rdbms-playground/src/dsl/grammar/mod.rs

//! Unified declarative grammar tree (ADR-0024).
//!
//! The grammar tree is the single source of truth for the DSL —
//! parsing, completion, syntax highlighting, parse-error usage
//! rendering, and hint-panel content all derive from this same
//! data structure (ADR-0023 institutional context).
//!
//! Phase A scope (ADR-0024 §migration): the framework lands
//! alongside the eleven app-lifecycle commands (quit, help,
//! rebuild, save, save as, new, load, export, import, mode,
//! messages). The chumsky parser still owns every other
//! command; the router in `dsl::parser` decides which path to
//! take per first-token. Schema-aware nodes (`IdentSource::Tables`
//! and friends) and `DynamicSubgrammar` are declared here but
//! not exercised until Phase B-D.
//!
//! The shape of `Node` mirrors ADR-0024 §node-taxonomy with one
//! pragmatic addition for Phase A: each `Ident` carries an
//! optional content validator, used today by the `mode <value>`
//! / `messages <value>` slots to surface friendly catalog
//! wording (`mode.unknown`, `messages.unknown`) on out-of-set
//! identifiers. The same hook generalises naturally to typed
//! value slots in Phase D.

pub mod app;
pub mod data;
pub mod ddl;
pub mod expr;
pub mod shared;
pub mod sql_expr;
pub mod sql_create_table;
pub mod sql_delete;
pub mod sql_insert;
pub mod sql_select;
pub mod sql_update;

use crate::dsl::command::Command;
use crate::dsl::walker::context::WalkContext;
use crate::dsl::walker::outcome::MatchedPath;

/// Highlight class assigned to a matched terminal.
///
/// Recorded on the `WalkResult::per_byte_class` slice and surfaced
/// by `walker::highlight_runs` to the input/echo-line renderers
/// (ADR-0024 §architecture).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum HighlightClass {
    Keyword,
    Identifier,
    /// Column data-type keyword (`int`, `serial`, `text`, …).
    /// Distinct from `Keyword` and `Identifier` so learners can
    /// tell "this is a type" from a clause keyword or a name they
    /// invented (ADR-0022 Amendment 4). Assigned via a type slot's
    /// `highlight_override`, not by byte shape.
    Type,
    Number,
    String,
    Punct,
    Flag,
    Error,
}

/// Where an `Ident` slot's candidates come from at completion time.
///
/// Drives both the walker's `Expectation::Ident { source }` (which
/// the parse-error bridge maps to a human label) and the
/// `SchemaCache` lookup the completion engine uses for Tab
/// candidates. The `Free` and `NewName` variants do not query the
/// schema — `NewName` is for slots where the user invents the
/// identifier, `Free` is the catch-all branch in `mode`/`messages`
/// that funnels unknown values into a friendly validator.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum IdentSource {
    /// User invents this name. No schema lookup; no completion
    /// candidates beyond the identifier shape itself.
    NewName,
    /// Existing table name.
    Tables,
    /// Existing column in the current table.
    Columns,
    /// Existing relationship name.
    Relationships,
    /// Existing index name.
    Indexes,
    /// Closed set from `Type::all()` — surfaced by the walker's
    /// content validator on column-type slots; not user-listable
    /// from the schema.
    Types,
    /// Any identifier shape; used by synthetic catch-all branches
    /// (e.g., the unknown-value branch of `mode <value>`).
    Free,
}

impl IdentSource {
    /// Whether this source can be completed from the schema
    /// cache (i.e. the candidate list comes from existing
    /// entities rather than user invention or a closed set).
    #[must_use]
    pub const fn completes_from_schema(self) -> bool {
        matches!(
            self,
            Self::Tables | Self::Columns | Self::Relationships | Self::Indexes
        )
    }

    /// Human-facing label used in parse-error wording
    /// ("expected table name") and in the completion engine's
    /// round-trip from a textual `expected` entry back to a
    /// source kind. `Free` and `Types` collapse to "identifier"
    /// and "type" respectively.
    #[must_use]
    pub const fn expected_label(self) -> &'static str {
        match self {
            Self::NewName | Self::Free => "identifier",
            Self::Tables => "table name",
            Self::Columns => "column name",
            Self::Relationships => "relationship name",
            Self::Indexes => "index name",
            Self::Types => "type",
        }
    }

    /// Inverse of `expected_label`. Used by the completion engine
    /// to recover the source kind from the `ParseError::Invalid::
    /// expected` strings the walker bridge produces. `"identifier"`
    /// maps to `NewName` (the only writeable label that uses that
    /// wording in production grammars today).
    #[must_use]
    pub fn from_expected_label(label: &str) -> Option<Self> {
        match label {
            "identifier" => Some(Self::NewName),
            "table name" => Some(Self::Tables),
            "column name" => Some(Self::Columns),
            "relationship name" => Some(Self::Relationships),
            "index name" => Some(Self::Indexes),
            "type" => Some(Self::Types),
            _ => None,
        }
    }
}

/// Hint-panel mode for an expected node (ADR-0024 §HintMode-per-node).
///
/// `Default` (today's behaviour) shows candidates if any, falls
/// back to a prose ladder otherwise. The other variants
/// override at slot positions where the candidate list would be
/// actively misleading or where the user benefits from format
/// guidance:
///
/// - `ProseOnly(catalog_key)` — show only prose from the
///   catalog; suppress Tab candidates. Used today by the
///   value-literal slot at empty prefix (the "null/true/false"
///   candidate trio is misleading at a slot that more often
///   takes a number / quoted text / date).
/// - `ForceProse(catalog_key)` — force this prose at the
///   catalog key regardless of candidates. Used today by
///   `NewName` ident slots ("Type a name, then `(`").
/// - `IntroProse(catalog_key)` — show prose at slot entry to
///   *introduce* a position whose first-class candidate is an
///   ident slot (which would be invisible in a pure-candidate
///   render) but whose keyword alternatives are also available.
///   Unlike `ProseOnly`, Tab candidates remain available — the
///   user still cycles through the keyword set. Used at the
///   advanced-mode CREATE TABLE element slot, where the
///   column-name `NewName` slot would otherwise be invisible
///   alongside the table-level constraint keywords (issue #4).
/// - `SuppressProse` — show only candidates; never fall back
///   to a prose ladder.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum HintMode {
    Default,
    ForceProse(&'static str),
    ProseOnly(&'static str),
    IntroProse(&'static str),
    SuppressProse,
}

/// A keyword node literal.
///
/// The `aliases` slice is empty for the app-lifecycle commands
/// today; the round-5 `q` removal remains intentional, and any
/// future re-introduction would be a one-line `aliases: &["q"]`
/// addition (ADR-0024 §aliases).
#[derive(Debug, Clone, Copy)]
pub struct Word {
    pub primary: &'static str,
    pub aliases: &'static [&'static str],
    pub highlight_override: Option<HighlightClass>,
}

impl Word {
    pub const fn keyword(primary: &'static str) -> Self {
        Self {
            primary,
            aliases: &[],
            highlight_override: None,
        }
    }

    /// A keyword that highlights as a column **type** rather than a
    /// clause keyword (ADR-0022 Amendment 4). The one user today is
    /// the two-word `double precision` SQL alias (ADR-0035 §3): it
    /// is matched as keyword tokens, not an `IdentSource::Types`
    /// `Ident`, so without this it would render keyword-coloured
    /// while its single-word synonyms (`float`, `real`) render as
    /// types.
    pub const fn type_keyword(primary: &'static str) -> Self {
        Self {
            primary,
            aliases: &[],
            highlight_override: Some(HighlightClass::Type),
        }
    }

    /// Case-insensitive match against the primary or any alias.
    pub fn matches(&self, candidate: &str) -> bool {
        if candidate.eq_ignore_ascii_case(self.primary) {
            return true;
        }
        self.aliases
            .iter()
            .any(|a| candidate.eq_ignore_ascii_case(a))
    }
}

/// Content-level validator for an `Ident` slot. Returns the
/// catalog key + arg list to surface as `WalkOutcome::ValidationFailed`
/// on mismatch.
pub type IdentValidator = fn(matched: &str) -> Result<(), ValidationError>;

/// Content-level validator for a `NumberLit` slot. Same shape
/// as `IdentValidator`; surfaces as `ValidationFailed` on Err.
pub type NumberValidator = fn(matched: &str) -> Result<(), ValidationError>;

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ValidationError {
    pub message_key: &'static str,
    pub args: Vec<(&'static str, String)>,
}

/// The grammar-tree node taxonomy (ADR-0024 §node-taxonomy).
///
/// Some variants carry data (`Word` literal, `Punct` char,
/// `Ident` source/role/validator); combinators reference their
/// children through `&'static [Node]` / `&'static Node` slices,
/// which lets the entire registry live in `const`s — no runtime
/// allocation, every command is one declaration block in its
/// grammar file.
pub enum Node {
    /// A keyword token. Case-insensitive match (ADR-0009).
    Word(Word),
    /// A single punctuation character. The exact set comes from
    /// the migrated commands' usage — Phase A only needs none of
    /// these (app-lifecycle commands are pure keyword + ident +
    /// path), but the variant is declared for Phase B+ use.
    #[allow(dead_code)]
    Punct(char),
    /// An identifier slot. `source` drives completion candidates;
    /// `role` names the slot for error wording / completion-engine
    /// dispatch; `validator` runs after a successful identifier-
    /// shape match and may reject the value with a catalog-driven
    /// message.
    ///
    /// `writes_table` (Phase D): when `true` and `source ==
    /// Tables`, the walker writes the matched ident to
    /// `WalkContext::current_table` and resolves
    /// `current_table_columns` from the schema cache (if any).
    /// `writes_column` (Phase D): when `true` and `source ==
    /// Columns`, the walker writes the matched ident's
    /// `TableColumn` to `WalkContext::current_column` (resolved
    /// against `current_table_columns`). Subsequent value slots
    /// dispatch on the column's type.
    Ident {
        source: IdentSource,
        role: &'static str,
        validator: Option<IdentValidator>,
        #[allow(dead_code)]
        highlight_override: Option<HighlightClass>,
        writes_table: bool,
        writes_column: bool,
        /// Append the matched text to
        /// `WalkContext::user_listed_columns` (Phase D). Used by
        /// the `insert into <T> (col1, col2, …)` column-list
        /// idents — when the walker sees these, the form is
        /// "Form A" and the inner values slot list mirrors the
        /// user's explicit selection instead of the
        /// auto-filtered schema default.
        writes_user_listed_column: bool,
        /// Set the matched text as the alias of the most-
        /// recently-pushed `TableBinding` on the top
        /// `ScopeFrame`'s `from_scope` (ADR-0032 §10.1). Used by
        /// the `[ AS ] alias` slot on `from_clause` /
        /// `join_clause` table sources in `sql_select.rs`; a
        /// no-op on `IdentSource::NewName` slots that do not
        /// follow a table-name push, or when the top frame's
        /// `from_scope` is empty.
        writes_table_alias: bool,
        /// Push a placeholder `CteBinding` (name only, empty
        /// columns) onto the top `ScopeFrame`'s `cte_bindings`
        /// (ADR-0032 §10.3 stage 1). Used by the CTE-name slot
        /// in `with_clause`; the placeholder is rewritten with
        /// derived output columns at the body's frame exit
        /// (§10.3 stage 2; harvest derivation rules pending).
        writes_cte_name: bool,
        /// Append the matched text to the top `ScopeFrame`'s
        /// `projection_aliases` (ADR-0032 §10.4). Used by the
        /// projection-list alias slot (both the bare and `AS`
        /// forms) so `ORDER BY` completion can offer aliases as
        /// candidates.
        writes_projection_alias: bool,
    },
    /// A number literal. The optional `validator` runs against
    /// the matched text (used by Phase D value slots to enforce
    /// per-type integer/decimal rules).
    NumberLit {
        validator: Option<NumberValidator>,
    },
    /// A literal byte sequence at this position — matches
    /// bytes verbatim (whitespace-skipped) with a lookahead so
    /// `1` doesn't half-match `12` and `n` doesn't half-match
    /// `name`. Used by Phase B's `add 1:n …` for the literal
    /// `1`. Surfaces in the expected-set as `` `<literal>` ``,
    /// matching chumsky's labelled-token rendering.
    Literal(&'static str),
    #[allow(dead_code)]
    StringLit,
    #[allow(dead_code)]
    BlobLit,
    /// A `--name` flag. Walker matches the flag shape and
    /// asserts the name matches the expected literal.
    Flag(&'static str),
    /// A non-whitespace run consumed verbatim from source. Per
    /// ADR-0024's path-bearing-commands UX change, paths with
    /// spaces use the quoted form (`StringLit`); `BarePath`
    /// terminates at the first whitespace byte.
    BarePath,
    /// Try each child in order. The first one that matches a
    /// non-empty prefix wins; if none match, the choice fails
    /// with the union of expectations.
    Choice(&'static [Self]),
    /// All children must match in order. Whitespace is implicitly
    /// allowed between siblings.
    Seq(&'static [Self]),
    /// The inner node may match or be skipped.
    Optional(&'static Self),
    /// `inner` matches at least `min` times, separated by
    /// `separator` (if any). Phase C+ uses this for `with pk`
    /// column lists.
    #[allow(dead_code)]
    Repeated {
        inner: &'static Self,
        separator: Option<&'static Self>,
        min: usize,
    },
    /// Walks the referenced `&'static Node` once, mandatory
    /// (ADR-0026 §2). The reference indirection is what lets a
    /// named `static` grammar fragment appear inside its own
    /// subtree: a `Seq` / `Choice` embeds its children by value
    /// and so cannot close a cycle, but a `&'static Node`
    /// reference can point back at an enclosing fragment. This
    /// is the mechanism the stratified WHERE-expression grammar
    /// recurses through — the `( or_expr )` branch and the
    /// `not_expr` self-reference.
    ///
    /// The walker counts active `Subgrammar` frames in
    /// `WalkContext::subgrammar_depth` and refuses past
    /// `walker::driver::MAX_SUBGRAMMAR_DEPTH`, so pathologically
    /// nested input (`((((…))))`) fails with a friendly error
    /// rather than overflowing the parser stack.
    ///
    /// The static counterpart of `DynamicSubgrammar`: that one
    /// builds a fresh node from the `WalkContext` at walk time;
    /// this one references a fixed fragment already in the
    /// grammar tree.
    Subgrammar(&'static Self),
    /// Like `Subgrammar`, but the walker additionally **pushes a
    /// new `ScopeFrame`** onto `WalkContext::from_scope_stack` on
    /// entry and pops it on exit (ADR-0032 §10.2). The
    /// `subgrammar_depth` counter increments uniformly across
    /// both variants — the depth cap applies the same way — so
    /// this variant introduces no new walker capability for
    /// grammar recursion; it only layers lexical-scope discipline
    /// on top.
    ///
    /// Used at every SQL `SELECT` recursion point: subqueries
    /// in `sql_expr.rs` (scalar `(SELECT …)`, `IN (SELECT …)`,
    /// `[NOT] EXISTS (SELECT …)`) and CTE bodies in
    /// `sql_select.rs` reference the compound-SELECT through
    /// `Node::ScopedSubgrammar(&SQL_SELECT_COMPOUND)`. DSL `Expr`
    /// recursion (ADR-0026) and the `sql_expr.rs` precedence-
    /// ladder recursion (ADR-0031) keep using the plain
    /// `Subgrammar` variant and never push a scope.
    ScopedSubgrammar(&'static Self),
    /// Resolves at walk time using the active `WalkContext`.
    /// Phase D+ uses this for `column_value_list`. The factory
    /// is pure in `ctx`, so the walker memoizes the resolution
    /// (one leak per distinct schema shape).
    #[allow(dead_code)]
    DynamicSubgrammar(fn(&WalkContext) -> Self),
    /// Like `DynamicSubgrammar` but the factory also sees the
    /// source and the current byte position, so it can look
    /// ahead. Used by the insert first-paren to discriminate
    /// Form A (`(cols) values (...)`) from Form C (`(vals)`)
    /// before walking the contents — Form C then routes through
    /// the typed `column_value_list` (ADR-0024 §Phase D, Form C
    /// type-awareness). Not memoized: the output depends on the
    /// source, not just `ctx`.
    Lookahead(fn(&WalkContext, &str, usize) -> Self),
    /// Zero-width node that *establishes the active column* for the
    /// value slot that follows it (ADR-0036 Phase 3b). Matches the
    /// empty string and, as a side effect, sets
    /// `WalkContext::current_column` to the referenced column and
    /// `pending_value_column` to its name — exactly as an
    /// `Ident { writes_column: true }` does, but without consuming a
    /// column identifier from the input.
    ///
    /// This is the primitive that gives `INSERT … VALUES (…)`
    /// positions a per-position column identity: the positions are
    /// positional (no per-position column ident to write
    /// `current_column`), so a `DynamicSubgrammar` factory
    /// (`sql_insert::sql_value_list`) emits `SetColumn(colᵢ)` before
    /// each value position, then the shared boundary-aware `SET_VALUE`
    /// slot routes a lone literal to that column's typed slot and any
    /// expression to `sql_expr`. The referenced `TableColumn` is
    /// leaked by the factory (bounded by the column count, like the
    /// `DynamicSubgrammar` `Box::leak`).
    SetColumn(&'static crate::completion::TableColumn),
    /// Typed value-literal slot (ADR-0024 §Phase D §typed-value-slots).
    ///
    /// Walks `inner` to consume the literal but records the
    /// column type in `WalkContext::pending_value_type` so the
    /// hint resolver can emit per-type catalog prose ("Type an
    /// integer", "Type a date as 'YYYY-MM-DD'", …) at empty
    /// prefix at this slot. When `column_name` is `Some`, the
    /// walker also writes `pending_value_column` so the hint
    /// can be rendered with the actual column name (e.g. "for
    /// `Email`: Type a quoted string …") rather than a generic
    /// type hint. The recorded values clear on a successful
    /// inner match — so positions BETWEEN typed slots
    /// (`insert into T values (1` mid-input) don't carry stale
    /// hint state.
    TypedValueSlot {
        ty: crate::dsl::types::Type,
        column_name: Option<&'static str>,
        inner: &'static Self,
    },
    /// Annotates `inner` with a hint-panel `HintMode` (ADR-0024
    /// §HintMode-per-node). On entry the walker records `mode`
    /// in `WalkContext::pending_hint_mode`; on a successful
    /// inner match the record clears (so positions past the
    /// slot don't carry stale hint state). Transparent to
    /// matching, highlighting and the expected-set otherwise —
    /// it walks `inner` and returns its result verbatim.
    ///
    /// This is the node-attached replacement for the hint
    /// resolver's earlier signature-matching: the grammar tree
    /// declares the hint mode at the slot, the walker
    /// propagates it, the resolver reads it. Used by the
    /// value-literal fallback slot (`ProseOnly`) and `NewName`
    /// ident slots (`ForceProse`).
    Hinted {
        mode: HintMode,
        inner: &'static Self,
    },
}

/// Which mode group a registered command belongs to (ADR-0030
/// §2, ADR-0033 Amendment 1).
///
/// Category is a *dispatcher* concern, not intrinsic to a
/// command's grammar, so it is attached at the `REGISTRY`
/// registration site rather than as a field on every
/// `CommandNode`. The dispatcher (`walker::walk`) uses it to
/// route a given input by the active input mode:
///
/// - `Simple` commands are the DSL surface; available in both
///   simple and advanced mode.
/// - `Advanced` commands are the SQL surface; available only in
///   advanced mode. In simple mode an advanced-only entry word
///   yields the "this is SQL" hint (`advanced_mode.sql_in_simple`).
///
/// A *shared* entry word (e.g. `insert`, from Phase 3 sub-phase
/// 3b on) carries a node in *both* groups — a `Simple` DSL node
/// and an `Advanced` SQL node. The dispatcher tries the SQL node
/// first in advanced mode and falls back to the DSL node when the
/// SQL shape does not match.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum CommandCategory {
    Simple,
    Advanced,
}

/// Top-level entry record. One per command. The `entry` keyword
/// alone identifies which command the walker dispatches to;
/// `shape` is what follows the entry word.
pub struct CommandNode {
    pub entry: Word,
    pub shape: Node,
    /// Builds the typed `Command` AST from the matched terminal
    /// path. May fail with a `ValidationError` for content-level
    /// rejections that are easier to express imperatively than
    /// as a per-node validator (Phase A: none — every app
    /// command's ast_builder is infallible).
    ///
    /// `source` is the full input line being parsed. Most builders
    /// reconstruct the `Command` from the matched `MatchedPath`
    /// alone and ignore it; SQL builders whose `Command` carries
    /// the validated SQL text (ADR-0030 §4/§6, ADR-0031 §2) read
    /// it.
    pub ast_builder: fn(&MatchedPath, &str) -> Result<Command, ValidationError>,
    /// Catalog key (`help.<id>`) for this command's in-app
    /// `help` entry. Consumed by `App::note_help`, which
    /// iterates the REGISTRY and translates each `help_id` —
    /// so a newly-registered command appears in `help`
    /// automatically (ADR-0024 §help_id).
    pub help_id: Option<&'static str>,
    /// Catalog keys under `parse.usage.*` to render in the
    /// "usage:" block when a parse error fires for this command
    /// (ADR-0021 §1, ADR-0024 §architecture). Multi-form families
    /// like `drop` (drop table / drop column / drop relationship)
    /// carry every variant so the user sees the full family on a
    /// generic-entry-word failure.
    pub usage_ids: &'static [&'static str],
}

/// Look up the usage catalog keys for the entry word at the start
/// of `source`.
///
/// Case-insensitive, whitespace-tolerant. Replaces
/// `dsl::usage::matched_entry` — the walker is the single source
/// of truth for which command a given input belongs to.
///
/// Returns the canonical (primary-form) entry literal and the
/// `usage_ids` list, or `None` if no entry word matches.
#[must_use]
pub fn usage_keys_for_input(source: &str) -> Option<(&'static str, Vec<&'static str>)> {
    usage_keys_for_input_in_mode(source, crate::mode::Mode::Simple)
}

/// Mode-aware variant of [`usage_keys_for_input`] (ADR-0042 G3).
///
/// A shared entry word (`create`, `drop`, `insert`, …) registers a
/// `Simple` DSL node *and* one or more `Advanced` SQL nodes. The
/// usage block must reflect the surface the user is actually typing:
/// the SQL forms in `Advanced` mode, the DSL forms in `Simple` mode
/// — otherwise advanced-mode `create` shows the DSL `create table …
/// with pk …` template, which is not valid SQL.
///
/// Selection prefers candidates whose [`CommandCategory`] matches
/// the mode; if the entry word has none in that category (an
/// app-lifecycle command is `Simple`-only yet usable in both modes),
/// every candidate is used. The returned keys are the union of the
/// selected nodes' `usage_ids`, de-duplicated in registry order — so
/// advanced `create` shows both `sql_create_table` and
/// `sql_create_index`.
#[must_use]
pub fn usage_keys_for_input_in_mode(
    source: &str,
    mode: crate::mode::Mode,
) -> Option<(&'static str, Vec<&'static str>)> {
    use crate::dsl::walker::lex_helpers::{consume_ident, skip_whitespace};
    let start = skip_whitespace(source, 0);
    let (kw_start, kw_end) = consume_ident(source, start)?;
    let word = &source[kw_start..kw_end];
    let candidates = commands_for_entry_word(word);
    if candidates.is_empty() {
        return None;
    }
    let union = |nodes: &[(usize, &'static CommandNode, CommandCategory)]| -> Vec<&'static str> {
        let mut keys: Vec<&'static str> = Vec::new();
        for (_, node, _) in nodes {
            for k in node.usage_ids {
                if !keys.contains(k) {
                    keys.push(*k);
                }
            }
        }
        keys
    };
    // Advanced mode: every candidate form is reachable — the SQL
    // nodes are primary, and the DSL nodes remain valid via fallback
    // (verified: `create table … with pk` and `drop column …` both
    // run in advanced mode). Show them all, mode-primary (Advanced)
    // first, so the usage hint never hides input that works. Simple
    // mode: only the DSL forms — the SQL-only forms hit the "this is
    // SQL" rail and are not reachable. (ADR-0042 G3.)
    let selected: Vec<(usize, &'static CommandNode, CommandCategory)> =
        if mode == crate::mode::Mode::Advanced {
            let mut v: Vec<_> = candidates
                .iter()
                .copied()
                .filter(|(_, _, c)| *c == CommandCategory::Advanced)
                .collect();
            v.extend(
                candidates
                    .iter()
                    .copied()
                    .filter(|(_, _, c)| *c != CommandCategory::Advanced),
            );
            v
        } else {
            candidates
                .iter()
                .copied()
                .filter(|(_, _, c)| *c == CommandCategory::Simple)
                .collect()
        };
    // Degenerate guard: an advanced-only word in simple mode (not
    // normally reachable — it hits the SQL rail first) leaves
    // `selected` empty; fall back to all candidates so a usage block
    // still renders rather than the available-commands fallback.
    let pick = if selected.is_empty() { candidates } else { selected };
    let keys = union(&pick);
    if keys.is_empty() {
        return None;
    }
    let entry = pick[0].1.entry.primary;
    Some((entry, keys))
}

/// The single usage template most relevant to `source`, when
/// one is determinable.
///
/// A single-form command resolves to its one usage key. A
/// multi-form command (`add`, `drop`) disambiguates by the
/// form word after the entry keyword — so a parse error in
/// `add index …` resolves to the `add index` usage rather than
/// the first-listed `add column`. Returns `None` for a bare
/// multi-form entry word (`add` with nothing after it), where
/// no form has been chosen — the caller decides whether to
/// show the whole family or nothing.
#[must_use]
pub fn usage_key_for_input(source: &str) -> Option<&'static str> {
    usage_key_for_input_in_mode(source, crate::mode::Mode::Simple)
}

/// Mode-aware variant of [`usage_key_for_input`] (ADR-0042 G3) —
/// disambiguates the single most-relevant usage key from the
/// mode-selected key set.
#[must_use]
pub fn usage_key_for_input_in_mode(
    source: &str,
    mode: crate::mode::Mode,
) -> Option<&'static str> {
    use crate::dsl::walker::lex_helpers::{consume_ident, skip_whitespace};
    let (_entry, keys) = usage_keys_for_input_in_mode(source, mode)?;
    let first = *keys.first()?;
    if keys.len() == 1 {
        return Some(first);
    }
    // Multi-form: the form is named by the token right after
    // the entry keyword.
    let start = skip_whitespace(source, 0);
    let (_, entry_end) = consume_ident(source, start)?;
    let after = skip_whitespace(source, entry_end);
    // The `add 1:n relationship` form opens with a digit.
    if source.as_bytes().get(after).is_some_and(u8::is_ascii_digit) {
        return keys.iter().copied().find(|k| k.ends_with("relationship"));
    }
    // The `create m:n relationship` form (ADR-0045) opens with `m:n`
    // — a letter, so the digit branch misses it, and its usage key ends
    // `…create_m2n` (not `relationship`).
    if source[after..].get(..3).is_some_and(|s| s.eq_ignore_ascii_case("m:n")) {
        return keys.iter().copied().find(|k| k.ends_with("m2n"));
    }
    // Otherwise the form word is an identifier — `column`,
    // `index`, `table`, `relationship` — matched against the
    // usage key's suffix.
    let (s, e) = consume_ident(source, after)?;
    let form = source[s..e].to_ascii_lowercase();
    keys.iter().copied().find(|k| k.ends_with(form.as_str()))
}

/// Every command-entry word in the registry, sorted alphabetically
/// by primary literal. Replaces `dsl::usage::entry_keywords_alphabetised`
/// which read the same data through the legacy `usage::REGISTRY`.
#[must_use]
pub fn entry_words_alphabetised() -> Vec<&'static str> {
    let mut words: Vec<&'static str> =
        REGISTRY.iter().map(|(c, _)| c.entry.primary).collect();
    words.sort_unstable();
    words.dedup();
    words
}

/// The active grammar registry, each command paired with its
/// dispatch [`CommandCategory`] (ADR-0033 Amendment 1).
///
/// Migrated commands route through this; everything else falls
/// through to the chumsky path in `dsl::parser`. `Advanced`
/// commands (`select`, `with`, and — from sub-phase 3b — the SQL
/// `insert` / `update` / `delete` nodes) are the SQL surface;
/// the rest are the DSL surface (`Simple`). A shared entry word
/// will appear twice (one `Simple`, one `Advanced` node); the
/// dispatcher selects by mode.
pub static REGISTRY: &[(&CommandNode, CommandCategory)] = &[
    (&app::QUIT, CommandCategory::Simple),
    (&app::HELP, CommandCategory::Simple),
    (&app::REBUILD, CommandCategory::Simple),
    (&app::SAVE, CommandCategory::Simple),
    (&app::NEW, CommandCategory::Simple),
    (&app::LOAD, CommandCategory::Simple),
    (&app::EXPORT, CommandCategory::Simple),
    (&app::IMPORT, CommandCategory::Simple),
    (&app::MODE, CommandCategory::Simple),
    (&app::MESSAGES, CommandCategory::Simple),
    (&app::UNDO, CommandCategory::Simple),
    (&app::REDO, CommandCategory::Simple),
    (&app::COPY, CommandCategory::Simple),
    (&ddl::DROP, CommandCategory::Simple),
    (&ddl::ADD, CommandCategory::Simple),
    (&ddl::RENAME, CommandCategory::Simple),
    (&ddl::CHANGE, CommandCategory::Simple),
    (&ddl::CREATE, CommandCategory::Simple),
    (&ddl::CREATE_M2N, CommandCategory::Simple),
    (&data::SHOW, CommandCategory::Simple),
    (&data::SEED, CommandCategory::Simple),
    (&data::INSERT, CommandCategory::Simple),
    (&data::UPDATE, CommandCategory::Simple),
    (&data::DELETE, CommandCategory::Simple),
    (&data::REPLAY, CommandCategory::Simple),
    (&data::EXPLAIN, CommandCategory::Simple),
    (&data::SELECT, CommandCategory::Advanced),
    (&data::WITH, CommandCategory::Advanced),
    // Shared entry words (sub-phase 3j, ADR-0033 §2 / Amendment 1):
    // `insert` / `update` / `delete` each appear twice — the
    // `Simple` DSL node above and this `Advanced` SQL node. The
    // dispatcher tries the SQL node first in Advanced mode and falls
    // back to the DSL node when the SQL shape does not match.
    (&data::SQL_INSERT, CommandCategory::Advanced),
    (&data::SQL_UPDATE, CommandCategory::Advanced),
    (&data::SQL_DELETE, CommandCategory::Advanced),
    // Shared entry word `explain` (ADR-0039): the `Simple` DSL
    // `data::EXPLAIN` (above) wraps `show data` / `update` / `delete`;
    // this `Advanced` node wraps the SQL `select` / `with` / `insert`
    // / `update` / `delete`. SQL-first / DSL-fallback in advanced mode
    // (so `explain show data …` and DSL-only `--all-rows` still reach
    // the DSL node); DSL-only in simple mode.
    (&data::EXPLAIN_SQL, CommandCategory::Advanced),
    // Shared entry word `create` (ADR-0035 §2): the simple
    // `ddl::CREATE` (above) and these advanced SQL nodes. The
    // dispatcher tries the advanced candidates first in advanced mode
    // and falls back to the `create table … with pk …` DSL node when no
    // SQL shape matches — the `insert` precedent. 4d adds
    // SQL_CREATE_INDEX, so `create` now has *two* advanced nodes;
    // `decide` tries both (`create table …` → SQL_CREATE_TABLE,
    // `create [unique] index …` → SQL_CREATE_INDEX).
    (&ddl::SQL_CREATE_TABLE, CommandCategory::Advanced),
    (&ddl::SQL_CREATE_INDEX, CommandCategory::Advanced),
    // `alter` is a new advanced-*only* DDL entry word (ADR-0035 §2/§4e),
    // like `select`/`with` — no simple node, so `is_advanced_only` is
    // true and simple-mode `alter …` gets the "this is SQL" hint.
    (&ddl::SQL_ALTER_TABLE, CommandCategory::Advanced),
    // Shared `drop` entry word: `ddl::DROP` (simple) and these advanced
    // SQL nodes. SQL-first in advanced mode; `drop table [if exists] T`
    // → SQL_DROP_TABLE, `drop index [if exists] <name>` → SQL_DROP_INDEX
    // (4d — `drop` now has *two* advanced nodes; the dispatcher's
    // `decide` tries all advanced candidates). `drop column`/`drop
    // relationship`/`drop index on T(…)` fall back to the simple `drop`
    // node.
    (&ddl::SQL_DROP_TABLE, CommandCategory::Advanced),
    (&ddl::SQL_DROP_INDEX, CommandCategory::Advanced),
];

/// Whether `entry` names an advanced-mode-only command (ADR-0030
/// §2, ADR-0033 Amendment 1). Case-insensitive, matching
/// keyword-matching elsewhere.
///
/// True when the entry word is registered and *every* candidate
/// for it is `Advanced` — i.e. there is no DSL (`Simple`) command
/// to fall back to. A shared entry word (a Simple DSL node plus
/// an Advanced SQL node) is therefore *not* advanced-only: it is
/// available in simple mode as DSL.
#[must_use]
pub fn is_advanced_only(entry: &str) -> bool {
    let mut found = false;
    for (c, category) in REGISTRY {
        if c.entry.matches(entry) {
            found = true;
            if *category == CommandCategory::Simple {
                return false;
            }
        }
    }
    found
}

/// Look up the first `CommandNode` registered for an entry word,
/// case-insensitively. Returns the index into `REGISTRY` so
/// callers can use it as a `WalkOutcome::Match { command_idx }`.
///
/// For shared entry words this returns whichever node is listed
/// first in `REGISTRY`; callers that must distinguish the Simple
/// from the Advanced candidate use [`commands_for_entry_word`].
pub fn command_for_entry_word(word: &str) -> Option<(usize, &'static CommandNode)> {
    REGISTRY
        .iter()
        .enumerate()
        .find(|(_, (c, _))| c.entry.matches(word))
        .map(|(i, (c, _))| (i, *c))
}

/// Every `CommandNode` registered for an entry word, with its
/// `REGISTRY` index and [`CommandCategory`], case-insensitively
/// (ADR-0033 Amendment 1).
///
/// A non-shared entry word returns a single candidate; a shared
/// entry word (`insert` / `update` / `delete` from sub-phase 3b)
/// returns its `Simple` DSL node and `Advanced` SQL node. The
/// dispatcher picks among them by the active input mode.
#[must_use]
pub fn commands_for_entry_word(
    word: &str,
) -> Vec<(usize, &'static CommandNode, CommandCategory)> {
    REGISTRY
        .iter()
        .enumerate()
        .filter(|(_, (c, _))| c.entry.matches(word))
        .map(|(i, (c, category))| (i, *c, *category))
        .collect()
}

#[cfg(test)]
mod usage_key_tests {
    use super::usage_key_for_input;

    /// Every multi-form command resolves a typed form to its
    /// own usage key — a parse error in one form must never
    /// show another form's usage (the handoff-18 `151ed08` fix;
    /// regression-locked here, including the `add 1:n
    /// relationship` digit-led form).
    #[test]
    fn multi_form_commands_resolve_to_the_typed_form() {
        let cases = [
            ("add column to T: c (int)", "parse.usage.add_column"),
            ("add index on T (c)", "parse.usage.add_index"),
            (
                "add constraint unique to T.c",
                "parse.usage.add_constraint",
            ),
            (
                "drop constraint check from T.c",
                "parse.usage.drop_constraint",
            ),
            (
                "add 1:n relationship from A.x to B.y",
                "parse.usage.add_relationship",
            ),
            // Trailing junk must not change the resolved form.
            (
                "add 1:n relationship from A.x to B.y --",
                "parse.usage.add_relationship",
            ),
            ("drop table T", "parse.usage.drop_table"),
            ("drop column from table T: c", "parse.usage.drop_column"),
            ("drop index i", "parse.usage.drop_index"),
            (
                "drop relationship r",
                "parse.usage.drop_relationship",
            ),
            ("show data T", "parse.usage.show_data"),
            ("show table T", "parse.usage.show_table"),
            // `create` is multi-form (table vs m:n, ADR-0045): each typed
            // form resolves to its own usage key.
            ("create table T with pk id(int)", "parse.usage.create_table"),
            (
                "create m:n relationship from A to B",
                "parse.usage.create_m2n",
            ),
        ];
        for (input, expected) in cases {
            assert_eq!(
                usage_key_for_input(input),
                Some(expected),
                "usage key for {input:?}",
            );
        }
    }

    #[test]
    fn a_bare_multi_form_entry_word_resolves_to_no_single_form() {
        // `add` / `drop` alone — no form chosen; the caller
        // shows the whole family rather than guessing.
        assert_eq!(usage_key_for_input("add "), None);
        assert_eq!(usage_key_for_input("drop "), None);
    }

    #[test]
    fn a_single_form_command_resolves_to_its_one_key() {
        assert_eq!(
            usage_key_for_input("create table T with pk"),
            Some("parse.usage.create_table"),
        );
    }

    #[test]
    fn no_two_registered_commands_share_a_help_id() {
        // `note_help` emits one help block per `help_id: Some(_)`
        // with no dedup, so a duplicate help_id prints the same
        // command twice in `help`. Shared-entry-word `Advanced`
        // nodes (SQL_INSERT, …, EXPLAIN_SQL) therefore carry
        // `help_id: None` and defer to their `Simple` sibling.
        let mut seen = std::collections::HashSet::new();
        for (command, _category) in super::REGISTRY {
            if let Some(id) = command.help_id {
                assert!(
                    seen.insert(id),
                    "duplicate help_id `{id}` in REGISTRY would print twice in `help`",
                );
            }
        }
    }
}