use itertools::Itertools;
use rustpython_parser::ast::{Constant, Expr, ExprKind, Location, Stmt, StmtKind};

use ruff_diagnostics::{AlwaysAutofixableViolation, Violation};
use ruff_diagnostics::{Diagnostic, Fix};
use ruff_macros::{derive_message_formats, violation};
use ruff_python_ast::helpers::{elif_else_range, end_of_statement};
use ruff_python_ast::types::Range;
use ruff_python_ast::visitor::Visitor;
use ruff_python_ast::whitespace::indentation;

use crate::checkers::ast::Checker;
use crate::registry::{AsRule, Rule};

use super::branch::Branch;
use super::helpers::result_exists;
use super::visitor::{ReturnVisitor, Stack};

#[violation]
pub struct UnnecessaryReturnNone;

impl AlwaysAutofixableViolation for UnnecessaryReturnNone {
    #[derive_message_formats]
    fn message(&self) -> String {
        format!(
            "Do not explicitly `return None` in function if it is the only possible return value"
        )
    }

    fn autofix_title(&self) -> String {
        "Remove explicit `return None`".to_string()
    }
}

#[violation]
pub struct ImplicitReturnValue;

impl AlwaysAutofixableViolation for ImplicitReturnValue {
    #[derive_message_formats]
    fn message(&self) -> String {
        format!("Do not implicitly `return None` in function able to return non-`None` value")
    }

    fn autofix_title(&self) -> String {
        "Add explicit `None` return value".to_string()
    }
}

#[violation]
pub struct ImplicitReturn;

impl AlwaysAutofixableViolation for ImplicitReturn {
    #[derive_message_formats]
    fn message(&self) -> String {
        format!("Missing explicit `return` at the end of function able to return non-`None` value")
    }

    fn autofix_title(&self) -> String {
        "Add explicit `return` statement".to_string()
    }
}

#[violation]
pub struct UnnecessaryAssign;

impl Violation for UnnecessaryAssign {
    #[derive_message_formats]
    fn message(&self) -> String {
        format!("Unnecessary variable assignment before `return` statement")
    }
}

#[violation]
pub struct SuperfluousElseReturn {
    pub branch: Branch,
}

impl Violation for SuperfluousElseReturn {
    #[derive_message_formats]
    fn message(&self) -> String {
        let SuperfluousElseReturn { branch } = self;
        format!("Unnecessary `{branch}` after `return` statement")
    }
}

#[violation]
pub struct SuperfluousElseRaise {
    pub branch: Branch,
}

impl Violation for SuperfluousElseRaise {
    #[derive_message_formats]
    fn message(&self) -> String {
        let SuperfluousElseRaise { branch } = self;
        format!("Unnecessary `{branch}` after `raise` statement")
    }
}

#[violation]
pub struct SuperfluousElseContinue {
    pub branch: Branch,
}

impl Violation for SuperfluousElseContinue {
    #[derive_message_formats]
    fn message(&self) -> String {
        let SuperfluousElseContinue { branch } = self;
        format!("Unnecessary `{branch}` after `continue` statement")
    }
}

#[violation]
pub struct SuperfluousElseBreak {
    pub branch: Branch,
}

impl Violation for SuperfluousElseBreak {
    #[derive_message_formats]
    fn message(&self) -> String {
        let SuperfluousElseBreak { branch } = self;
        format!("Unnecessary `{branch}` after `break` statement")
    }
}

/// RET501
fn unnecessary_return_none(checker: &mut Checker, stack: &Stack) {
    for (stmt, expr) in &stack.returns {
        let Some(expr) = expr else {
            continue;
        };
        if !matches!(
            expr.node,
            ExprKind::Constant {
                value: Constant::None,
                ..
            }
        ) {
            continue;
        }
        let mut diagnostic = Diagnostic::new(UnnecessaryReturnNone, Range::from(*stmt));
        if checker.patch(diagnostic.kind.rule()) {
            diagnostic.amend(Fix::replacement(
                "return".to_string(),
                stmt.location,
                stmt.end_location.unwrap(),
            ));
        }
        checker.diagnostics.push(diagnostic);
    }
}

/// RET502
fn implicit_return_value(checker: &mut Checker, stack: &Stack) {
    for (stmt, expr) in &stack.returns {
        if expr.is_some() {
            continue;
        }
        let mut diagnostic = Diagnostic::new(ImplicitReturnValue, Range::from(*stmt));
        if checker.patch(diagnostic.kind.rule()) {
            diagnostic.amend(Fix::replacement(
                "return None".to_string(),
                stmt.location,
                stmt.end_location.unwrap(),
            ));
        }
        checker.diagnostics.push(diagnostic);
    }
}

const NORETURN_FUNCS: &[&[&str]] = &[
    // builtins
    &["", "exit"],
    &["", "quit"],
    // stdlib
    &["builtins", "exit"],
    &["builtins", "quit"],
    &["os", "_exit"],
    &["os", "abort"],
    &["posix", "_exit"],
    &["posix", "abort"],
    &["sys", "exit"],
    &["typing", "assert_never"],
    &["typing_extensions", "assert_never"],
    &["_thread", "exit"],
    &["_winapi", "ExitProcess"],
    // third-party modules
    &["pytest", "exit"],
    &["pytest", "fail"],
    &["pytest", "skip"],
    &["pytest", "xfail"],
];

/// Return `true` if the `func` is a known function that never returns.
fn is_noreturn_func(checker: &Checker, func: &Expr) -> bool {
    checker
        .ctx
        .resolve_call_path(func)
        .map_or(false, |call_path| {
            NORETURN_FUNCS
                .iter()
                .any(|target| call_path.as_slice() == *target)
        })
}

/// RET503
fn implicit_return(checker: &mut Checker, stmt: &Stmt) {
    match &stmt.node {
        StmtKind::If { body, orelse, .. } => {
            if let Some(last_stmt) = body.last() {
                implicit_return(checker, last_stmt);
            }
            if let Some(last_stmt) = orelse.last() {
                implicit_return(checker, last_stmt);
            } else {
                let mut diagnostic = Diagnostic::new(ImplicitReturn, Range::from(stmt));
                if checker.patch(diagnostic.kind.rule()) {
                    if let Some(indent) = indentation(checker.locator, stmt) {
                        let mut content = String::new();
                        content.push_str(checker.stylist.line_ending().as_str());
                        content.push_str(indent);
                        content.push_str("return None");
                        // This is the last statement in the function. So it has to be followed by
                        // a newline, or comments, or nothing.
                        diagnostic.amend(Fix::insertion(
                            content,
                            end_of_statement(stmt, checker.locator),
                        ));
                    }
                }
                checker.diagnostics.push(diagnostic);
            }
        }
        StmtKind::Assert { test, .. }
            if matches!(
                test.node,
                ExprKind::Constant {
                    value: Constant::Bool(false),
                    ..
                }
            ) => {}
        StmtKind::While { test, .. }
            if matches!(
                test.node,
                ExprKind::Constant {
                    value: Constant::Bool(true),
                    ..
                }
            ) => {}
        StmtKind::For { orelse, .. }
        | StmtKind::AsyncFor { orelse, .. }
        | StmtKind::While { orelse, .. } => {
            if let Some(last_stmt) = orelse.last() {
                implicit_return(checker, last_stmt);
            } else {
                let mut diagnostic = Diagnostic::new(ImplicitReturn, Range::from(stmt));
                if checker.patch(diagnostic.kind.rule()) {
                    if let Some(indent) = indentation(checker.locator, stmt) {
                        let mut content = String::new();
                        content.push_str(checker.stylist.line_ending().as_str());
                        content.push_str(indent);
                        content.push_str("return None");
                        diagnostic.amend(Fix::insertion(
                            content,
                            end_of_statement(stmt, checker.locator),
                        ));
                    }
                }
                checker.diagnostics.push(diagnostic);
            }
        }
        StmtKind::Match { cases, .. } => {
            for case in cases {
                if let Some(last_stmt) = case.body.last() {
                    implicit_return(checker, last_stmt);
                }
            }
        }
        StmtKind::With { body, .. } | StmtKind::AsyncWith { body, .. } => {
            if let Some(last_stmt) = body.last() {
                implicit_return(checker, last_stmt);
            }
        }
        StmtKind::Return { .. }
        | StmtKind::Raise { .. }
        | StmtKind::Try { .. }
        | StmtKind::TryStar { .. } => {}
        StmtKind::Expr { value, .. }
            if matches!(
                &value.node,
                ExprKind::Call { func, ..  }
                    if is_noreturn_func(checker, func)
            ) => {}
        _ => {
            let mut diagnostic = Diagnostic::new(ImplicitReturn, Range::from(stmt));
            if checker.patch(diagnostic.kind.rule()) {
                if let Some(indent) = indentation(checker.locator, stmt) {
                    let mut content = String::new();
                    content.push_str(checker.stylist.line_ending().as_str());
                    content.push_str(indent);
                    content.push_str("return None");
                    diagnostic.amend(Fix::insertion(
                        content,
                        end_of_statement(stmt, checker.locator),
                    ));
                }
            }
            checker.diagnostics.push(diagnostic);
        }
    }
}

fn has_multiple_assigns(id: &str, stack: &Stack) -> bool {
    if let Some(assigns) = stack.assigns.get(&id) {
        if assigns.len() > 1 {
            return true;
        }
    }
    false
}

fn has_refs_before_next_assign(id: &str, return_location: Location, stack: &Stack) -> bool {
    let mut before_assign: &Location = &Location::default();
    let mut after_assign: Option<&Location> = None;
    if let Some(assigns) = stack.assigns.get(&id) {
        for location in assigns.iter().sorted() {
            if location.row() > return_location.row() {
                after_assign = Some(location);
                break;
            }
            if location.row() <= return_location.row() {
                before_assign = location;
            }
        }
    }

    if let Some(refs) = stack.refs.get(&id) {
        for location in refs {
            if location.row() == return_location.row() {
                continue;
            }
            if let Some(after_assign) = after_assign {
                if before_assign.row() < location.row() && location.row() <= after_assign.row() {
                    return true;
                }
            } else if before_assign.row() < location.row() {
                return true;
            }
        }
    }
    false
}

fn has_refs_or_assigns_within_try_or_loop(id: &str, stack: &Stack) -> bool {
    if let Some(refs) = stack.refs.get(&id) {
        for location in refs {
            for (try_location, try_end_location) in &stack.tries {
                if try_location.row() < location.row() && location.row() <= try_end_location.row() {
                    return true;
                }
            }
            for (loop_location, loop_end_location) in &stack.loops {
                if loop_location.row() < location.row() && location.row() <= loop_end_location.row()
                {
                    return true;
                }
            }
        }
    }
    if let Some(refs) = stack.assigns.get(&id) {
        for location in refs {
            for (try_location, try_end_location) in &stack.tries {
                if try_location.row() < location.row() && location.row() <= try_end_location.row() {
                    return true;
                }
            }
            for (loop_location, loop_end_location) in &stack.loops {
                if loop_location.row() < location.row() && location.row() <= loop_end_location.row()
                {
                    return true;
                }
            }
        }
    }
    false
}

/// RET504
fn unnecessary_assign(checker: &mut Checker, stack: &Stack, expr: &Expr) {
    if let ExprKind::Name { id, .. } = &expr.node {
        if !stack.assigns.contains_key(id.as_str()) {
            return;
        }

        if !stack.refs.contains_key(id.as_str()) {
            checker
                .diagnostics
                .push(Diagnostic::new(UnnecessaryAssign, Range::from(expr)));
            return;
        }

        if has_multiple_assigns(id, stack)
            || has_refs_before_next_assign(id, expr.location, stack)
            || has_refs_or_assigns_within_try_or_loop(id, stack)
        {
            return;
        }

        if stack.non_locals.contains(id.as_str()) {
            return;
        }

        checker
            .diagnostics
            .push(Diagnostic::new(UnnecessaryAssign, Range::from(expr)));
    }
}

/// RET505, RET506, RET507, RET508
fn superfluous_else_node(checker: &mut Checker, stmt: &Stmt, branch: Branch) -> bool {
    let StmtKind::If { body, .. } = &stmt.node else {
        return false;
    };
    for child in body {
        if matches!(child.node, StmtKind::Return { .. }) {
            let diagnostic = Diagnostic::new(
                SuperfluousElseReturn { branch },
                elif_else_range(stmt, checker.locator).unwrap_or_else(|| Range::from(stmt)),
            );
            if checker.settings.rules.enabled(diagnostic.kind.rule()) {
                checker.diagnostics.push(diagnostic);
            }
            return true;
        }
        if matches!(child.node, StmtKind::Break) {
            let diagnostic = Diagnostic::new(
                SuperfluousElseBreak { branch },
                elif_else_range(stmt, checker.locator).unwrap_or_else(|| Range::from(stmt)),
            );
            if checker.settings.rules.enabled(diagnostic.kind.rule()) {
                checker.diagnostics.push(diagnostic);
            }
            return true;
        }
        if matches!(child.node, StmtKind::Raise { .. }) {
            let diagnostic = Diagnostic::new(
                SuperfluousElseRaise { branch },
                elif_else_range(stmt, checker.locator).unwrap_or_else(|| Range::from(stmt)),
            );
            if checker.settings.rules.enabled(diagnostic.kind.rule()) {
                checker.diagnostics.push(diagnostic);
            }
            return true;
        }
        if matches!(child.node, StmtKind::Continue) {
            let diagnostic = Diagnostic::new(
                SuperfluousElseContinue { branch },
                elif_else_range(stmt, checker.locator).unwrap_or_else(|| Range::from(stmt)),
            );
            if checker.settings.rules.enabled(diagnostic.kind.rule()) {
                checker.diagnostics.push(diagnostic);
            }
            return true;
        }
    }
    false
}

/// RET505, RET506, RET507, RET508
fn superfluous_elif(checker: &mut Checker, stack: &Stack) -> bool {
    for stmt in &stack.elifs {
        if superfluous_else_node(checker, stmt, Branch::Elif) {
            return true;
        }
    }
    false
}

/// RET505, RET506, RET507, RET508
fn superfluous_else(checker: &mut Checker, stack: &Stack) -> bool {
    for stmt in &stack.elses {
        if superfluous_else_node(checker, stmt, Branch::Else) {
            return true;
        }
    }
    false
}

/// Run all checks from the `flake8-return` plugin.
pub fn function(checker: &mut Checker, body: &[Stmt]) {
    // Skip empty functions.
    if body.is_empty() {
        return;
    }

    // Find the last statement in the function.
    let last_stmt = body.last().unwrap();

    // Skip functions that consist of a single return statement.
    if body.len() == 1 && matches!(last_stmt.node, StmtKind::Return { .. }) {
        return;
    }

    // Traverse the function body, to collect the stack.
    let stack = {
        let mut visitor = ReturnVisitor::default();
        for stmt in body {
            visitor.visit_stmt(stmt);
        }
        visitor.stack
    };

    // Avoid false positives for generators.
    if !stack.yields.is_empty() {
        return;
    }

    if checker.settings.rules.enabled(&Rule::SuperfluousElseReturn)
        || checker.settings.rules.enabled(&Rule::SuperfluousElseRaise)
        || checker
            .settings
            .rules
            .enabled(&Rule::SuperfluousElseContinue)
        || checker.settings.rules.enabled(&Rule::SuperfluousElseBreak)
    {
        if superfluous_elif(checker, &stack) {
            return;
        }
        if superfluous_else(checker, &stack) {
            return;
        }
    }

    // Skip any functions without return statements.
    if stack.returns.is_empty() {
        return;
    }

    if !result_exists(&stack.returns) {
        if checker.settings.rules.enabled(&Rule::UnnecessaryReturnNone) {
            unnecessary_return_none(checker, &stack);
        }
        return;
    }

    if checker.settings.rules.enabled(&Rule::ImplicitReturnValue) {
        implicit_return_value(checker, &stack);
    }
    if checker.settings.rules.enabled(&Rule::ImplicitReturn) {
        implicit_return(checker, last_stmt);
    }

    if checker.settings.rules.enabled(&Rule::UnnecessaryAssign) {
        for (_, expr) in &stack.returns {
            if let Some(expr) = expr {
                unnecessary_assign(checker, &stack, expr);
            }
        }
    }
}
