fire-planner/frontend/src/pages/WhatIf.tsx

/**
 * What-If — interactive Monte Carlo. Form on the left, fan chart on the
 * right. Hits POST /simulate (no DB write); ~1-3s for 5k paths.
 */
import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
import { useEffect, useState } from 'react';
import { useNavigate } from 'react-router-dom';

import { api, type SimulateRequest, type SimulateResult } from '@/api/client';
import { FanChart } from '@/components/FanChart';
import { gbp, pct } from '@/lib/format';

const JURISDICTIONS = ['uk', 'cyprus', 'bulgaria', 'malaysia', 'thailand', 'uae', 'nomad'];
const STRATEGIES = ['trinity', 'guyton_klinger', 'vpw', 'vpw_floor'];
const GLIDES = ['rising', 'static_60_40'];

// Plain-English notes shown next to each dropdown so the user knows
// what each option does without leaving the page. Same content gets
// reused in the "About the model" panel at the bottom.
const STRATEGY_NOTES: Record<string, string> = {
  trinity:
    'Withdraw 4% of the starting portfolio in year 1, then keep that real-£ amount fixed. Simple and famous, but rigid — never adapts to market crashes.',
  guyton_klinger:
    'Start higher (~5.5%) and follow guardrail rules: cut spending if the portfolio drops too far, raise it if it grows enough. Adapts to markets, sustainable on long horizons.',
  vpw:
    'Variable Percentage Withdrawal — each year, withdraw a percentage based on years left and expected real return (annuity-style). Mathematically can\'t fail, but income swings can be wide.',
  vpw_floor:
    'VPW with a hard real-£ floor: never withdraw less than the floor, even if VPW says you should. Trades guaranteed lifestyle against ruin risk in bad sequences.',
};

const GLIDE_NOTES: Record<string, string> = {
  rising:
    'Rising-equity glide path (Pfau/Kitces 2014): start ~30% stocks, ramp to ~70% over 15 years. Reduces sequence-of-returns risk in early retirement.',
  static_60_40:
    'Constant 60/40 stocks/bonds for the whole horizon. The classic baseline.',
};

const JURISDICTION_NOTES: Record<string, string> = {
  uk: 'UK 2026/27 PAYE + NI + CGT + dividend rules. Personal allowance tapers above £100k; pension withdrawals 25% tax-free.',
  cyprus: 'Cyprus 60-day non-dom: 17-year exemption on foreign dividends + interest. 2.65% GeSY healthcare levy capped at €180k.',
  bulgaria: 'Flat 10% on worldwide income. EU/EEA capital gains exempt (we apply 10% conservatively).',
  malaysia: 'Foreign-sourced income exempt through 2036. Effective 0% on a typical retiree withdrawal.',
  thailand: 'Foreign-sourced income exempt (v1; the 2024 remittance rule is deferred in this model).',
  uae: 'No personal income tax, no levy. Effective 0%.',
  nomad: 'Tax-free baseline + a 1% regulatory-risk premium to hedge against OECD/CRS rules tightening.',
};

const DEFAULTS: SimulateRequest = {
  jurisdiction: 'cyprus',
  strategy: 'guyton_klinger',
  leave_uk_year: 2,
  glide_path: 'rising',
  spending_gbp: '60000',
  nw_seed_gbp: '1500000',
  savings_per_year_gbp: '0',
  horizon_years: 60,
  floor_gbp: null,
  n_paths: 5000,
  seed: 42,
};

export function WhatIf() {
  const [form, setForm] = useState<SimulateRequest>(DEFAULTS);
  const [nwAutoFilled, setNwAutoFilled] = useState(false);
  const navigate = useNavigate();
  const qc = useQueryClient();

  // Pre-fill NW seed from the latest Wealthfolio snapshot the first
  // time it loads, so opening /what-if always starts from real numbers.
  // The user can still edit; we won't clobber their input on later
  // refetches.
  const nw = useQuery({ queryKey: ['networth', 'current'], queryFn: api.networth.current });
  useEffect(() => {
    if (nwAutoFilled || !nw.data || nw.data.accounts.length === 0) return;
    // Round to whole pounds — pence from the API don't survive HTML5
    // step validation, and the user doesn't think in pence at this scale.
    const rounded = String(Math.round(Number(nw.data.total_gbp)));
    setForm((f) => ({ ...f, nw_seed_gbp: rounded }));
    setNwAutoFilled(true);
  }, [nw.data, nwAutoFilled]);

  const sim = useMutation({
    mutationFn: (req: SimulateRequest) => api.simulate(req),
  });

  const save = useMutation({
    mutationFn: (name: string) =>
      api.scenarios.create({
        name,
        jurisdiction: form.jurisdiction,
        strategy: form.strategy,
        leave_uk_year: form.leave_uk_year,
        glide_path: form.glide_path,
        spending_gbp: form.spending_gbp,
        nw_seed_gbp: form.nw_seed_gbp,
        savings_per_year_gbp: form.savings_per_year_gbp,
        horizon_years: form.horizon_years,
      }),
    onSuccess: (s) => {
      qc.invalidateQueries({ queryKey: ['scenarios'] });
      navigate(`/scenarios/${s.id}`);
    },
  });

  const onSubmit = (e: React.FormEvent) => {
    e.preventDefault();
    sim.mutate({
      ...form,
      floor_gbp: form.strategy === 'vpw_floor' ? form.floor_gbp : null,
    });
  };

  const onSaveAs = () => {
    const suggested = `${form.jurisdiction}-${form.strategy}-leave-y${form.leave_uk_year}`;
    const name = prompt('Save as scenario — name:', suggested);
    if (!name?.trim()) return;
    save.mutate(name.trim());
  };

  const update = <K extends keyof SimulateRequest>(key: K, value: SimulateRequest[K]) =>
    setForm((f) => ({ ...f, [key]: value }));

  return (
    <section className="space-y-6">
      <header>
        <h1 className="text-3xl font-semibold tracking-tight">What if…</h1>
        <p className="text-sm text-slate-500">
          Run a single Monte Carlo against the engine. No data persisted.
        </p>
      </header>

      <div className="grid grid-cols-1 lg:grid-cols-[320px_1fr] gap-6 items-start">
        <form
          onSubmit={onSubmit}
          className="rounded-lg border border-slate-200 bg-white p-5 space-y-4 sticky top-4"
        >
          <Field label="Jurisdiction" hint={JURISDICTION_NOTES[form.jurisdiction]}>
            <Select
              value={form.jurisdiction}
              onChange={(v) => update('jurisdiction', v)}
              options={JURISDICTIONS}
            />
          </Field>
          <Field label="Strategy" hint={STRATEGY_NOTES[form.strategy]}>
            <Select
              value={form.strategy}
              onChange={(v) => update('strategy', v)}
              options={STRATEGIES}
            />
          </Field>
          <Field label="Glide path" hint={GLIDE_NOTES[form.glide_path]}>
            <Select
              value={form.glide_path}
              onChange={(v) => update('glide_path', v)}
              options={GLIDES}
            />
          </Field>
          <Field label="Years until leaving UK">
            <NumberInput
              value={form.leave_uk_year}
              onChange={(v) => update('leave_uk_year', v)}
              min={0}
              max={60}
            />
          </Field>
          <Field label="Annual spending (£)">
            <NumberInput
              value={Number(form.spending_gbp)}
              onChange={(v) => update('spending_gbp', String(v))}
              min={0}
            />
          </Field>
          <Field label="NW seed (£)">
            <NumberInput
              value={Number(form.nw_seed_gbp)}
              onChange={(v) => update('nw_seed_gbp', String(v))}
              min={0}
            />
          </Field>
          <Field label="Annual savings (£)">
            <NumberInput
              value={Number(form.savings_per_year_gbp ?? 0)}
              onChange={(v) => update('savings_per_year_gbp', String(v))}
              min={0}
            />
          </Field>
          <Field label="Horizon (years)">
            <NumberInput
              value={form.horizon_years ?? 60}
              onChange={(v) => update('horizon_years', v)}
              min={5}
              max={100}
            />
          </Field>
          {form.strategy === 'vpw_floor' && (
            <Field label="Floor (£/yr)">
              <NumberInput
                value={Number(form.floor_gbp ?? 40000)}
                onChange={(v) => update('floor_gbp', String(v))}
                min={0}
              />
            </Field>
          )}
          <Field label="Monte Carlo paths">
            <NumberInput
              value={form.n_paths ?? 5000}
              onChange={(v) => update('n_paths', v)}
              min={100}
              max={50000}
              step={100}
            />
          </Field>

          <button
            type="submit"
            disabled={sim.isPending}
            className="w-full rounded-md bg-slate-900 text-white text-sm font-medium px-4 py-2 hover:bg-slate-800 disabled:opacity-60 disabled:cursor-not-allowed"
          >
            {sim.isPending ? 'Running…' : 'Run simulation'}
          </button>
        </form>

        <div className="space-y-4 min-w-0">
          {sim.isError && (
            <div className="rounded-md border border-red-200 bg-red-50 p-4 text-red-800 text-sm">
              {String((sim.error as Error)?.message ?? sim.error)}
            </div>
          )}
          {!sim.data && !sim.isPending && !sim.isError && (
            <div className="rounded-lg border border-slate-200 bg-white p-12 text-center text-slate-500">
              Set parameters on the left and run a simulation.
            </div>
          )}
          {sim.isPending && (
            <div className="rounded-lg border border-slate-200 bg-white p-12 text-center text-slate-500">
              Running Monte Carlo…
            </div>
          )}
          {sim.data && (
            <>
              <Results result={sim.data} horizon={form.horizon_years ?? 60} />
              <div className="flex items-center gap-3">
                <button
                  type="button"
                  onClick={onSaveAs}
                  disabled={save.isPending}
                  className="rounded-md border border-slate-300 bg-white text-sm px-4 py-2 hover:bg-slate-50 disabled:opacity-60"
                >
                  {save.isPending ? 'Saving…' : 'Save as scenario'}
                </button>
                {save.isError && (
                  <span className="text-xs text-red-700">
                    {String((save.error as Error)?.message ?? save.error)}
                  </span>
                )}
              </div>
            </>
          )}

          <AboutTheModel />
        </div>
      </div>
    </section>
  );
}

function AboutTheModel() {
  return (
    <details className="rounded-lg border border-slate-200 bg-white p-5 group">
      <summary className="cursor-pointer text-base font-semibold flex items-center justify-between">
        <span>About the model</span>
        <span className="text-xs text-slate-500 group-open:hidden">Click to expand</span>
      </summary>
      <div className="mt-4 space-y-5 text-sm text-slate-700">
        <Section title="Withdrawal strategies">
          <Term name="Trinity 4%">{STRATEGY_NOTES.trinity}</Term>
          <Term name="Guyton-Klinger guardrails">{STRATEGY_NOTES.guyton_klinger}</Term>
          <Term name="VPW">{STRATEGY_NOTES.vpw}</Term>
          <Term name="VPW + floor">{STRATEGY_NOTES.vpw_floor}</Term>
        </Section>
        <Section title="Glide paths (stock/bond mix over time)">
          <Term name="Rising equity">{GLIDE_NOTES.rising}</Term>
          <Term name="Static 60/40">{GLIDE_NOTES.static_60_40}</Term>
        </Section>
        <Section title="Tax jurisdictions">
          <Term name="UK">{JURISDICTION_NOTES.uk}</Term>
          <Term name="Cyprus">{JURISDICTION_NOTES.cyprus}</Term>
          <Term name="Bulgaria">{JURISDICTION_NOTES.bulgaria}</Term>
          <Term name="Malaysia">{JURISDICTION_NOTES.malaysia}</Term>
          <Term name="Thailand">{JURISDICTION_NOTES.thailand}</Term>
          <Term name="UAE">{JURISDICTION_NOTES.uae}</Term>
          <Term name="Nomad (no fixed residency)">{JURISDICTION_NOTES.nomad}</Term>
        </Section>
        <Section title="How the engine treats tax">
          <p>
            Each year the strategy proposes a real-£ withdrawal <code>w</code>; the chosen
            jurisdiction's tax engine computes <code>tax(w)</code>; the portfolio drops by
            <code> w + tax(w)</code>. Higher-tax jurisdictions therefore drain the portfolio
            faster and lower the success rate — switching jurisdiction visibly moves the fan,
            not just the lifetime-tax cell.
          </p>
        </Section>
        <Section title="Returns">
          <p>
            Real returns are sampled by 5-year block bootstrap from the Shiller 1871+ series
            (or a synthetic Shiller-calibrated stream). 60/40 long-run real ≈ 4.6%; equities
            are ~9.5% nominal / 17% volatility, bonds ~5%/8%. Each path resamples blocks
            independently so sequence-of-returns risk is preserved.
          </p>
        </Section>
        <Section title="Success rate">
          <p>
            A path counts as a success if the portfolio stays positive through every interim
            year. The very last year-end is excluded because VPW deliberately drains to ~0
            at the horizon by construction.
          </p>
        </Section>
      </div>
    </details>
  );
}

function Section({ title, children }: { title: string; children: React.ReactNode }) {
  return (
    <div>
      <h3 className="text-sm font-semibold text-slate-900 mb-2">{title}</h3>
      <div className="space-y-2">{children}</div>
    </div>
  );
}

function Term({ name, children }: { name: string; children: React.ReactNode }) {
  return (
    <div>
      <span className="font-medium">{name}.</span>{' '}
      <span className="text-slate-600">{children}</span>
    </div>
  );
}

function Results({ result, horizon }: { result: SimulateResult; horizon: number }) {
  return (
    <>
      <div className="grid grid-cols-2 md:grid-cols-4 gap-3">
        <Stat label="Success rate" value={pct(result.success_rate)} accent />
        <Stat label="Median ending NW" value={gbp(result.p50_ending_gbp)} />
        <Stat label="P10 ending" value={gbp(result.p10_ending_gbp)} />
        <Stat label="P90 ending" value={gbp(result.p90_ending_gbp)} />
        <Stat label="Median lifetime tax" value={gbp(result.median_lifetime_tax_gbp)} />
        <Stat
          label="Median years to ruin"
          value={result.median_years_to_ruin ?? 'never'}
        />
        <Stat label="Horizon" value={`${horizon} years`} />
        <Stat label="Engine time" value={`${result.elapsed_seconds}s`} />
      </div>
      <div className="rounded-lg border border-slate-200 bg-white p-5">
        <h2 className="text-lg font-semibold mb-2">Portfolio fan</h2>
        <FanChart yearly={result.yearly} height={420} showWithdrawal />
      </div>
    </>
  );
}

function Stat({
  label,
  value,
  accent,
}: {
  label: string;
  value: string | number;
  accent?: boolean;
}) {
  return (
    <div className="rounded-md border border-slate-200 bg-white p-3">
      <div className="text-xs uppercase tracking-wide text-slate-500">{label}</div>
      <div
        className={`text-xl font-semibold tabular-nums mt-1 ${
          accent ? 'text-emerald-700' : ''
        }`}
      >
        {value}
      </div>
    </div>
  );
}

function Field({
  label,
  hint,
  children,
}: {
  label: string;
  hint?: string;
  children: React.ReactNode;
}) {
  return (
    <label className="block">
      <span className="text-xs uppercase tracking-wide text-slate-500">{label}</span>
      <div className="mt-1">{children}</div>
      {hint && <p className="mt-1 text-xs text-slate-500 leading-snug">{hint}</p>}
    </label>
  );
}

function Select({
  value,
  onChange,
  options,
}: {
  value: string;
  onChange: (v: string) => void;
  options: string[];
}) {
  return (
    <select
      value={value}
      onChange={(e) => onChange(e.target.value)}
      className="w-full rounded-md border border-slate-300 bg-white px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-slate-400"
    >
      {options.map((o) => (
        <option key={o} value={o}>
          {o}
        </option>
      ))}
    </select>
  );
}

function NumberInput({
  value,
  onChange,
  min,
  max,
  step = 1,
}: {
  value: number;
  onChange: (v: number) => void;
  min?: number;
  max?: number;
  step?: number;
}) {
  return (
    <input
      type="number"
      value={value}
      min={min}
      max={max}
      step={step}
      onChange={(e) => {
        const n = Number(e.target.value);
        if (Number.isFinite(n)) onChange(n);
      }}
      className="w-full rounded-md border border-slate-300 bg-white px-3 py-2 text-sm tabular-nums focus:outline-none focus:ring-2 focus:ring-slate-400"
    />
  );
}