// h3d-carve-12: T-section (arpeggio inference) extracted from screen.js.
// VERBATIM-MOVE: all function bodies are byte-for-byte identical to their
// screen.js originals except for the single DI rewire noted below.
// No logic changes, no new guards, no structural additions.
//
// Beyond-subst changes:
//   1. arpeggioLaneDividerXYScaleMatchFrameRim: `nStr` → `getNStr()`
//      (the explicit argument `sY(nStr - 1)` — sY itself is a plain shorthand
//       that already captures live state internally, but the argument needs getNStr())
//   2. _resetStringDependentCaches: STAYS in screen.js; this module exports
//      `resetChordShapeCache()` instead, which screen.js calls to reset
//      _chordShapeCache (the only cache that moved here).
//
// lowerBoundT imported directly from ./geometry.js — not in DI surface.
// Total DI params: 19 (18 plain const shorthand + 1 live getter).
// Missed in contract survey (corrected before GO): NEXT_ON_STRING_T_EPS (line 248).

import { lowerBoundT } from './geometry.js';

export function createArp({
    // ── plain const shorthand (fn refs or number consts, never reassigned) ──
    validString,            // IIFE fn decl ~line 3736
    filterValidNotes,       // IIFE fn decl ~line 3767 — used by arpeggioLaneDividerFrameAccentMul
    sY,                     // factory-scope fn: s => S_BASE + (...) * S_GAP (captures live vars)
    K,                      // module-level const line 124
    S_GAP,                  // module-level const line 203
    BEHIND,                 // module-level const line 206
    CHORD_FRAME_RIM_MIN,    // line 610
    CHORD_FRAME_RIM_FRAC_H, // line 611
    ARP_FRAME_ONSET_PAD_S,          // line 620
    ARP_FRAME_ONSET_CLUSTER_S,      // line 621
    ARP_INFER_MIN_HAND_SHAPE_SPAN_S,     // line 628
    ARP_INFER_STRUM_VS_ARP_SPREAD_MIN_S, // line 634
    ARP_INFER_MULTI_STRUM_HIT_SLACK,     // line 640
    ARP_INFER_MULTI_STRUM_WIN_MIN_S,     // line 642
    ARP_INFER_MIN_HITS_VS_SHAPE_CAP,     // line 653
    ARP_HWY_RAIL_END_TAIL_S,    // line 263
    ARP_HWY_RAIL_START_LEAD_S,  // line 265
    NEXT_ON_STRING_T_EPS,       // line 248 — used in chordShapeCoveredByStandaloneNotes
    // ── live getter (let var, reassigned per arrangement/frame) ──
    getNStr,    // () => nStr — used in arpeggioLaneDividerXYScaleMatchFrameRim
}) {

        // ── Pre-arp utilities ─────────────────────────────────────────────
        function truthyChartFlag(v) {
            if (v === true || v === 1) return true;
            if (v === '1') return true;
            return typeof v === 'string' && v.toLowerCase() === 'true';
        }

        /** RS / sloppak `hd` (highDensity); tolerate occasional string forms. */
        function chordWireHighDensity(ch) {
            return truthyChartFlag(ch && ch.hd);
        }

        /**
         * Per spec, `displayName` is the UI label for a chord template
         * (defaulting to `name` when the chart didn't set it). Always go
         * through this helper so name vs. displayName drift can't surface
         * the wrong label or break displayName-based dedupe heuristics.
         */
        function chordTemplateLabel(tmpl) {
            if (!tmpl) return '';
            const d = tmpl.displayName;
            if (typeof d === 'string' && d.length > 0) return d;
            const n = tmpl.name;
            return typeof n === 'string' ? n : '';
        }

        /**
         * Arpeggio styling is driven by authored metadata, not by post-hoc
         * note-stream inference. Prefer explicit hand-shape flags and fall back
         * to template markers when present.
         */
        function chordTemplateMarkedArpeggio(cid, chordTemplates) {
            if (cid == null || !chordTemplates) return false;
            const tmpl = chordTemplates[cid] ?? chordTemplates[Number(cid)];
            if (!tmpl) return false;
            if (truthyChartFlag(tmpl.arp) || truthyChartFlag(tmpl.arpeggio)) return true;
            const displayName = typeof tmpl.displayName === 'string' ? tmpl.displayName.toLowerCase() : '';
            if (displayName.includes('-arp')) return true;
            const name = typeof tmpl.name === 'string' ? tmpl.name.toLowerCase() : '';
            return name.endsWith('(arp)') || name.includes(' arpeggio');
        }

        function handShapeMarkedArpeggio(hs, chordTemplates) {
            if (!hs) return false;
            if (truthyChartFlag(hs.arp) || truthyChartFlag(hs.arpeggio)) return true;
            return chordTemplateMarkedArpeggio(hsChordIdNorm(hs), chordTemplates);
        }

        // ── Hint cache ───────────────────────────────────────────────────
        /**
         * Matching hand-shape metadata for a chord onset. ``explicit`` follows
         * authored arpeggio markers only; note inference is handled separately
         * by the callers that still need it for non-visual behavior.
         *
         * Cached per chord: result depends only on (ch, hss, chordTemplates),
         * all chart-static for the lifetime of an arrangement. The cache is
         * swapped on (hss, templates) ref change so an arrangement switch
         * cannot resurrect stale entries. Empty-input case bypasses the cache
         * — it returns a fresh sentinel anyway and isn't hot enough to share.
         */
        const _HINT_NONE = Object.freeze({ explicit: false, covered: false, hs: null });
        let _hintCache = new WeakMap();
        let _hintCacheHsRef = null;
        let _hintCacheTplRef = null;
        function chordHandShapeArpeggioHint(ch, hss, chordTemplates) {
            if (!hss || hss.length === 0) return _HINT_NONE;
            if (_hintCacheHsRef !== hss || _hintCacheTplRef !== chordTemplates) {
                _hintCache = new WeakMap();
                _hintCacheHsRef = hss;
                _hintCacheTplRef = chordTemplates;
            }
            const cached = _hintCache.get(ch);
            if (cached !== undefined) return cached;
            const t = ch.t;
            const cid = ch.id;
            let result = _HINT_NONE;
            for (let i = 0; i < hss.length; i++) {
                const hs = hss[i];
                const tLo = hsStart(hs);
                const tHi = hsEnd(hs);
                if (Number.isNaN(tLo) || Number.isNaN(tHi)) continue;
                if (t + 1e-4 < tLo || t > tHi + 1e-4) continue;
                const hsCid = hsChordIdNorm(hs);
                if (hsCid !== cid && Number(hsCid) !== Number(cid)) continue;
                const explicit = handShapeMarkedArpeggio(hs, chordTemplates);
                result = { explicit, covered: true, hs };
                break;
            }
            _hintCache.set(ch, result);
            return result;
        }

        /** Build ``ch.notes`` from ``chordTemplates[cid].frets`` (-1 omitted). */
        function chordNotesFromTemplate(cid, templates) {
            if (templates == null || cid == null) return [];
            const tmpl = templates[cid] ?? templates[Number(cid)];
            if (!tmpl || !Array.isArray(tmpl.frets)) return [];
            const out = [];
            for (let si = 0; si < tmpl.frets.length; si++) {
                const f = tmpl.frets[si];
                if (f >= 0 && validString(si)) out.push({ s: si, f, sus: 0 });
            }
            return out;
        }

        /**
         * Chart-format fingerpicking passages often have ``<handShape>`` + per-string
         * ``<note>`` rows but **no** ``<chord>`` events. The 3D chord frame / arp
         * styling only runs over ``bundle.chords``, so synthesize minimal chord
         * rows at each hand-shape onset when the chart omits them.
         */
        function mergeHandShapeSynthChords(realChords, handShapes, chordTemplates) {
            if (!handShapes || handShapes.length === 0) return realChords;
            const reals = realChords && realChords.length ? realChords : [];
            const synth = [];
            const seenSynth = new Set();
            const tol = 0.028;
            /**
             * Suppress a synth chord box when a real chord with the **same trimmed
             * display name** played within this window — Custom songs commonly authors
             * several ``<chordTemplate>`` rows that share a display name (with
             * trailing-whitespace IDs) for fingering variants. The follow-up
             * hand-shape with no chord row is a fingering hint, not a new strum
             * (e.g. Jackson 5 "I Want You Back" ~0:27 — Fm7 cid=18 strum followed
             * by Fm7 cid=19 hand-shape, which earlier produced a stacked second
             * "Fm7" label and an extra chord frame).
             */
            const SAME_NAME_RUN_S = 0.5;
            const trimmedTemplateName = (cid) => {
                if (cid == null || !chordTemplates) return '';
                const tmpl = chordTemplates[cid] ?? chordTemplates[Number(cid)];
                // custom songs commonly authors several <chordTemplate> rows that share
                // a displayName for fingering variants; the suppression
                // heuristic in the surrounding code dedupes on the *label*,
                // not the underlying name, so go through chordTemplateLabel.
                return chordTemplateLabel(tmpl).trim();
            };
            outer: for (let i = 0; i < handShapes.length; i++) {
                const hs = handShapes[i];
                const cid = hs.chord_id != null ? hs.chord_id : hs.chordId;
                const st = hs.start_time != null ? hs.start_time : hs.startTime;
                if (cid == null || st == null || Number.isNaN(Number(st))) continue;
                const key = `${cid}|${Number(st).toFixed(3)}`;
                if (seenSynth.has(key)) continue;
                seenSynth.add(key);
                const myName = trimmedTemplateName(cid);
                for (let j = 0; j < reals.length; j++) {
                    const ch = reals[j];
                    const rid = ch.id;
                    const sameId = rid === cid || Number(rid) === Number(cid);
                    if (sameId && Math.abs(ch.t - st) <= tol) continue outer;
                    // A real strum at the same onset already represents this
                    // chord — never synthesize a phantom on top of it. The
                    // id/name checks alone miss hand-shapes whose template
                    // differs from (or shares no name with) the coincident real
                    // chord — e.g. an edited chart that left a stale hand-shape
                    // template pointing at the pre-edit shape, which then drew a
                    // spurious second power chord beside the real one.
                    if (Math.abs(ch.t - st) <= tol) continue outer;
                    if (!sameId && myName !== '') {
                        const otherName = trimmedTemplateName(rid);
                        if (otherName === myName
                            && st > ch.t
                            && st - ch.t <= SAME_NAME_RUN_S) {
                            continue outer;
                        }
                    }
                }
                const notes = chordNotesFromTemplate(cid, chordTemplates);
                if (notes.length === 0) continue;
                const et = hs.end_time != null ? hs.end_time : hs.endTime;
                synth.push({
                    t: st,
                    id: cid,
                    // `hd` is the chart-format `highDensity` wire field (gallops /
                    // repeated strums), not an arpeggio carrier — arpeggio
                    // intent is read directly from the hand-shape via
                    // chordHandShapeArpeggioHint() downstream. Keep `hd` false
                    // so chordWireHighDensity() / label-suppression behave the
                    // same as for any other non-gallop chord row.
                    hd: false,
                    notes,
                    /** Hand-shape fill-in (no authored chord row) — skip note-stream arp frame. */
                    h3dSynth: true,
                    /** Hand-shape end time — used to draw the shape-sustain border for non-arp cases. */
                    h3dSynthEnd: et != null ? Number(et) : null,
                });
            }
            if (synth.length === 0) return reals;
            const merged = reals.concat(synth);
            merged.sort((a, b) => {
                const dt = a.t - b.t;
                if (Math.abs(dt) > 1e-6) return dt;
                const ia = Number(a.id);
                const ib = Number(b.id);
                return (ia - ib) || 0;
            });
            return merged;
        }

        // ── Chord-shape cache ─────────────────────────────────────────────
        /**
         * Merge chart-format ``chordTemplates[id].frets`` with live ``chordNote`` rows.
         * Cached via WeakMap on the chord object — chord data never changes after
         * chart load, so the Map is computed once and reused every frame.
         * The init-time callers (fillArpeggioGhostInferFlags) pass ephemeral `fakeCh`
         * objects that are never seen again, so they bypass the cache naturally.
         */
        let _chordShapeCache = new WeakMap();
        function mergeChordShape(ch, chordNotes, templates) {
            if (_chordShapeCache.has(ch)) return _chordShapeCache.get(ch);
            const shape = new Map();
            const tid = ch && ch.id != null ? ch.id : null;
            const tmpl = (tid != null && templates)
                ? (templates[tid] ?? templates[Number(tid)])
                : null;
            if (tmpl && Array.isArray(tmpl.frets)) {
                for (let si = 0; si < tmpl.frets.length; si++) {
                    if (!validString(si)) continue;
                    const f = tmpl.frets[si];
                    if (f >= 0) shape.set(si, f);
                }
            }
            for (let i = 0; i < chordNotes.length; i++) {
                const cn = chordNotes[i];
                if (!validString(cn.s)) continue;
                if (cn.f < 0) shape.delete(cn.s);
                else shape.set(cn.s, cn.f);
            }
            _chordShapeCache.set(ch, shape);
            return shape;
        }

        // h3d-carve-12: screen.js's _resetStringDependentCaches() calls this
        // instead of directly assigning `_chordShapeCache = new WeakMap()`.
        // Reset the validString()/nStr-dependent chord caches. Called when nStr
        // changes so a string count discovered after the first frame (e.g. a
        // 7-string chart whose stringCount arrives in song_info) doesn't leave
        // string-6+ notes filtered out of cached chord shapes/signatures.
        function resetChordShapeCache() {
            _chordShapeCache = new WeakMap();
        }

        function hitTimesQualifyArpeggioSpread(hitTimes) {
            if (hitTimes.length < 2) return false;
            hitTimes.sort((a, b) => a - b);
            const spread = hitTimes[hitTimes.length - 1] - hitTimes[0];
            if (spread >= 0.03) return true;
            return hitTimes.length >= 4 && spread >= 0.016;
        }

        /** RS XML / IPC payloads use snake_case or camelCase field names. */
        function hsStart(hs) {
            if (!hs) return NaN;
            const v = hs.start_time != null ? hs.start_time : hs.startTime;
            if (v == null) return NaN;
            const n = Number(v);
            return Number.isNaN(n) ? NaN : n;
        }
        function hsEnd(hs) {
            if (!hs) return NaN;
            const v = hs.end_time != null ? hs.end_time : hs.endTime;
            if (v == null) return NaN;
            const n = Number(v);
            return Number.isNaN(n) ? NaN : n;
        }
        function hsChordIdNorm(hs) {
            if (!hs) return null;
            const v = hs.chord_id != null ? hs.chord_id : hs.chordId;
            return v == null ? null : v;
        }

        /** ``<handShape>`` chart duration in seconds (snake_case or camelCase XML). */
        function handShapeChartSpanSec(hs) {
            const a = hsStart(hs), b = hsEnd(hs);
            if (Number.isNaN(a) || Number.isNaN(b)) return 0;
            return Math.max(0, b - a);
        }

        // ── Infer-pattern cache ───────────────────────────────────────────
        /**
         * When ``hd`` is missing/false, detect arpeggio from the **note** stream
         * using the **full voicing** (template ∪ chord notes). RS often stores the
         * plucks only in ``notes[]``, not as duplicate chord rows.
         *
         * @param {{ tLo: number, tHi: number } | null} [timeWin]
         *        When set (e.g. from ``<handShape>`` span), scan staggered picks
         *        across the whole held-shape window — RS often omits ``arp`` and ``hd``.
         */
        // Cached per chord: result depends on (ch, shape, notesArr) and an
        // optional timeWin which itself is a function of the chord's matching
        // <handShape>. Both inputs are chart-static, so the cache invalidates
        // on (notesArr, hss) ref change — `hss` is threaded in purely as the
        // invalidation key for the chord-loop caller, which passes a stable
        // `ch` (reused across frames) and a timeWin that is null until
        // bundle.handShapes arrives over the WS; without the hss check the
        // null-timeWin result would stick once handShapes loaded late. shape
        // comes from mergeChordShape(ch) which is also chart-static, so it
        // doesn't enter the invalidation key directly. The cache deliberately
        // stores boolean results; a sentinel distinguishes "not computed"
        // from "false".
        let _arpInferCache = new WeakMap();
        let _arpInferCacheNotesRef = null;
        let _arpInferCacheHssRef = null;
        function inferArpeggioFromNotePattern(ch, shape, notesArr, timeWin, hss = null) {
            if (!notesArr || notesArr.length === 0 || shape.size < 2) return false;
            if (_arpInferCacheNotesRef !== notesArr || _arpInferCacheHssRef !== hss) {
                _arpInferCache = new WeakMap();
                _arpInferCacheNotesRef = notesArr;
                _arpInferCacheHssRef = hss;
            }
            const cached = _arpInferCache.get(ch);
            if (cached !== undefined) return cached;
            const result = _inferArpeggioFromNotePatternUncached(ch, shape, notesArr, timeWin);
            _arpInferCache.set(ch, result);
            return result;
        }
        function _inferArpeggioFromNotePatternUncached(ch, shape, notesArr, timeWin) {
            const tHi = timeWin ? timeWin.tHi : ch.t + 2.35;
            const tLo = timeWin ? timeWin.tLo : ch.t - 0.28;
            let i2 = lowerBoundT(notesArr, tLo - 0.02);
            const hitTimes = [];
            const hitStrings = new Set();
            for (; i2 < notesArr.length; i2++) {
                const n = notesArr[i2];
                if (n.t > tHi) break;
                if (n.t < tLo) continue;
                if (!validString(n.s)) continue;
                const ef = shape.get(n.s);
                if (ef === undefined || ef !== n.f) continue;
                hitTimes.push(n.t);
                hitStrings.add(n.s);
            }
            if (!hitTimesQualifyArpeggioSpread(hitTimes)) return false;
            // A genuine arpeggio SWEEPS across the held shape, so its standalone
            // notes land on MULTIPLE strings of the shape. When every matching
            // hit is on a single string, this is a repeated single-string run
            // (e.g. a palm-muted gallop hammering the chord's root) that happens
            // to share one string/fret with the chord — NOT an arpeggio. Inferring
            // one here deferred the chord's gems and made the power chord render as
            // just that one repeated note (bar 25 of starlight). Require ≥2 strings.
            if (hitStrings.size < 2) return false;
            // Strumming/gallop rejection — far more hits than the shape has
            // strings means the chord's notes are being re-struck repeatedly
            // (a riff/gallop reusing both power-chord notes), not swept once as
            // an arpeggio. This guard used to live inside `if (timeWin)`, so it
            // was skipped for charts with no hand-shapes (timeWin null) — which
            // let dense two-string gallops over a power chord infer a bogus
            // arpeggio and defer the chord's gems (bar 88 of starlight: a
            // (s5:4,s6:2) chord whose root+fifth recur ~16x over 2 s). Apply it
            // with the actual window span whether or not a hand-shape is present.
            const winSpan = timeWin ? (timeWin.tHi - timeWin.tLo) : (tHi - tLo);
            if (winSpan > ARP_INFER_MULTI_STRUM_WIN_MIN_S
                && hitTimes.length > shape.size + ARP_INFER_MULTI_STRUM_HIT_SLACK) {
                return false;
            }
            if (timeWin) {
                if (winSpan < 0.70 && hitTimes.length < 4) {
                    const spread = hitTimes[hitTimes.length - 1] - hitTimes[0];
                    if (spread < ARP_INFER_STRUM_VS_ARP_SPREAD_MIN_S) return false;
                }
                // Reject when too few staggered hits for a genuine sweep across
                // the held shape — see ARP_INFER_MIN_HITS_VS_SHAPE_CAP.
                const minHits = Math.min(shape.size, ARP_INFER_MIN_HITS_VS_SHAPE_CAP);
                if (hitTimes.length < minHits) return false;
            }
            return true;
        }

        /**
         * True when standalone note rows already cover every string/fret in the
         * arpeggio shape, so drawing the chord gems too would duplicate the same
         * authored passage.
         */
        // Cached per chord: result depends on (ch, shape, notesArr) — chart-
        // static; the cache invalidates on notesArr ref change. The same
        // ``ch`` may be queried multiple times per frame from the chord
        // render loop (deferChordGems / _deferFallback / suppressSynthChord),
        // so survival across frames is also useful.
        let _arpCoverCache = new WeakMap();
        let _arpCoverCacheNotesRef = null;
        function chordShapeCoveredByStandaloneNotes(ch, shape, notesArr, timeWin) {
            if (!notesArr || notesArr.length === 0 || !shape || shape.size === 0) return false;
            if (_arpCoverCacheNotesRef !== notesArr) {
                _arpCoverCache = new WeakMap();
                _arpCoverCacheNotesRef = notesArr;
            }
            const cached = _arpCoverCache.get(ch);
            if (cached !== undefined) return cached;
            const tLo = (timeWin ? timeWin.tLo : ch.t - ARP_FRAME_ONSET_PAD_S) - NEXT_ON_STRING_T_EPS;
            const tHi = (timeWin ? timeWin.tHi : ch.t + ARP_FRAME_ONSET_CLUSTER_S) + NEXT_ON_STRING_T_EPS;
            let i2 = lowerBoundT(notesArr, tLo);
            const matchedStrings = new Set();
            let result = false;
            for (; i2 < notesArr.length; i2++) {
                const n = notesArr[i2];
                if (n.t > tHi) break;
                if (!validString(n.s) || matchedStrings.has(n.s)) continue;
                const ef = shape.get(n.s);
                if (ef === undefined || ef !== n.f) continue;
                matchedStrings.add(n.s);
                if (matchedStrings.size >= shape.size) { result = true; break; }
            }
            _arpCoverCache.set(ch, result);
            return result;
        }

        /**
         * Notes in an inferred arpeggio passage are charted in ``notes[]`` with
         * staggered times; treat them like chord-cluster notes for chart-format-style
         * board-ghost fret digits (``fromChord`` + template column).
         */
        function arpeggioChordIdForNote(n, handShapes, chordTemplates, notesArr) {
            if (!handShapes || handShapes.length === 0 || !notesArr || notesArr.length === 0) return null;
            if (!validString(n.s)) return null;
            for (let i = 0; i < handShapes.length; i++) {
                const hs = handShapes[i];
                const hsLo = hsStart(hs);
                const hsHi = hsEnd(hs);
                if (Number.isNaN(hsLo) || Number.isNaN(hsHi)) continue;
                if (n.t + 1e-4 < hsLo || n.t > hsHi + 1e-4) continue;
                const cid = hsChordIdNorm(hs);
                if (cid == null) continue;
                const tmpl = chordTemplates?.[cid] ?? chordTemplates?.[Number(cid)];
                if (!tmpl || !Array.isArray(tmpl.frets)) continue;
                const tf = tmpl.frets[n.s];
                if (typeof tf !== 'number' || tf < 0 || n.f !== tf) continue;
                const synthNotes = chordNotesFromTemplate(cid, chordTemplates);
                if (synthNotes.length === 0) continue;
                const fakeCh = { t: hsLo, id: cid, notes: synthNotes };
                const shape = mergeChordShape(fakeCh, synthNotes, chordTemplates);
                const tw = { tLo: hsLo - 0.06, tHi: hsHi + 0.06 };
                if (handShapeChartSpanSec(hs) < ARP_INFER_MIN_HAND_SHAPE_SPAN_S) continue;
                if (inferArpeggioFromNotePattern(fakeCh, shape, notesArr, tw, handShapes)) return cid;
            }
            return null;
        }

        /**
         * Per-frame warmup: ``inferArpeggioFromNotePattern`` depends only on
         * ``handShape × chart``, not on the candidate note — the old path
         * recomputed it for every visible note (O(notecount × hs × notescan)).
         * Fill ``outFlags[i]`` with the boolean once per ``handShapes[i]``.
         */
        function fillArpeggioGhostInferFlags(handShapes, chordTemplates, notesArr, outFlags, outSynthOnsetSet = null) {
            for (let i = 0; i < handShapes.length; i++) {
                let infer = false;
                const hs = handShapes[i];
                if (handShapeChartSpanSec(hs) < ARP_INFER_MIN_HAND_SHAPE_SPAN_S) {
                    outFlags[i] = false;
                    continue;
                }
                const cid = hsChordIdNorm(hs);
                if (cid != null && notesArr.length > 0) {
                    const tmpl = chordTemplates?.[cid] ?? chordTemplates?.[Number(cid)];
                    if (tmpl && Array.isArray(tmpl.frets)) {
                        const synthNotes = chordNotesFromTemplate(cid, chordTemplates);
                        if (synthNotes.length > 0) {
                            const hsLo = hsStart(hs);
                            const hsHi = hsEnd(hs);
                            const fakeCh = { t: hsLo, id: cid, notes: synthNotes };
                            const shape = mergeChordShape(fakeCh, synthNotes, chordTemplates);
                            const tw = { tLo: hsLo - 0.06, tHi: hsHi + 0.06 };
                            infer = inferArpeggioFromNotePattern(fakeCh, shape, notesArr, tw, handShapes);
                            // Chord-hold gate: inferArpeggioFromNotePattern can fire true
                            // when open-string notes coincidentally match the template's
                            // open positions but only a SINGLE fretted (f>0) string is
                            // actually played at the handshape onset. Treat that as a
                            // chord hold (not an arpeggio) — clear the arp flag, no
                            // brackets. The original implementation also intended to
                            // record a synthetic sustain extending to hsEnd for the
                            // onset note, but that read-side was never wired up; the
                            // visual decay-before-handshape-end is benign.
                            if (infer) {
                                let _frettedCount = 0;
                                let _onsetNote = null;
                                const _fSeen = new Set();
                                let _ci = lowerBoundT(notesArr, tw.tLo - 0.02);
                                for (; _ci < notesArr.length; _ci++) {
                                    const _cn = notesArr[_ci];
                                    if (_cn.t > tw.tHi + 0.02) break;
                                    if (_cn.t < tw.tLo) continue;
                                    if (!validString(_cn.s)) continue;
                                    if (shape.get(_cn.s) !== _cn.f) continue;
                                    if (_cn.f > 0 && !_fSeen.has(_cn.s)) {
                                        _frettedCount++;
                                        _fSeen.add(_cn.s);
                                        if (_onsetNote === null) _onsetNote = _cn;
                                    }
                                }
                                if (_frettedCount <= 1 && _onsetNote !== null) {
                                    outFlags[i] = false;
                                    continue; // chord hold handled — skip onset-match and outFlags assignment
                                }
                            }
                            // Non-arp template inferred as arpeggio: suppress brackets.
                            // Only explicit arp-marked templates (arp:true / displayName "-arp")
                            // should show [ ] / < > bracket markers.
                            if (infer && outSynthOnsetSet != null
                                && !handShapeMarkedArpeggio(hs, chordTemplates)) {
                                outSynthOnsetSet.add(hsLo);
                            }
                            // Also treat as arp ghost when the hs generated a suppressed
                            // synth chord: any standalone note in the onset window matches
                            // any shape string. Handles patterns where inferArpeggioFromNotePattern
                            // returns false (e.g. repeated arpeggio across a long hs span
                            // triggers the multi-strum rejection), but the player still
                            // needs the "hold this shape" ghost fret numbers on the board.
                            if (!infer) {
                                const _oLo = hsLo - ARP_FRAME_ONSET_PAD_S;
                                const _oHi = hsLo + ARP_FRAME_ONSET_CLUSTER_S;
                                let _oi = lowerBoundT(notesArr, _oLo - 0.02);
                                for (; _oi < notesArr.length; _oi++) {
                                    const _on = notesArr[_oi];
                                    if (_on.t > _oHi) break;
                                    if (_on.t < _oLo) continue;
                                    if (shape.get(_on.s) === _on.f) {
                                        infer = true;
                                        // Only suppress brackets when the handshape is NOT an
                                        // explicit arpeggio (arp:true template / displayName "-arp").
                                        // Genuine arp handshapes reached via onset-match still need
                                        // the [ ] bracket markers — only non-arp synth chords are
                                        // "false positives" that should hide the brackets.
                                        if (outSynthOnsetSet != null
                                            && !handShapeMarkedArpeggio(hs, chordTemplates)) {
                                            outSynthOnsetSet.add(hsLo);
                                        }
                                        break;
                                    }
                                }
                            }
                        }
                    }
                }
                outFlags[i] = infer;
            }
        }

        // Chart-static WeakMap cache: note object → chord-id (or null sentinel).
        // The result depends only on the note's (t, s, f) and the chart's handShapes
        // + chordTemplates, which never change after load. Keyed by note object so
        // switching songs/arrangements drops the entries with the old array.
        const _ARP_CID_NULL = Object.freeze({});
        const _arpCidCache = new WeakMap();
        function arpeggioChordIdForNoteWithInferCache(n, handShapes, chordTemplates, notesArr, hsInferFlags) {
            const cached = _arpCidCache.get(n);
            if (cached !== undefined) return cached === _ARP_CID_NULL ? null : cached;
            let result = null;
            if (!handShapes || handShapes.length === 0 || !notesArr || notesArr.length === 0 || !hsInferFlags) {
                result = arpeggioChordIdForNote(n, handShapes, chordTemplates, notesArr);
            } else if (validString(n.s)) {
                for (let i = 0; i < handShapes.length; i++) {
                    if (!hsInferFlags[i]) continue;
                    const hs = handShapes[i];
                    const hsLo = hsStart(hs);
                    const hsHi = hsEnd(hs);
                    if (Number.isNaN(hsLo) || Number.isNaN(hsHi)) continue;
                    if (n.t + 1e-4 < hsLo || n.t > hsHi + 1e-4) continue;
                    const cid = hsChordIdNorm(hs);
                    if (cid == null) continue;
                    const tmpl = chordTemplates?.[cid] ?? chordTemplates?.[Number(cid)];
                    if (!tmpl || !Array.isArray(tmpl.frets)) continue;
                    const tf = tmpl.frets[n.s];
                    if (typeof tf !== 'number' || tf < 0 || n.f !== tf) continue;
                    result = cid;
                    break;
                }
            }
            _arpCidCache.set(n, result === null ? _ARP_CID_NULL : result);
            return result;
        }

        /** Returns {start, end} chart-time bounds of the arpeggio handshape that contains
         *  this note, or null when not found.  Uses hsInferFlags to skip ruled-out
         *  handshapes; falls back to a full scan when hsInferFlags is null. */
        // WeakMap cache — arpHsBoundsForNote result is chart-static (note, handShapes,
        // and hsInferFlags never change after chart load). Each renderer instance has
        // its own WeakMap, so splitscreen panels don't interfere.
        // Sentinel: _ARP_BOUNDS_NULL = {} distinguishes "no matching hs" from "uncached".
        const _ARP_BOUNDS_NULL = Object.freeze({});
        const _arpBoundsCache = new WeakMap();
        function arpHsBoundsForNote(n, handShapes, hsInferFlags) {
            if (!handShapes || handShapes.length === 0) return null;
            const cached = _arpBoundsCache.get(n);
            if (cached !== undefined) return cached === _ARP_BOUNDS_NULL ? null : cached;
            let result = null;
            for (let i = 0; i < handShapes.length; i++) {
                if (hsInferFlags && !hsInferFlags[i]) continue;
                const hs = handShapes[i];
                const lo = hsStart(hs);
                const hi = hsEnd(hs);
                if (Number.isNaN(lo) || Number.isNaN(hi)) continue;
                if (n.t + 1e-4 < lo || n.t > hi + 1e-4) continue;
                result = { start: lo, end: hi };
                break;
            }
            _arpBoundsCache.set(n, result === null ? _ARP_BOUNDS_NULL : result);
            return result;
        }

        /** Cache the authored arpeggio marker per hand shape. */
        function handShapeIsArpeggioForLaneRail(hs, chordTemplates) {
            return handShapeMarkedArpeggio(hs, chordTemplates);
        }

        /**
         * Chart-time window for purple rails: hand-shape span clipped to matching
         * ``chords[].t`` and template notes in the passage — same times that drive
         * the 3D arpeggio frame (``ch.t`` + note stream), avoiding rails that start
         * before the box or end before the last arpeggiated note.
         */
        function effectiveArpRailChartBoundsForHandShape(hs, chords, chordTemplates, notesArr) {
            let shapeLo = hsStart(hs);
            const _hsEndOrig = hsEnd(hs);
            let shapeHi = _hsEndOrig;
            const cid = hsChordIdNorm(hs);
            if (Number.isNaN(shapeLo) || Number.isNaN(shapeHi)) {
                return { shapeLo: 1e9, shapeHi: -1e9 };
            }
            if (notesArr && notesArr.length > 0 && chordTemplates && cid != null) {
                const tmpl = chordTemplates[cid] ?? chordTemplates[Number(cid)];
                if (tmpl && Array.isArray(tmpl.frets)) {
                    let tFirst = null;
                    let tLast = null;
                    for (let i = 0; i < notesArr.length; i++) {
                        const n = notesArr[i];
                        if (n.t + 1e-4 < shapeLo - 0.18 || n.t > shapeHi + 0.45) continue;
                        if (!validString(n.s)) continue;
                        const tf = tmpl.frets[n.s];
                        if (typeof tf !== 'number' || tf < 0 || n.f !== tf) continue;
                        if (tFirst === null || n.t < tFirst) tFirst = n.t;
                        if (tLast === null || n.t > tLast) tLast = n.t;
                    }
                    if (tFirst != null) shapeLo = Math.max(shapeLo, tFirst);
                    if (tLast != null) shapeHi = Math.max(shapeHi, tLast);
                }
            }
            if (chords && chords.length && cid != null) {
                let tMinC = null;
                let tMaxC = null;
                for (let j = 0; j < chords.length; j++) {
                    const ch = chords[j];
                    if (ch.id !== cid && Number(ch.id) !== Number(cid)) continue;
                    if (ch.t + 1e-4 < shapeLo || ch.t > shapeHi + 0.28) continue;
                    if (tMinC === null || ch.t < tMinC) tMinC = ch.t;
                    if (tMaxC === null || ch.t > tMaxC) tMaxC = ch.t;
                }
                if (tMinC != null) shapeLo = Math.max(shapeLo, tMinC);
                if (tMaxC != null) shapeHi = Math.max(shapeHi, tMaxC);
            }
            shapeLo -= ARP_HWY_RAIL_START_LEAD_S;
            // Only extend past the handshape end when notes/chords genuinely reach
            // beyond it — otherwise the tail would make the rail visually larger
            // than the actual handshape duration (e.g. 0.38 s / 1.3 s ≈ 29% extra).
            if (shapeHi > _hsEndOrig) shapeHi += ARP_HWY_RAIL_END_TAIL_S;
            return { shapeLo, shapeHi };
        }

        /** Cache the authored arpeggio marker per hand shape. */
        function fillLaneRailHandShapeFlags(handShapes, chordTemplates, outFlags) {
            const nHs = handShapes.length;
            for (let i = 0; i < nHs; i++) {
                outFlags[i] = handShapeIsArpeggioForLaneRail(handShapes[i], chordTemplates);
            }
        }

        function fillArpeggioRailShapeBoundsCaches(
            handShapes, chords, chordTemplates, notesArr, laneRailFlags, loOut, hiOut,
        ) {
            const nHs = handShapes.length;
            for (let i = 0; i < nHs; i++) {
                if (!laneRailFlags[i]) continue;
                const b = effectiveArpRailChartBoundsForHandShape(
                    handShapes[i], chords, chordTemplates, notesArr,
                );
                loOut[i] = b.shapeLo;
                hiOut[i] = b.shapeHi;
            }
        }

        /** ``[tChartLo,tChartHi]`` chart times that a lane slice covers (see module ``BEHIND`` / approach ``dt``). */
        function arpeggioLaneOuterRailChartIntervalOverlaps(
            tChartLo,
            tChartHi,
            handShapes,
            boundLo,
            boundHi,
            laneRailFlags,
        ) {
            if (!handShapes || handShapes.length === 0) return false;
            if (!laneRailFlags) return false;
            if (tChartHi < tChartLo) {
                const s = tChartLo;
                tChartLo = tChartHi;
                tChartHi = s;
            }
            for (let i = 0; i < handShapes.length; i++) {
                if (!laneRailFlags[i]) continue;
                const shapeLo = boundLo[i];
                const shapeHi = boundHi[i];
                if (tChartHi < shapeLo - 1e-4 || tChartLo > shapeHi + 1e-4) continue;
                return true;
            }
            return false;
        }

        function arpeggioLaneOuterRailLaneSlice(
            dt0, dt1, nowClock,
            handShapes, boundLo, boundHi, laneRailFlags,
        ) {
            const tLo = nowClock + Math.min(dt0, dt1) - BEHIND;
            const tHi = nowClock + Math.max(dt0, dt1) - BEHIND;
            return arpeggioLaneOuterRailChartIntervalOverlaps(
                tLo, tHi, handShapes, boundLo, boundHi, laneRailFlags,
            );
        }

        /**
         * True when **chart time** ``chartT`` falls inside an arpeggio hand-shape.
         * Uses a short end tail only — no ``CHORD_HWY_LINGER_S`` — so purple lane
         * rails match visible highway slices and do not leak after shapes end.
         */
        function arpeggioLaneOuterRailAtChartTime(
            chartT, handShapes, boundLo, boundHi, laneRailFlags,
        ) {
            return arpeggioLaneOuterRailChartIntervalOverlaps(
                chartT, chartT, handShapes, boundLo, boundHi, laneRailFlags,
            );
        }

        /**
         * Same ``chordAccent ? ft *= 1.22`` as the 3D arpeggio chord rim so lane
         * rails match an accented frame when the active hand shape links to a
         * chord row that carries ``.ac`` notes.
         */
        function arpeggioLaneDividerFrameAccentMul(nowT, handShapes, chords, boundLo, boundHi, laneRailFlags) {
            if (!handShapes || handShapes.length === 0 || !chords || chords.length === 0) return 1;
            if (!laneRailFlags) return 1;
            for (let i = 0; i < handShapes.length; i++) {
                if (!laneRailFlags[i]) continue;
                const shapeLo = boundLo[i];
                const shapeHi = boundHi[i];
                if (nowT + 1e-4 < shapeLo || nowT > shapeHi + 1e-4) continue;

                const cid = hsChordIdNorm(handShapes[i]);
                if (cid == null) return 1;
                for (let j = 0; j < chords.length; j++) {
                    const ch = chords[j];
                    if (ch.id !== cid && Number(ch.id) !== Number(cid)) continue;
                    if (Math.abs(ch.t - hsStart(handShapes[i])) > 0.12) continue;
                    const chordNotes = ch.notes ? filterValidNotes(ch.notes) : [];
                    if (chordNotes.some(cn => cn.ac)) return 1.22;
                    return 1;
                }
                return 1;
            }
            return 1;
        }

        /** World-scale XY for purple lane rails = arpeggio ``ftSide`` / ``gLaneDivider`` edge (0.15×K). */
        function arpeggioLaneDividerXYScaleMatchFrameRim(accentMul = 1) {
            const yA = sY(0), yB = sY(getNStr() - 1); // DI: nStr → getNStr()
            const yMinF = Math.min(yA, yB) - S_GAP * 0.8;
            const yMaxF = Math.max(yA, yB) + S_GAP * 0.8;
            const fullChordBoxH = yMaxF - yMinF;
            let ft = Math.max(CHORD_FRAME_RIM_MIN * K, fullChordBoxH * CHORD_FRAME_RIM_FRAC_H);
            if (accentMul !== 1 && accentMul > 0) ft *= accentMul;
            const ftSide = ft * 1.55;
            return ftSide / (0.15 * K);
        }

    return {
        // ── exported (called from outside T-section) ──────────────────────
        chordWireHighDensity,                   // callers: 9155, 9384, 9791, 9815
        chordTemplateLabel,                     // callers: 9790, 10801, 10852
        chordTemplateMarkedArpeggio,            // callers: 9474, 10045
        chordHandShapeArpeggioHint,             // caller:  9112
        mergeHandShapeSynthChords,              // caller:  7965
        mergeChordShape,                        // caller:  8982
        resetChordShapeCache,                   // caller:  _resetStringDependentCaches (screen.js)
        inferArpeggioFromNotePattern,           // caller:  9126
        chordShapeCoveredByStandaloneNotes,     // caller:  9134
        hsStart,                                // callers: 8008, 9114, 9152, 9239, 9423, 10040
        hsEnd,                                  // callers: 8008, 9114, 9240, 9423, 10040
        handShapeChartSpanSec,                  // caller:  9125
        fillArpeggioGhostInferFlags,            // caller:  7987
        arpeggioChordIdForNoteWithInferCache,   // caller:  8811
        arpHsBoundsForNote,                     // caller:  8819
        fillLaneRailHandShapeFlags,             // caller:  8101
        fillArpeggioRailShapeBoundsCaches,      // caller:  8110
        arpeggioLaneOuterRailLaneSlice,         // caller:  10267
        arpeggioLaneOuterRailAtChartTime,       // caller:  10124
        arpeggioLaneDividerFrameAccentMul,      // callers: 10128, 10366
        arpeggioLaneDividerXYScaleMatchFrameRim, // callers: 10133, 10371
        // ── private (T-internal only, not in return) ──────────────────────
        // truthyChartFlag            — only used by T-internal fns
        // handShapeMarkedArpeggio    — only used by T-internal fns
        // chordNotesFromTemplate     — only used by T-internal fns
        // hitTimesQualifyArpeggioSpread — only called by _inferArpeggioFromNotePatternUncached
        // _inferArpeggioFromNotePatternUncached — only called by inferArpeggioFromNotePattern
        // hsChordIdNorm              — only used by T-internal fns
        // arpeggioChordIdForNote     — only called by arpeggioChordIdForNoteWithInferCache (line 7378)
        // handShapeIsArpeggioForLaneRail — only called by fillLaneRailHandShapeFlags (line 7490)
        // effectiveArpRailChartBoundsForHandShape — only called by fillArpeggioRailShapeBoundsCaches (line 7500)
        // arpeggioLaneOuterRailChartIntervalOverlaps — only called by Slice/AtChartTime (lines 7540, 7553)
    };
}