Session 28: Parlay builder, line movement tracker, book comparison — 3 features, zero credits (1623 tests)

2026-06-13 12:37:08 -04:00
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+'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);
+}
+
+/**
+ * Compare one grouped prop across its books for a given side.
+ * Returns null when there are no usable book lines.
+ */
+function compareProp(prop, side = 'over') {
+  const books = (prop?.lines || prop?.books || []).filter((b) => b && b.book);
+  const priced = books.filter((b) => Number.isFinite(oddsForSide(b, side)));
+  if (priced.length === 0) return null;
+
+  let best = priced[0];
+  for (const b of priced) {
+    if (americanToDecimal(oddsForSide(b, side)) > americanToDecimal(oddsForSide(best, side))) best = b;
+  }
+
+  const bestDecimal = americanToDecimal(oddsForSide(best, side));
+  const avgDecimal = average(priced.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,
+    books: priced.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,
+    })),
+    bestBook: best.book,
+    bestOdds: oddsForSide(best, side),
+    bookCount: priced.length,
+    savings,
+  };
+}
+
+/**
+ * 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 } = {}) {
+  if (!Array.isArray(props)) return [];
+  const out = [];
+  for (const prop of props) {
+    const cmp = compareProp(prop, side);
+    if (cmp && 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 },
+};