alright its fast now

nine/src/bruteforce.js

@@ -3,8 +3,10 @@ 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 mul = (a, b) => a * b
 const toInt = n => parseInt(n, 10)
+const sum = (list, init = 0) => list.reduce(add, init)
+
 
 function* scanLeft(xs) {
     for (let i = 0, j = 0; i < xs.length; i++) {
@@ -19,7 +21,7 @@ function* scanLeft(xs) {
 
 function* scanRight(xs) {
     const len = xs.length - 1
-    for (let i = len; i > 0; i--) {
+    for (let i = len; i >= 0; i--) {
         const token = xs[i]
         const value = i % 2 ? -1 : i / 2
 
@@ -32,10 +34,10 @@ function* scanRight(xs) {
 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))]
+    return [scanLeft(ints), scanRight(ints), sum(tokens.map(BigInt), 0n)]
 }
 
-function* defrag([fwd, bwd, len]) {
+function* enfrag([fwd, bwd, len]) {
     const swaps = new Set()
     let i = 0n
     let j = len - 1n
@@ -45,7 +47,7 @@ function* defrag([fwd, bwd, len]) {
     for (i, a; !a.done; a = fwd.next(), i += 1n) {
         const x = a.value
         if (swaps.has(i)) {
-            yield -1
+            yield - 1
             swaps.delete(i)
             continue
         } else if (x !== -1) {
@@ -75,23 +77,72 @@ function* defrag([fwd, bwd, len]) {
 }
 
 const checksum = iter => {
+    return iter.map(mul)
+        .filter(x => x > 0)
+        .reduce(add)
+}
+
+const expand3 = x => x.split('').map(toInt).map((x, i) => ([x, i % 2, i % 2 ? -1 : i / 2]))
+
+const defrag2 = blocks => {
+    const len = blocks.length - 1
+    for (let i = len; i > 0; i--) {
+        const right = blocks[i]
+        if (right[1]) { continue }
+
+        for (let j = 0; j < len; j++) {
+            if (i < j) { break }
+            const left = blocks[j]
+            if (!left[1]) { continue }
+            if (left[0] >= right[0]) {
+                blocks.splice(i, 1)
+                blocks.splice(j, 0, right)
+                left[0] -= right[0]
+                blocks.splice(i, 0, [right[0], 1, -1])
+                break
+            }
+        }
+    }
+    return blocks
+}
+
+const naturalSum = n => (n * (n + 1)) / 2
+const sumRange = (x, y) => naturalSum(y) - naturalSum(x - 1)
+
+const checksum2 = blocks => {
+    let index = 0
     let acc = 0
-
-    let j = 0
-    for (const value of iter) {
-        const skip = value === -1
-        acc += skip ? 0 : j * value
-
-        if (!skip) {
-            j++
+    for (let i = 0; i < blocks.length; i++) {
+        const block = blocks[i]
+        const start = index
+        index += block[0]
+        if (block[1] > 0) { continue }
+        acc += block[2] * sumRange(start, start + block[0] - 1)
     }
-    }
-
     return acc
 }
 
-const main = pipe(expand, defrag, checksum)
+const part1 = pipe(expand, enfrag, checksum)
+const part3 = pipe(expand3, defrag2, checksum2)
+
+//const tests = [
+//    ['1313165', 169],
+//    ['12345', 132],
+//    ['54321', 31],
+//    ['2333133121414131402', 2858],
+//    ['1010101010101010101010', 385],
+//    ['111111111111111111111', 290],
+//    ['10101010101010101010101', 506],
+//    ['90909', 513]
+//]
+//
+//tests.forEach(([input, expected], i) => {
+//    //console.log(`input length is ${input.length}`)
+//    const result = part3(input)
+//    console.log(i, result === expected, result)
+//})
 
 readFile(resolve('./input'), { encoding: 'utf8' })
-    .then(main)
+    .then(part3)
     .then(console.log)
+