7 files changed, 585 insertions, 0 deletions
diff --git a/just_template/Cargo.lock b/just_template/Cargo.lock
new file mode 100644
index 0000000..d6a39f0
--- /dev/null
+++ b/just_template/Cargo.lock
@@ -0,0 +1,16 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "just_fmt"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5454cda0d57db59778608d7a47bff5b16c6705598265869fb052b657f66cf05e"
+
+[[package]]
+name = "just_template"
+version = "0.2.0"
+dependencies = [
+ "just_fmt",
+]
diff --git a/just_template/Cargo.toml b/just_template/Cargo.toml
new file mode 100644
index 0000000..66217b4
--- /dev/null
+++ b/just_template/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "just_template"
+version.workspace = true
+description.workspace = true
+authors.workspace = true
+edition.workspace = true
+
+readme = "README.md"
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/catilgrass/just_template"
+
+[dependencies]
+just_fmt = "0.1.2"
diff --git a/just_template/src/deprecated.rs b/just_template/src/deprecated.rs
new file mode 100644
index 0000000..b218969
--- /dev/null
+++ b/just_template/src/deprecated.rs
@@ -0,0 +1,45 @@
+#[macro_export]
+macro_rules! tmpl_param {
+    ($template:ident, $($key:ident = $value:expr),* $(,)?) => {{
+        $(
+            $template.insert_param(stringify!($key).to_string(), $value.to_string());
+        )*
+    }};
+}
+
+#[macro_export]
+macro_rules! tmpl {
+    ($template:ident, $($name:ident {
+        $($key:ident = $value:expr),* $(,)?
+    }),* $(,)?) => {{
+        $(
+            let $name = $template.add_impl(stringify!($name).to_string());
+            $(
+                $name.push({
+                    let mut params = std::collections::HashMap::new();
+                    params.insert(stringify!($key).to_string(), $value.to_string());
+                    params
+                });
+            )*
+        )*
+    }};
+
+    // Old syntax
+    ($template:ident += {
+        $($name:ident {
+            $(($($key:ident = $value:expr),* $(,)?)),*
+            $(,)?
+        }),*
+    }) => {{
+        $(
+            let $name = $template.add_impl(stringify!($name).to_string());
+            $(
+                $name.push({
+                    let mut params = std::collections::HashMap::new();
+                    $(params.insert(stringify!($key).to_string(), $value.to_string());)*
+                    params
+                });
+            )*
+        )*
+    }};
+}
diff --git a/just_template/src/expand.rs b/just_template/src/expand.rs
new file mode 100644
index 0000000..b609113
--- /dev/null
+++ b/just_template/src/expand.rs
@@ -0,0 +1,212 @@
+use std::collections::HashMap;
+
+use just_fmt::snake_case;
+
+use crate::template::Template;
+
+const DISPLAY_BLOCK_BEGIN: &str = "??? >>> ";
+const DISPLAY_BLOCK_END: &str = "??? <<<";
+
+const IMPL_AREA_BEGIN: &str = "@@@ >>> ";
+const IMPL_AREA_END: &str = "@@@ <<<";
+
+const IMPL_BEGIN: &str = ">>>>>>>>>>";
+
+const PARAM_BEGIN: &str = "<<<";
+const PARAM_BEND: &str = ">>>";
+
+impl Template {
+    pub fn expand(mut self) -> Option<String> {
+        // Extract template text
+        let expanded = std::mem::take(&mut self.template_str);
+
+        let (expanded, impl_areas) = read_impl_areas(expanded)?;
+        let expanded = apply_impls(&self, expanded, impl_areas)?;
+        let expanded = apply_display_blocks(&self.params, expanded);
+        let expanded = apply_param(&self, expanded)?;
+        Some(expanded.trim().to_string())
+    }
+}
+
+/// Read all ImplAreas (HashMap<Name, Codes>)
+fn read_impl_areas(content: String) -> Option<(String, HashMap<String, String>)> {
+    let mut striped_content = String::new();
+    let mut impl_areas: HashMap<String, String> = HashMap::new();
+
+    let mut current_area_name = String::default();
+    let mut current_area_codes: Vec<String> = Vec::new();
+
+    for line in content.split("\n") {
+        let trimmed_line = line.trim();
+
+        // Implementation block end
+        if trimmed_line.starts_with(IMPL_AREA_END) {
+            // If the current ImplArea name length is less than 1, it means no block is being matched,
+            // so matching fails, exit early
+            if current_area_name.is_empty() {
+                return None;
+            }
+
+            // Submit Impl Area
+            let name = std::mem::take(&mut current_area_name);
+            impl_areas.insert(name, current_area_codes.join("\n"));
+            current_area_codes.clear();
+            continue;
+        }
+
+        // Implementation block start
+        if trimmed_line.starts_with(IMPL_AREA_BEGIN) {
+            // If the current ImplArea name length is greater than 0, it means we are already inside a block,
+            // since nesting is not allowed, matching fails, exit early
+            if !current_area_name.is_empty() {
+                return None;
+            }
+
+            // Get a snake_case name
+            let snake_name = snake_case!(line.trim_start_matches(IMPL_AREA_BEGIN).trim());
+            current_area_name = snake_name;
+
+            // Continue to next line
+            continue;
+        }
+
+        // During implementation block
+        if !current_area_name.is_empty() {
+            // Add to current block code
+            current_area_codes.push(line.to_string());
+            continue;
+        } else {
+            // Add to remaining content
+            striped_content += "\n";
+            striped_content += line;
+        }
+    }
+
+    Some((striped_content, impl_areas))
+}
+
+/// Apply Template parameters to implementation block areas
+fn apply_impls(
+    template: &Template,
+    content: String,
+    impl_areas: HashMap<String, String>,
+) -> Option<String> {
+    let mut applied_content = String::new();
+
+    let mut impled_areas: HashMap<String, Vec<String>> = HashMap::new();
+    for (impl_area_name, impl_area_template) in impl_areas {
+        // Get user-provided parameters
+        let impl_items = template.impl_params.get(&impl_area_name);
+
+        // No parameters, return early
+        let Some(impl_items) = impl_items else {
+            impled_areas.insert(impl_area_name, Vec::new());
+            continue;
+        };
+
+        let mut impled_area_code_applied = Vec::new();
+
+        // Split items
+        for item in impl_items {
+            // Get base template
+            let mut applied = impl_area_template.clone();
+
+            // Merge global params with arm-specific params for display block check
+            let mut display_params = template.params.clone();
+            for (k, v) in item {
+                display_params.insert(k.clone(), v.clone());
+            }
+            applied = apply_display_blocks(&display_params, applied);
+
+            // Extract parameters
+            for (param_name, param_value) in item {
+                // Apply parameter
+                applied = applied.replace(
+                    &format!("{}{}{}", PARAM_BEGIN, param_name, PARAM_BEND),
+                    param_value,
+                );
+            }
+
+            // Add applied template
+            impled_area_code_applied.push(applied);
+        }
+
+        impled_areas.insert(impl_area_name, impled_area_code_applied);
+    }
+
+    for line in content.split("\n") {
+        let trimmed_line = line.trim();
+
+        // Recognize implementation line
+        if trimmed_line.starts_with(IMPL_BEGIN) {
+            let impl_name = snake_case!(trimmed_line.trim_start_matches(IMPL_BEGIN).trim());
+
+            // Try to get implementation code block
+            let Some(impled_code) = impled_areas.get(&impl_name) else {
+                continue;
+            };
+
+            if !impled_code.is_empty() {
+                applied_content += "\n";
+                applied_content += impled_code.join("\n").as_str();
+            }
+        } else {
+            // Other content directly appended
+            applied_content += "\n";
+            applied_content += line;
+        }
+    }
+
+    Some(applied_content)
+}
+
+/// Process display blocks (`??? >>> name` / `??? <<<`).
+///
+/// If `params` contains a key matching the block name, the block content is
+/// included (with markers removed). Otherwise the entire block is omitted.
+fn apply_display_blocks(params: &HashMap<String, String>, content: String) -> String {
+    let mut result = String::new();
+    let lines: Vec<&str> = content.split("\n").collect();
+    let mut i = 0;
+    let mut first = true;
+
+    while i < lines.len() {
+        let line = lines[i];
+        let trimmed = line.trim();
+
+        if trimmed.starts_with(DISPLAY_BLOCK_BEGIN) {
+            let block_name = trimmed.trim_start_matches(DISPLAY_BLOCK_BEGIN).trim();
+            let show = params.contains_key(block_name);
+            i += 1;
+
+            while i < lines.len() && !lines[i].trim().starts_with(DISPLAY_BLOCK_END) {
+                if show {
+                    if !first {
+                        result += "\n";
+                    }
+                    result += lines[i];
+                    first = false;
+                }
+                i += 1;
+            }
+        } else if !trimmed.starts_with(DISPLAY_BLOCK_END) {
+            if !first {
+                result += "\n";
+            }
+            result += line;
+            first = false;
+        }
+
+        i += 1;
+    }
+
+    result
+}
+
+fn apply_param(template: &Template, content: String) -> Option<String> {
+    let mut content = content;
+    for (k, v) in template.params.iter() {
+        content = content.replace(&format!("{}{}{}", PARAM_BEGIN, k, PARAM_BEND), v);
+    }
+    Some(content)
+}
diff --git a/just_template/src/lib.rs b/just_template/src/lib.rs
new file mode 100644
index 0000000..7ff77f5
--- /dev/null
+++ b/just_template/src/lib.rs
@@ -0,0 +1,58 @@
+//! Template struct for storing template strings and their parameters.
+//!
+//! The template supports two types of parameters:
+//! - Simple parameters: key-value pairs used to replace simple placeholders (`<<<key>>>` format) in the template.
+//! - Implementation parameters: for implementation blocks (`>>>>>>>>> block_name` and `@@@ >>> block_name` format),
+//!   can contain multiple parameter sets, each corresponding to an implementation instance.
+//!
+//! # Examples
+//! ```
+//! use just_template::Template;
+//!
+//! let mut tmpl = Template::from("Hello, <<<name>>>!".to_string());
+//! tmpl.insert_param("name".to_string(), "World".to_string());
+//! assert_eq!(tmpl.to_string(), "Hello, World!");
+//! ```
+//!
+//! Using the `tmpl_param!` macro makes it easier to add simple parameters:
+//! ```
+//! use just_template::{Template, tmpl_param};
+//!
+//! let mut tmpl = Template::from("<<<a>>> + <<<b>>> = <<<c>>>".to_string());
+//! tmpl_param!(tmpl, a = 1, b = 2, c = 3);
+//! assert_eq!(tmpl.to_string(), "1 + 2 = 3");
+//! ```
+//!
+//! Using the `tmpl!` macro adds implementation block parameters:
+//! ```
+//! use just_template::{Template, tmpl};
+//!
+//! let mut tmpl = Template::from("
+//! >>>>>>>>>> arms
+//! @@@ >>> arms
+//!     <<<crate_name>>> => Some(<<<crate_name>>>::exec(data, params).await),
+//! @@@ <<<
+//! ".trim().to_string());
+//! tmpl!(tmpl,
+//!     arms {
+//!         crate_name = "my",
+//!         crate_name = "you",
+//!     }
+//! );
+//! // Output the expanded template
+//! let expanded = tmpl.to_string();
+//! assert_eq!(expanded, "
+//!     my => Some(my::exec(data, params).await),
+//!     you => Some(you::exec(data, params).await),
+//! ".trim().to_string());
+//! ```
+mod template;
+pub use template::*; // Re-export template to just_template
+
+pub mod expand;
+
+#[cfg(test)]
+pub mod test;
+
+#[deprecated]
+pub mod deprecated;
diff --git a/just_template/src/template.rs b/just_template/src/template.rs
new file mode 100644
index 0000000..e368917
--- /dev/null
+++ b/just_template/src/template.rs
@@ -0,0 +1,54 @@
+use std::collections::HashMap;
+
+#[derive(Default, Clone)]
+pub struct Template {
+    pub(crate) template_str: String,
+    pub(crate) params: HashMap<String, String>,
+    pub(crate) impl_params: HashMap<String, Vec<HashMap<String, String>>>,
+}
+
+impl Template {
+    /// Add a parameter
+    pub fn insert_param(&mut self, name: String, value: String) {
+        self.params.insert(name, value);
+    }
+
+    /// Add an implementation block and return a HashMap to set its parameters
+    pub fn add_impl(&mut self, impl_name: String) -> &mut Vec<HashMap<String, String>> {
+        self.impl_params.entry(impl_name).or_default()
+    }
+}
+
+impl From<String> for Template {
+    fn from(s: String) -> Self {
+        Template {
+            template_str: s,
+            ..Default::default()
+        }
+    }
+}
+
+impl<'a> From<&'a str> for Template {
+    fn from(s: &'a str) -> Self {
+        Template {
+            template_str: s.to_string(),
+            ..Default::default()
+        }
+    }
+}
+
+impl<'a> From<std::borrow::Cow<'a, str>> for Template {
+    fn from(s: std::borrow::Cow<'a, str>) -> Self {
+        Template {
+            template_str: s.into_owned(),
+            ..Default::default()
+        }
+    }
+}
+
+impl std::fmt::Display for Template {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let cloned = self.clone();
+        write!(f, "{}", cloned.expand().unwrap_or_default())
+    }
+}
diff --git a/just_template/src/test.rs b/just_template/src/test.rs
new file mode 100644
index 0000000..ca7b357
--- /dev/null
+++ b/just_template/src/test.rs
@@ -0,0 +1,187 @@
+use std::collections::HashMap;
+
+use crate::template::Template;
+
+#[test]
+fn basic_param() {
+    let mut tmpl = Template::from("Hello, <<<name>>>!".to_string());
+    tmpl.insert_param("name".to_string(), "World".to_string());
+    assert_eq!(tmpl.expand().unwrap(), "Hello, World!");
+}
+
+#[test]
+fn multi_param() {
+    let mut tmpl = Template::from("<<<a>>> + <<<b>>> = <<<c>>>".to_string());
+    tmpl.insert_param("a".to_string(), "1".to_string());
+    tmpl.insert_param("b".to_string(), "2".to_string());
+    tmpl.insert_param("c".to_string(), "3".to_string());
+    assert_eq!(tmpl.expand().unwrap(), "1 + 2 = 3");
+}
+
+#[test]
+fn impl_blocks() {
+    let mut tmpl = Template::from(
+        r#"
+>>>>>>>>>> arms
+@@@ >>> arms
+    "<<<crate_name>>>" => Some(<<<crate_name>>>::exec(data, params).await),
+@@@ <<<
+"#
+        .trim()
+        .to_string(),
+    );
+
+    let arms = tmpl.add_impl("arms".to_string());
+    arms.push(HashMap::from([(
+        "crate_name".to_string(),
+        "my".to_string(),
+    )]));
+    arms.push(HashMap::from([(
+        "crate_name".to_string(),
+        "you".to_string(),
+    )]));
+
+    let expanded = tmpl.expand().unwrap();
+    assert!(expanded.contains(r#""my" => Some(my::exec(data, params).await)"#));
+    assert!(expanded.contains(r#""you" => Some(you::exec(data, params).await)"#));
+}
+
+#[test]
+fn display_block_global_hidden_by_default() {
+    let tmpl = Template::from(
+        r#"
+visible line
+??? >>> debug
+    hidden line
+??? <<<
+visible end
+"#
+        .trim()
+        .to_string(),
+    );
+
+    let expanded = tmpl.expand().unwrap();
+    assert!(expanded.contains("visible line"));
+    assert!(expanded.contains("visible end"));
+    assert!(!expanded.contains("hidden line"));
+}
+
+#[test]
+fn display_block_global_shown_via_param() {
+    let mut tmpl = Template::from(
+        r#"
+visible line
+??? >>> debug
+    shown line
+??? <<<
+visible end
+"#
+        .trim()
+        .to_string(),
+    );
+
+    tmpl.insert_param("debug".to_string(), "".to_string());
+
+    let expanded = tmpl.expand().unwrap();
+    assert!(expanded.contains("visible line"));
+    assert!(expanded.contains("visible end"));
+    assert!(expanded.contains("shown line"));
+}
+
+#[test]
+fn display_block_inside_impl_area_hidden_by_default() {
+    let mut tmpl = Template::from(
+        r#"
+>>>>>>>>>> arms
+@@@ >>> arms
+    <<<crate_name>>> => exec,
+??? >>> extra
+    <<<crate_name>>> => metrics,
+??? <<<
+@@@ <<<
+"#
+        .trim()
+        .to_string(),
+    );
+
+    let arms = tmpl.add_impl("arms".to_string());
+    arms.push(HashMap::from([(
+        "crate_name".to_string(),
+        "my".to_string(),
+    )]));
+
+    let expanded = tmpl.expand().unwrap();
+    assert!(expanded.contains(r#"my => exec"#));
+    assert!(!expanded.contains(r#"my => metrics"#));
+}
+
+#[test]
+fn display_block_inside_impl_area_shown_by_global_param() {
+    let mut tmpl = Template::from(
+        r#"
+>>>>>>>>>> arms
+@@@ >>> arms
+    <<<crate_name>>> => exec,
+??? >>> extra
+    <<<crate_name>>> => metrics,
+??? <<<
+@@@ <<<
+"#
+        .trim()
+        .to_string(),
+    );
+
+    // Enable via global param — shows for ALL arms
+    tmpl.insert_param("extra".to_string(), "".to_string());
+
+    let arms = tmpl.add_impl("arms".to_string());
+    arms.push(HashMap::from([(
+        "crate_name".to_string(),
+        "my".to_string(),
+    )]));
+    arms.push(HashMap::from([(
+        "crate_name".to_string(),
+        "you".to_string(),
+    )]));
+
+    let expanded = tmpl.expand().unwrap();
+    assert!(expanded.contains(r#"my => exec"#));
+    assert!(expanded.contains(r#"my => metrics"#));
+    assert!(expanded.contains(r#"you => exec"#));
+    assert!(expanded.contains(r#"you => metrics"#));
+}
+
+#[test]
+fn display_block_inside_impl_area_shown_by_arm_param() {
+    let mut tmpl = Template::from(
+        r#"
+>>>>>>>>>> arms
+@@@ >>> arms
+    <<<crate_name>>> => exec,
+??? >>> extra
+    <<<crate_name>>> => metrics,
+??? <<<
+@@@ <<<
+"#
+        .trim()
+        .to_string(),
+    );
+
+    let arms = tmpl.add_impl("arms".to_string());
+    // Arm 1: no "extra" → hidden
+    arms.push(HashMap::from([(
+        "crate_name".to_string(),
+        "my".to_string(),
+    )]));
+    // Arm 2: has "extra" → shown for this arm only
+    arms.push(HashMap::from([
+        ("crate_name".to_string(), "you".to_string()),
+        ("extra".to_string(), "".to_string()),
+    ]));
+
+    let expanded = tmpl.expand().unwrap();
+    assert!(expanded.contains(r#"my => exec"#));
+    assert!(!expanded.contains(r#"my => metrics"#));
+    assert!(expanded.contains(r#"you => exec"#));
+    assert!(expanded.contains(r#"you => metrics"#));
+}