered = render(            "literal open: {{{{, literal close: }}}}, value: {{ name }}",            [("name", "Codex")],        )        .unwrap();        assert_eq!(            rendered,            "literal open: {{, literal close: }}, value: Codex"        );    }    #[test]    fn parse_errors_when_placeholder_is_empty() {        let err = Template::parse("Hello, {{   }}.").unwrap_err();        assert_eq!(err, TemplateParseError::EmptyPlaceholder { start: 7 });    }    #[test]    fn parse_errors_when_placeholder_is_unterminated() {        let err = Template::parse("Hello, {{ name.").unwrap_err();        assert_eq!(            err,            TemplateParseError::UnterminatedPlaceholder { start: 7 }        );    }    #[test]    fn parse_errors_when_placeholder_is_nested() {        let err = Template::parse("Hello, {{ outer {{ inner }} }}.").unwrap_err();        assert_eq!(err, TemplateParseError::NestedPlaceholder { start: 7 });    }    #[test]    fn parse_errors_when_closing_delimiter_is_unmatched() {        let err = Template::parse("Hello, }} world.").unwrap_err();        assert_eq!(            err,            TemplateParseError::UnmatchedClosingDelimiter { start: 7 }        );    }    #[test]    fn render_errors_when_placeholder_is_missing() {        let template = Template::parse("Hello, {{ name }}.").unwrap();        assert_eq!(            template.render(Vec::<(&str, &str)>::new()),            Err(TemplateRenderError::MissingValue {                name: "name".to_string()            })        );    }    #[test]    fn render_errors_when_extra_value_is_provided() {        let template = Template::parse("Hello, {{ name }}.").unwrap();        assert_eq!(            template.render([("name", "Codex"), ("unused", "extra")]),            Err(TemplateRenderError::ExtraValue {                name: "unused".to_string()            })        );    }    #[test]    fn render_errors_when_duplicate_value_is_provided() {        let template = Template::parse("Hello, {{ name }}.").unwrap();        assert_eq!(            template.render([("name", "Codex"), ("name", "other")]),            Err(TemplateRenderError::DuplicateValue {                name: "name".to_string()            })        );    }    #[test]    fn render_function_wraps_parse_errors() {        let err = render("Hello, }} world.", [("name", "Codex")]).unwrap_err();        assert_eq!(            err,            TemplateError::Parse(TemplateParseError::UnmatchedClosingDelimiter { start: 7 })        );    }    #[test]    fn render_function_wraps_render_errors() {        let err = render("Hello, {{ name }}.", [("extra", "Codex")]).unwrap_err();        assert_eq!(            err,            TemplateError::Render(TemplateRenderError::MissingValue {                name: "name".to_string()            })        );    }}