//! ZLS as a LIBRARY, not a language server.
//!
//! There is no subprocess, no JSON-RPC, no `initialize` handshake and no
//! `Server`. `gd` runs ZLS's own analyser on this thread and comes back with a
//! token; the whole "protocol" is a struct field. ZLS's build.zig already
//! publishes its guts as an importable module (`b.modules.put("zls", ...)`),
//! and everything under `features/` that is not welded to `*Server` is a plain
//! function over (analyser, arena, handle, offset) — so the parts we want are
//! callable and the parts we don't want cost nothing.
//!
//! What that buys and what it costs:
//!
//! - Cold IS warm. There is no server to start, no workspace to index, no
//!   handshake to wait for. The first `gd` after launch pays for parsing this
//!   file and whatever it imports, and nothing else.
//! - It also means NOTHING IS CACHED between queries. Every call builds a
//!   DocumentStore, resolves imports, and throws it all away, because `query`
//!   is handed an arena that dies on return and a `req` that is a snapshot of
//!   a buffer the user is still typing into. A cache would have to be a global
//!   with its own mutex and its own invalidation story; that is a real design,
//!   not a line of code, and it is the obvious next step rather than something
//!   smuggled in here.
//! - `zig_exe_path` is null on purpose: shelling out to the compiler is the
//!   thing this backend exists to avoid. `zig_lib_dir` is the build's own
//!   (`b.graph.zig_lib_directory`), which is what makes `std` resolvable —
//!   `gd` on `std.mem.count` lands in the real `mem.zig`.
//!
//!   The price is DEPENDENCIES, and it is worth being exact about how far it
//!   goes. ZLS resolves a non-`std`, non-relative import only through a
//!   `BuildFile` whose config came from running `zig build --build-runner`, so
//!   with no compiler it resolves none of them. We paper over ONE case of
//!   that: `gd` on the import string itself (`moduleRoot`, fed by build.zig).
//!   That is a fallback in OUR handler and ZLS cannot see it, so the analyser
//!   still cannot type `vaxis`, and `gd` on `vaxis.init` still finds nothing.
//!   Making member access work means giving ZLS a real build config — either
//!   by letting it run the build runner (a subprocess, and with no cross-query
//!   cache that is once PER KEYPRESS) or by synthesizing a BuildConfig and
//!   reaching into `BuildFile.impl`. Both are real work; neither is smuggled
//!   in here. `@import("builtin")` and `@cImport` are unresolved for the same
//!   reason.
const std = @import("std");
const zls = @import("zls");
const lsp = @import("lsp.zig");
const cfg = @import("pardes_config");

const Analyser = zls.Analyser;
const DocumentStore = zls.DocumentStore;
const InternPool = zls.analyser.InternPool;
const offsets = zls.offsets;
const Uri = zls.Uri;
const types = zls.lsp.types;
const Ast = std.zig.Ast;

/// BYTE offsets. ZLS defaults to utf-16 because that is what the wire protocol
/// says; we are not on a wire, and `+Search` rows are byte columns.
const enc: offsets.Encoding = .@"utf-8";

/// Guards on the workspace walks (workspace symbols / workspace diagnostics).
/// A language query runs on every keypress of `SPC S`; it may not turn into an
/// unbounded tree walk because someone opened a file in `/`.
const max_files = 512;
const max_rows = 2000;

pub const supports: std.EnumSet(lsp.Kind) = .initMany(&.{
    .definition,
    .declaration,
    .type_definition,
    .implementation,
    .references,
    .select_refs,
    .hover,
    .document_symbols,
    .workspace_symbols,
    .diagnostics,
    .workspace_diagnostics,
    .rename,
    .format,
    .code_action,
    .status,
    .explain,
});

// ------------------------------------------------------------ introspection
//
// Everything below exists because of the contract two lines down: this backend
// never fails loudly. That is right for an editor — a thrown analyser must not
// take the process with it — but it means a broken backend and a correct one
// that found nothing look identical from the outside. So every query leaves a
// record, and `SPC l i` reads them back.

/// One finished query. Fixed-size and inline: this array is process-global and
/// lives forever, so it must not own heap.
const LogEntry = struct {
    used: bool = false,
    kind: lsp.Kind = .definition,
    us: u64 = 0,
    rows: usize = 0,
    offset: u32 = 0,
    /// the error `run` returned, or empty when it returned normally. THIS is
    /// the thing you cannot see any other way.
    err: [40]u8 = @splat(0),
    err_len: u8 = 0,
    /// basename only — a full path would need most of this struct
    file: [64]u8 = @splat(0),
    file_len: u8 = 0,

    fn errName(e: *const LogEntry) []const u8 {
        return e.err[0..e.err_len];
    }
    fn fileName(e: *const LogEntry) []const u8 {
        return e.file[0..e.file_len];
    }
};

const log_cap = 24;
var log_buf: [log_cap]LogEntry = @splat(.{});
var log_next: usize = 0;
var log_total: u64 = 0;
/// 0.16 has no std.Thread.Mutex; the critical sections here are a memcpy, so
/// spinning on the lock-free one is enough — the same call gui.zig's pty queue
/// makes. Only WORKER threads touch this; the core never does.
var log_mu: std.atomic.Mutex = .unlocked;

fn logLock() void {
    while (!log_mu.tryLock()) std.atomic.spinLoopHint();
}

fn record(req: lsp.Req, us: u64, rows: usize, err: []const u8) void {
    logLock();
    defer log_mu.unlock();
    const e = &log_buf[log_next];
    e.* = .{ .used = true, .kind = req.kind, .us = us, .rows = rows, .offset = req.offset };
    const base = std.fs.path.basename(req.path);
    e.file_len = @intCast(@min(base.len, e.file.len));
    @memcpy(e.file[0..e.file_len], base[0..e.file_len]);
    e.err_len = @intCast(@min(err.len, e.err.len));
    @memcpy(e.err[0..e.err_len], err[0..e.err_len]);
    log_next = (log_next + 1) % log_cap;
    log_total += 1;
}

/// A duration cannot go in a snapshot golden — it differs every run. The
/// snapshot harness sets PARDES_LSP_NOTIME so the two introspection views
/// print `-` instead, the same way it pins PARDES_DUMP for the dump tests.
/// Nothing else reads it, and unset (i.e. every real run) shows real numbers.
fn hideTime() bool {
    const v = std.c.getenv("PARDES_LSP_NOTIME") orelse return false;
    return std.mem.span(v).len != 0;
}

fn nowUs() u64 {
    var ts: std.c.timespec = undefined;
    _ = std.c.clock_gettime(.MONOTONIC, &ts);
    return @as(u64, @intCast(ts.sec)) *| 1_000_000 +| @as(u64, @intCast(ts.nsec)) / 1000;
}

/// The narration `SPC l w` prints. It is threaded through the REAL resolution
/// path rather than re-derived beside it: a debug view that reimplements the
/// logic is a debug view that can disagree with it, and the whole point is to
/// find out what actually happened.
const Trace = struct {
    on: bool = false,
    arena: std.mem.Allocator,
    buf: std.ArrayList(u8) = .empty,

    fn note(t: *Trace, comptime fmt: []const u8, args: anytype) void {
        if (!t.on) return;
        t.buf.print(t.arena, fmt ++ "\n", args) catch {};
    }
};

/// The seam's contract: never fail, never panic, no rows is a legal answer.
/// Every error path in here — OOM, a cancelled io, a file that vanished —
/// collapses to "appended nothing", which the core already treats as "no
/// result". There is deliberately no error rendering.
pub fn query(gpa: std.mem.Allocator, arena: std.mem.Allocator, req: lsp.Req, out: *std.Io.Writer) void {
    // status reads the log; recording it would push a real query out of a
    // 24-entry ring every time you looked at it.
    if (req.kind == .status) return status(arena, req, out) catch {};

    // Rows land in a scratch buffer first, for two reasons a plain Writer
    // cannot serve: the log wants an exact row count, and `explain` throws the
    // rows away and prints the narration in their place — neither a count nor
    // a rewind exists on a Writer you do not own.
    var scratch: std.Io.Writer.Allocating = .init(gpa);
    defer scratch.deinit();

    var tr: Trace = .{ .arena = arena, .on = req.kind == .explain };
    const t0 = nowUs();
    var err_name: []const u8 = "";
    run(gpa, arena, req, &scratch.writer, &tr) catch |e| {
        err_name = @errorName(e);
        tr.note("ERROR: {s} — the query threw; the editor shows this as 'no result'", .{err_name});
    };
    const us = nowUs() -| t0;
    const rows = std.mem.count(u8, scratch.written(), "\n");
    record(req, us, rows, err_name);

    if (req.kind != .explain) {
        out.writeAll(scratch.written()) catch {};
        return;
    }
    // the question was never "where is it", it was "what did you do"
    out.print("lsp explain — the definition query at byte {d} of {s}\n\n", .{
        req.offset, if (req.path.len == 0) "(no file)" else std.fs.path.basename(req.path),
    }) catch {};
    out.writeAll(tr.buf.items) catch {};
    if (hideTime())
        out.print("\n{d} row(s)\n", .{rows}) catch {}
    else
        out.print("\n{d} row(s) in {d}us\n", .{ rows, us }) catch {};
}

fn run(gpa: std.mem.Allocator, arena: std.mem.Allocator, req: lsp.Req, out: *std.Io.Writer, tr: *Trace) !void {
    // The blocking, single-threaded std.Io — the same one look.zig walks
    // directories with. ZLS wants an `Io` for every file read; a worker thread
    // that is allowed to block wants the one that just does the syscall.
    const io = std.Io.Threaded.global_single_threaded.io();

    // The two workspace kinds are about the PROJECT, not about this buffer, so
    // they answer from any pane and never need the analyser, the store or the
    // intern pool — a walk and a parse each.
    // `explain` narrates the definition query; everywhere below dispatches on
    // the effective kind, so the trace follows the code `gd` really runs.
    const kind: lsp.Kind = if (req.kind == .explain) .definition else req.kind;

    switch (kind) {
        .workspace_symbols => return workspaceSymbols(gpa, arena, io, req, out),
        .workspace_diagnostics => return workspaceDiagnostics(gpa, arena, io, req, out),
        // Everything else is about THIS file, and this backend speaks Zig. The
        // core does not gate the keymap by file type (a pane is a pane), so
        // the gate is here: `gd` in a README must find nothing rather than
        // parse prose as Zig and confidently resolve a word out of it.
        else => if (!std.mem.endsWith(u8, req.path, ".zig")) {
            tr.note("STOP: not a .zig file ({s}) — this backend only reads Zig", .{
                if (req.path.len == 0) "no path: this pane has no file behind it" else req.path,
            });
            return;
        },
    }
    // Pure tree, no analyser.
    if (kind == .format) return formatQuery(arena, req, out);

    var env: std.process.Environ.Map = .init(arena);
    var lib_dir: ?std.Build.Cache.Directory = null;
    if (zigLibPath()) |p| {
        if (std.Io.Dir.cwd().openDir(io, p, .{})) |h| {
            lib_dir = .{ .handle = h, .path = p };
            tr.note("zig lib dir: {s} (open) — `std` imports resolve", .{p});
        } else |e| tr.note("zig lib dir: {s} FAILED to open ({s}) — nothing in `std` will resolve", .{ p, @errorName(e) });
    } else tr.note("zig lib dir: NOT SET — nothing in `std` will resolve", .{});
    defer if (lib_dir) |d| d.handle.close(io);

    var ip: InternPool = try .init(io, gpa);
    defer ip.deinit(gpa);

    var diags: zls.DiagnosticsCollection = .{ .io = io, .allocator = gpa };
    defer diags.deinit();

    var store: DocumentStore = .{
        .io = io,
        .allocator = gpa,
        .config = .{
            .environ_map = &env,
            // no compiler subprocess: that is the entire point of this backend
            .zig_exe_path = null,
            .zig_lib_dir = lib_dir,
            .build_runner_path = null,
            .builtin_path = null,
            .global_cache_dir = null,
            .wasi_preopens = {},
        },
        .diagnostics_collection = &diags,
    };
    defer store.deinit();

    // The buffer the user is looking at, not the file on disk: `source` is the
    // shell's snapshot, so unsaved edits are what gets analysed.
    const uri: Uri = try .fromPath(arena, req.path);
    try store.openLspSyncedDocument(uri, req.source);
    const handle = store.getHandle(uri) orelse {
        tr.note("STOP: the document store would not take this buffer", .{});
        return;
    };
    if (handle.tree.mode == .zon) {
        tr.note("STOP: parsed as .zon, not Zig code — there is nothing to resolve", .{});
        return;
    }
    tr.note("parsed {d} bytes, {d} syntax error(s){s}", .{
        req.source.len,
        handle.tree.errors.len,
        if (handle.tree.errors.len == 0) "" else " — a broken tree resolves badly or not at all",
    });

    var analyser: Analyser = .init(gpa, arena, &store, &ip, handle);
    defer analyser.deinit();

    const off: usize = @min(req.offset, req.source.len);

    switch (kind) {
        .definition, .implementation => try goto(arena, &analyser, handle, off, .definition, out, tr),
        .declaration => try goto(arena, &analyser, handle, off, .declaration, out, tr),
        .type_definition => try goto(arena, &analyser, handle, off, .type_definition, out, tr),
        .hover => try hover(arena, &analyser, handle, off, out),
        .document_symbols => try documentSymbols(gpa, arena, req.path, &handle.tree, out),
        .references, .select_refs => try references(arena, &analyser, handle, off, null, out),
        .rename => try references(arena, &analyser, handle, off, req.arg, out),
        .diagnostics => try diagnostics(gpa, arena, req.path, &handle.tree, out),
        .code_action => try codeActions(gpa, arena, &analyser, handle, off, out),
        else => {},
    }
}

/// `SPC l i`. What the backend IS: which ZLS, which stdlib, what it will and
/// will not answer, and the last two dozen queries with their timings and —
/// the point of the whole exercise — the errors that `query` swallowed.
fn status(arena: std.mem.Allocator, req: lsp.Req, out: *std.Io.Writer) !void {
    const io = std.Io.Threaded.global_single_threaded.io();
    try out.print("backend: {s}\nzls: {s} (compiled in — no server process, no JSON-RPC)\n", .{
        lsp.backend_name, cfg.zls_version,
    });

    // The single most common cause of "gd does nothing in std": say whether it
    // is actually openable, not just what it is set to.
    if (zigLibPath()) |p| {
        if (std.Io.Dir.cwd().openDir(io, p, .{})) |h| {
            var d = h;
            d.close(io);
            try out.print("zig lib dir: {s}  [OK]\n", .{p});
        } else |e| try out.print("zig lib dir: {s}  [CANNOT OPEN: {s}] — nothing in `std` will resolve\n", .{ p, @errorName(e) });
    } else try out.print("zig lib dir: NOT SET — nothing in `std` will resolve\n", .{});
    try out.print("offsets: {s}   walk caps: {d} files, {d} rows\n", .{ @tagName(enc), max_files, max_rows });
    // The dependency module map, which is the answer to "why does gd work on
    // std and not on @import(\"vaxis\")" — if a name is not on this list, the
    // build did not import it and nothing can resolve it.
    try out.print("\ndependency imports gd can follow ({d}):\n", .{cfg.module_names.len});
    for (cfg.module_names, cfg.module_roots) |n, r| {
        try out.print("  {s:<14} {s}\n", .{ n, r });
    }
    try out.print("asked from: {s}\n", .{if (req.path.len == 0) "a pane with no file" else req.path});

    try out.print("\nanswers:", .{});
    var it = supports.iterator();
    var n: usize = 0;
    while (it.next()) |k| : (n += 1) {
        if (k == .status or k == .explain) continue; // these two are the debugger, not a feature
        try out.print("{s}{s}", .{ if (n == 0) " " else ", ", @tagName(k) });
    }
    try out.print("\nrefuses: ", .{});
    n = 0;
    for (std.enums.values(lsp.Kind)) |k| {
        if (supports.contains(k)) continue;
        try out.print("{s}{s}", .{ if (n == 0) "" else ", ", @tagName(k) });
        n += 1;
    }
    if (n == 0) try out.print("(nothing)", .{});
    try out.print("\n", .{});

    // The log. Oldest first, so it reads like a transcript.
    logLock();
    defer log_mu.unlock();
    try out.print("\nlast queries ({d} total, keeping {d}):\n", .{ log_total, log_cap });
    var shown: usize = 0;
    for (0..log_cap) |i| {
        const e = &log_buf[(log_next + i) % log_cap];
        if (!e.used) continue;
        shown += 1;
        if (hideTime()) {
            try out.print("  {s:<22} {s:<20} @{d:<7} {d:>4} row(s){s}{s}\n", .{
                @tagName(e.kind),                        e.fileName(), e.offset, e.rows,
                if (e.err_len == 0) "" else "  ERROR: ", e.errName(),
            });
        } else {
            try out.print("  {s:<22} {s:<20} @{d:<7} {d:>7}us  {d:>4} row(s){s}{s}\n", .{
                @tagName(e.kind),                        e.fileName(), e.offset, e.us, e.rows,
                if (e.err_len == 0) "" else "  ERROR: ", e.errName(),
            });
        }
    }
    if (shown == 0) try out.print("  (none yet — press gd somewhere, then ask again)\n", .{});
    _ = arena;
}

/// The root source file of a module this build imports, or null.
///
/// ZLS answers `@import("std")` from `zig_lib_dir` and a relative `.zig` path
/// from the filesystem, but ANY OTHER name — every dependency in
/// build.zig.zon — it can only resolve by running `zig build --build-runner`
/// to discover the module graph. This backend sets `zig_exe_path = null` on
/// purpose, so that branch always returned nothing: `gd` worked perfectly into
/// `std` and silently did nothing on `@import("vaxis")`.
///
/// We never needed the compiler for this. build.zig IS the module graph, so it
/// folds its own `root_mod.import_table` into these two parallel arrays at
/// configure time. Correct by construction: a dependency added or renamed in
/// build.zig cannot forget to appear here.
///
/// LIMITS, in the order you will hit them:
///  1. This resolves the IMPORT STRING only. It is consulted from our own
///     `goto`, not from inside ZLS, so the analyser still cannot type the
///     `vaxis` const — `gd` on `vaxis.init` finds nothing. Fixing that means
///     giving ZLS a BuildConfig; see the note at the top of this file.
///  2. These are THIS build's imports. A file inside a dependency importing
///     that dependency's own internal module name is a miss — that would mean
///     running ITS build.zig.
///  3. A module whose root source is a GENERATED file is absent from the
///     table: it has no path until make() runs.
fn moduleRoot(name: []const u8) ?[]const u8 {
    comptime std.debug.assert(cfg.module_names.len == cfg.module_roots.len);
    for (cfg.module_names, cfg.module_roots) |n, r| {
        if (std.mem.eql(u8, n, name)) return r;
    }
    return null;
}

/// Why a field access came back empty. Worth its own function because ONE of
/// the reasons is a known, explainable hole rather than "no idea": if the left
/// side starts with a dependency module name, the analyser could not type it
/// because ZLS has no build config, and no amount of staring at the code will
/// reveal that.
fn explainFieldMiss(tr: *Trace, lhs: []const u8) void {
    var head: usize = 0;
    while (head < lhs.len and (std.ascii.isAlphanumeric(lhs[head]) or lhs[head] == '_')) head += 1;
    if (head != 0 and moduleRoot(lhs[0..head]) != null) {
        tr.note("      `{s}` IS a build.zig dependency, and this is the known hole: the module", .{lhs[0..head]});
        tr.note("      map is consulted by our @import handler, not by ZLS, so the analyser cannot", .{});
        tr.note("      type it. `gd` on the import string works; `gd` on a member does not.", .{});
        return;
    }
    tr.note("      (a value whose type needs comptime, an unresolved @import, or genuinely absent)", .{});
}

/// The zig lib directory, which is the one thing this backend cannot work out
/// for itself: it is baked in at build time from `b.graph.zig_lib_directory`,
/// i.e. the exact stdlib pardes was compiled against, and ZIG_LIB_DIR wins if
/// the user moved it. Without this, `gd` on anything in `std` finds nothing.
fn zigLibPath() ?[]const u8 {
    if (std.c.getenv("ZIG_LIB_DIR")) |p| {
        const s = std.mem.span(p);
        if (s.len != 0) return s;
    }
    if (cfg.zig_lib_dir.len != 0) return cfg.zig_lib_dir;
    return null;
}

// ---------------------------------------------------------------- rows

/// Byte offset -> line, without rescanning the file per row. Built once per
/// query over the file we emit many rows from (symbols, references,
/// diagnostics); a goto that emits two rows just scans.
const Lines = struct {
    starts: []const u32,
    src: []const u8,

    fn build(arena: std.mem.Allocator, src: []const u8) !Lines {
        var starts: std.ArrayList(u32) = .empty;
        try starts.append(arena, 0);
        for (src, 0..) |c, i| if (c == '\n') try starts.append(arena, @intCast(i + 1));
        return .{ .starts = starts.items, .src = src };
    }

    fn line(l: Lines, n: usize) []const u8 {
        if (n >= l.starts.len) return "";
        const a = l.starts[n];
        const b = if (n + 1 < l.starts.len) l.starts[n + 1] - 1 else l.src.len;
        return l.src[a..@min(b, l.src.len)];
    }
};

/// The source line at a 0-based line number, scanned. For the one-or-two-row
/// answers (a goto lands in a file we opened only to read one line out of).
fn lineAt(src: []const u8, want: usize) []const u8 {
    var it = std.mem.splitScalar(u8, src, '\n');
    var i: usize = 0;
    while (it.next()) |l| : (i += 1) if (i == want) return l;
    return "";
}

/// Emit one row for a token in whatever file it lives in — which for a goto
/// into `std` is not the file the cursor was in, so the path comes off the
/// handle's uri rather than off `req`.
fn rowForToken(arena: std.mem.Allocator, th: Analyser.TokenWithHandle, out: *std.Io.Writer) !void {
    const tree = &th.handle.tree;
    if (th.token >= tree.tokens.len) return;
    const r = offsets.tokenToRange(tree, th.token, enc);
    const path = th.handle.uri.toFsPath(arena) catch return;
    lsp.row(out, path, r.start.line, r.start.character, lineAt(tree.source, r.start.line));
}

// ---------------------------------------------------------------- goto

const GotoKind = enum { declaration, definition, type_definition };

/// `goto.zig:gotoHandler` minus the Server, minus the LSP types, minus the
/// linkSupport branch: dispatch on the position context, resolve to a decl,
/// resolve the decl to a token, print the token's line. The five helix gotos
/// collapse onto three ZLS kinds (`gi` behaves as `gd`, which is what ZLS's
/// own implementation handler does too).
fn goto(
    arena: std.mem.Allocator,
    analyser: *Analyser,
    handle: *DocumentStore.Handle,
    off: usize,
    kind: GotoKind,
    out: *std.Io.Writer,
    tr: *Trace,
) !void {
    const tree = &handle.tree;
    const ctx = try Analyser.getPositionContext(arena, tree, off, true);
    tr.note("position context: .{s}", .{@tagName(ctx)});

    var decls: std.ArrayList(Analyser.DeclWithHandle) = .empty;
    switch (ctx) {
        .var_access, .test_doctest_name => {
            const name_loc = offsets.identifierLocFromIndex(tree, off) orelse {
                tr.note("STOP: no identifier under the cursor", .{});
                return;
            };
            const name = offsets.locToSlice(tree.source, name_loc);
            tr.note("identifier: `{s}` — a plain name, so: innermost scope outward", .{name});
            if (std.mem.eql(u8, name, "_")) {
                tr.note("STOP: `_` is the discard, it declares nothing", .{});
                return;
            }
            if (std.zig.isPrimitive(name)) {
                tr.note("STOP: `{s}` is a builtin primitive — it has no declaration in any file", .{name});
                return;
            }
            const d = try analyser.lookupSymbolGlobal(handle, name, off) orelse {
                tr.note("STOP: lookupSymbolGlobal found no `{s}` in scope here", .{name});
                return;
            };
            try decls.append(arena, d);
        },
        .field_access => |loc| {
            const name_loc = offsets.identifierLocFromIndex(tree, off) orelse {
                tr.note("STOP: no identifier under the cursor", .{});
                return;
            };
            const name = offsets.locToSlice(tree.source, name_loc);
            const held = offsets.locMerge(loc, name_loc);
            tr.note("field access: `{s}` on `{s}` — needs the TYPE of the left side", .{
                name, offsets.locToSlice(tree.source, loc),
            });
            // NOTE both misses: a null result AND an empty one. ZLS returns an
            // empty slice when it typed the left side but found no such member,
            // and null when it could not type it at all — from the outside both
            // are "gd did nothing", so both have to be explained.
            const found = try analyser.getSymbolFieldAccesses(arena, handle, off, held, name) orelse {
                tr.note("STOP: could not resolve the left side to a type at all", .{});
                explainFieldMiss(tr, offsets.locToSlice(tree.source, loc));
                return;
            };
            if (found.len == 0) {
                tr.note("STOP: the left side resolved, but nothing named `{s}` was found on it", .{name});
                explainFieldMiss(tr, offsets.locToSlice(tree.source, loc));
                return;
            }
            try decls.appendSlice(arena, found);
        },
        .label_access, .label_decl => {
            const name_loc = offsets.identifierLocFromIndex(tree, off) orelse return;
            const name = offsets.locToSlice(tree.source, name_loc);
            const d = try Analyser.lookupLabel(handle, name, off) orelse return;
            try decls.append(arena, d);
        },
        .enum_literal => {
            const name_loc = offsets.identifierLocFromIndex(tree, off) orelse return;
            const name = offsets.locToSlice(tree.source, name_loc);
            const d = try analyser.getSymbolEnumLiteral(handle, off, name) orelse return;
            try decls.append(arena, d);
        },
        // `gd` on `@import("foo.zig")` opens foo.zig. Only relative imports
        // resolve — a package name needs the build graph we do not run.
        .import_string_literal, .embedfile_string_literal => {
            const loc = ctx.stringLiteralContentLoc(tree.source);
            if (loc.start == loc.end) return;
            const str = offsets.locToSlice(tree.source, loc);
            tr.note("import string: \"{s}\"", .{str});
            const res = try analyser.store.uriFromImportStr(arena, handle, str);
            switch (res) {
                .none => {
                    // ZLS gave up, which for a DEPENDENCY name is not a real
                    // dead end — it just means it wanted the module graph and
                    // we would not start a compiler to get one. Our build.zig
                    // knows the graph and baked it in; consult that.
                    if (moduleRoot(str)) |path| {
                        tr.note("`{s}` is a build.zig dependency; resolved from the compiled-in module map", .{str});
                        lsp.row(out, path, 0, 0, str);
                        return;
                    }
                    tr.note("STOP: `{s}` does not resolve to a file. Relative paths, `std` and this", .{str});
                    tr.note("      build's own dependencies work; `builtin`, `root` and a dependency's", .{});
                    tr.note("      OWN internal module names need the build graph we do not run.", .{});
                    return;
                },
                .one => |u| lsp.row(out, u.toFsPath(arena) catch return, 0, 0, str),
                .many => |us| for (us) |u| lsp.row(out, u.toFsPath(arena) catch continue, 0, 0, str),
            }
            return;
        },
        else => {
            tr.note("STOP: nothing to resolve in a .{s} context — `gd` only answers on", .{@tagName(ctx)});
            tr.note("      an identifier, a field access, a label, an enum literal or an @import string.", .{});
            return;
        },
    }

    tr.note("resolved to {d} declaration(s); asking each for its {s} token", .{ decls.items.len, @tagName(kind) });
    // One decl that fails to resolve must not swallow the others: a field
    // access can name several and a partial answer beats none.
    for (decls.items) |d| {
        switch (kind) {
            .declaration => try rowForToken(arena, d.definitionToken(analyser, false) catch continue, out),
            .definition => try rowForToken(arena, d.definitionToken(analyser, true) catch continue, out),
            .type_definition => {
                // peel error unions / pointers / optionals the way ZLS does,
                // then take the container's own name token
                if (try d.resolveType(analyser)) |ty| {
                    var r = ty;
                    while (true) {
                        r = try analyser.resolveUnwrapErrorUnionType(r, .payload) orelse
                            try analyser.resolveDerefType(r) orelse
                            try analyser.resolveOptionalUnwrap(r) orelse break;
                    }
                    if (r.typeDefinitionToken()) |th| {
                        try rowForToken(arena, th, out);
                        continue;
                    }
                }
                const nd = try d.typeDeclarationNode() orelse continue;
                const t2 = &nd.handle.tree;
                const rr = offsets.nodeToRange(t2, nd.node, enc);
                const path = nd.handle.uri.toFsPath(arena) catch continue;
                lsp.row(out, path, rr.start.line, rr.start.character, lineAt(t2.source, rr.start.line));
            },
        }
    }
}

// ---------------------------------------------------------------- hover

/// `hover.hover` is already Server-free, so this is a call and a reformat: the
/// `+Hover` buffer holds prose, not locations, so the markdown comes through
/// as lines and nothing arms the n/N stepper. Fenced-code markers are dropped
/// because a plain-text pane has nothing to do with them.
fn hover(
    arena: std.mem.Allocator,
    analyser: *Analyser,
    handle: *DocumentStore.Handle,
    off: usize,
    out: *std.Io.Writer,
) !void {
    const h = try zls.hover.hover(analyser, arena, handle, off, .markdown, enc) orelse return;
    const text = switch (h.contents) {
        .markup_content => |m| m.value,
        else => return,
    };
    var it = std.mem.splitScalar(u8, text, '\n');
    while (it.next()) |ln| {
        if (std.mem.startsWith(u8, ln, "```")) continue;
        out.print("{s}\n", .{std.mem.trimEnd(u8, ln, " \t\r")}) catch return;
    }
}

// ---------------------------------------------------------------- symbols

/// `document_symbol.getDocumentSymbols` needs an arena, a tree and an encoding
/// — no store, no analyser, no intern pool. That makes `SPC s` the cheapest
/// thing in this file: a parse and a walk.
fn documentSymbols(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    path: []const u8,
    tree: *const Ast,
    out: *std.Io.Writer,
) !void {
    const syms = try zls.document_symbol.getDocumentSymbols(arena, tree, enc);
    var n: usize = 0;
    try emitSymbols(gpa, arena, path, syms, "", &n, out);
}

/// Depth-first so the rows read in source order, with the parent's name as a
/// qualifier — `Kind.jumpsWhenSingle` rather than a bare `jumpsWhenSingle`,
/// because a flat list of method names is not navigable.
fn emitSymbols(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    path: []const u8,
    syms: []const types.DocumentSymbol,
    prefix: []const u8,
    n: *usize,
    out: *std.Io.Writer,
) !void {
    for (syms) |s| {
        if (n.* >= max_rows) return;
        n.* += 1;
        const name = if (prefix.len == 0)
            s.name
        else
            try std.fmt.allocPrint(arena, "{s}.{s}", .{ prefix, s.name });
        const text = if (s.detail) |d|
            try std.fmt.allocPrint(arena, "{s}  {s}", .{ name, d })
        else
            name;
        lsp.row(out, path, s.selectionRange.start.line, s.selectionRange.start.character, text);
        if (s.children) |kids| try emitSymbols(gpa, arena, path, kids, name, n, out);
    }
}

/// `SPC S`: every `.zig` under the pane's directory, parsed, walked, filtered
/// by substring. ZLS's own workspace symbols would only see files already in
/// the DocumentStore (which is one), so this walks the tree itself — and
/// because `getDocumentSymbols` needs nothing but a tree, it never has to open
/// a store or resolve an import to do it.
fn workspaceSymbols(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    io: std.Io,
    req: lsp.Req,
    out: *std.Io.Writer,
) !void {
    if (req.arg.len == 0) return;
    var files: std.ArrayList([]const u8) = .empty;
    try collectZigFiles(arena, io, req.root, &files);
    var n: usize = 0;
    for (files.items) |path| {
        if (n >= max_rows) return;
        const src = readFileZ(arena, io, path) catch continue;
        var tree: Ast = Ast.parse(arena, src, .zig) catch continue;
        defer tree.deinit(arena);
        const syms = zls.document_symbol.getDocumentSymbols(arena, &tree, enc) catch continue;
        try filterSymbols(gpa, arena, path, syms, "", req.arg, &n, out);
    }
}

fn filterSymbols(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    path: []const u8,
    syms: []const types.DocumentSymbol,
    prefix: []const u8,
    needle: []const u8,
    n: *usize,
    out: *std.Io.Writer,
) !void {
    for (syms) |s| {
        if (n.* >= max_rows) return;
        const name = if (prefix.len == 0)
            s.name
        else
            try std.fmt.allocPrint(arena, "{s}.{s}", .{ prefix, s.name });
        if (containsIgnoreCase(s.name, needle)) {
            n.* += 1;
            lsp.row(out, path, s.selectionRange.start.line, s.selectionRange.start.character, name);
        }
        if (s.children) |kids| try filterSymbols(gpa, arena, path, kids, name, needle, n, out);
    }
}

fn containsIgnoreCase(hay: []const u8, needle: []const u8) bool {
    if (needle.len == 0) return true;
    if (needle.len > hay.len) return false;
    var i: usize = 0;
    outer: while (i + needle.len <= hay.len) : (i += 1) {
        for (needle, 0..) |c, j|
            if (std.ascii.toLower(hay[i + j]) != std.ascii.toLower(c)) continue :outer;
        return true;
    }
    return false;
}

// ------------------------------------------------------------- references

/// `gr` / `SPC h` / `SPC r`. ZLS's `symbolReferences` is private and its
/// `referencesHandler` takes a `*Server`, so this is the same algorithm from
/// the outside: find the decl under the cursor, then offer every identifier
/// token in the file that spells the same name back to the analyser and keep
/// the ones that resolve to the same decl. Resolving rather than matching is
/// what makes a shadowed local not a reference to the outer one.
///
/// LIMIT: this file only. A workspace-wide answer means loading every project
/// file into the store and running the analyser over each — seconds, not
/// milliseconds, on every keypress, and the store's own workspace iteration
/// has the same restriction (it can only see handles that were loaded).
///
/// `new_name` non-null makes it a rename PREVIEW: the same rows, annotated
/// with the replacement. The seam returns rows, not edits, so `SPC r` shows
/// what would change and changes nothing — an honest half of rename.
fn references(
    arena: std.mem.Allocator,
    analyser: *Analyser,
    handle: *DocumentStore.Handle,
    off: usize,
    new_name: ?[]const u8,
    out: *std.Io.Writer,
) !void {
    const tree = &handle.tree;
    const target = try declAt(arena, analyser, handle, off) orelse return;
    // The decl's name token indexes ITS OWN file, which after an alias resolve
    // is not necessarily this one — and on a half-typed file it is not
    // necessarily an identifier either, which `identifierTokenToNameSlice`
    // asserts. Both of those are crashes on a keystroke, so both are checked.
    const decl_tree = &target.handle.tree;
    const name_tok = target.nameToken();
    if (name_tok >= decl_tree.tokens.len) return;
    if (decl_tree.tokenTag(name_tok) != .identifier) return;
    const want = offsets.identifierTokenToNameSlice(decl_tree, name_tok);
    if (want.len == 0) return;

    const lines: Lines = try .build(arena, tree.source);
    var n: usize = 0;
    for (0..tree.tokens.len) |i| {
        if (n >= max_rows) return;
        const tok: Ast.TokenIndex = @intCast(i);
        if (tree.tokenTag(tok) != .identifier) continue;
        if (!std.mem.eql(u8, offsets.identifierTokenToNameSlice(tree, tok), want)) continue;
        const at = tree.tokenStart(tok);
        const d = (declAt(arena, analyser, handle, at) catch continue) orelse continue;
        if (!d.eql(target)) continue;
        n += 1;
        const r = offsets.tokenToRange(tree, tok, enc);
        const text = if (new_name) |nn|
            try std.fmt.allocPrint(arena, "{s} -> {s}   {s}", .{ want, nn, std.mem.trim(u8, lines.line(r.start.line), " \t") })
        else
            lines.line(r.start.line);
        lsp.row(out, handle.uri.toFsPath(arena) catch return, r.start.line, r.start.character, text);
    }
}

/// The decl under a byte offset, whatever the surrounding syntax is. Shared by
/// references and by rename, which is references with a label on it.
fn declAt(
    arena: std.mem.Allocator,
    analyser: *Analyser,
    handle: *DocumentStore.Handle,
    off: usize,
) !?Analyser.DeclWithHandle {
    const tree = &handle.tree;
    const ctx = try Analyser.getPositionContext(arena, tree, off, true);
    const name_loc = offsets.identifierLocFromIndex(tree, off) orelse return null;
    const name = offsets.locToSlice(tree.source, name_loc);
    const d = switch (ctx) {
        .var_access, .test_doctest_name => try analyser.lookupSymbolGlobal(handle, name, off),
        .field_access => |loc| blk: {
            const found = try analyser.getSymbolFieldAccesses(arena, handle, off, offsets.locMerge(loc, name_loc), name) orelse break :blk null;
            break :blk if (found.len != 0) found[0] else null;
        },
        .label_access, .label_decl => try Analyser.lookupLabel(handle, name, off),
        .enum_literal => try analyser.getSymbolEnumLiteral(handle, off, name),
        else => null,
    } orelse return null;
    return try analyser.resolveVarDeclAlias(d) orelse d;
}

// ------------------------------------------------------------ diagnostics

/// `SPC d` and the list `]d`/`[d` step. ZLS's `getAstCheckDiagnostics` prefers
/// spawning `zig ast-check`; its OTHER branch runs `std.zig.AstGen` in this
/// process, which is the branch a subprocess-free backend wants, so that is
/// what is inlined here (it is ten lines and needs a `*Server` only for the
/// config it reads).
///
/// A clean file emits ONE row saying so. Silence already means "no backend" in
/// this seam — `lspResponse` opens nothing for zero rows — so a `SPC d` that
/// checked and found nothing has to be able to say the difference.
fn diagnostics(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    path: []const u8,
    tree: *const Ast,
    out: *std.Io.Writer,
) !void {
    const n = try treeDiagnostics(gpa, arena, path, tree, out);
    if (n == 0) lsp.row(out, path, 0, 0, "no diagnostics");
}

/// `zig ast-check`, in this process. ZLS spawns the compiler for this when it
/// has one; its fallback branch is `std.zig.AstGen` directly, and that fallback
/// is this backend's only branch. Caller owns the bundle.
fn astCheck(gpa: std.mem.Allocator, path: []const u8, tree: *const Ast) !std.zig.ErrorBundle {
    if (tree.errors.len != 0) return .empty;
    var zir = std.zig.AstGen.generate(gpa, tree.*) catch return .empty;
    defer zir.deinit(gpa);
    if (!zir.hasCompileErrors()) return .empty;
    var wip: std.zig.ErrorBundle.Wip = undefined;
    try wip.init(gpa);
    defer wip.deinit();
    wip.addZirErrorMessages(zir, tree.*, tree.source, path) catch return .empty;
    return wip.toOwnedBundle("") catch .empty;
}

/// Parse errors first (AstGen cannot run on a tree that did not parse), then
/// the AstGen pass. Returns how many rows were emitted.
fn treeDiagnostics(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    path: []const u8,
    tree: *const Ast,
    out: *std.Io.Writer,
) !usize {
    var n: usize = 0;
    if (tree.errors.len != 0) {
        const lines: Lines = try .build(arena, tree.source);
        for (tree.errors) |e| {
            if (n >= max_rows) return n;
            var buf: std.ArrayList(u8) = .empty;
            var w: std.Io.Writer.Allocating = .fromArrayList(arena, &buf);
            tree.renderError(e, &w.writer) catch continue;
            const at = tree.tokenStart(e.token);
            const lc = lsp.lineCol(tree.source, at);
            n += 1;
            lsp.row(out, path, lc.line, lc.col, try std.fmt.allocPrint(arena, "error: {s}   {s}", .{
                w.written(), std.mem.trim(u8, lines.line(lc.line), " \t"),
            }));
        }
        return n;
    }

    var bundle = try astCheck(gpa, path, tree);
    defer bundle.deinit(gpa);
    if (bundle.errorMessageCount() == 0) return n;
    for (bundle.getMessages()) |m| {
        if (n >= max_rows) return n;
        const em = bundle.getErrorMessage(m);
        if (em.src_loc == .none) continue;
        const sl = bundle.getSourceLocation(em.src_loc);
        n += 1;
        lsp.row(out, path, sl.line, sl.column, try std.fmt.allocPrint(arena, "error: {s}", .{
            bundle.nullTerminatedString(em.msg),
        }));
    }
    return n;
}

/// `SPC D`: the same check over every `.zig` under the pane's directory. No
/// analyser and no store — AstGen is a per-file pass — so this is bounded by
/// how fast Zig can parse the tree, which for pardes's own `src/` is tens of
/// milliseconds.
fn workspaceDiagnostics(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    io: std.Io,
    req: lsp.Req,
    out: *std.Io.Writer,
) !void {
    var files: std.ArrayList([]const u8) = .empty;
    try collectZigFiles(arena, io, req.root, &files);
    var total: usize = 0;
    for (files.items) |path| {
        const src = readFileZ(arena, io, path) catch continue;
        var tree: Ast = Ast.parse(arena, src, .zig) catch continue;
        defer tree.deinit(arena);
        total += treeDiagnostics(gpa, arena, path, &tree, out) catch continue;
        if (total >= max_rows) return;
    }
    if (total == 0) lsp.row(out, req.path, 0, 0, try std.fmt.allocPrint(arena, "no diagnostics in {d} file(s)", .{files.items.len}));
}

// ----------------------------------------------------------- code actions

/// `SPC a`. `code_actions.Builder` is one of the Server-free ones — it wants
/// an arena, an analyser, a handle and an ErrorBundle — so the only missing
/// piece is the bundle, which `astCheck` above produces without a compiler.
///
/// It lists what could be done, and does none of it: the seam hands back rows,
/// not edits, so there is no channel through which a backend could apply a
/// fix. `+Lsp` is prose, so these are lines rather than locations.
fn codeActions(
    gpa: std.mem.Allocator,
    arena: std.mem.Allocator,
    analyser: *Analyser,
    handle: *DocumentStore.Handle,
    off: usize,
    out: *std.Io.Writer,
) !void {
    const tree = &handle.tree;
    if (tree.errors.len != 0) {
        out.print("no code actions: file does not parse\n", .{}) catch {};
        return;
    }
    var bundle = try astCheck(gpa, "", tree);
    defer bundle.deinit(gpa);

    var builder: zls.code_actions.Builder = .{
        .arena = arena,
        .analyser = analyser,
        .handle = handle,
        .offset_encoding = enc,
        .only_kinds = null,
    };
    builder.generateCodeAction(bundle) catch {};
    const at = offsets.locToRange(tree.source, .{ .start = off, .end = off }, enc);
    builder.generateCodeActionsInRange(at) catch {};

    for (builder.actions.items) |a| {
        out.print("{s}\n", .{a.title}) catch return;
    }
    if (builder.actions.items.len == 0) out.print("no code actions\n", .{}) catch {};
}

// ---------------------------------------------------------------- format

/// `=`. The seam returns ROWS, not edits — there is no channel through which a
/// backend can rewrite the buffer — so format reports rather than formats: one
/// row per line `zig fmt` would change, in the `+Lsp` buffer. That is the
/// honest shape of "format" behind a read-only seam, and it is still the
/// answer to the question the keypress asks.
fn formatQuery(arena: std.mem.Allocator, req: lsp.Req, out: *std.Io.Writer) !void {
    var tree: Ast = try .parse(arena, req.source, .zig);
    defer tree.deinit(arena);
    if (tree.errors.len != 0) {
        lsp.row(out, req.path, 0, 0, "cannot format: file does not parse");
        return;
    }
    var buf: std.ArrayList(u8) = .empty;
    var w: std.Io.Writer.Allocating = .fromArrayList(arena, &buf);
    tree.render(arena, &w.writer, .{}) catch return;
    const formatted = w.written();
    if (std.mem.eql(u8, formatted, req.source)) {
        lsp.row(out, req.path, 0, 0, "already formatted");
        return;
    }
    var a = std.mem.splitScalar(u8, req.source, '\n');
    var b = std.mem.splitScalar(u8, formatted, '\n');
    var ln: usize = 0;
    var n: usize = 0;
    while (n < max_rows) : (ln += 1) {
        const x = a.next();
        const y = b.next();
        if (x == null and y == null) break;
        const xs = x orelse "";
        const ys = y orelse "";
        if (std.mem.eql(u8, xs, ys)) continue;
        n += 1;
        lsp.row(out, req.path, ln, 0, try std.fmt.allocPrint(arena, "- {s}   + {s}", .{
            std.mem.trim(u8, xs, " \t"), std.mem.trim(u8, ys, " \t"),
        }));
    }
}

// ------------------------------------------------------------------ files

/// Every `.zig` under `root`, skipping the directories a source walk has no
/// business in. Capped, because this runs on a keypress.
fn collectZigFiles(arena: std.mem.Allocator, io: std.Io, root: []const u8, out: *std.ArrayList([]const u8)) !void {
    if (root.len == 0) return;
    var dir = std.Io.Dir.cwd().openDir(io, root, .{ .iterate = true }) catch return;
    defer dir.close(io);
    var w = dir.walkSelectively(arena) catch return;
    defer w.deinit();
    while (out.items.len < max_files) {
        const e = (w.next(io) catch break) orelse break;
        switch (e.kind) {
            .directory => {
                if (e.basename.len != 0 and e.basename[0] == '.') continue;
                if (std.mem.eql(u8, e.basename, "zig-out")) continue;
                if (std.mem.eql(u8, e.basename, "zig-cache")) continue;
                w.enter(io, e) catch {};
            },
            .file => {
                if (!std.mem.endsWith(u8, e.basename, ".zig")) continue;
                const p = std.fs.path.join(arena, &.{ root, e.path }) catch continue;
                try out.append(arena, p);
            },
            else => {},
        }
    }
}

fn readFileZ(arena: std.mem.Allocator, io: std.Io, path: []const u8) ![:0]u8 {
    return std.Io.Dir.cwd().readFileAllocOptions(io, path, arena, .limited(4 * 1024 * 1024), .of(u8), 0);
}