use std::collections::{HashMap, HashSet};
use std::fs::read_to_string;
use std::path::{Path, PathBuf};

use annotate_snippets::{Renderer, Snippet};
use fluent_bundle::{FluentBundle, FluentError, FluentResource};
use fluent_syntax::ast::{
    Attribute, Entry, Expression, Identifier, InlineExpression, Message, Pattern, PatternElement,
};
use fluent_syntax::parser::ParserError;
use proc_macro::tracked_path::path;
use proc_macro::{Diagnostic, Level, Span};
use proc_macro2::TokenStream;
use quote::quote;
use syn::{Ident, LitStr, parse_macro_input};
use unic_langid::langid;

/// Helper function for returning an absolute path for macro-invocation relative file paths.
///
/// If the input is already absolute, then the input is returned. If the input is not absolute,
/// then it is appended to the directory containing the source file with this macro invocation.
fn invocation_relative_path_to_absolute(span: Span, path: &str) -> PathBuf {
    let path = Path::new(path);
    if path.is_absolute() {
        path.to_path_buf()
    } else {
        // `/a/b/c/foo/bar.rs` contains the current macro invocation
        let mut source_file_path = span.local_file().unwrap();
        // `/a/b/c/foo/`
        source_file_path.pop();
        // `/a/b/c/foo/../locales/en-US/example.ftl`
        source_file_path.push(path);
        source_file_path
    }
}

/// Final tokens.
fn finish(body: TokenStream, resource: TokenStream) -> proc_macro::TokenStream {
    quote! {
        /// Raw content of Fluent resource for this crate, generated by `fluent_messages` macro,
        /// imported by `rustc_driver` to include all crates' resources in one bundle.
        pub static DEFAULT_LOCALE_RESOURCE: &'static str = #resource;

        #[allow(non_upper_case_globals)]
        #[doc(hidden)]
        /// Auto-generated constants for type-checked references to Fluent messages.
        pub(crate) mod fluent_generated {
            #body

            /// Constants expected to exist by the diagnostic derive macros to use as default Fluent
            /// identifiers for different subdiagnostic kinds.
            pub mod _subdiag {
                /// Default for `#[help]`
                pub const help: rustc_errors::SubdiagMessage =
                    rustc_errors::SubdiagMessage::FluentAttr(std::borrow::Cow::Borrowed("help"));
                /// Default for `#[note]`
                pub const note: rustc_errors::SubdiagMessage =
                    rustc_errors::SubdiagMessage::FluentAttr(std::borrow::Cow::Borrowed("note"));
                /// Default for `#[warn]`
                pub const warn: rustc_errors::SubdiagMessage =
                    rustc_errors::SubdiagMessage::FluentAttr(std::borrow::Cow::Borrowed("warn"));
                /// Default for `#[label]`
                pub const label: rustc_errors::SubdiagMessage =
                    rustc_errors::SubdiagMessage::FluentAttr(std::borrow::Cow::Borrowed("label"));
                /// Default for `#[suggestion]`
                pub const suggestion: rustc_errors::SubdiagMessage =
                    rustc_errors::SubdiagMessage::FluentAttr(std::borrow::Cow::Borrowed("suggestion"));
            }
        }
    }
    .into()
}

/// Tokens to be returned when the macro cannot proceed.
fn failed(crate_name: &Ident) -> proc_macro::TokenStream {
    finish(quote! { pub mod #crate_name {} }, quote! { "" })
}

/// See [rustc_fluent_macro::fluent_messages].
pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let crate_name = std::env::var("CARGO_CRATE_NAME")
        // If `CARGO_CRATE_NAME` is missing, then we're probably running in a test, so use
        // `no_crate`.
        .unwrap_or_else(|_| "no_crate".to_string())
        .replace("rustc_", "");

    // Cannot iterate over individual messages in a bundle, so do that using the
    // `FluentResource` instead. Construct a bundle anyway to find out if there are conflicting
    // messages in the resources.
    let mut bundle = FluentBundle::new(vec![langid!("en-US")]);

    // Set of Fluent attribute names already output, to avoid duplicate type errors - any given
    // constant created for a given attribute is the same.
    let mut previous_attrs = HashSet::new();

    let resource_str = parse_macro_input!(input as LitStr);
    let resource_span = resource_str.span().unwrap();
    let relative_ftl_path = resource_str.value();
    let absolute_ftl_path = invocation_relative_path_to_absolute(resource_span, &relative_ftl_path);

    let crate_name = Ident::new(&crate_name, resource_str.span());

    path(absolute_ftl_path.to_str().unwrap());
    let resource_contents = match read_to_string(absolute_ftl_path) {
        Ok(resource_contents) => resource_contents,
        Err(e) => {
            Diagnostic::spanned(
                resource_span,
                Level::Error,
                format!("could not open Fluent resource: {e}"),
            )
            .emit();
            return failed(&crate_name);
        }
    };
    let mut bad = false;
    for esc in ["\\n", "\\\"", "\\'"] {
        for _ in resource_contents.matches(esc) {
            bad = true;
            Diagnostic::spanned(resource_span, Level::Error, format!("invalid escape `{esc}` in Fluent resource"))
                .note("Fluent does not interpret these escape sequences (<https://projectfluent.org/fluent/guide/special.html>)")
                .emit();
        }
    }
    if bad {
        return failed(&crate_name);
    }

    let resource = match FluentResource::try_new(resource_contents) {
        Ok(resource) => resource,
        Err((this, errs)) => {
            Diagnostic::spanned(resource_span, Level::Error, "could not parse Fluent resource")
                .help("see additional errors emitted")
                .emit();
            for ParserError { pos, slice: _, kind } in errs {
                let mut err = kind.to_string();
                // Entirely unnecessary string modification so that the error message starts
                // with a lowercase as rustc errors do.
                err.replace_range(0..1, &err.chars().next().unwrap().to_lowercase().to_string());

                let message = annotate_snippets::Level::Error.title(&err).snippet(
                    Snippet::source(this.source())
                        .origin(&relative_ftl_path)
                        .fold(true)
                        .annotation(annotate_snippets::Level::Error.span(pos.start..pos.end - 1)),
                );
                let renderer = Renderer::plain();
                eprintln!("{}\n", renderer.render(message));
            }

            return failed(&crate_name);
        }
    };

    let mut constants = TokenStream::new();
    let mut previous_defns = HashMap::new();
    let mut message_refs = Vec::new();
    for entry in resource.entries() {
        if let Entry::Message(msg) = entry {
            let Message { id: Identifier { name }, attributes, value, .. } = msg;
            let _ = previous_defns.entry(name.to_string()).or_insert(resource_span);
            if name.contains('-') {
                Diagnostic::spanned(
                    resource_span,
                    Level::Error,
                    format!("name `{name}` contains a '-' character"),
                )
                .help("replace any '-'s with '_'s")
                .emit();
            }

            if let Some(Pattern { elements }) = value {
                for elt in elements {
                    if let PatternElement::Placeable {
                        expression:
                            Expression::Inline(InlineExpression::MessageReference { id, .. }),
                    } = elt
                    {
                        message_refs.push((id.name, *name));
                    }
                }
            }

            // `typeck_foo_bar` => `foo_bar` (in `typeck.ftl`)
            // `const_eval_baz` => `baz` (in `const_eval.ftl`)
            // `const-eval-hyphen-having` => `hyphen_having` (in `const_eval.ftl`)
            // The last case we error about above, but we want to fall back gracefully
            // so that only the error is being emitted and not also one about the macro
            // failing.
            let crate_prefix = format!("{crate_name}_");

            let snake_name = name.replace('-', "_");
            if !snake_name.starts_with(&crate_prefix) {
                Diagnostic::spanned(
                    resource_span,
                    Level::Error,
                    format!("name `{name}` does not start with the crate name"),
                )
                .help(format!(
                    "prepend `{crate_prefix}` to the slug name: `{crate_prefix}{snake_name}`"
                ))
                .emit();
            };
            let snake_name = Ident::new(&snake_name, resource_str.span());

            if !previous_attrs.insert(snake_name.clone()) {
                continue;
            }

            let docstr =
                format!("Constant referring to Fluent message `{name}` from `{crate_name}`");
            constants.extend(quote! {
                #[doc = #docstr]
                pub const #snake_name: rustc_errors::DiagMessage =
                    rustc_errors::DiagMessage::FluentIdentifier(
                        std::borrow::Cow::Borrowed(#name),
                        None
                    );
            });

            for Attribute { id: Identifier { name: attr_name }, .. } in attributes {
                let snake_name = Ident::new(
                    &format!("{crate_prefix}{}", attr_name.replace('-', "_")),
                    resource_str.span(),
                );
                if !previous_attrs.insert(snake_name.clone()) {
                    continue;
                }

                if attr_name.contains('-') {
                    Diagnostic::spanned(
                        resource_span,
                        Level::Error,
                        format!("attribute `{attr_name}` contains a '-' character"),
                    )
                    .help("replace any '-'s with '_'s")
                    .emit();
                }

                let msg = format!(
                    "Constant referring to Fluent message `{name}.{attr_name}` from `{crate_name}`"
                );
                constants.extend(quote! {
                    #[doc = #msg]
                    pub const #snake_name: rustc_errors::SubdiagMessage =
                        rustc_errors::SubdiagMessage::FluentAttr(std::borrow::Cow::Borrowed(#attr_name));
                });
            }

            // Record variables referenced by these messages so we can produce
            // tests in the derive diagnostics to validate them.
            let ident = quote::format_ident!("{snake_name}_refs");
            let vrefs = variable_references(msg);
            constants.extend(quote! {
                #[cfg(test)]
                pub const #ident: &[&str] = &[#(#vrefs),*];
            })
        }
    }

    for (mref, name) in message_refs.into_iter() {
        if !previous_defns.contains_key(mref) {
            Diagnostic::spanned(
                resource_span,
                Level::Error,
                format!("referenced message `{mref}` does not exist (in message `{name}`)"),
            )
            .help(&format!("you may have meant to use a variable reference (`{{${mref}}}`)"))
            .emit();
        }
    }

    if let Err(errs) = bundle.add_resource(resource) {
        for e in errs {
            match e {
                FluentError::Overriding { kind, id } => {
                    Diagnostic::spanned(
                        resource_span,
                        Level::Error,
                        format!("overrides existing {kind}: `{id}`"),
                    )
                    .emit();
                }
                FluentError::ResolverError(_) | FluentError::ParserError(_) => unreachable!(),
            }
        }
    }

    finish(constants, quote! { include_str!(#relative_ftl_path) })
}

fn variable_references<'a>(msg: &Message<&'a str>) -> Vec<&'a str> {
    let mut refs = vec![];
    if let Some(Pattern { elements }) = &msg.value {
        for elt in elements {
            if let PatternElement::Placeable {
                expression: Expression::Inline(InlineExpression::VariableReference { id }),
            } = elt
            {
                refs.push(id.name);
            }
        }
    }
    for attr in &msg.attributes {
        for elt in &attr.value.elements {
            if let PatternElement::Placeable {
                expression: Expression::Inline(InlineExpression::VariableReference { id }),
            } = elt
            {
                refs.push(id.name);
            }
        }
    }
    refs
}