use glob::{glob, Pattern};
use proc_macro::TokenStream;
use proc_macro2::Span;
use quote::{format_ident, quote};
use std::borrow::Cow;
use std::collections::BTreeMap;
use std::env;
use std::path::{Component, PathBuf};
use syn::parse::{Parse, ParseStream};
use syn::punctuated::Punctuated;
use syn::spanned::Spanned;
use syn::{bracketed, parse_macro_input, parse_quote, Error, FnArg, ItemFn, LitStr, Pat, Token};

#[derive(Debug)]
struct FixtureConfiguration {
    pattern: Pattern,
    pattern_span: Span,
    exclude: Vec<Pattern>,
}

struct Arg {
    name: syn::Ident,
    value: ArgValue,
}

impl Parse for Arg {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let name = input.parse()?;
        let _equal_token: Token![=] = input.parse()?;
        let value = input.parse()?;

        Ok(Self { name, value })
    }
}

enum ArgValue {
    LitStr(LitStr),
    List(Punctuated<LitStr, Token![,]>),
}

impl Parse for ArgValue {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let value = if input.peek(syn::token::Bracket) {
            let inner;
            _ = bracketed!(inner in input);

            let values = inner.parse_terminated(
                |parser| {
                    let value: LitStr = parser.parse()?;
                    Ok(value)
                },
                Token![,],
            )?;
            ArgValue::List(values)
        } else {
            ArgValue::LitStr(input.parse()?)
        };

        Ok(value)
    }
}

impl Parse for FixtureConfiguration {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let args: Punctuated<_, Token![,]> = input.parse_terminated(Arg::parse, Token![,])?;

        let mut pattern = None;
        let mut exclude = None;

        for arg in args {
            match arg.name.to_string().as_str() {
                "pattern" => match arg.value {
                    ArgValue::LitStr(value) => {
                        pattern = Some(try_parse_pattern(&value)?);
                    }
                    ArgValue::List(list) => {
                        return Err(Error::new(
                            list.span(),
                            "The pattern must be a string literal",
                        ))
                    }
                },

                "exclude" => {
                    match arg.value {
                        ArgValue::LitStr(lit) => return Err(Error::new(
                            lit.span(),
                            "The exclude argument must be an array of globs: 'exclude=[\"a.py\"]",
                        )),
                        ArgValue::List(list) => {
                            let mut exclude_patterns = Vec::with_capacity(list.len());

                            for pattern in list {
                                let (pattern, _) = try_parse_pattern(&pattern)?;
                                exclude_patterns.push(pattern);
                            }

                            exclude = Some(exclude_patterns);
                        }
                    }
                }

                _ => {
                    return Err(Error::new(
                        arg.name.span(),
                        format!("Unknown argument {}.", arg.name),
                    ));
                }
            }
        }

        let exclude = exclude.unwrap_or_default();

        match pattern {
            None => Err(Error::new(
                input.span(),
                "'fixture' macro must have a pattern attribute",
            )),
            Some((pattern, pattern_span)) => Ok(Self {
                pattern,
                pattern_span,
                exclude,
            }),
        }
    }
}

fn try_parse_pattern(pattern_lit: &LitStr) -> syn::Result<(Pattern, Span)> {
    let raw_pattern = pattern_lit.value();
    match Pattern::new(&raw_pattern) {
        Ok(pattern) => Ok((pattern, pattern_lit.span())),
        Err(err) => Err(Error::new(
            pattern_lit.span(),
            format!("'{raw_pattern}' is not a valid glob pattern: '{}'", err.msg),
        )),
    }
}

/// Generates a test for each file that matches the specified pattern.
///
/// The attributed function must have exactly one argument of the type `&Path`.
/// The `#[test]` attribute must come after the `#[fixture]` argument or `test` will complain
/// that your function can not have any arguments.
///
/// ## Examples
///
/// Creates a test for every python file file in the `fixtures` directory.
///
/// ```ignore
/// #[fixture(pattern="fixtures/*.py")]
/// #[test]
/// fn my_test(path: &Path) -> std::io::Result<()> {
///     // ... implement the test
///     Ok(())
/// }
/// ```
///
/// ### Excluding Files
///
/// You can exclude files by specifying optional `exclude` patterns.
///
/// ```ignore
/// #[fixture(pattern="fixtures/*.py", exclude=["a_*.py"])]
/// #[test]
/// fn my_test(path: &Path) -> std::io::Result<()> {
///     // ... implement the test
///     Ok(())
/// }
/// ```
///
/// Creates tests for each python file in the `fixtures` directory except for files matching the `a_*.py` pattern.
#[proc_macro_attribute]
pub fn fixture(attribute: TokenStream, item: TokenStream) -> TokenStream {
    let test_fn = parse_macro_input!(item as ItemFn);
    let configuration = parse_macro_input!(attribute as FixtureConfiguration);

    let result = generate_fixtures(test_fn, &configuration);

    let stream = match result {
        Ok(output) => output,
        Err(err) => err.to_compile_error(),
    };

    TokenStream::from(stream)
}

fn generate_fixtures(
    mut test_fn: ItemFn,
    configuration: &FixtureConfiguration,
) -> syn::Result<proc_macro2::TokenStream> {
    // Remove the fixtures attribute
    test_fn
        .attrs
        .retain(|attr| !attr.path().is_ident("fixtures"));

    // Extract the name of the only argument of the test function.
    let last_arg = test_fn.sig.inputs.last();
    let path_ident = match (test_fn.sig.inputs.len(), last_arg) {
        (1, Some(last_arg)) => match last_arg {
            FnArg::Typed(typed) => match typed.pat.as_ref() {
                Pat::Ident(ident) => ident.ident.clone(),
                pat => {
                    return Err(Error::new(
                        pat.span(),
                        "#[fixture] function argument name must be an identifier",
                    ));
                }
            },
            FnArg::Receiver(receiver) => {
                return Err(Error::new(
                    receiver.span(),
                    "#[fixture] function argument name must be an identifier",
                ));
            }
        },
        _ => {
            return Err(Error::new(
                test_fn.sig.inputs.span(),
                "#[fixture] function must have exactly one argument with the type '&Path'",
            ));
        }
    };

    // Remove all arguments
    test_fn.sig.inputs.clear();

    let crate_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").expect(
        "#[fixture] requires CARGO_MANIFEST_DIR to be set during the build to resolve the relative paths to the test files.",
    ));

    let pattern = if configuration.pattern.as_str().starts_with('/') {
        Cow::from(configuration.pattern.as_str())
    } else {
        Cow::from(format!(
            "{}/{}",
            crate_dir
                .to_str()
                .expect("CARGO_MANIFEST_DIR must point to a directory with a UTF8 path"),
            configuration.pattern.as_str()
        ))
    };

    let files = glob(&pattern).expect("Pattern to be valid").flatten();
    let mut modules = Modules::default();

    for file in files {
        if configuration
            .exclude
            .iter()
            .any(|exclude| exclude.matches_path(&file))
        {
            continue;
        }

        let mut test_fn = test_fn.clone();

        let test_name = file
            .file_name()
            // SAFETY: Glob only matches on file names.
            .unwrap()
            .to_str()
            .expect("Expected path to be valid UTF8")
            .replace('.', "_");

        test_fn.sig.ident = format_ident!("{test_name}");

        let path = file.as_os_str().to_str().unwrap();

        test_fn.block.stmts.insert(
            0,
            parse_quote!(let #path_ident = std::path::Path::new(#path);),
        );

        modules.push_test(Test {
            path: file,
            test_fn,
        });
    }

    if modules.is_empty() {
        return Err(Error::new(
            configuration.pattern_span,
            "No file matches the specified glob pattern",
        ));
    }

    let root = find_highest_common_ancestor_module(&modules.root);

    root.generate(&test_fn.sig.ident.to_string())
}

fn find_highest_common_ancestor_module(module: &Module) -> &Module {
    let children = &module.children;

    if children.len() == 1 {
        let (_, child) = children.iter().next().unwrap();

        match child {
            Child::Module(common_child) => find_highest_common_ancestor_module(common_child),
            Child::Test(_) => module,
        }
    } else {
        module
    }
}

#[derive(Debug)]
struct Test {
    path: PathBuf,
    test_fn: ItemFn,
}

impl Test {
    fn generate(&self, _: &str) -> proc_macro2::TokenStream {
        let test_fn = &self.test_fn;
        quote!(#test_fn)
    }
}

#[derive(Debug, Default)]
struct Module {
    children: BTreeMap<String, Child>,
}

impl Module {
    fn generate(&self, name: &str) -> syn::Result<proc_macro2::TokenStream> {
        let mut inner = Vec::with_capacity(self.children.len());

        for (name, child) in &self.children {
            inner.push(child.generate(name)?);
        }

        let module_ident = format_ident!("{name}");

        Ok(quote!(
            mod #module_ident {
                use super::*;

                #(#inner)*
            }
        ))
    }
}

#[derive(Debug)]
enum Child {
    Module(Module),
    Test(Test),
}

impl Child {
    fn generate(&self, name: &str) -> syn::Result<proc_macro2::TokenStream> {
        match self {
            Child::Module(module) => module.generate(name),
            Child::Test(test) => Ok(test.generate(name)),
        }
    }
}

#[derive(Debug, Default)]
struct Modules {
    root: Module,
}

impl Modules {
    fn push_test(&mut self, test: Test) {
        let mut components = test
            .path
            .as_path()
            .components()
            .skip_while(|c| matches!(c, Component::RootDir))
            .peekable();

        let mut current = &mut self.root;
        while let Some(component) = components.next() {
            let name = component.as_os_str().to_str().unwrap();
            // A directory
            if components.peek().is_some() {
                let name = component.as_os_str().to_str().unwrap();
                let entry = current.children.entry(name.to_owned());

                match entry.or_insert_with(|| Child::Module(Module::default())) {
                    Child::Module(module) => {
                        current = module;
                    }
                    Child::Test(_) => {
                        unreachable!()
                    }
                }
            } else {
                // We reached the final component, insert the test
                drop(components);
                current.children.insert(name.to_owned(), Child::Test(test));
                break;
            }
        }
    }

    fn is_empty(&self) -> bool {
        self.root.children.is_empty()
    }
}