Day 6 solutions 🔧🔧🔧

day_6/part_1.ts

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+import * as fs from "fs";
+import { Race, solve_all_races } from "./solver";
+
+export const read_file = () => {
+  const input = fs.readFileSync("input", "ascii");
+  const [times_str, distances_str] = input.split("\n");
+  const times = times_str
+    .split(" ")
+    .map((token) => Number.parseInt(token))
+    .filter((token) => !isNaN(token));
+  const distances = distances_str
+    .split(" ")
+    .map((token) => Number.parseInt(token))
+    .filter((token) => !isNaN(token));
+
+  const races: Race[] = times.map((time, i) => {
+    return { time: time, distance: distances[i] };
+  });
+
+  return races;
+};
+
+console.log(solve_all_races(read_file()));

day_6/part_2.ts

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+import * as fs from "fs";
+import { Race, solve_all_races } from "./solver";
+
+export const read_file = () => {
+  const input = fs.readFileSync("input", "ascii");
+  const [times_str, distances_str] = input.split("\n");
+  const times = times_str
+    .replace(/ /g, "")
+    .split(":")
+    .map((token) => Number.parseInt(token))
+    .filter((token) => !isNaN(token));
+  const distances = distances_str
+    .replace(/ /g, "")
+    .split(":")
+    .map((token) => Number.parseInt(token))
+    .filter((token) => !isNaN(token));
+
+  const races: Race[] = times.map((time, i) => {
+    return { time: time, distance: distances[i] };
+  });
+
+  return races;
+};
+
+console.log(solve_all_races(read_file()));

day_6/solver.ts

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+export interface Race {
+  time: number;
+  distance: number;
+}
+
+const try_soln = (time: number, race: Race) =>
+  time * (race.time - time) > race.distance;
+
+// In[60]:= distance[ta_, tmax_] := ta*(tmax - ta)
+//
+// In[103]:=
+// solns[d_, tmax_] := Reduce[distance[ta, tmax] >= d + 1, ta, Integers]
+//
+// In[104]:= solns[d, t]
+//
+// Out[104]= (d | t | ta) \[Element]
+//   Integers && ((d < 1/4 (-4 + t^2) &&
+//      t/2 - 1/2 Sqrt[-4 - 4 d + t^2] <= ta <=
+//       t/2 + 1/2 Sqrt[-4 - 4 d + t^2]) || (d == 1/4 (-4 + t^2) &&
+//      ta == t/2))
+
+const get_interval_length = (race: Race) => {
+  var low = Math.ceil(
+    race.time / 2 - (1 / 2) * Math.sqrt(race.time ** 2 - 4 * race.distance)
+  );
+  var high = Math.floor(
+    race.time / 2 + (1 / 2) * Math.sqrt(race.time ** 2 - 4 * race.distance)
+  );
+
+  //   lol, lmao even.
+  if (!try_soln(low, race)) {
+    low += 1;
+  }
+  if (try_soln(low - 1, race)) {
+    low -= 1;
+  }
+  if (!try_soln(high, race)) {
+    high -= 1;
+  }
+  if (try_soln(high + 1, race)) {
+    high += 1;
+  }
+  return high - low + 1;
+};
+
+export const solve_all_races = (races: Race[]) =>
+  races
+    .map((race) => get_interval_length(race))
+    .reduce((prod, v) => prod * v, 1);