Day 15 (late, lol)

solutions/day15/solution.js

@@ -0,0 +1,44 @@
+const path = require('path')
+const { read, position } = require('promise-path')
+const fromHere = position(__dirname)
+const report = (...messages) => console.log(`[${require(fromHere('../../package.json')).logName} / ${__dirname.split(path.sep).pop()}]`, ...messages)
+
+async function run () {
+  const input = (await read(fromHere('input.txt'), 'utf8')).trim().split(',').map(x => 1 * x)
+
+  await solveForFirstStar(input)
+  await solveForSecondStar(input)
+}
+
+async function solveForFirstStar (input) {
+  const seq = [...input]
+  const next = () => {
+    for (let i = 1; i < seq.length; i++) {
+      if (seq[seq.length - 1 - i] === seq[seq.length - 1]) {
+        return i
+      }
+    }
+    return 0
+  }
+  while (seq.length < 2020) {
+    seq.push(next())
+  }
+
+  const solution = seq[2019]
+  //  report('Input:', input);
+  report('Solution 1:', solution)
+}
+
+async function solveForSecondStar (input) {
+  const lastSeenIndex = new Map(input.slice(0, -1).map((n, i) => [n, i]))
+  let last = input[input.length - 1]
+  for (let i = input.length; i < 30000000; i++) {
+    const next = i - 1 - lastSeenIndex.get(last) || 0
+    lastSeenIndex.set(last, i - 1)
+    last = next
+  }
+
+  report('Solution 2:', last)
+}
+
+run()