cpp_ls 0.1.3

//! # `textDocument/implementation` — the classes and functions that implement a declaration
//!
//! The protocol's "go to implementations" is a question about **overrides**: given a class, the classes deriving from
//! it; given a member function, the functions in those classes that redeclare it. C++ spells both with the same two
//! facts the analysis already stores — [`DeclFact::bases`] on the deriving class, and the member's own `scope` — so
//! neither needs a new index query.
//!
//! # What is answered, precisely
//!
//! ```text
//! a class          every class whose `bases` names it          → those classes' declarations
//! a member         every member of the same name whose scope
//!                  is one of those derived classes             → those members' declarations
//! ```
//!
//! A base is matched by its **last segment**: a declaration writes `Base` in one file and `ns::Base` in another, and
//! both name the same class. `scope` is matched the same way, for the same reason.
//!
//! # What is not answered, and why that is deliberate
//!
//! * **Nothing** for a name with no class or member behind it — a local, a namespace, a macro.
//!
//! "Implementations of a local" is not a question with a wrong answer, it is a question with no
//! answer, and `null` says so. **Nothing** from classes the index has not read yet: the scan is
//! over the summaries the session holds, and a partial index gives a partial list rather than a
//! guess.

use cpp_code_analysis::{FileProvider, Known, Session};
use lsp_types::{
    ClientCapabilities, ImplementationProviderCapability, Location, Range, ServerCapabilities,
    request::{GotoImplementationParams, GotoImplementationResponse},
};
use tokio_util::sync::CancellationToken;

use super::RegisterCapabilities;
use crate::context::{RequestOutcome, ServerContextSnapshot, snapshot_query};
use crate::util::{offset_at_position, path_to_uri, position_in_file, uri_to_file_path};

pub async fn on_goto_implementation(
    context: ServerContextSnapshot,
    params: GotoImplementationParams,
    cancel_token: CancellationToken,
) -> RequestOutcome<GotoImplementationResponse> {
    let uri = params.text_document_position_params.text_document.uri;
    let position = params.text_document_position_params.position;

    if let Some(path) = uri_to_file_path(&uri) {
        context.analysis().prepare(&path).await;
    }

    snapshot_query(context.analysis(), cancel_token, move |session| {
        let path = uri_to_file_path(&uri)?;
        // **The cursor is read from the file's own text, not from the rendering**, for the reason Go to Definition
        // states at the same line: a rendering has the directives resolved and their line breaks deleted, so a line and a
        // column from a client name a *different* place in it. Measured on this handler over a real project: the
        // position of `void UpdateCodeStyle(…)` in `codeformat.h` resolved one declaration away in the rendering,
        // `definitions` answered the class rather than the function, and the feature answered `null` — while the same
        // question asked of the written text answered `CodeFormat.cpp:18` first time
        // (`tests/implementation_real.rs`).
        let view = session.view_of_the_file(&path)?;
        let offset = offset_at_position(&view, position)?;

        // **The name comes from the declaration the cursor is on**, not from the text: the analysis already answered
        // "what does this name declare", and reading the identifier would be a second answer free to disagree.
        let Known::Yes(defined) = session.definitions(&view, offset) else {
            return None;
        };
        let name = defined.found.first()?.fact.name.clone();

        // **The body, then the overrides.** A cursor on a declaration is asking "where is the code"; a cursor on a
        // base class or a virtual member is asking "what implements this". Both are answered, the definition first
        // because it is the one a reader on a header declaration means.
        let mut found = definitions_of(session, &view, offset.get());
        found.extend(implementations_of(session, &name));
        let mut seen: Vec<(String, String)> = Vec::new();
        found.retain(|location| {
            let key = (location.uri.to_string(), format!("{:?}", location.range));
            if seen.contains(&key) {
                false
            } else {
                seen.push(key);
                true
            }
        });
        if found.is_empty() {
            return None;
        }
        Some(GotoImplementationResponse::Array(found))
    })
    .await
}

/// **The definitions of the declaration at the cursor** — `void Base::run() { … }` for `void run();` in a header.
///
/// This is the question a reader asks when the cursor is on a *declaration*: where is the code? C++ puts the
/// declaration in a header and the body in a `.cpp`, and until the index recorded out-of-line definitions it answered
/// nothing at all — the fact for `void CodeFormat::UpdateCodeStyle(…) { … }` did not exist (see `qualified_declarations`
/// in `sema::declarations` for why, and what it cost).
///
/// # Why this is a scan and not a `definitions` query
///
/// [`Session::definitions`] answers "what does this **name** resolve to **from here**", and a `.cpp` that defines a
/// header's function is not visible from the header — it includes the header, not the other way round. So the
/// direction `definitions` answers is the call site's, and the direction this answers is the declaration's, which is a
/// scan over the summaries for the same **qualified** name.
///
/// Only functions: a class's implementations are its derived classes, which [`implementations_of`] answers.
pub fn definitions_of<F: FileProvider + Clone>(
    session: &Session<F>,
    view: &cpp_code_analysis::FileView,
    offset: usize,
) -> Vec<Location> {
    let Known::Yes(found) = session.definitions(
        view,
        cpp_code_analysis::file::coordinates::ReadingOffset::new(offset),
    ) else {
        return Vec::new();
    };

    // **The class the cursor is in, not the fact's `scope`.** `definitions` answers with facts whose `scope` is
    // rewritten for the asking file (`qualified_name()` of the header's `run` came back as `run`, not `Base::run`), and
    // the *definition* in the `.cpp` carries `Base` — so a comparison on the fact's own scope would never match. The
    // cursor's enclosing class is the same answer both declarations were written under, and it is what Find
    // References and Go to Definition use for the same reason.
    let owner =
        crate::handlers::references::the_class_the_cursor_is_in(&view.scopes, &view.root, offset);
    let mut wanted: Vec<(String, Option<String>)> = found
        .found
        .iter()
        .filter(|definition| definition.fact.kind == cpp_code_analysis::DeclKind::Function)
        .map(|definition| (definition.fact.name.clone(), owner.clone()))
        .collect();
    wanted.sort();
    wanted.dedup();
    if wanted.is_empty() {
        return Vec::new();
    }

    let mut locations: Vec<Location> = Vec::new();
    let summaries = session.index().all_summaries();

    for (path, summary) in &summaries {
        for fact in &summary.declarations {
            if fact.kind != cpp_code_analysis::DeclKind::Function {
                continue;
            }
            if !wanted.iter().any(|(name, scope)| {
                fact.name == *name && fact.scope.as_deref() == scope.as_deref()
            }) {
                continue;
            }
            // **A definition is the one with a body.** The fact does not record whether it has one (a declaration and
            // its definition are the same seven fields), so the text is asked: a declaration ends with `;`, a
            // definition with `}`. One read per candidate file, and only for the name the cursor is on.
            if !ends_with_a_body(session, path, fact.range) {
                continue;
            }
            push_location(session, path, fact.name_range, &mut locations);
        }
    }
    locations
}

/// Does the text at `range` in `path` end with a `}` — that is, is this declaration a definition?
fn ends_with_a_body<F: FileProvider + Clone>(
    session: &Session<F>,
    path: &std::path::Path,
    range: cpp_parser::SourceRange,
) -> bool {
    let text = session
        .files()
        .held(path)
        .map(|held| held.text.to_string())
        .or_else(|| std::fs::read_to_string(path).ok());
    let Some(text) = text else {
        return false;
    };
    text.get(range.start_offset..range.end_offset())
        .is_some_and(|declaration| declaration.trim_end().ends_with('}'))
}
/// Every declaration that implements `name` — the derived classes, and their members of that name.
///
/// A free function so that a test can ask it without a client, and generic over the provider for the same reason
/// [`crate::handlers::references::locations`] is: the fixture that reproduces the question is a session, not a server.
pub fn implementations_of<F: FileProvider + Clone>(
    session: &Session<F>,
    name: &str,
) -> Vec<Location> {
    let summaries = session.index().all_summaries();

    // **The classes the name belongs to.** A class name belongs to no class (it *is* one), a member to its scope —
    // and the distinction is what makes one function answer both questions: derive from `name` itself for a class,
    // from the class that declares `name` for a member.
    let mut owners: Vec<String> = Vec::new();
    for (_, summary) in &summaries {
        for fact in &summary.declarations {
            if fact.name == name
                && let Some(scope) = fact.scope.as_deref()
            {
                owners.push(last_segment(scope).to_string());
            }
        }
    }

    let mut found: Vec<Location> = Vec::new();
    let mut derived: Vec<String> = Vec::new();
    for (path, summary) in &summaries {
        for fact in &summary.declarations {
            let derives_from_the_name = fact.bases.iter().any(|base| {
                let segment = last_segment(base);
                segment == name || owners.iter().any(|owner| owner == segment)
            });
            if derives_from_the_name {
                derived.push(fact.name.clone());
                push_location(session, path, fact.name_range, &mut found);
            }
        }
    }

    if derived.is_empty() {
        return found;
    }

    // …and the members of that name written in those classes: the overrides themselves.
    for (path, summary) in &summaries {
        for fact in &summary.declarations {
            if fact.name != name {
                continue;
            }
            let Some(scope) = fact.scope.as_deref() else {
                continue;
            };
            if derived.iter().any(|class| class == last_segment(scope)) {
                push_location(session, path, fact.name_range, &mut found);
            }
        }
    }

    found
}

/// The last `::`-separated segment of a spelling, for a caller in another handler — `std::vector` -> `vector`.
pub(crate) fn last_segment_of(spelling: &str) -> &str {
    last_segment(spelling)
}
/// The last `::`-separated segment of a spelling — `std::vector` -> `vector`.
fn last_segment(spelling: &str) -> &str {
    spelling.rsplit("::").next().unwrap_or(spelling).trim()
}

/// The declaration's position in its own file, or the top of that file when it cannot be read.
fn push_location<F: FileProvider + Clone>(
    session: &Session<F>,
    path: &std::path::Path,
    range: cpp_parser::SourceRange,
    found: &mut Vec<Location>,
) {
    let Some(uri) = path_to_uri(path) else {
        return;
    };
    let at_the_top = Location {
        uri: uri.clone(),
        range: Range::default(),
    };
    let position = session.files().held(path).and_then(|file| {
        let start = position_in_file(file, range.start_offset)?;
        let end = position_in_file(file, range.end_offset())?;
        Some(Range { start, end })
    });
    let location = match position {
        Some(range) => Location { uri, range },
        None => at_the_top,
    };
    if !found
        .iter()
        .any(|held| held.uri == location.uri && held.range == location.range)
    {
        found.push(location);
    }
}

pub struct ImplementationCapabilities;

impl RegisterCapabilities for ImplementationCapabilities {
    fn register_capabilities(server: &mut ServerCapabilities, _client: &ClientCapabilities) {
        server.implementation_provider = Some(ImplementationProviderCapability::Simple(true));
    }
}

#[cfg(test)]
mod tests {
    use std::path::PathBuf;

    use cpp_code_analysis::{
        CompilerConfig, MemoryFiles, OpenDocuments, Session, SessionFiles, WatchFilter,
    };

    use super::*;

    fn an_indexed_project() -> Session<MemoryFiles> {
        let files = MemoryFiles::new()
            .with_file("/p/base.h", "struct Base {\n    virtual void run();\n};\n")
            .with_file(
                "/p/derived.h",
                "struct Derived : public Base {\n    void run() override;\n};\n",
            );
        let providers = SessionFiles::new(OpenDocuments::new(), files);
        let mut session = Session::with_config(
            "/p",
            providers,
            WatchFilter::new("/p"),
            CompilerConfig::default(),
        );
        session.add_project_files([PathBuf::from("/p/base.h"), PathBuf::from("/p/derived.h")]);
        session.index_everything();
        session
    }

    /// **A class is implemented by the classes deriving from it, and a member by the members that override it.**
    ///
    /// The two questions share one function because they share one relation — the base of a class — but they find it
    /// from different ends: the class from its own name, the member from the name of the class that declares it. A
    /// version that looked for "classes deriving from `run`" would answer the first question only, and would answer it
    /// with nothing.
    #[test]
    fn a_derived_class_and_its_override_are_the_implementations() {
        let session = an_indexed_project();

        let classes = implementations_of(&session, "Base");
        assert_eq!(classes.len(), 1, "one class derives from Base: {classes:?}");
        assert!(
            classes[0].uri.path().ends_with("derived.h"),
            "{:?}",
            classes[0]
        );

        let overrides = implementations_of(&session, "run");
        assert!(
            !overrides.is_empty(),
            "the member `run` of the derived class implements the base's"
        );
        assert!(
            overrides
                .iter()
                .any(|found| found.uri.path().ends_with("derived.h")),
            "the implementation is in the derived class: {overrides:?}"
        );
    }
}
#[cfg(test)]
mod a_declaration_in_a_header {
    use std::path::PathBuf;

    use cpp_code_analysis::{
        CompilerConfig, MemoryFiles, OpenDocuments, Session, SessionFiles, WatchFilter,
    };

    use super::definitions_of;

    /// **A header's declaration answers the `.cpp` that defines it.**
    ///
    /// The layout every C++ project has, and the answer a reader means by "go to implementation" on a declaration:
    /// `void run();` in `base.h`, `void Base::run() { … }` in `base.cpp`. It answered **nothing** until the index
    /// recorded out-of-line definitions at all (`qualified_declarations` in `sema::declarations`).
    #[test]
    fn answers_the_body_in_the_source_file() {
        let files = MemoryFiles::new()
            .with_file("/p/base.h", "struct Base {\n    virtual void run();\n};\n")
            .with_file("/p/base.cpp", "#include \"base.h\"\nvoid Base::run() { }\n");
        let providers = SessionFiles::new(OpenDocuments::new(), files);
        let mut session = Session::with_config(
            "/p",
            providers,
            WatchFilter::new("/p"),
            CompilerConfig::default(),
        );
        session.add_project_files([PathBuf::from("/p/base.h"), PathBuf::from("/p/base.cpp")]);
        session.index_everything();

        let path = PathBuf::from("/p/base.h");
        let held = session.files().held(&path).expect("the header is held");
        let view =
            cpp_code_analysis::FileView::parse_with(held, &cpp_parser::MacroEnvironment::default());
        let at = "struct Base {\n    virtual void ".len();

        let found = definitions_of(&session, &view, at);
        println!(
            "definitions: {:?}",
            found
                .iter()
                .map(|location| location.uri.path().to_string())
                .collect::<Vec<_>>()
        );
        assert_eq!(found.len(), 1, "the body, and only it: {found:?}");
        assert!(
            found[0].uri.path().ends_with("base.cpp"),
            "the implementation is in the source file: {found:?}"
        );
    }
}