use std::{
    ffi::OsString,
    path::{Component, Path},
};

use rustc_ast::ast;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_lint::{EarlyContext, EarlyLintPass, Level, LintContext};
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::{FileName, RealFileName, SourceFile, Span, SyntaxContext};

declare_clippy_lint! {
    /// ### What it does
    /// Checks that module layout uses only self named module files, bans mod.rs files.
    ///
    /// ### Why is this bad?
    /// Having multiple module layout styles in a project can be confusing.
    ///
    /// ### Example
    /// ```text
    /// src/
    ///   stuff/
    ///     stuff_files.rs
    ///     mod.rs
    ///   lib.rs
    /// ```
    /// Use instead:
    /// ```text
    /// src/
    ///   stuff/
    ///     stuff_files.rs
    ///   stuff.rs
    ///   lib.rs
    /// ```
    pub MOD_MODULE_FILES,
    restriction,
    "checks that module layout is consistent"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks that module layout uses only mod.rs files.
    ///
    /// ### Why is this bad?
    /// Having multiple module layout styles in a project can be confusing.
    ///
    /// ### Example
    /// ```text
    /// src/
    ///   stuff/
    ///     stuff_files.rs
    ///   stuff.rs
    ///   lib.rs
    /// ```
    /// Use instead:
    /// ```text
    /// src/
    ///   stuff/
    ///     stuff_files.rs
    ///     mod.rs
    ///   lib.rs
    /// ```

    pub SELF_NAMED_MODULE_FILES,
    restriction,
    "checks that module layout is consistent"
}

pub struct ModStyle;

impl_lint_pass!(ModStyle => [MOD_MODULE_FILES, SELF_NAMED_MODULE_FILES]);

impl EarlyLintPass for ModStyle {
    fn check_crate(&mut self, cx: &EarlyContext<'_>, _: &ast::Crate) {
        if cx.builder.lint_level(MOD_MODULE_FILES).0 == Level::Allow
            && cx.builder.lint_level(SELF_NAMED_MODULE_FILES).0 == Level::Allow
        {
            return;
        }

        let files = cx.sess.source_map().files();

        let trim_to_src = if let RealFileName::LocalPath(p) = &cx.sess.opts.working_dir {
            p.to_string_lossy()
        } else {
            return;
        };

        // `folder_segments` is all unique folder path segments `path/to/foo.rs` gives
        // `[path, to]` but not foo
        let mut folder_segments = FxHashSet::default();
        // `mod_folders` is all the unique folder names that contain a mod.rs file
        let mut mod_folders = FxHashSet::default();
        // `file_map` maps file names to the full path including the file name
        // `{ foo => path/to/foo.rs, .. }
        let mut file_map = FxHashMap::default();
        for file in files.iter() {
            match &file.name {
                FileName::Real(RealFileName::LocalPath(lp))
                    if lp.to_string_lossy().starts_with(trim_to_src.as_ref()) =>
                {
                    let p = lp.to_string_lossy();
                    let path = Path::new(p.trim_start_matches(trim_to_src.as_ref()));
                    if let Some(stem) = path.file_stem() {
                        file_map.insert(stem.to_os_string(), (file, path.to_owned()));
                    }
                    process_paths_for_mod_files(path, &mut folder_segments, &mut mod_folders);
                    check_self_named_mod_exists(cx, path, file);
                }
                _ => {},
            }
        }

        for folder in &folder_segments {
            if !mod_folders.contains(folder) {
                if let Some((file, path)) = file_map.get(folder) {
                    let mut correct = path.clone();
                    correct.pop();
                    correct.push(folder);
                    correct.push("mod.rs");
                    cx.struct_span_lint(
                        SELF_NAMED_MODULE_FILES,
                        Span::new(file.start_pos, file.start_pos, SyntaxContext::root(), None),
                        |build| {
                            let mut lint =
                                build.build(&format!("`mod.rs` files are required, found `{}`", path.display()));
                            lint.help(&format!("move `{}` to `{}`", path.display(), correct.display(),));
                            lint.emit();
                        },
                    );
                }
            }
        }
    }
}

/// For each `path` we add each folder component to `folder_segments` and if the file name
/// is `mod.rs` we add it's parent folder to `mod_folders`.
fn process_paths_for_mod_files(
    path: &Path,
    folder_segments: &mut FxHashSet<OsString>,
    mod_folders: &mut FxHashSet<OsString>,
) {
    let mut comp = path.components().rev().peekable();
    let _ = comp.next();
    if path.ends_with("mod.rs") {
        mod_folders.insert(comp.peek().map(|c| c.as_os_str().to_owned()).unwrap_or_default());
    }
    let folders = comp
        .filter_map(|c| {
            if let Component::Normal(s) = c {
                Some(s.to_os_string())
            } else {
                None
            }
        })
        .collect::<Vec<_>>();
    folder_segments.extend(folders);
}

/// Checks every path for the presence of `mod.rs` files and emits the lint if found.
fn check_self_named_mod_exists(cx: &EarlyContext<'_>, path: &Path, file: &SourceFile) {
    if path.ends_with("mod.rs") {
        let mut mod_file = path.to_path_buf();
        mod_file.pop();
        mod_file.set_extension("rs");

        cx.struct_span_lint(
            MOD_MODULE_FILES,
            Span::new(file.start_pos, file.start_pos, SyntaxContext::root(), None),
            |build| {
                let mut lint = build.build(&format!("`mod.rs` files are not allowed, found `{}`", path.display()));
                lint.help(&format!("move `{}` to `{}`", path.display(), mod_file.display(),));
                lint.emit();
            },
        );
    }
}