//! The unified sx name/type resolver — the shared author-collection layer.
//!
//! A read-only facade over the borrowed Phase A import facts on a
//! `*ProgramIndex` (`module_decls` / `namespace_edges`) and the existing
//! `import_graph` / `flat_import_graph` views. It OWNS nothing import-derived;
//! those maps live in `imports.zig`/`core.zig` and are borrowed here.
//!
//! Two collectors sit on top of these facts (R5 §1 #1):
//!   - `collectVisibleAuthors` — own author ∪ the flat-import edge walk. THE one
//!     graph-walk; the permanent flat-import F-series root.
//!   - `collectNamespaceAuthors` — a single already-selected namespace target's
//!     members. NO graph walk.
//!
//! Both are RAW and verdict-free: they return who authors a name, not which
//! author wins. Per-domain selectors (Phase C+) decide eligibility. Nothing
//! routes resolution through these collectors yet.
//!
//! Falsifiable invariant (R5 §1 #1): there is EXACTLY ONE iterator over
//! `flat_import_graph`/`import_graph` in this file — inside
//! `collectVisibleAuthors`. `collectNamespaceAuthors` iterates one
//! `NamespaceTarget.own_decls` slice and touches no graph. This is what keeps
//! 0102 (callable) and 0105 (type) the SAME cross-module edge-walk.

const std = @import("std");
const ast = @import("../ast.zig");
const imports = @import("../imports.zig");
const program_index = @import("program_index.zig");
const ProgramIndex = program_index.ProgramIndex;

// ── Raw-fact aliases (defined in imports.zig by buildImportFacts, Phase A) ──
pub const RawDeclRef = imports.RawDeclRef;
pub const RawAuthor = imports.RawAuthor;
pub const NamespaceTarget = imports.NamespaceTarget;

/// Author multiplicity for ONE name as seen from ONE querying module: the
/// own-module author (tier-2) plus the distinct flat-import authors (tier-3),
/// diamond-deduped by author identity. RAW — no verdict, no domain, no pick.
pub const AuthorSet = struct {
    /// The author declared in the querying module itself, if any.
    own: ?RawAuthor,
    /// Distinct flat-import authors. Diamond imports of the SAME author (same
    /// AST node reached over two edges, e.g. a directory aggregate and one of
    /// its member files) collapse to a single entry. Always disjoint from `own`.
    flat: []const RawAuthor,

    /// own + flat, counted by author identity. `flat` is already deduped and
    /// disjoint from `own`, so this is a plain sum.
    pub fn distinctCount(self: AuthorSet) usize {
        return (if (self.own != null) @as(usize, 1) else 0) + self.flat.len;
    }
};

/// How a name's cross-module visibility is computed. The author collector and
/// the lowering-side visibility predicate (`Lowering.isVisible`) both switch on
/// this single vocabulary.
pub const VisibilityMode = enum {
    /// own scope ∪ `flat_import_graph`. The PERMANENT core for bare-name lookup
    /// under flat imports (Agra constraint) — never a transitional path.
    user_bare_flat,
    /// `user_bare_flat` plus the foreign-C gate (today's `isCImportVisible`):
    /// only C-import `fn_decl`s without a `library_ref` are policed; everything
    /// else is unconditionally visible.
    c_import_bare,
    /// own scope ∪ the TRANSITIVE import relation (specs.md:793-801). Owned by
    /// `ProtocolResolver.findVisibleImpls`; the single-hop author collector
    /// never serves it.
    impl_transitive,
    /// Registration / lazy lowering: falls open (visible), emits no user
    /// diagnostic, performs no graph walk.
    lowering_internal,
};

/// Read-only facade over the borrowed import facts. `alloc` backs the
/// `AuthorSet.flat` slices the collectors return (the caller owns + frees them).
pub const Resolver = struct {
    index: *ProgramIndex,
    alloc: std.mem.Allocator,

    pub fn init(index: *ProgramIndex, alloc: std.mem.Allocator) Resolver {
        return .{ .index = index, .alloc = alloc };
    }

    /// THE single graph-walk in this file (falsifiable invariant, R5 §1 #1):
    /// the own author declared in `from` ∪ the flat-import authors reachable
    /// over the edge set `vis` chooses. RAW — selectors decide eligibility, not
    /// this. `from` is the querying module's source path.
    ///
    /// Edge set by mode: `flat_import_graph` for `user_bare_flat`/
    /// `c_import_bare`. `impl_transitive` (a transitive closure owned by
    /// `findVisibleImpls`) and `lowering_internal` (no graph walk) are not
    /// single-hop author walks — reaching them here is a wiring bug, so we trip
    /// loudly.
    pub fn collectVisibleAuthors(
        self: *Resolver,
        name: []const u8,
        from: []const u8,
        vis: VisibilityMode,
    ) AuthorSet {
        const decls = self.index.module_decls orelse return .{ .own = null, .flat = &.{} };

        const own: ?RawAuthor = blk: {
            const mod = decls.get(from) orelse break :blk null;
            const ref = mod.names.get(name) orelse break :blk null;
            break :blk .{ .raw = ref, .source = mod.source };
        };

        const graph = (switch (vis) {
            .user_bare_flat, .c_import_bare => self.index.flat_import_graph,
            // findVisibleImpls owns transitive visibility; lowering_internal
            // performs no graph walk. Neither selects a single-hop edge set.
            .impl_transitive, .lowering_internal => @panic(
                "collectVisibleAuthors: vis mode performs no single-hop author walk",
            ),
        }) orelse return .{ .own = own, .flat = &.{} };

        const direct = graph.get(from) orelse return .{ .own = own, .flat = &.{} };

        var flat = std.ArrayList(RawAuthor).empty;
        var it = direct.iterator(); // ← the one graph iterator (invariant)
        while (it.next()) |kv| {
            const dep = decls.get(kv.key_ptr.*) orelse continue;
            const ref = dep.names.get(name) orelse continue;
            const cand = RawAuthor{ .raw = ref, .source = dep.source };
            if (sameAuthor(own, cand)) continue; // keep flat disjoint from own
            if (containsAuthor(flat.items, cand)) continue; // diamond dedup
            flat.append(self.alloc, cand) catch @panic("collectVisibleAuthors: OOM");
        }
        return .{
            .own = own,
            .flat = flat.toOwnedSlice(self.alloc) catch @panic("collectVisibleAuthors: OOM"),
        };
    }

    /// Container collector for ONE already-selected namespace target. Iterates
    /// the target's `own_decls` and touches NO import graph (R5 §1 #1). A
    /// namespace's `own_decls` is name-deduped, so a name has at most one author
    /// here — returned as `own`, sourced to the target's module path.
    pub fn collectNamespaceAuthors(
        self: *Resolver,
        target: NamespaceTarget,
        name: []const u8,
    ) AuthorSet {
        _ = self;
        for (target.own_decls) |decl| {
            const dn = decl.data.declName() orelse continue;
            if (!std.mem.eql(u8, dn, name)) continue;
            const ref = imports.rawDeclRefOf(decl) orelse continue;
            return .{ .own = .{ .raw = ref, .source = target.target_module_path }, .flat = &.{} };
        }
        return .{ .own = null, .flat = &.{} };
    }
};

/// Author identity is the AST node pointer the `RawDeclRef` wraps; every variant
/// holds a pointer, so a single `inline else` extracts it.
fn authorNodePtr(ref: RawDeclRef) usize {
    return switch (ref) {
        inline else => |p| @intFromPtr(p),
    };
}

fn sameAuthor(a: ?RawAuthor, b: RawAuthor) bool {
    const aa = a orelse return false;
    return authorNodePtr(aa.raw) == authorNodePtr(b.raw);
}

fn containsAuthor(list: []const RawAuthor, b: RawAuthor) bool {
    for (list) |x| {
        if (authorNodePtr(x.raw) == authorNodePtr(b.raw)) return true;
    }
    return false;
}

// ── The owning resolution pass (Fork C S2.1a) ───────────────────────────────
//
// `resolve` turns this module from a raw author-collection FACADE into the
// OWNING pass: ONE exhaustive recursive walk of the resolved AST that populates a
// `ResolvedProgram` — node-keyed side tables binding each user spelling to its RAW
// author identity. ADDITIVE / PARALLEL / UNCONSUMED: lowering still reads the old
// selectors, so this changes no generated byte. The walk switches over EVERY
// `ast.Node.Data` kind with NO `else` arm, so a newly added node kind is a compile
// error here rather than a silently unvisited subtree (it structurally cannot be
// "half-populated").
//
// S2.1a populates the three BARE-NAME domains (type / value-const / callable) via
// `collectVisibleAuthors`, and records generic-param references ($T, ..$Ts,
// $pack[i]) SYMBOLICALLY (template/pack ids, never TypeIds). S2.1b adds the
// namespace-qualified table (`alias.member` resolved via `collectNamespaceAuthors`)
// and the three HEAD tables (generic-struct / type-fn / protocol), binned by the
// resolved author's decl kind at `parameterized_type_expr` heads. S2.1c closes the
// set with the final three: a bare reference whose author is a `foreign_class_decl`
// is routed to `foreign_class_refs`; a `Type.CONST` field access whose base author
// is a struct carrying that const member fills `struct_const_refs`; and a UFCS
// alias (`alias :: ufcs target`) plus its rewrite call sites fill `ufcs_refs`. All
// ten domains are now populated — still PARALLEL / UNCONSUMED / RAW.

/// A symbolic id for one enclosing generic TYPE/VALUE param (`$T`, `$N`), assigned
/// by the pass and indexing `ResolvedProgram.template_params`. Process-local.
pub const TemplateParamId = enum(u32) { _ };

/// A symbolic id for one enclosing type pack (`..$Ts`, referenced as `$Ts` /
/// `$Ts[i]`), assigned by the pass and indexing `ResolvedProgram.pack_params`.
pub const PackParamId = enum(u32) { _ };

/// One generic param, identified symbolically — NOT a TypeId (the concrete binding
/// is instantiation-time, owned by S2.2+).
pub const TemplateParamInfo = struct {
    id: TemplateParamId,
    name: []const u8,
    /// The decl node that introduced the param (fn / struct / lambda / protocol /
    /// impl) — the param's identity is its address, this is its scope owner.
    owner: *const ast.Node,
    /// `$N: u32` (value) vs `$T: Type` (type), read off the param's constraint.
    is_value: bool,
};

/// One type pack, identified symbolically.
pub const PackParamInfo = struct {
    id: PackParamId,
    name: []const u8,
    owner: *const ast.Node,
};

/// A reference to an enclosing pack — the whole pack (`$Ts`) or one element
/// (`$Ts[i]`). Symbolic.
pub const PackRef = struct {
    id: PackParamId,
    /// `$Ts[i]` literal index, or null for a whole-pack reference (`$Ts`).
    index: ?u32 = null,
};

/// What ONE reference site resolves to — the S2.1 RAW form. `authors` carries the
/// collected author identity (own ∪ flat, diamond-deduped) with NO verdict:
/// own-wins / direct-flat ambiguity selection is S2.2. `template` / `pack` are
/// symbolic generic-param references.
pub const ResolvedRef = union(enum) {
    authors: AuthorSet,
    template: TemplateParamId,
    pack: PackRef,
};

/// Node-keyed side table: an AST reference node → its `ResolvedRef`. Keyed by node
/// IDENTITY (the `*const ast.Node` pointer), so two textually-identical spellings
/// at different sites are distinct entries.
pub const NodeRefTable = std.AutoHashMap(*const ast.Node, ResolvedRef);

/// The output of the owning resolution pass: ten node-keyed side tables (one per
/// reference domain) plus the symbolic template/pack registries. OWNS every
/// allocation it holds — the maps, the registry lists, and each `AuthorSet.flat`
/// slice the collector returned — and frees them in `deinit`. Owned by
/// `Compilation`; borrowed by `ProgramIndex.resolved_program`.
pub const ResolvedProgram = struct {
    alloc: std.mem.Allocator,

    // ── bare-name domains (S2.1a populates these) ──
    type_refs: NodeRefTable,
    value_refs: NodeRefTable,
    callable_refs: NodeRefTable,
    // ── namespace-qualified + head domains (S2.1b) ──
    namespace_refs: NodeRefTable,
    generic_struct_heads: NodeRefTable,
    type_fn_heads: NodeRefTable,
    protocol_heads: NodeRefTable,
    // ── foreign-class / struct-const / UFCS (S2.1c) ──
    foreign_class_refs: NodeRefTable,
    struct_const_refs: NodeRefTable,
    ufcs_refs: NodeRefTable,

    // ── symbolic generic-param registries ──
    template_params: std.ArrayList(TemplateParamInfo) = .empty,
    template_by_ptr: std.AutoHashMap(usize, TemplateParamId),
    pack_params: std.ArrayList(PackParamInfo) = .empty,
    pack_by_ptr: std.AutoHashMap(usize, PackParamId),

    pub fn init(alloc: std.mem.Allocator) ResolvedProgram {
        return .{
            .alloc = alloc,
            .type_refs = NodeRefTable.init(alloc),
            .value_refs = NodeRefTable.init(alloc),
            .callable_refs = NodeRefTable.init(alloc),
            .namespace_refs = NodeRefTable.init(alloc),
            .generic_struct_heads = NodeRefTable.init(alloc),
            .type_fn_heads = NodeRefTable.init(alloc),
            .protocol_heads = NodeRefTable.init(alloc),
            .foreign_class_refs = NodeRefTable.init(alloc),
            .struct_const_refs = NodeRefTable.init(alloc),
            .ufcs_refs = NodeRefTable.init(alloc),
            .template_by_ptr = std.AutoHashMap(usize, TemplateParamId).init(alloc),
            .pack_by_ptr = std.AutoHashMap(usize, PackParamId).init(alloc),
        };
    }

    pub fn deinit(self: *ResolvedProgram) void {
        for (self.allTables()) |t| {
            var it = t.valueIterator();
            while (it.next()) |ref| switch (ref.*) {
                .authors => |a| if (a.flat.len > 0) self.alloc.free(a.flat),
                .template, .pack => {},
            };
            t.deinit();
        }
        self.template_params.deinit(self.alloc);
        self.template_by_ptr.deinit();
        self.pack_params.deinit(self.alloc);
        self.pack_by_ptr.deinit();
    }

    fn allTables(self: *ResolvedProgram) [10]*NodeRefTable {
        return .{
            &self.type_refs,            &self.value_refs,
            &self.callable_refs,        &self.namespace_refs,
            &self.generic_struct_heads, &self.type_fn_heads,
            &self.protocol_heads,       &self.foreign_class_refs,
            &self.struct_const_refs,    &self.ufcs_refs,
        };
    }
};

/// Run the owning resolution pass over `root` (the resolved program root), using
/// `index`'s borrowed import facts (`module_decls` / `flat_import_graph`) for
/// author collection. `main_file` is the ambient-source fallback for nodes that
/// carry no `source_file` stamp. Returns a fully-owned `ResolvedProgram` (the
/// caller stores it and calls `deinit`). One pass, no AST mutation, no diagnostics
/// — parallel/unconsumed, so generated output is unaffected.
pub fn resolve(
    root: *const ast.Node,
    index: *ProgramIndex,
    main_file: []const u8,
    alloc: std.mem.Allocator,
) ResolvedProgram {
    var pass = ResolvePass{
        .res = Resolver.init(index, alloc),
        .out = ResolvedProgram.init(alloc),
        .ufcs_aliases = std.StringHashMap([]const u8).init(alloc),
    };
    defer pass.ufcs_aliases.deinit();
    pass.seedTopLevelUfcsAliases(root);
    pass.visit(root, .{ .source = main_file, .scope = null });
    return pass.out;
}

/// One frame of generic params introduced by an enclosing decl (fn / struct /
/// lambda / protocol / impl). Lives on the Zig call stack (no allocation), chained
/// to its parent — a reference resolves a name against the NEAREST enclosing frame.
const Frame = struct {
    params: []const ast.StructTypeParam,
    owner: *const ast.Node,
    parent: ?*const Frame,
};

/// Ambient walk context: the querying module's source path (`collectVisibleAuthors`'s
/// `from`) and the enclosing generic-param scope.
const Ctx = struct {
    source: []const u8,
    scope: ?*const Frame,
    /// True only while visiting declarations that were already covered by the
    /// top-level UFCS alias pre-scan. Their decl nodes still get recorded into
    /// `ufcs_refs`, but the alias map keeps the scanDecls-style final state.
    preseeded_decl: bool = false,
};

/// A resolved generic-param reference: the matched param (its address is its
/// identity) plus the scope owner that declared it.
const GenericMatch = struct {
    param: *const ast.StructTypeParam,
    owner: *const ast.Node,
};

/// `$N: u32` is a value param; `$T: Type` (or a variadic / non-type constraint) is
/// a type param. Read off the param's constraint type-expr name.
fn paramIsValue(p: ast.StructTypeParam) bool {
    if (p.is_variadic) return false;
    return switch (p.constraint.data) {
        .type_expr => |te| !std.mem.eql(u8, te.name, "Type"),
        else => false,
    };
}

/// Nearest enclosing generic param named `name`, or null when the name is not a
/// generic in scope (→ it is an ordinary bare-name reference).
fn lookupGeneric(scope: ?*const Frame, name: []const u8) ?GenericMatch {
    var cur = scope;
    while (cur) |f| : (cur = f.parent) {
        for (f.params) |*p| {
            if (std.mem.eql(u8, p.name, name)) return .{ .param = p, .owner = f.owner };
        }
    }
    return null;
}

/// Bin ONE raw author by the head kind it can author: a struct with type params (a
/// generic-struct head), a fn / const-wrapped fn with type params (a type-function
/// head), or a protocol. The `type_params.len > 0` gate is the head test — a
/// non-generic struct or a zero-type-param fn authors no head kind and sets
/// nothing. The `const_decl` arm unwraps a `Name :: struct/fn(...)` const exactly
/// as `structDeclOfRaw` / `fnDeclOfRaw` do.
fn classifyHeadKind(raw: RawDeclRef, gs: *bool, tf: *bool, pr: *bool) void {
    switch (raw) {
        .struct_decl => |sd| if (sd.type_params.len > 0) {
            gs.* = true;
        },
        .fn_decl => |fd| if (fd.type_params.len > 0) {
            tf.* = true;
        },
        .const_decl => |cd| switch (cd.value.data) {
            .struct_decl => |*sd| if (sd.type_params.len > 0) {
                gs.* = true;
            },
            .fn_decl => |*fd| if (fd.type_params.len > 0) {
                tf.* = true;
            },
            else => {},
        },
        .protocol_decl => pr.* = true,
        else => {},
    }
}

/// True when an author set resolves to a `foreign_class_decl` — the own author
/// decides when present, else any flat author. Such a reference is routed to
/// `foreign_class_refs` (its own domain) instead of the bare type/value table.
fn authorSetIsForeignClass(set: AuthorSet) bool {
    if (set.own) |a| return std.meta.activeTag(a.raw) == .foreign_class_decl;
    for (set.flat) |a| {
        if (std.meta.activeTag(a.raw) == .foreign_class_decl) return true;
    }
    return false;
}

/// A struct author carrying a `const_decl` member named `field` — the RAW shape
/// `Type.CONST` field access resolves to. Mirrors the lowering `struct_const_map`
/// domain, which is struct-level constants only; enums / other decls carry no
/// const members, so only `struct_decl` matches.
fn structHasConstMember(raw: RawDeclRef, field: []const u8) bool {
    return switch (raw) {
        .struct_decl => |sd| blk: {
            for (sd.constants) |c| {
                if (c.data == .const_decl and std.mem.eql(u8, c.data.const_decl.name, field))
                    break :blk true;
            }
            break :blk false;
        },
        else => false,
    };
}

/// Any author in the set (own or flat) is a struct with a const member `field`.
fn authorSetHasStructConst(set: AuthorSet, field: []const u8) bool {
    if (set.own) |a| {
        if (structHasConstMember(a.raw, field)) return true;
    }
    for (set.flat) |a| {
        if (structHasConstMember(a.raw, field)) return true;
    }
    return false;
}

/// The single owning traversal. Holds the author collector + the `ResolvedProgram`
/// it populates; threads `Ctx` (ambient source + generic scope) down the tree.
const ResolvePass = struct {
    res: Resolver,
    out: ResolvedProgram,
    /// `alias name → target name` for every `ufcs_alias` seen so far on the walk.
    /// Global (not block-scoped) and populated in traversal order, mirroring
    /// lowering's flat `ufcs_alias_map`; lets a UFCS-rewrite call site resolve to
    /// the alias target's author. Scratch — freed when `resolve` returns, NOT part
    /// of the owned `ResolvedProgram`.
    ufcs_aliases: std.StringHashMap([]const u8),

    /// Visit ONE node, then recurse into its children. A stamped
    /// `node.source_file` (top-level decls, and cross-module fn bodies whose bare
    /// names must resolve in their DEFINING module) overrides the ambient source
    /// for this subtree; an unstamped node inherits its parent's.
    fn visit(self: *ResolvePass, node: *const ast.Node, ctx: Ctx) void {
        const here = Ctx{
            .source = node.source_file orelse ctx.source,
            .scope = ctx.scope,
            .preseeded_decl = ctx.preseeded_decl,
        };
        switch (node.data) {
            // ── declarations that open a generic-param scope ──
            .fn_decl => |*fd| {
                var frame = Frame{ .params = fd.type_params, .owner = node, .parent = here.scope };
                const inner = Ctx{ .source = here.source, .scope = &frame };
                self.visitTypeParamConstraints(fd.type_params, inner);
                for (fd.params) |p| {
                    self.visit(p.type_expr, inner);
                    if (p.default_expr) |d| self.visit(d, inner);
                }
                if (fd.return_type) |rt| self.visit(rt, inner);
                self.visit(fd.body, inner);
            },
            .lambda => |*l| {
                var frame = Frame{ .params = l.type_params, .owner = node, .parent = here.scope };
                const inner = Ctx{ .source = here.source, .scope = &frame };
                self.visitTypeParamConstraints(l.type_params, inner);
                for (l.params) |p| {
                    self.visit(p.type_expr, inner);
                    if (p.default_expr) |d| self.visit(d, inner);
                }
                if (l.return_type) |rt| self.visit(rt, inner);
                self.visit(l.body, inner);
            },
            .struct_decl => |*sd| {
                var frame = Frame{ .params = sd.type_params, .owner = node, .parent = here.scope };
                const inner = Ctx{ .source = here.source, .scope = &frame };
                self.visitTypeParamConstraints(sd.type_params, inner);
                self.visitAll(sd.field_types, inner);
                self.visitAllOpt(sd.field_defaults, inner);
                self.visitAll(sd.methods, inner);
                self.visitAll(sd.constants, inner);
            },
            .protocol_decl => |*pd| {
                var frame = Frame{ .params = pd.type_params, .owner = node, .parent = here.scope };
                const inner = Ctx{ .source = here.source, .scope = &frame };
                self.visitTypeParamConstraints(pd.type_params, inner);
                for (pd.methods) |m| {
                    self.visitAll(m.params, inner);
                    if (m.return_type) |rt| self.visit(rt, inner);
                    if (m.default_body) |b| self.visit(b, inner);
                }
            },
            .impl_block => |*ib| {
                var frame = Frame{ .params = ib.target_type_params, .owner = node, .parent = here.scope };
                const inner = Ctx{ .source = here.source, .scope = &frame };
                self.visitTypeParamConstraints(ib.target_type_params, inner);
                if (ib.target_type_expr) |tt| self.visit(tt, inner);
                self.visitAll(ib.protocol_type_args, inner);
                self.visitAll(ib.methods, inner);
            },
            .foreign_class_decl => |*fc| {
                for (fc.members) |m| switch (m) {
                    .method => |meth| {
                        self.visitAll(meth.params, here);
                        if (meth.return_type) |rt| self.visit(rt, here);
                        if (meth.body) |b| self.visit(b, here);
                    },
                    .field => |fld| self.visit(fld.field_type, here),
                    .extends, .implements => {},
                };
            },

            // ── the three bare-name domains + symbolic generic refs ──
            .type_expr => self.classifyType(node, here),
            .identifier => self.classifyValue(node, here),
            .call => |*c| {
                if (c.callee.data == .identifier) {
                    const cname = c.callee.data.identifier.name;
                    // a UFCS-alias callee (`alias(args)`, incl. the parser's
                    // pipe-desugared `x |> alias()`) resolves to the alias TARGET's
                    // author → ufcs_refs (S2.1c). Any other bare callee is an
                    // ordinary callable HEAD — recorded here, not re-walked as a
                    // value ref.
                    if (self.ufcs_aliases.get(cname)) |target| {
                        self.recordAuthorsInto(&self.out.ufcs_refs, c.callee, target, here.source);
                    } else {
                        self.recordAuthors(&self.out.callable_refs, c.callee, cname, here.source);
                    }
                } else {
                    self.visit(c.callee, here);
                }
                self.visitAll(c.args, here);
            },
            .field_access => |*fa| {
                // `alias.member` whose base is a namespace import edge of the
                // ambient source resolves via `collectNamespaceAuthors` into the
                // namespace-qualified table (S2.1b). Otherwise `Type.CONST` — a base
                // resolving to a struct author that carries the named const member —
                // fills `struct_const_refs` (S2.1c). A receiver that is neither (a
                // local value / instance field access) records nothing and is not
                // walked as a value ref; a compound receiver is recursed so its
                // inner refs are collected.
                if (fa.object.data == .identifier) {
                    const base = fa.object.data.identifier.name;
                    if (!self.classifyNamespaceQualified(node, base, fa.field, here.source))
                        _ = self.classifyStructConst(node, base, fa.field, here.source);
                } else {
                    self.visit(fa.object, here);
                }
            },
            .pack_index_type_expr => |*p| self.recordPack(&self.out.type_refs, node, p.pack_name, p.index, here.scope),
            .comptime_pack_ref => |*p| self.recordPack(&self.out.value_refs, node, p.pack_name, null, here.scope),
            .error_type_expr => |*e| {
                if (e.name) |name| self.recordAuthors(&self.out.type_refs, node, name, here.source);
            },
            .parameterized_type_expr => |*p| {
                // the head (`Name(args)`) is binned by its resolved author's decl
                // kind into the generic-struct / type-fn / protocol head tables
                // (S2.1b); the type args are ordinary references, collected now.
                self.classifyHead(node, p.name, p.is_raw, here);
                self.visitAll(p.args, here);
            },

            // ── structural recursion (no classification of their own) ──
            .root => |*r| {
                // each top-level decl carries its own ambient source stamp.
                const decl_ctx = Ctx{ .source = here.source, .scope = here.scope, .preseeded_decl = true };
                self.visitAll(r.decls, decl_ctx);
            },
            .block => |*b| self.visitAll(b.stmts, here),
            .binary_op => |*b| {
                self.visit(b.lhs, here);
                self.visit(b.rhs, here);
            },
            .chained_comparison => |*c| self.visitAll(c.operands, here),
            .unary_op => |*u| self.visit(u.operand, here),
            .if_expr => |*e| {
                self.visit(e.condition, here);
                self.visit(e.then_branch, here);
                if (e.else_branch) |b| self.visit(b, here);
            },
            .match_expr => |*e| {
                self.visit(e.subject, here);
                for (e.arms) |arm| {
                    if (arm.pattern) |pat| self.visit(pat, here);
                    self.visit(arm.body, here);
                }
            },
            .match_arm => |*arm| {
                if (arm.pattern) |pat| self.visit(pat, here);
                self.visit(arm.body, here);
            },
            .const_decl => |*cd| {
                if (cd.type_annotation) |ta| self.visit(ta, here);
                self.visit(cd.value, here);
            },
            .var_decl => |*vd| {
                if (vd.type_annotation) |ta| self.visit(ta, here);
                if (vd.value) |v| self.visit(v, here);
            },
            .assignment => |*a| {
                self.visit(a.target, here);
                self.visit(a.value, here);
            },
            .multi_assign => |*m| {
                self.visitAll(m.targets, here);
                self.visitAll(m.values, here);
            },
            .destructure_decl => |*d| self.visit(d.value, here),
            .enum_decl => |*ed| {
                self.visitAllOpt(ed.variant_types, here);
                self.visitAllOpt(ed.variant_values, here);
                if (ed.backing_type) |bt| self.visit(bt, here);
            },
            .union_decl => |*ud| self.visitAll(ud.field_types, here),
            .struct_literal => |*sl| {
                if (sl.type_expr) |te| self.visit(te, here);
                for (sl.field_inits) |fi| self.visit(fi.value, here);
                if (sl.init_block) |ib| self.visit(ib, here);
            },
            .param => |*p| {
                self.visit(p.type_expr, here);
                if (p.default_expr) |d| self.visit(d, here);
            },
            .defer_stmt => |*d| self.visit(d.expr, here),
            .push_stmt => |*p| {
                self.visit(p.context_expr, here);
                self.visit(p.body, here);
            },
            .comptime_expr => |*c| self.visit(c.expr, here),
            .insert_expr => |*i| self.visit(i.expr, here),
            .return_stmt => |*r| if (r.value) |v| self.visit(v, here),
            .array_type_expr => |*a| {
                self.visit(a.length, here);
                self.visit(a.element_type, here);
            },
            .slice_type_expr => |*s| self.visit(s.element_type, here),
            .array_literal => |*a| {
                if (a.type_expr) |te| self.visit(te, here);
                self.visitAll(a.elements, here);
            },
            .index_expr => |*i| {
                self.visit(i.object, here);
                self.visit(i.index, here);
            },
            .slice_expr => |*s| {
                self.visit(s.object, here);
                if (s.start) |st| self.visit(st, here);
                if (s.end) |en| self.visit(en, here);
            },
            .pointer_type_expr => |*p| self.visit(p.pointee_type, here),
            .many_pointer_type_expr => |*p| self.visit(p.element_type, here),
            .optional_type_expr => |*o| self.visit(o.inner_type, here),
            .raise_stmt => |*r| self.visit(r.tag, here),
            .try_expr => |*t| self.visit(t.operand, here),
            .catch_expr => |*c| {
                self.visit(c.operand, here);
                self.visit(c.body, here);
            },
            .onfail_stmt => |*o| self.visit(o.body, here),
            .force_unwrap => |*f| self.visit(f.operand, here),
            .null_coalesce => |*n| {
                self.visit(n.lhs, here);
                self.visit(n.rhs, here);
            },
            .deref_expr => |*d| self.visit(d.operand, here),
            .while_expr => |*w| {
                self.visit(w.condition, here);
                self.visit(w.body, here);
            },
            .for_expr => |*f| {
                self.visit(f.iterable, here);
                if (f.range_end) |re| self.visit(re, here);
                self.visit(f.body, here);
            },
            .spread_expr => |*s| self.visit(s.operand, here),
            .function_type_expr => |*ft| {
                self.visitAll(ft.param_types, here);
                if (ft.return_type) |rt| self.visit(rt, here);
            },
            .closure_type_expr => |*ct| {
                self.visitAll(ct.param_types, here);
                if (ct.return_type) |rt| self.visit(rt, here);
            },
            .tuple_type_expr => |*tt| self.visitAll(tt.field_types, here),
            .tuple_literal => |*tl| {
                for (tl.elements) |el| self.visit(el.value, here);
            },
            .ffi_intrinsic_call => |*f| {
                self.visit(f.return_type, here);
                self.visitAll(f.args, here);
            },
            .jni_env_block => |*j| {
                self.visit(j.env, here);
                self.visit(j.body, here);
            },

            // ── leaves: no child node, no bare-name reference of their own ──
            // `namespace_decl` is a leaf HERE: its members belong to another
            // module and are reached via `collectNamespaceAuthors` in S2.1b, not
            // re-walked through the importing root.
            .int_literal,
            .float_literal,
            .bool_literal,
            .string_literal,
            .enum_literal,
            .caller_location,
            .null_literal,
            .break_expr,
            .continue_expr,
            .undef_literal,
            .inferred_type,
            .builtin_expr,
            .compiler_expr,
            .import_decl,
            .namespace_decl,
            .error_set_decl,
            .foreign_expr,
            .library_decl,
            .framework_decl,
            .c_import_decl,
            => {},

            // `alias :: ufcs target` — record the alias→target binding (the
            // target's RAW author) keyed by the decl node, and remember the alias
            // name so its rewrite call sites resolve to the same target. The map is
            // global / traversal-ordered, mirroring lowering's flat `ufcs_alias_map`.
            .ufcs_alias => |ua| {
                if (!here.preseeded_decl) {
                    self.ufcs_aliases.put(ua.name, ua.target) catch @panic("resolve: OOM");
                }
                self.recordAuthorsInto(&self.out.ufcs_refs, node, ua.target, here.source);
            },
        }
    }

    /// Mirror lowering's declaration pre-scan for UFCS aliases: top-level roots
    /// and namespace declaration lists are scanned before function bodies are
    /// visited, so a call can resolve through an alias declared later in the file.
    /// Function/lambda/block bodies are intentionally not entered here; local
    /// aliases keep normal statement-order behavior on the owning walk.
    fn seedTopLevelUfcsAliases(self: *ResolvePass, node: *const ast.Node) void {
        switch (node.data) {
            .root => |*r| self.seedTopLevelUfcsAliasDecls(r.decls),
            .namespace_decl => |*ns| self.seedTopLevelUfcsAliasDecls(ns.decls),
            else => {},
        }
    }

    fn seedTopLevelUfcsAliasDecls(self: *ResolvePass, decls: []const *ast.Node) void {
        for (decls) |decl| switch (decl.data) {
            .ufcs_alias => |ua| {
                self.ufcs_aliases.put(ua.name, ua.target) catch @panic("resolve: OOM");
            },
            .namespace_decl => self.seedTopLevelUfcsAliases(decl),
            else => {},
        };
    }

    fn visitAll(self: *ResolvePass, nodes: anytype, ctx: Ctx) void {
        for (nodes) |n| self.visit(n, ctx);
    }

    fn visitAllOpt(self: *ResolvePass, nodes: anytype, ctx: Ctx) void {
        for (nodes) |n| if (n) |nn| self.visit(nn, ctx);
    }

    fn visitTypeParamConstraints(self: *ResolvePass, params: []const ast.StructTypeParam, ctx: Ctx) void {
        for (params) |p| self.visit(p.constraint, ctx);
    }

    /// A type-position reference: a generic param in scope → symbolic template ref;
    /// otherwise a user type, collected RAW. Builtins / undeclared names collect to
    /// an empty set and are simply not recorded.
    fn classifyType(self: *ResolvePass, node: *const ast.Node, ctx: Ctx) void {
        const te = node.data.type_expr;
        if (!te.is_raw) {
            if (lookupGeneric(ctx.scope, te.name)) |m| {
                self.recordTemplate(&self.out.type_refs, node, m);
                return;
            }
        }
        self.recordAuthors(&self.out.type_refs, node, te.name, ctx.source);
    }

    /// A value-position identifier: a generic value/type param in scope (shadowing)
    /// → symbolic template ref; otherwise a module value/const, collected RAW.
    fn classifyValue(self: *ResolvePass, node: *const ast.Node, ctx: Ctx) void {
        const id = node.data.identifier;
        if (!id.is_raw) {
            if (lookupGeneric(ctx.scope, id.name)) |m| {
                self.recordTemplate(&self.out.value_refs, node, m);
                return;
            }
        }
        self.recordAuthors(&self.out.value_refs, node, id.name, ctx.source);
    }

    /// RAW author collection for a bare name. Only recorded when the name has ≥1
    /// visible author (own or flat); a builtin / local / undeclared spelling has
    /// none and is omitted — this is what keeps the tables to genuine authors. A
    /// name whose author is a `foreign_class_decl` is routed to `foreign_class_refs`
    /// (its own S2.1c domain) instead of the passed bare type/value/callable table.
    fn recordAuthors(self: *ResolvePass, table: *NodeRefTable, node: *const ast.Node, name: []const u8, from: []const u8) void {
        const set = self.res.collectVisibleAuthors(name, from, .user_bare_flat);
        if (set.distinctCount() == 0) return;
        const dest = if (authorSetIsForeignClass(set)) &self.out.foreign_class_refs else table;
        self.replaceRef(dest, node, .{ .authors = set });
    }

    /// RAW author collection into an explicit table, with NO foreign-class routing —
    /// the destination domain is already chosen by the caller (UFCS rewrite sites
    /// and alias decls, whose target is always a free function).
    fn recordAuthorsInto(self: *ResolvePass, table: *NodeRefTable, node: *const ast.Node, name: []const u8, from: []const u8) void {
        const set = self.res.collectVisibleAuthors(name, from, .user_bare_flat);
        if (set.distinctCount() == 0) return;
        self.replaceRef(table, node, .{ .authors = set });
    }

    /// `alias.member`: when `alias` is a namespace import edge of `from`, resolve
    /// `member` against that already-selected target via `collectNamespaceAuthors`
    /// (NO graph walk) and record it into the namespace-qualified table. Returns
    /// whether it recorded — a base that is not a namespace alias records nothing
    /// here and lets the caller try the struct-const path.
    fn classifyNamespaceQualified(self: *ResolvePass, node: *const ast.Node, alias: []const u8, member: []const u8, from: []const u8) bool {
        const edges = self.res.index.namespace_edges orelse return false;
        const aliases = edges.get(from) orelse return false;
        const target = aliases.get(alias) orelse return false;
        const set = self.res.collectNamespaceAuthors(target, member);
        if (set.distinctCount() == 0) return false;
        self.replaceRef(&self.out.namespace_refs, node, .{ .authors = set });
        return true;
    }

    /// `Type.CONST`: when `base` resolves to a struct author carrying a const member
    /// named `member`, record the base's RAW author set into `struct_const_refs`
    /// (keyed by the field-access node) — the owning-type identity the constant
    /// lives on. A base with authors but no matching const member, or no author at
    /// all (a local value receiver), records nothing and releases its allocation.
    /// Returns whether it recorded.
    fn classifyStructConst(self: *ResolvePass, node: *const ast.Node, base: []const u8, member: []const u8, from: []const u8) bool {
        const set = self.res.collectVisibleAuthors(base, from, .user_bare_flat);
        if (set.distinctCount() == 0) return false;
        if (!authorSetHasStructConst(set, member)) {
            if (set.flat.len > 0) self.out.alloc.free(set.flat);
            return false;
        }
        self.replaceRef(&self.out.struct_const_refs, node, .{ .authors = set });
        return true;
    }

    /// A parameterized head (`Name(args)`) binned by its resolved author's decl
    /// kind: a generic struct (struct with type params) → `generic_struct_heads`;
    /// a type-function (fn / const-wrapped fn with type params) → `type_fn_heads`; a
    /// protocol → `protocol_heads`. RAW — the whole author set is recorded with no
    /// winner picked, so a name authored as more than one head kind across modules
    /// lands a distinct entry in every matching table. A head naming a generic param
    /// in scope is symbolic (not an author); a name with no user author (builtins
    /// like `Vector`, undeclared) or only non-head authors is omitted.
    fn classifyHead(self: *ResolvePass, node: *const ast.Node, name: []const u8, is_raw: bool, ctx: Ctx) void {
        if (std.mem.indexOfScalar(u8, name, '.')) |first_dot| {
            const set = self.collectQualifiedHeadAuthors(name, first_dot, ctx.source) orelse return;
            self.classifyHeadSet(node, set);
            return;
        }

        if (!is_raw and lookupGeneric(ctx.scope, name) != null) return;
        const set = self.res.collectVisibleAuthors(name, ctx.source, .user_bare_flat);
        if (set.distinctCount() == 0) return;

        self.classifyHeadSet(node, set);
    }

    /// `alias.Member(args)` reaches this pass as one `parameterized_type_expr`
    /// named `alias.Member`. Split like the old lowering path: alias before the
    /// first dot, member after the last dot, then collect from the namespace
    /// target's own declarations only.
    fn collectQualifiedHeadAuthors(self: *ResolvePass, name: []const u8, first_dot: usize, from: []const u8) ?AuthorSet {
        const alias = name[0..first_dot];
        const last_dot = std.mem.lastIndexOfScalar(u8, name, '.') orelse first_dot;
        const member = name[last_dot + 1 ..];
        if (alias.len == 0 or member.len == 0) return null;

        const edges = self.res.index.namespace_edges orelse return null;
        const aliases = edges.get(from) orelse return null;
        const target = aliases.get(alias) orelse return null;
        const set = self.res.collectNamespaceAuthors(target, member);
        if (set.distinctCount() == 0) return null;
        return set;
    }

    fn classifyHeadSet(self: *ResolvePass, node: *const ast.Node, set: AuthorSet) void {
        var gs = false;
        var tf = false;
        var pr = false;
        if (set.own) |a| classifyHeadKind(a.raw, &gs, &tf, &pr);
        for (set.flat) |a| classifyHeadKind(a.raw, &gs, &tf, &pr);

        var tables: [3]*NodeRefTable = undefined;
        var n: usize = 0;
        if (gs) {
            tables[n] = &self.out.generic_struct_heads;
            n += 1;
        }
        if (tf) {
            tables[n] = &self.out.type_fn_heads;
            n += 1;
        }
        if (pr) {
            tables[n] = &self.out.protocol_heads;
            n += 1;
        }
        if (n == 0) {
            // an author exists but is not a head kind (e.g. a non-generic struct or
            // a zero-type-param fn) — own this set's allocation, then drop it.
            if (set.flat.len > 0) self.out.alloc.free(set.flat);
            return;
        }
        // each table OWNS its `AuthorSet.flat`; give the first match the collected
        // slice and a fresh copy to every subsequent table so `deinit` frees each
        // exactly once.
        self.replaceRef(tables[0], node, .{ .authors = set });
        var i: usize = 1;
        while (i < n) : (i += 1) {
            self.replaceRef(tables[i], node, .{ .authors = self.dupAuthorSet(set) });
        }
    }

    /// A shallow copy of an `AuthorSet` with its own freshly-allocated `flat` slice
    /// (the `RawAuthor` elements are borrowed AST pointers + source strings, so the
    /// copy is shallow). Lets one head reference be recorded into several head
    /// tables without aliasing the owned slice.
    fn dupAuthorSet(self: *ResolvePass, set: AuthorSet) AuthorSet {
        return .{
            .own = set.own,
            .flat = if (set.flat.len > 0)
                (self.out.alloc.dupe(RawAuthor, set.flat) catch @panic("resolve: OOM"))
            else
                &.{},
        };
    }

    fn recordTemplate(self: *ResolvePass, table: *NodeRefTable, node: *const ast.Node, m: GenericMatch) void {
        self.replaceRef(table, node, .{ .template = self.internTemplate(m) });
    }

    fn recordPack(self: *ResolvePass, table: *NodeRefTable, node: *const ast.Node, name: []const u8, index: ?u32, scope: ?*const Frame) void {
        const m = lookupGeneric(scope, name) orelse return;
        self.replaceRef(table, node, .{ .pack = .{ .id = self.internPack(m), .index = index } });
    }

    fn replaceRef(self: *ResolvePass, table: *NodeRefTable, node: *const ast.Node, ref: ResolvedRef) void {
        const entry = table.getOrPut(node) catch @panic("resolve: OOM");
        if (entry.found_existing) self.releaseRef(entry.value_ptr.*);
        entry.value_ptr.* = ref;
    }

    fn releaseRef(self: *ResolvePass, ref: ResolvedRef) void {
        switch (ref) {
            .authors => |a| if (a.flat.len > 0) self.out.alloc.free(a.flat),
            .template, .pack => {},
        }
    }

    fn internTemplate(self: *ResolvePass, m: GenericMatch) TemplateParamId {
        const key = @intFromPtr(m.param);
        if (self.out.template_by_ptr.get(key)) |id| return id;
        const id: TemplateParamId = @enumFromInt(@as(u32, @intCast(self.out.template_params.items.len)));
        self.out.template_params.append(self.out.alloc, .{
            .id = id,
            .name = m.param.name,
            .owner = m.owner,
            .is_value = paramIsValue(m.param.*),
        }) catch @panic("resolve: OOM");
        self.out.template_by_ptr.put(key, id) catch @panic("resolve: OOM");
        return id;
    }

    fn internPack(self: *ResolvePass, m: GenericMatch) PackParamId {
        const key = @intFromPtr(m.param);
        if (self.out.pack_by_ptr.get(key)) |id| return id;
        const id: PackParamId = @enumFromInt(@as(u32, @intCast(self.out.pack_params.items.len)));
        self.out.pack_params.append(self.out.alloc, .{
            .id = id,
            .name = m.param.name,
            .owner = m.owner,
        }) catch @panic("resolve: OOM");
        self.out.pack_by_ptr.put(key, id) catch @panic("resolve: OOM");
        return id;
    }
};