use ide_db::assists::{AssistId, AssistKind};
use stdx::to_lower_snake_case;
use syntax::{
    ast::{self, edit::IndentLevel, HasDocComments, HasName},
    AstNode,
};

use crate::assist_context::{AssistContext, Assists};

// Assist: generate_documentation_template
//
// Adds a documentation template above a function definition / declaration.
//
// ```
// fn my_$0func(a: i32, b: i32) -> Result<(), std::io::Error> {
//     unimplemented!()
// }
// ```
// ->
// ```
// /// .
// ///
// /// # Examples
// ///
// /// ```
// /// use test::my_func;
// ///
// /// assert_eq!(my_func(a, b), );
// /// ```
// ///
// /// # Errors
// ///
// /// This function will return an error if .
// fn my_func(a: i32, b: i32) -> Result<(), std::io::Error> {
//     unimplemented!()
// }
// ```
pub(crate) fn generate_documentation_template(
    acc: &mut Assists,
    ctx: &AssistContext,
) -> Option<()> {
    let name = ctx.find_node_at_offset::<ast::Name>()?;
    let ast_func = ast::Fn::cast(name.syntax().parent()?)?;
    if is_in_trait_impl(&ast_func) {
        return None;
    }
    // TODO disable at least examples if function not public, as the example will fail to build on
    // `cargo test`. What is the exact criteria of `pub`ness? All parent modules must be `pub`, for
    // `impl { fn }` both `fn` and `struct`* must be public.
    //
    // What about `pub(crate)`?
    //
    // *: Seems complex but maybe ignoring this criteria can be ignored.

    let parent_syntax = ast_func.syntax();
    let text_range = parent_syntax.text_range();
    let indent_level = IndentLevel::from_node(&parent_syntax);
    let krate_name =
        ctx.sema.scope(&parent_syntax).module()?.krate().display_name(ctx.db())?.to_string();

    acc.add(
        AssistId("generate_documentation_template", AssistKind::Generate),
        "Generate a documentation template",
        text_range,
        |builder| {
            let mut doc_lines = Vec::new();
            // Introduction / short function description before the sections
            doc_lines.push(introduction_builder(&ast_func));
            // Then come the sections
            if let Some(mut lines) = examples_builder(&ast_func, krate_name) {
                doc_lines.push("".into());
                doc_lines.append(&mut lines);
            }
            for section_builder in [panics_builder, errors_builder, safety_builder] {
                if let Some(mut lines) = section_builder(&ast_func) {
                    doc_lines.push("".into());
                    doc_lines.append(&mut lines);
                }
            }
            if ast_func.doc_comments().next().is_some() {
                doc_lines.push("--- OLD VERSION BELOW ---".into());
            }
            builder.insert(text_range.start(), documentation_from_lines(doc_lines, indent_level));
        },
    )
}

/// Builds an introduction, trying to be smart if the function is `::new()`
fn introduction_builder(ast_func: &ast::Fn) -> String {
    let is_new = ast_func.name().map(|name| &name.to_string() == "new").unwrap_or(false);
    if is_new {
        let ret_type = return_type(ast_func).map(|ret_type| ret_type.to_string());
        let self_type = self_type(ast_func);
        if ret_type.as_deref() == Some("Self") || ret_type == self_type {
            if let Some(self_type) = self_type {
                return format!("Creates a new [`{}`].", self_type);
            }
        }
    }
    ".".into()
}

/// Builds an `# Examples` section. An option is returned to be able to manage an error in the AST.
fn examples_builder(ast_func: &ast::Fn, krate_name: String) -> Option<Vec<String>> {
    let (no_panic_ex, panic_ex) = if is_in_trait_def(ast_func) {
        let message = "// Example template not implemented for trait functions";
        (Some(vec![message.into()]), Some(vec![message.into()]))
    } else {
        let panic_ex = match can_panic(ast_func) {
            Some(true) => gen_panic_ex_template(ast_func, krate_name.clone()),
            _ => None,
        };
        let no_panic_ex = gen_ex_template(ast_func, krate_name);
        (no_panic_ex, panic_ex)
    };

    let mut lines = string_vec_from(&["# Examples", "", "```"]);
    lines.append(&mut no_panic_ex?);
    lines.push("```".into());
    if let Some(mut ex) = panic_ex {
        lines.push("".into());
        lines.push("```should_panic".into());
        lines.append(&mut ex);
        lines.push("```".into());
    }
    Some(lines)
}

/// Builds an optional `# Panics` section
fn panics_builder(ast_func: &ast::Fn) -> Option<Vec<String>> {
    match can_panic(ast_func) {
        Some(true) => Some(string_vec_from(&["# Panics", "", "Panics if ."])),
        _ => None,
    }
}

/// Builds an optional `# Errors` section
fn errors_builder(ast_func: &ast::Fn) -> Option<Vec<String>> {
    match return_type(ast_func)?.to_string().contains("Result") {
        true => Some(string_vec_from(&["# Errors", "", "This function will return an error if ."])),
        false => None,
    }
}

/// Builds an optional `# Safety` section
fn safety_builder(ast_func: &ast::Fn) -> Option<Vec<String>> {
    let is_unsafe = ast_func.unsafe_token().is_some();
    match is_unsafe {
        true => Some(string_vec_from(&["# Safety", "", "."])),
        false => None,
    }
}

/// Generate an example template which should not panic
/// `None` if the function has a `self` parameter but is not in an `impl`.
fn gen_ex_template(ast_func: &ast::Fn, krate_name: String) -> Option<Vec<String>> {
    let (mut lines, ex_helper) = gen_ex_start_helper(ast_func, krate_name)?;
    // Call the function, check result
    if returns_a_value(ast_func) {
        if count_parameters(&ex_helper.param_list) < 3 {
            lines.push(format!("assert_eq!({}, );", ex_helper.function_call));
        } else {
            lines.push(format!("let result = {};", ex_helper.function_call));
            lines.push("assert_eq!(result, );".into());
        }
    } else {
        lines.push(format!("{};", ex_helper.function_call));
    }
    // Check the mutated values
    if is_ref_mut_self(ast_func) == Some(true) {
        lines.push(format!("assert_eq!({}, );", ex_helper.self_name?));
    }
    for param_name in &ex_helper.ref_mut_params {
        lines.push(format!("assert_eq!({}, );", param_name));
    }
    Some(lines)
}

/// Generate an example template which should panic
/// `None` if the function has a `self` parameter but is not in an `impl`.
fn gen_panic_ex_template(ast_func: &ast::Fn, krate_name: String) -> Option<Vec<String>> {
    let (mut lines, ex_helper) = gen_ex_start_helper(ast_func, krate_name)?;
    match returns_a_value(ast_func) {
        true => lines.push(format!("let _ = {}; // panics", ex_helper.function_call)),
        false => lines.push(format!("{}; // panics", ex_helper.function_call)),
    }
    Some(lines)
}

/// Intermediary results of the start of example generation
struct ExHelper {
    function_call: String,
    param_list: ast::ParamList,
    ref_mut_params: Vec<String>,
    self_name: Option<String>,
}

/// Build the start of the example and transmit the useful intermediary results.
/// `None` if the function has a `self` parameter but is not in an `impl`.
fn gen_ex_start_helper(ast_func: &ast::Fn, krate_name: String) -> Option<(Vec<String>, ExHelper)> {
    let mut lines = Vec::new();
    let is_unsafe = ast_func.unsafe_token().is_some();
    let param_list = ast_func.param_list()?;
    let ref_mut_params = ref_mut_params(&param_list);
    let self_name: Option<String> = self_name(ast_func);

    lines.push(format!("use {};", build_path(ast_func, krate_name)));
    lines.push("".into());
    if let Some(self_definition) = self_definition(ast_func, self_name.as_deref()) {
        lines.push(self_definition);
    }
    for param_name in &ref_mut_params {
        lines.push(format!("let mut {} = ;", param_name))
    }
    let function_call = function_call(ast_func, &param_list, self_name.as_deref(), is_unsafe)?;
    let ex_helper = ExHelper { function_call, param_list, ref_mut_params, self_name };
    Some((lines, ex_helper))
}

/// `None` if function without a body; some bool to guess if function can panic
fn can_panic(ast_func: &ast::Fn) -> Option<bool> {
    let body = ast_func.body()?.to_string();
    let can_panic = body.contains("panic!(")
        || body.contains("assert!(")
        || body.contains(".unwrap()")
        || body.contains(".expect(");
    Some(can_panic)
}

/// Helper function to get the name that should be given to `self` arguments
fn self_name(ast_func: &ast::Fn) -> Option<String> {
    self_partial_type(ast_func).map(|name| to_lower_snake_case(&name))
}

/// Heper function to get the name of the type of `self`
fn self_type(ast_func: &ast::Fn) -> Option<String> {
    ast_func
        .syntax()
        .ancestors()
        .find_map(ast::Impl::cast)
        .and_then(|i| i.self_ty())
        .map(|t| (t.to_string()))
}

/// Heper function to get the name of the type of `self` without generic arguments
fn self_partial_type(ast_func: &ast::Fn) -> Option<String> {
    let mut self_type = self_type(ast_func)?;
    if let Some(idx) = self_type.find(|c| ['<', ' '].contains(&c)) {
        self_type.truncate(idx);
    }
    Some(self_type)
}

/// Helper function to determine if the function is in a trait implementation
fn is_in_trait_impl(ast_func: &ast::Fn) -> bool {
    ast_func
        .syntax()
        .ancestors()
        .find_map(ast::Impl::cast)
        .and_then(|impl_| impl_.trait_())
        .is_some()
}

/// Helper function to determine if the function definition is in a trait definition
fn is_in_trait_def(ast_func: &ast::Fn) -> bool {
    ast_func.syntax().ancestors().find_map(ast::Trait::cast).is_some()
}

/// Returns `None` if no `self` at all, `Some(true)` if there is `&mut self` else `Some(false)`
fn is_ref_mut_self(ast_func: &ast::Fn) -> Option<bool> {
    let self_param = ast_func.param_list()?.self_param()?;
    Some(self_param.mut_token().is_some() && self_param.amp_token().is_some())
}

/// Helper function to define an variable to be the `self` argument
fn self_definition(ast_func: &ast::Fn, self_name: Option<&str>) -> Option<String> {
    let definition = match is_ref_mut_self(ast_func)? {
        true => format!("let mut {} = ;", self_name?),
        false => format!("let {} = ;", self_name?),
    };
    Some(definition)
}

/// Helper function to determine if a parameter is `&mut`
fn is_a_ref_mut_param(param: &ast::Param) -> bool {
    match param.ty() {
        Some(ast::Type::RefType(param_ref)) => param_ref.mut_token().is_some(),
        _ => false,
    }
}

/// Helper function to build the list of `&mut` parameters
fn ref_mut_params(param_list: &ast::ParamList) -> Vec<String> {
    param_list
        .params()
        .filter_map(|param| match is_a_ref_mut_param(&param) {
            // Maybe better filter the param name (to do this maybe extract a function from
            // `arguments_from_params`?) in case of a `mut a: &mut T`. Anyway managing most (not
            // all) cases might be enough, the goal is just to produce a template.
            true => Some(param.pat()?.to_string()),
            false => None,
        })
        .collect()
}

/// Helper function to build the comma-separated list of arguments of the function
fn arguments_from_params(param_list: &ast::ParamList) -> String {
    let args_iter = param_list.params().map(|param| match param.pat() {
        // To avoid `mut` in the function call (which would be a nonsense), `Pat` should not be
        // written as is so its variants must be managed independently. Other variants (for
        // instance `TuplePat`) could be managed later.
        Some(ast::Pat::IdentPat(ident_pat)) => match ident_pat.name() {
            Some(name) => match is_a_ref_mut_param(&param) {
                true => format!("&mut {}", name.to_string()),
                false => name.to_string(),
            },
            None => "_".to_string(),
        },
        _ => "_".to_string(),
    });
    intersperse_string(args_iter, ", ")
}

/// Helper function to build a function call. `None` if expected `self_name` was not provided
fn function_call(
    ast_func: &ast::Fn,
    param_list: &ast::ParamList,
    self_name: Option<&str>,
    is_unsafe: bool,
) -> Option<String> {
    let name = ast_func.name()?;
    let arguments = arguments_from_params(&param_list);
    let function_call = if param_list.self_param().is_some() {
        format!("{}.{}({})", self_name?, name, arguments)
    } else if let Some(implementation) = self_partial_type(ast_func) {
        format!("{}::{}({})", implementation, name, arguments)
    } else {
        format!("{}({})", name, arguments)
    };
    match is_unsafe {
        true => Some(format!("unsafe {{ {} }}", function_call)),
        false => Some(function_call),
    }
}

/// Helper function to count the parameters including `self`
fn count_parameters(param_list: &ast::ParamList) -> usize {
    param_list.params().count() + if param_list.self_param().is_some() { 1 } else { 0 }
}

/// Helper function to transform lines of documentation into a Rust code documentation
fn documentation_from_lines(doc_lines: Vec<String>, indent_level: IndentLevel) -> String {
    let mut result = String::new();
    for doc_line in doc_lines {
        result.push_str("///");
        if !doc_line.is_empty() {
            result.push(' ');
            result.push_str(&doc_line);
        }
        result.push('\n');
        result.push_str(&indent_level.to_string());
    }
    result
}

/// Helper function to transform an array of borrowed strings to an owned `Vec<String>`
fn string_vec_from(string_array: &[&str]) -> Vec<String> {
    string_array.iter().map(|&s| s.to_owned()).collect()
}

/// Helper function to build the path of the module in the which is the node
fn build_path(ast_func: &ast::Fn, krate_name: String) -> String {
    let mut path: Vec<String> = ast_func
        .syntax()
        .ancestors()
        .filter_map(|m| ast::Module::cast(m).and_then(|m| m.name()))
        .map(|m| m.to_string())
        .collect();
    path.push(krate_name);
    path.reverse();
    path.push(
        self_partial_type(ast_func)
            .or_else(|| ast_func.name().map(|n| n.to_string()))
            .unwrap_or_else(|| "*".into()),
    );
    intersperse_string(path.into_iter(), "::")
}

/// Helper function to get the return type of a function
fn return_type(ast_func: &ast::Fn) -> Option<ast::Type> {
    ast_func.ret_type()?.ty()
}

/// Helper function to determine if the function returns some data
fn returns_a_value(ast_func: &ast::Fn) -> bool {
    match return_type(ast_func) {
        Some(ret_type) => !["()", "!"].contains(&ret_type.to_string().as_str()),
        None => false,
    }
}

/// Helper function to concatenate string with a separator between them
fn intersperse_string(mut iter: impl Iterator<Item = String>, separator: &str) -> String {
    let mut result = String::new();
    if let Some(first) = iter.next() {
        result.push_str(&first);
    }
    for string in iter {
        result.push_str(separator);
        result.push_str(&string);
    }
    result
}

#[cfg(test)]
mod tests {
    use crate::tests::{check_assist, check_assist_not_applicable};

    use super::*;

    #[test]
    fn not_applicable_on_function_calls() {
        check_assist_not_applicable(
            generate_documentation_template,
            r#"
fn hello_world() {}
fn calls_hello_world() {
    hello_world$0();
}
"#,
        )
    }

    #[test]
    fn not_applicable_in_trait_impl() {
        check_assist_not_applicable(
            generate_documentation_template,
            r#"
trait MyTrait {}
struct MyStruct;
impl MyTrait for MyStruct {
    fn hello_world$0();
}
"#,
        )
    }

    #[test]
    fn supports_noop_function() {
        check_assist(
            generate_documentation_template,
            r#"
fn no$0op() {}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::noop;
///
/// noop();
/// ```
fn noop() {}
"#,
        );
    }

    #[test]
    fn supports_a_parameter() {
        check_assist(
            generate_documentation_template,
            r#"
fn no$0op_with_param(_a: i32) {}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::noop_with_param;
///
/// noop_with_param(_a);
/// ```
fn noop_with_param(_a: i32) {}
"#,
        );
    }

    #[test]
    fn detects_unsafe_function() {
        check_assist(
            generate_documentation_template,
            r#"
unsafe fn no$0op_unsafe() {}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::noop_unsafe;
///
/// unsafe { noop_unsafe() };
/// ```
///
/// # Safety
///
/// .
unsafe fn noop_unsafe() {}
"#,
        );
    }

    #[test]
    fn guesses_panic_macro_can_panic() {
        check_assist(
            generate_documentation_template,
            r#"
fn panic$0s_if(a: bool) {
    if a {
        panic!();
    }
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::panics_if;
///
/// panics_if(a);
/// ```
///
/// ```should_panic
/// use test::panics_if;
///
/// panics_if(a); // panics
/// ```
///
/// # Panics
///
/// Panics if .
fn panics_if(a: bool) {
    if a {
        panic!();
    }
}
"#,
        );
    }

    #[test]
    fn guesses_assert_macro_can_panic() {
        check_assist(
            generate_documentation_template,
            r#"
fn $0panics_if_not(a: bool) {
    assert!(a == true);
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::panics_if_not;
///
/// panics_if_not(a);
/// ```
///
/// ```should_panic
/// use test::panics_if_not;
///
/// panics_if_not(a); // panics
/// ```
///
/// # Panics
///
/// Panics if .
fn panics_if_not(a: bool) {
    assert!(a == true);
}
"#,
        );
    }

    #[test]
    fn guesses_unwrap_can_panic() {
        check_assist(
            generate_documentation_template,
            r#"
fn $0panics_if_none(a: Option<()>) {
    a.unwrap();
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::panics_if_none;
///
/// panics_if_none(a);
/// ```
///
/// ```should_panic
/// use test::panics_if_none;
///
/// panics_if_none(a); // panics
/// ```
///
/// # Panics
///
/// Panics if .
fn panics_if_none(a: Option<()>) {
    a.unwrap();
}
"#,
        );
    }

    #[test]
    fn guesses_expect_can_panic() {
        check_assist(
            generate_documentation_template,
            r#"
fn $0panics_if_none2(a: Option<()>) {
    a.expect("Bouh!");
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::panics_if_none2;
///
/// panics_if_none2(a);
/// ```
///
/// ```should_panic
/// use test::panics_if_none2;
///
/// panics_if_none2(a); // panics
/// ```
///
/// # Panics
///
/// Panics if .
fn panics_if_none2(a: Option<()>) {
    a.expect("Bouh!");
}
"#,
        );
    }

    #[test]
    fn checks_output_in_example() {
        check_assist(
            generate_documentation_template,
            r#"
fn returns_a_value$0() -> i32 {
    0
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::returns_a_value;
///
/// assert_eq!(returns_a_value(), );
/// ```
fn returns_a_value() -> i32 {
    0
}
"#,
        );
    }

    #[test]
    fn detects_result_output() {
        check_assist(
            generate_documentation_template,
            r#"
fn returns_a_result$0() -> Result<i32, std::io::Error> {
    Ok(0)
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::returns_a_result;
///
/// assert_eq!(returns_a_result(), );
/// ```
///
/// # Errors
///
/// This function will return an error if .
fn returns_a_result() -> Result<i32, std::io::Error> {
    Ok(0)
}
"#,
        );
    }

    #[test]
    fn checks_ref_mut_in_example() {
        check_assist(
            generate_documentation_template,
            r#"
fn modifies_a_value$0(a: &mut i32) {
    *a = 0;
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::modifies_a_value;
///
/// let mut a = ;
/// modifies_a_value(&mut a);
/// assert_eq!(a, );
/// ```
fn modifies_a_value(a: &mut i32) {
    *a = 0;
}
"#,
        );
    }

    #[test]
    fn stores_result_if_at_least_3_params() {
        check_assist(
            generate_documentation_template,
            r#"
fn sum3$0(a: i32, b: i32, c: i32) -> i32 {
    a + b + c
}
"#,
            r#"
/// .
///
/// # Examples
///
/// ```
/// use test::sum3;
///
/// let result = sum3(a, b, c);
/// assert_eq!(result, );
/// ```
fn sum3(a: i32, b: i32, c: i32) -> i32 {
    a + b + c
}
"#,
        );
    }

    #[test]
    fn supports_fn_in_mods() {
        check_assist(
            generate_documentation_template,
            r#"
mod a {
    mod b {
        fn no$0op() {}
    }
}
"#,
            r#"
mod a {
    mod b {
        /// .
        ///
        /// # Examples
        ///
        /// ```
        /// use test::a::b::noop;
        ///
        /// noop();
        /// ```
        fn noop() {}
    }
}
"#,
        );
    }

    #[test]
    fn supports_fn_in_impl() {
        check_assist(
            generate_documentation_template,
            r#"
struct MyStruct;
impl MyStruct {
    fn no$0op() {}
}
"#,
            r#"
struct MyStruct;
impl MyStruct {
    /// .
    ///
    /// # Examples
    ///
    /// ```
    /// use test::MyStruct;
    ///
    /// MyStruct::noop();
    /// ```
    fn noop() {}
}
"#,
        );
    }

    #[test]
    fn detects_new() {
        check_assist(
            generate_documentation_template,
            r#"
#[derive(Debug, PartialEq)]
pub struct MyGenericStruct<T> {
    pub x: T,
}
impl<T> MyGenericStruct<T> {
    pub fn new$0(x: T) -> MyGenericStruct<T> {
        MyGenericStruct { x }
    }
}
"#,
            r#"
#[derive(Debug, PartialEq)]
pub struct MyGenericStruct<T> {
    pub x: T,
}
impl<T> MyGenericStruct<T> {
    /// Creates a new [`MyGenericStruct<T>`].
    ///
    /// # Examples
    ///
    /// ```
    /// use test::MyGenericStruct;
    ///
    /// assert_eq!(MyGenericStruct::new(x), );
    /// ```
    pub fn new(x: T) -> MyGenericStruct<T> {
        MyGenericStruct { x }
    }
}
"#,
        );
    }

    #[test]
    fn detects_new_with_self() {
        check_assist(
            generate_documentation_template,
            r#"
#[derive(Debug, PartialEq)]
pub struct MyGenericStruct2<T> {
    pub x: T,
}
impl<T> MyGenericStruct2<T> {
    pub fn new$0(x: T) -> Self {
        MyGenericStruct2 { x }
    }
}
"#,
            r#"
#[derive(Debug, PartialEq)]
pub struct MyGenericStruct2<T> {
    pub x: T,
}
impl<T> MyGenericStruct2<T> {
    /// Creates a new [`MyGenericStruct2<T>`].
    ///
    /// # Examples
    ///
    /// ```
    /// use test::MyGenericStruct2;
    ///
    /// assert_eq!(MyGenericStruct2::new(x), );
    /// ```
    pub fn new(x: T) -> Self {
        MyGenericStruct2 { x }
    }
}
"#,
        );
    }

    #[test]
    fn supports_method_call() {
        check_assist(
            generate_documentation_template,
            r#"
impl<T> MyGenericStruct<T> {
    pub fn co$0nsume(self) {}
}
"#,
            r#"
impl<T> MyGenericStruct<T> {
    /// .
    ///
    /// # Examples
    ///
    /// ```
    /// use test::MyGenericStruct;
    ///
    /// let my_generic_struct = ;
    /// my_generic_struct.consume();
    /// ```
    pub fn consume(self) {}
}
"#,
        );
    }

    #[test]
    fn checks_modified_self_param() {
        check_assist(
            generate_documentation_template,
            r#"
impl<T> MyGenericStruct<T> {
    pub fn modi$0fy(&mut self, new_value: T) {
        self.x = new_value;
    }
}
"#,
            r#"
impl<T> MyGenericStruct<T> {
    /// .
    ///
    /// # Examples
    ///
    /// ```
    /// use test::MyGenericStruct;
    ///
    /// let mut my_generic_struct = ;
    /// my_generic_struct.modify(new_value);
    /// assert_eq!(my_generic_struct, );
    /// ```
    pub fn modify(&mut self, new_value: T) {
        self.x = new_value;
    }
}
"#,
        );
    }
}