day 9 part 1

src/days/9/__snapshots__/index.test.ts.snap

@@ -1,68 +1,15 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`expandWithDifferences handles neg values 1`] = `
+exports[`Day 9 Mirage maintenance reduceHistories (recursive) generates prev histories 1`] = `
 [
   [
-    -9,
-    -13,
-    -17,
-    -21,
-    -25,
-    -29,
-    -33,
-    -37,
-    -41,
-    -45,
-    -49,
-    -53,
-    -57,
-    -61,
-    -65,
-    -69,
-    -73,
-    -77,
-    -81,
-    -85,
-    -89,
+    3,
+    3,
+    3,
+    3,
+    3,
   ],
   [
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-    -4,
-  ],
-  [
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
-    0,
     0,
     0,
     0,

src/days/9/index.test.ts

@@ -1,32 +1,56 @@
-import { explode } from '@/lib/utils'
-import { calculateDifferences, expandWithDifferences } from '.'
+import {
+  calculateDifferences,
+  calculateNext,
+  isAllZeros,
+  makeHistory,
+  parseInput,
+  reduceHistories,
+} from '.'
 
-describe('calculateDifferences', () => {
-  it('creates a new Array with differences', () => {
-    const result = calculateDifferences([0, 3, 6, 9, 12, 15])
-    expect(result).toEqual([3, 3, 3, 3, 3])
-    const result2 = calculateDifferences(result)
-    expect(result2).toEqual([0, 0, 0, 0])
+describe('Day 9 Mirage maintenance', () => {
+  describe('parseInput', () => {
+    it('generates array of histories', () => {
+      expect(parseInput('0 3 6 9 12 15\n1 2 3 4')).toEqual([
+        [0, 3, 6, 9, 12, 15],
+        [1, 2, 3, 4],
+      ])
+    })
   })
-})
-
-describe('expandWithDifferences', () => {
-  it('creates an Array of dimishing differences from original row', () => {
-    const result = expandWithDifferences([0, 3, 6, 9, 12, 15])
-    expect(result).toEqual([
-      [0, 3, 6, 9, 12, 15],
-      [3, 3, 3, 3, 3],
-      [0, 0, 0, 0],
-    ])
+  describe('isAllZeros', () => {
+    it('returns true if array is composed of only 0s', () => {
+      expect(isAllZeros([0])).toBe(true)
+      expect(isAllZeros([])).toBe(true)
+      expect(isAllZeros([1, 0])).toBe(false)
+    })
+  })
+  describe('calculateDifferences', () => {
+    it('creates a new Array with differences', () => {
+      const result = calculateDifferences([0, 3, 6, 9, 12, 15])
+      expect(result).toEqual([3, 3, 3, 3, 3])
+      const result2 = calculateDifferences(result)
+      expect(result2).toEqual([0, 0, 0, 0])
+    })
   })
 
-  it('handles neg values', () => {
-    const result = expandWithDifferences(
-      explode(
-        '-9 -13 -17 -21 -25 -29 -33 -37 -41 -45 -49 -53 -57 -61 -65 -69 -73 -77 -81 -85 -89',
-        ' '
-      ).map(Number)
-    )
-    expect(result).toMatchSnapshot()
+  describe('reduceHistories (recursive)', () => {
+    it('generates prev histories', () => {
+      const result = reduceHistories([0, 3, 6, 9, 12, 15])
+      expect(result).toMatchSnapshot()
+    })
+  })
+  describe('calculateNext', () => {
+    it('returns the next value in the history', () => {
+      expect(calculateNext([0, 3, 6, 9, 12, 15])).toBe(18)
+      expect(calculateNext([1, 3, 6, 10, 15, 21])).toBe(28)
+      expect(calculateNext([10, 13, 16, 21, 30, 45])).toBe(68)
+      expect(
+        calculateNext(
+          makeHistory('9 8 7 6 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11')
+        )
+      ).toBe(-12)
+      expect(calculateNext(makeHistory('-9 -13 -17 -21 -25 -29 -33 -37'))).toBe(
+        -41
+      )
+    })
   })
 })

src/days/9/index.ts

@@ -1,63 +1,53 @@
 import { explode, splitLines, sum } from '@/lib/utils'
 import assert from 'assert'
-import { createPipe, map, pipe, tap } from 'remeda'
+import { createPipe, equals, last, map, pipe } from 'remeda'
 
-const parseInput = createPipe(
-  splitLines,
-  map((l) => explode(l, ' ').map(Number))
-)
+export const makeHistory = (line: string) => explode(line, ' ').map(Number)
+
+export const parseInput = createPipe(splitLines, map(makeHistory))
+
+export const isAllZeros = (xs: number[]) =>
+  xs.length === 0 || xs.every(equals(0))
+
+export const printPyramid = (hs: number[][]) =>
+  hs.reduce((acc, h, i) => {
+    acc += '  '.repeat(i) + h.join('   ') + '\n'
+    return acc
+  }, '')
 
 export const calculateDifferences = (seq: number[]) =>
   seq.reduce((acc, n, index) => {
-    if (index < seq.length - 1) {
-      const next = seq[index + 1]
-      if (next) {
-        acc.push(next - n)
-      }
+    if (index > 0) {
+      acc.push(n - (seq.at(index - 1) as number))
     }
     return acc
   }, [] as number[])
 
-export const expandWithDifferences = (seq: number[]) => {
-  return seq.reduce(
-    (acc, _seq, index) => {
-      if (acc.at(-1)?.every((x) => x === 0)) return acc
-      const differences = calculateDifferences(acc[index] as number[])
-      acc.push(differences)
-      return acc
-    },
-    [seq]
-  )
+/**
+ * Returns the differences until a [0...0] array is found.
+ * Does not include the original
+ * @param from initial history
+ * @returns
+ */
+export function reduceHistories(from: number[]): number[][] {
+  if (isAllZeros(from)) return []
+  const next = calculateDifferences(from)
+  return [next].concat(reduceHistories(next))
 }
 
-export const extrapolateNextValue = (seqs: number[][]): number => {
-  const reversed = seqs.reverse()
-  reversed[0] = Array.from({ length: seqs.at(-1)?.length ?? 0 }).fill(
-    0
-  ) as number[]
-  return reversed
-    .reduce(
-      (acc, _seq, index) => {
-        const nextRow = reversed[index + 1]
-        if (!nextRow) return acc
-        acc.push((nextRow.at(-1) as number) + (acc.at(-1) as number))
-        return acc
-      },
-      [0]
-    )
-    .at(-1) as number
+export const calculateNext = (history: number[], debug = false) => {
+  assert(history.length, 'empty history')
+  const histories = reduceHistories(history)
+  if (debug) {
+    console.log(printPyramid([history].concat(histories)))
+  }
+  const next = histories.reduceRight((acc, h) => (last(h) ?? 0) + acc, 0)
+  return next + (last(history) ?? 0)
 }
 
 export function day9PartOne(sample: string, input: string) {
   assert(sample && input, 'Bad input data')
-  const solution = pipe(
-    input,
-    parseInput,
-    map(expandWithDifferences),
-    tap(console.log),
-    map(extrapolateNextValue),
-    sum
-  )
+  const solution = pipe(input, parseInput, map(calculateNext), sum)
   console.log('Part One', solution)
 }