// 🤖 The app-schema engine: a small interpreter for the declarative app schema
// (docs/app-schema.md). A schema describes an app's state, how events fold into
// it, and how the folded state presents as a searchable, sectioned list with
// actions — in plain JSON that a human, an LLM, and BOTH runtimes (this JS
// engine and the Swift twin in ios/LChatCore/Sources/LChatCore/SchemaEngine.swift)
// can read. The two implementations are drift-gated by the shared event vectors
// (test/event-vectors/*) and by test/schema-engine.test.mjs — change behaviour
// here and the Swift twin must change in the same PR.
//
// Deliberately dependency-free and DOM-free so it runs in Node tests unchanged.

// ---- values: `$refs`, literals, and tiny numeric ops ----------------------
// A schema value is JSON. Strings starting with "$" are references into the
// evaluation scope ("$item.done", "$event.text", "$now"); "$$" escapes a
// literal leading dollar. Objects of the shape {"add":[a,b]} / {"sub":[a,b]}
// are the only computed values. Everything else is itself.

function lookupPath(scope, path) {
  let cur = scope;
  for (const part of path.split(".")) {
    if (cur == null || typeof cur !== "object") return null;
    cur = cur[part];
  }
  return cur === undefined ? null : cur;
}

export function evalValue(v, scope) {
  if (typeof v === "string") {
    if (v.startsWith("$$")) return v.slice(1);
    if (v.startsWith("$")) return lookupPath(scope, v.slice(1));
    return v;
  }
  if (Array.isArray(v)) return v.map((x) => evalValue(x, scope));
  if (v && typeof v === "object") {
    if (Array.isArray(v.add) && v.add.length === 2) return num(evalValue(v.add[0], scope)) + num(evalValue(v.add[1], scope));
    if (Array.isArray(v.sub) && v.sub.length === 2) return num(evalValue(v.sub[0], scope)) - num(evalValue(v.sub[1], scope));
    const out = {};
    for (const [k, x] of Object.entries(v)) out[k] = evalValue(x, scope);
    return out;
  }
  return v;
}

function num(v) {
  const n = typeof v === "number" ? v : typeof v === "string" ? parseFloat(v) : v === true ? 1 : 0;
  return isNaN(n) ? 0 : n;
}

function truthy(v) {
  if (Array.isArray(v)) return v.length > 0;
  return !!v;
}

// ---- conditions -----------------------------------------------------------
// {"truthy": v} {"not": c} {"all": [c…]} {"any": [c…]} {"eq": [a,b]}
// {"empty": v} {"gt"|"gte"|"lt"|"lte": [a,b]}

export function evalCond(c, scope) {
  if (c == null) return true;
  if ("truthy" in c) return truthy(evalValue(c.truthy, scope));
  if ("not" in c) return !evalCond(c.not, scope);
  if ("all" in c) return c.all.every((x) => evalCond(x, scope));
  if ("any" in c) return c.any.some((x) => evalCond(x, scope));
  if ("eq" in c) return evalValue(c.eq[0], scope) === evalValue(c.eq[1], scope);
  if ("empty" in c) return !truthy(evalValue(c.empty, scope));
  if ("gt" in c) return num(evalValue(c.gt[0], scope)) > num(evalValue(c.gt[1], scope));
  if ("gte" in c) return num(evalValue(c.gte[0], scope)) >= num(evalValue(c.gte[1], scope));
  if ("lt" in c) return num(evalValue(c.lt[0], scope)) < num(evalValue(c.lt[1], scope));
  if ("lte" in c) return num(evalValue(c.lte[0], scope)) <= num(evalValue(c.lte[1], scope));
  return false;
}

// ---- templates ------------------------------------------------------------
// Display strings interpolate "{path}" or "{path|filter}" against the scope;
// "{{" escapes a literal brace. Filters: count (array length), reltime
// ("in 2h" / "5m ago" / "now" — needs $now in scope), abstime (locale
// "Wed, Jul 9, 14:30"), number (String(n)).

export function relTime(when, now) {
  let diff = when - now;
  const past = diff < 0;
  diff = Math.abs(diff);
  const mins = Math.round(diff / 60000);
  let s;
  if (mins < 1) return "now";
  if (mins < 60) s = mins + "m";
  else if (mins < 1440) s = Math.round(mins / 60) + "h";
  else s = Math.round(mins / 1440) + "d";
  return past ? s + " ago" : "in " + s;
}

export function absTime(when) {
  try {
    return new Date(when).toLocaleString([], { weekday: "short", month: "short", day: "numeric", hour: "2-digit", minute: "2-digit" });
  } catch {
    return "";
  }
}

function applyFilter(v, filter, scope) {
  switch (filter) {
    case "count":
      return Array.isArray(v) ? String(v.length) : "0";
    case "reltime":
      return relTime(num(v), num(scope.now));
    case "abstime":
      return absTime(num(v));
    case "number":
      return String(num(v));
    default:
      return str(v);
  }
}

function str(v) {
  if (v == null) return "";
  if (typeof v === "string") return v;
  if (typeof v === "number") return String(v);
  if (typeof v === "boolean") return v ? "true" : "false";
  return "";
}

export function evalTemplate(tpl, scope) {
  let out = "";
  let i = 0;
  while (i < tpl.length) {
    const ch = tpl[i];
    if (ch === "{") {
      if (tpl[i + 1] === "{") {
        out += "{";
        i += 2;
        continue;
      }
      const end = tpl.indexOf("}", i);
      if (end < 0) {
        out += tpl.slice(i);
        break;
      }
      const expr = tpl.slice(i + 1, end);
      const bar = expr.indexOf("|");
      const path = (bar >= 0 ? expr.slice(0, bar) : expr).trim();
      const filter = bar >= 0 ? expr.slice(bar + 1).trim() : "";
      const v = lookupPath(scope, path);
      out += filter ? applyFilter(v, filter, scope) : str(v);
      i = end + 1;
    } else {
      out += ch;
      i++;
    }
  }
  return out;
}

// ---- state & fold ---------------------------------------------------------

// The declared zero value for a field type ("string" | "bool" | "number" | "time").
function zeroFor(type) {
  return type === "string" ? "" : type === "bool" ? false : 0;
}

// Coerce a value to a declared field type — missing/null becomes the zero
// value, so "$event.ts" on an event with no ts folds to 0, exactly like the
// hand-written reducers' `e.ts || 0`.
function coerce(v, type) {
  if (v == null) return zeroFor(type);
  switch (type) {
    case "string":
      return typeof v === "string" ? v : str(v);
    case "bool":
      return truthy(v);
    case "number":
    case "time":
      return num(v);
    default:
      return v;
  }
}

export function initialState(schema) {
  const s = {};
  for (const key of Object.keys(schema.state || {})) s[key] = [];
  return s;
}

// foldSchemaEvent applies one event to state via the schema's declarative fold
// ops. Unknown types and failed `require` guards return state unchanged. Pure:
// always returns fresh arrays, never mutates. `add` is idempotent by the
// collection key (Invariant #1 in apps/README.md).
export function foldSchemaEvent(schema, state, event) {
  const spec = (schema.events || {})[event && event.type];
  if (!spec) return state;
  const scope = { event, state };
  for (const r of spec.require || []) {
    if (!truthy(evalValue(r, scope))) return state;
  }
  let next = state;
  for (const op of spec.fold || []) next = applyFoldOp(schema, next, op, event);
  return next;
}

function collectionDecl(schema, name) {
  return (schema.state || {})[name] || { key: "id", fields: {} };
}

function applyFoldOp(schema, state, op, event) {
  const scope = { event, state };
  if (op.add) {
    const decl = collectionDecl(schema, op.add.to);
    const list = state[op.add.to] || [];
    const item = {};
    for (const [field, v] of Object.entries(op.add.item || {})) {
      item[field] = coerce(evalValue(v, scope), decl.fields[field]);
    }
    const keyField = decl.key || "id";
    if (list.some((i) => i[keyField] === item[keyField])) return { ...state, [op.add.to]: list.slice() };
    return { ...state, [op.add.to]: [...list, item] };
  }
  if (op.update) {
    const decl = collectionDecl(schema, op.update.in);
    const keyField = decl.key || "id";
    const id = evalValue(op.update.id, scope);
    const list = (state[op.update.in] || []).map((i) => {
      if (i[keyField] !== id) return i;
      const patch = {};
      for (const [field, v] of Object.entries(op.update.set || {})) {
        patch[field] = coerce(evalValue(v, scope), decl.fields[field]);
      }
      return { ...i, ...patch };
    });
    return { ...state, [op.update.in]: list };
  }
  if (op.remove) {
    const decl = collectionDecl(schema, op.remove.from);
    const keyField = decl.key || "id";
    const id = evalValue(op.remove.id, scope);
    return { ...state, [op.remove.from]: (state[op.remove.from] || []).filter((i) => i[keyField] !== id) };
  }
  if (op.move) {
    const decl = collectionDecl(schema, op.move.in);
    const keyField = decl.key || "id";
    const id = evalValue(op.move.id, scope);
    const list = state[op.move.in] || [];
    const from = list.findIndex((i) => i[keyField] === id);
    if (from < 0) return { ...state, [op.move.in]: list.slice() };
    // `after` names the item this one now follows; ""/missing = the list's
    // start. An unknown `after` id (it was removed concurrently) is a no-op —
    // both runtimes must agree, so the rule is pinned here.
    const after = str(evalValue(op.move.after, scope));
    const next = list.slice();
    const [moved] = next.splice(from, 1);
    let to = 0;
    if (after) {
      const anchor = next.findIndex((i) => i[keyField] === after);
      if (anchor < 0) return { ...state, [op.move.in]: list.slice() };
      to = anchor + 1;
    }
    next.splice(to, 0, moved);
    return { ...state, [op.move.in]: next };
  }
  if (op.sort) {
    const by = op.sort.by;
    const dir = op.sort.dir === "desc" ? -1 : 1;
    // Decorate with the index so the sort is stable on every runtime (Swift's
    // sort isn't guaranteed stable; JS's is — the tiebreak keeps them twins).
    const list = (state[op.sort.in] || []).map((item, idx) => ({ item, idx }));
    list.sort((a, b) => {
      const av = num(a.item[by]);
      const bv = num(b.item[by]);
      if (av !== bv) return (av - bv) * dir;
      return a.idx - b.idx;
    });
    return { ...state, [op.sort.in]: list.map((e) => e.item) };
  }
  return state;
}

// ---- partition: the grace-period buckets ----------------------------------
// Splits the primary collection into { active, grace, archived } from the
// clock — the reducer only records the completion time; the linger-then-tuck
// behaviour is DERIVED here on every render (never a timer mutating state).
// A flagged item with no timestamp (legacy data) reads as long-archived.

export function partitionState(schema, state, now) {
  const p = schema.partition;
  const items = (state || {})[schema.collection] || [];
  const active = [];
  const grace = [];
  const archived = [];
  if (!p) return { active: items.slice(), grace, archived };
  for (const item of items) {
    const at = item[p.flag] ? item[p.at] || 1 : 0;
    if (!at) active.push(item);
    else if (now - at < p.graceMs) grace.push(item);
    else archived.push(item);
  }
  return { active, grace, archived };
}

// ---- the list render model -------------------------------------------------
// listModel resolves the main view against state + the clock + transient UI
// state (collapsed sections, a search query) into a plain description a
// renderer draws directly: summary line, sections with ordered rows (each row
// knowing whether it's in its grace window), and the empty-state line. Both
// renderers (schema-render.js on the web, SchemaAppView in SwiftUI) consume
// this same shape.

export function listModel(schema, state, now, ui = {}) {
  const view = schema.views && schema.views.main;
  if (!view) return null;
  // A never-used app thread has no folded state at all (the host only seeds on
  // the first event), so `state` can arrive undefined — treat it as empty.
  state = state || initialState(schema);
  const items = state[schema.collection] || [];
  const buckets = partitionState(schema, state, now);
  const scope = { state, items, buckets, now };

  let summary = "";
  for (const c of view.summary || []) {
    if (evalCond(c.if, scope)) {
      summary = evalTemplate(c.text, scope);
      break;
    }
  }

  const query = (ui.query || "").trim().toLowerCase();
  const searchFields = (view.search && view.search.fields) || [];
  const matches = (item) => {
    if (!query) return true;
    return searchFields.some((f) => str(item[f]).toLowerCase().includes(query));
  };

  const graceIds = new Set(buckets.grace.map((i) => i[collectionDecl(schema, schema.collection).key || "id"]));
  const indexOf = new Map(items.map((i, idx) => [i, idx]));

  const sections = [];
  for (const s of view.sections || []) {
    const names = Array.isArray(s.source) ? s.source : [s.source];
    let rows = names.flatMap((n) => buckets[n] || []).filter(matches);
    if (s.order && s.order.by) {
      const by = s.order.by;
      const dir = s.order.dir === "desc" ? -1 : 1;
      const decorated = rows.map((item) => ({ item, idx: indexOf.get(item) || 0 }));
      decorated.sort((a, b) => {
        const av = by === "_index" ? a.idx : num(a.item[by]);
        const bv = by === "_index" ? b.idx : num(b.item[by]);
        if (av !== bv) return (av - bv) * dir;
        return a.idx - b.idx;
      });
      rows = decorated.map((d) => d.item);
    }
    const collapsed = s.collapsible ? (ui.collapsed && s.id in ui.collapsed ? !!ui.collapsed[s.id] : !!s.collapsed) : false;
    const keyField = collectionDecl(schema, schema.collection).key || "id";
    sections.push({
      id: s.id,
      title: s.title ? evalTemplate(s.title, { ...scope, count: rows.length }) : "",
      collapsible: !!s.collapsible,
      collapsed,
      // Manual reorder: the action a drag runs (with $item + $after bound by
      // the renderer). Suppressed while searching — a filtered list has no
      // meaningful drop position. descending tells the renderer how a visual
      // drop maps to the array anchor.
      reorder: (!query && s.reorder && s.reorder.action) || "",
      descending: !!(s.order && s.order.dir === "desc"),
      rows: rows.map((item) => ({ item, grace: graceIds.has(item[keyField]) })),
    });
  }

  // The empty line shows when every non-collapsible section has no rows.
  let empty = "";
  const visibleRows = sections.filter((s) => !s.collapsible).reduce((n, s) => n + s.rows.length, 0);
  if (!visibleRows) {
    for (const c of view.empty || []) {
      if (evalCond(c.if, scope)) {
        empty = evalTemplate(c.text, scope);
        break;
      }
    }
  }

  return { summary, sections, empty, buckets };
}

// rowModel resolves one row's presentation (title/subtitle/note text + flags)
// for an item — the per-row half of listModel, split out so a renderer can
// re-resolve a single row on its refresh tick without rebuilding the list.
export function rowModel(schema, item, now) {
  const view = schema.views && schema.views.main;
  const row = (view && view.row) || {};
  const scope = { item, now };
  const out = { title: "", strike: false, muted: false, subtitle: "", accent: false, note: "" };
  if (row.title) {
    out.title = evalTemplate(row.title.text || "", scope);
    out.strike = row.title.strike ? evalCond(row.title.strike, scope) : false;
    out.muted = row.title.muted ? evalCond(row.title.muted, scope) : false;
  }
  for (const c of row.subtitle || []) {
    if (evalCond(c.if, scope)) {
      out.subtitle = evalTemplate(c.text, scope);
      out.accent = !!c.accent && evalCond(c.accent, scope);
      break;
    }
  }
  for (const c of row.note || []) {
    if (evalCond(c.if, scope)) {
      out.note = evalTemplate(c.text, scope);
      break;
    }
  }
  return out;
}

// detailModel resolves the detail ("show") view for one item: title +
// sections (definition rows / text blocks / buttons), conditions applied.
export function detailModel(schema, item, now) {
  const view = schema.views && schema.views.detail;
  if (!view) return null;
  const scope = { item, now };
  const sections = [];
  for (const s of view.sections || []) {
    if (s.kind === "definitions") {
      const rows = (s.rows || []).filter((r) => evalCond(r.if, scope)).map((r) => ({ term: r.term, value: evalTemplate(r.value || "", scope) }));
      if (rows.length) sections.push({ kind: "definitions", rows });
    } else if (s.kind === "text") {
      if (evalCond(s.if, scope)) {
        const body = evalTemplate(s.body || "", scope);
        if (body) sections.push({ kind: "text", body });
      }
    } else if (s.kind === "buttons") {
      const items = (s.items || []).filter((b) => evalCond(b.if, scope)).map((b) => ({ label: b.label, action: b.action, role: b.role || "", dismiss: !!b.dismiss }));
      if (items.length) sections.push({ kind: "buttons", items });
    }
  }
  return { title: evalTemplate(view.title || "", scope), sections };
}

// ---- chat projection -------------------------------------------------------
// The schema's `chat` map projects an app event into the Chat view: a bot
// bubble (string) or a /me-style action line ({ text, action: true }), or
// null for silence — the same contract as an app's chatMessage hook.

export function chatProjection(schema, event, opts) {
  const spec = (schema.chat || {})[event && event.type];
  if (!spec) return null;
  const scope = { event };
  if (!evalCond(spec.if, scope)) return null;
  const tpl = opts && opts.group && spec.groupText ? spec.groupText : spec.text;
  const text = evalTemplate(tpl, scope);
  return spec.action ? { text, action: true } : text;
}

// ---- actions ---------------------------------------------------------------
// runAction executes a named action against an io surface the host provides:
//   io.emit(event)            — encrypt + send an event into the thread
//   io.schedule(event, at)    — server-schedule a future event; resolves sid|null
//   io.unschedule(sid)        — cancel a pending schedule
//   io.newId()                — a fresh unique entity id
//   io.parse(kind, text)      — run a named parser; null = parse failure
//   io.hint(text)             — surface inline feedback (a failed parse's nudge)
// scope carries the trigger's bindings: { item } for a row action, { input }
// for the composer (already trimmed). `$newId` is minted once per run, so a
// schedule step and the emit that records its sid share the same id.

export async function runAction(schema, name, scope, io) {
  const action = (schema.actions || {})[name];
  if (!action) return false;
  let mintedId = null;
  const ctx = {
    ...scope,
    get newId() {
      if (mintedId === null) mintedId = io.newId();
      return mintedId;
    },
    sid: "",
    parsed: null,
  };
  for (const r of action.require || []) {
    if (!truthy(evalValue(r, ctx))) return false;
  }
  for (const step of action.steps || []) {
    if (step.if && !evalCond(step.if, ctx)) continue;
    if (step.parse) {
      const text = evalValue(step.parse.text, ctx);
      const parsed = io.parse ? io.parse(step.parse.kind, str(text)) : null;
      if (!parsed) {
        if (step.parse.onFail && step.parse.onFail.hint && io.hint) io.hint(step.parse.onFail.hint);
        return false;
      }
      ctx.parsed = parsed;
    } else if (step.schedule) {
      const at = num(evalValue(step.schedule.at, ctx));
      const sid = await io.schedule(evalValue(step.schedule.event, ctx), at);
      ctx.sid = sid || "";
    } else if (step.unschedule) {
      await io.unschedule(str(evalValue(step.unschedule, ctx)));
    } else if (step.emit) {
      await io.emit(evalValue(step.emit, ctx));
    }
  }
  return true;
}

// tickerRules evaluates the schema's per-item timed rules (e.g. Reminders'
// client-side fallback firing for sid-less items) and returns the events due
// to emit now. `fired` is the host's per-session dedupe set for `once` rules —
// pass the same Set every tick; this function adds to it.
export function tickerRules(schema, state, now, fired) {
  const t = schema.ticker;
  if (!t) return [];
  const items = (state || {})[schema.collection] || [];
  const keyField = collectionDecl(schema, schema.collection).key || "id";
  const out = [];
  for (const rule of t.perItem || []) {
    for (const item of items) {
      const scope = { item, now };
      if (!evalCond(rule.if, scope)) continue;
      const dedupe = item[keyField];
      if (rule.once) {
        if (fired && fired.has(dedupe)) continue;
        if (fired) fired.add(dedupe);
      }
      out.push(evalValue(rule.emit, scope));
    }
  }
  return out;
}