vyndr/src/services/bookComparisonService.js

'use strict';

/**
 * Book comparison (Session 28).
 *
 * Surfaces the same prop across every available sportsbook with the best
 * line highlighted. Data source is the grouped odds-api prop shape, which
 * already carries book-by-book lines:
 *   { player, stat_type, lines: [{ book, line, over_odds, under_odds }] }
 *
 * "Best line" = highest decimal payout for the selected side. Savings is
 * the per-$100 payout edge of the best book over the field average — the
 * concrete dollars a user leaves on the table by not line-shopping.
 *
 * Zero credits: it only reads odds already fetched/cached.
 */

const { __internals } = require('./parlayService');
const { americanToDecimal } = __internals;

function average(nums) {
  const valid = nums.filter((n) => Number.isFinite(n));
  if (valid.length === 0) return 0;
  return valid.reduce((a, b) => a + b, 0) / valid.length;
}

function oddsForSide(line, side) {
  if (!line) return null;
  const raw = side === 'under' ? line.under_odds : line.over_odds;
  return raw == null ? null : Number(raw);
}

// Book Comparison order — the crown is PRE-REGISTERED and threshold-gated. It
// stays OFF until Phase 2's spread measurement (scripts/measure-book-spread.js)
// confirms the median same-line spread clears the bar (≥8¢ OR ≥2 implied-prob
// pts). A crown over a flat market is a claim of edge that does not exist.
function crownEnabled(opts) {
  if (opts && opts.crownEnabled != null) return !!opts.crownEnabled;
  return process.env.BOOK_CROWN_ENABLED === '1';
}

// Render every book row honestly, with NO crown. Used for single-book props,
// props with no shared line, and whenever the crown flag is off.
function honestGrid(prop, side, books) {
  const rows = books.map((b) => ({
    book: b.book,
    line: b.line ?? null,
    over_odds: b.over_odds ?? null,
    under_odds: b.under_odds ?? null,
    isBest: false,
  }));
  return {
    player: prop.player,
    stat: prop.stat_type || prop.stat,
    line: prop.line ?? (books[0] && books[0].line) ?? null,
    side,
    books: rows,
    bestBook: null,
    bestOdds: null,
    bookCount: new Set(books.map((b) => b.book)).size,
    savings: 0,
    crowned: false,
  };
}

/**
 * Compare one grouped prop across its books for a given side.
 * Returns null when there are no usable book rows at all.
 *
 * HONEST-ABSENT: a single-book prop renders that one book — no crown, no implied
 * second price, no savings claim. The crown fires ONLY among ≥2 books posting
 * the SAME line with DIFFERING prices, and ONLY when the flag is enabled (Phase 2
 * gate). Comparing prices across different lines is meaningless (Data Semantics).
 */
function compareProp(prop, side = 'over', opts = {}) {
  const books = (prop?.lines || prop?.books || []).filter((b) => b && b.book);
  if (books.length === 0) return null;

  const priced = books.filter((b) => Number.isFinite(oddsForSide(b, side)));
  // Single priced book, or crown disabled → honest grid, no crown.
  if (priced.length < 2 || !crownEnabled(opts)) return honestGrid(prop, side, books);

  // Crown only among books at the SAME line (the shared-line comparison set).
  const byLine = {};
  for (const b of priced) { const L = String(b.line); (byLine[L] = byLine[L] || []).push(b); }
  let shared = [];
  for (const rows of Object.values(byLine)) { if (rows.length > shared.length) shared = rows; }
  const distinctBooks = new Set(shared.map((b) => b.book));
  const distinctPrices = new Set(shared.map((b) => oddsForSide(b, side)));
  // Fewer than 2 books at one line, or all books post the identical price →
  // nothing to crown. Render honestly.
  if (distinctBooks.size < 2 || distinctPrices.size < 2) return honestGrid(prop, side, books);

  let best = shared[0];
  for (const b of shared) {
    if (americanToDecimal(oddsForSide(b, side)) > americanToDecimal(oddsForSide(best, side))) best = b;
  }
  const bestDecimal = americanToDecimal(oddsForSide(best, side));
  const avgDecimal = average(shared.map((b) => americanToDecimal(oddsForSide(b, side))));
  const savings = Math.round((bestDecimal - avgDecimal) * 100 * 100) / 100; // per $100, 2dp

  return {
    player: prop.player,
    stat: prop.stat_type || prop.stat,
    line: best.line ?? prop.line ?? null,
    side,
    // Show all books; crown only the best AT THE SHARED LINE.
    books: books.map((b) => ({
      book: b.book,
      line: b.line ?? null,
      over_odds: b.over_odds ?? null,
      under_odds: b.under_odds ?? null,
      isBest: b.book === best.book && Number(b.line) === Number(best.line),
    })),
    bestBook: best.book,
    bestOdds: oddsForSide(best, side),
    bookCount: distinctBooks.size,
    savings,
    crowned: true,
  };
}

/**
 * Best lines across a list of props, sorted by savings desc (where line-
 * shopping matters most). Drops props with a single book (nothing to
 * compare). `limit` caps the result.
 */
function bestLines(props, { side = 'over', limit = 20, crownEnabled: ce } = {}) {
  if (!Array.isArray(props)) return [];
  // "Best lines tonight" is inherently a crown claim — when the crown is gated
  // off (Phase 2 not yet cleared) it makes no such claim.
  if (!crownEnabled({ crownEnabled: ce })) return [];
  const out = [];
  for (const prop of props) {
    const cmp = compareProp(prop, side, { crownEnabled: ce });
    if (cmp && cmp.crowned && cmp.bookCount >= 2) out.push(cmp);
  }
  out.sort((a, b) => b.savings - a.savings);
  return limit > 0 ? out.slice(0, limit) : out;
}

module.exports = {
  compareProp,
  bestLines,
  __internals: { average, oddsForSide },
};