do boring version

nine/src/bruteforce.js

@@ -0,0 +1,97 @@
+import { readFile } from 'node:fs/promises'
+import { resolve } from 'node:path'
+
+const pipe = (...fns) => x => fns.reduce((val, fn) => fn(val), x)
+const add = (a, b) => a + b
+const sum = list => list.reduce(add, 0n)
+const toInt = n => parseInt(n, 10)
+
+function* scanLeft(xs) {
+    for (let i = 0, j = 0; i < xs.length; i++) {
+        const token = xs[i]
+        const value = i % 2 ? -1 : i / 2
+
+        for (let k = 0; k < token; k++, j++) {
+            yield value
+        }
+    }
+}
+
+function* scanRight(xs) {
+    const len = xs.length - 1
+    for (let i = len; i > 0; i--) {
+        const token = xs[i]
+        const value = i % 2 ? -1 : i / 2
+
+        for (let j = token; j > 0; j--) {
+            yield value
+        }
+    }
+}
+
+const expand = input => {
+    const tokens = Array.isArray(input) ? input : input.split('')
+    const ints = tokens.map(toInt)
+    return [scanLeft(ints), scanRight(ints), sum(tokens.map(BigInt))]
+}
+
+function* defrag([fwd, bwd, len]) {
+    const swaps = new Set()
+    let i = 0n
+    let j = len - 1n
+    let a = fwd.next()
+    let b = bwd.next()
+
+    for (i, a; !a.done; a = fwd.next(), i += 1n) {
+        const x = a.value
+        if (swaps.has(i)) {
+            yield -1
+            swaps.delete(i)
+            continue
+        } else if (x !== -1) {
+            yield x
+            continue
+        } else if (j <= i) {
+            yield x
+            continue
+        }
+
+        while (!b.done) {
+            const y = b.value
+            if (j <= i) { break }
+
+            if (y > -1) {
+                swaps.add(j)
+                yield y
+                b = bwd.next()
+                j -= 1n
+                break
+            }
+
+            b = bwd.next()
+            j -= 1n
+        }
+    }
+}
+
+const checksum = iter => {
+    let acc = 0
+
+    let j = 0
+    for (const value of iter) {
+        const skip = value === -1
+        acc += skip ? 0 : j * value
+
+        if (!skip) {
+            j++
+        }
+    }
+
+    return acc
+}
+
+const main = pipe(expand, defrag, checksum)
+
+readFile(resolve('./input'), { encoding: 'utf8' })
+    .then(main)
+    .then(console.log)