feat(2018 day-12): find checksum at generation 50,000,000,000

Author: developher-net

2018/day-12/solution.js

@@ -30,8 +30,35 @@ const init = (data) => {
   // 500 generations takes about 30s, so running 50B generations isn't possible. It's
   // clear looking at the log there's a "Game of Life"-style glider.
   // See output/500-generations.txt and output/500-generations.png
+  // This probably is reflected in a pattern in the checksum.
+  let prevCheckSum = 0
+  let prevDelta = 0
+  const stableThreshold = 5 // The number of sequentially identical deltas necessary to determine stabilization
+  const stableDeltas = Array(stableThreshold).fill(0)
+  let stableGeneration = 0
+  let stableCheckSum = 0
+  for (let gen = 0; gen <= generations2; gen++) {
+    const checkSum = plantTracker.getCheckSum(gen)
+    const delta = checkSum - prevCheckSum
+    console.log(`Generation ${gen} checksum: ${plantTracker.getCheckSum(gen)} which is Δ of ${delta})`)
 
-  const answer2 = ''
+    // When delta matches previous generation, we may have reached stabilization
+    if (delta === prevDelta) {
+      stableDeltas.shift()
+      stableDeltas.push(delta)
+      // Reached true stable point when there are N deltas in a row that are the same.
+      if (stableDeltas.filter((Δ) => Δ === delta).length === stableDeltas.length) {
+        stableCheckSum = checkSum
+        stableGeneration = gen
+        break
+      }
+    }
+    prevCheckSum = checkSum
+    prevDelta = delta
+  }
+  console.log(`At generation ${stableGeneration} the Δ is ${stableDeltas[0]} and the checksum is ${stableCheckSum}.`)
+  // Calculate the checksum for 50B generations (minus the generation we're already at)
+  const answer2 = (stableDeltas[0] * (50000000000 - stableGeneration - 1)) + stableCheckSum
 
   console.log(`-- Part 1 --`)
   console.log(`Answer: ${answer}`)