Day 5 solution

05.ts

@@ -0,0 +1,73 @@
+import fs from 'fs'
+
+const INPUT = fs.readFileSync('05_input.txt', 'utf8')
+
+function strArrToNumArr(arr: string[]): number[] {
+  return arr.filter(str => str.length).map(str => parseInt(str))
+}
+
+function determineRulePlacement(update: number[], selectedPage: number, comparePage: number): number {
+  const afterPages = PAGE_ORDERING_RULES.filter((rule) => rule[0] === selectedPage).map((rule) => rule[1])
+  const beforePages = PAGE_ORDERING_RULES.filter((rule) => rule[1] === selectedPage).map((rule) => rule[0])
+  const selectedIndex = update.indexOf(selectedPage)
+  const comparedIndex = update.indexOf(comparePage)
+  const comparedIsBefore = comparedIndex < selectedIndex
+  const comparedIsAfter = selectedIndex < comparedIndex
+  if (afterPages.includes(comparePage) && comparedIsBefore) {
+    // selected page should come before
+    return -1
+  } else if (beforePages.includes(comparePage) && comparedIsAfter) {
+    // selected page should come after
+    return 1
+  }
+  return 0
+}
+
+// TODO: Rewrite follows rules so it uses determineRulePlacement algorithm
+function followsRules(update: number[]) {
+  for (let i = 0; i < update.length; i++) {
+    const selectedPage = update[i]
+    const afterPageRules = PAGE_ORDERING_RULES.filter((rule) => rule[0] === selectedPage).map((rule) => rule[1])
+    const beforePageRules = PAGE_ORDERING_RULES.filter((rule) => rule[1] === selectedPage).map((rule) => rule[0])
+    for (let j = 0; j < update.length; j++) {
+      if (i === j) {
+        continue
+      }
+      const comparePage = update[j]
+      if (afterPageRules.includes(comparePage) && j < i) {
+        // Rule broken because compared page is before current page
+        return false
+      } else if (beforePageRules.includes(comparePage) && j > i) {
+        // Rule broken because compared page is after current page
+        return false
+      }
+    }
+  }
+
+  return true
+}
+
+function fixUpdate(update: number[]) {
+  const output = [...update]
+  output.sort((a, b) => determineRulePlacement(update, a, b))
+  return output
+}
+
+const INPUT_PARTS = INPUT.split('\n\n')
+const PAGE_ORDERING_RULES = INPUT_PARTS[0].split('\n').map(rule => strArrToNumArr(rule.split('|')))
+const UPDATES = INPUT_PARTS[1].split('\n').map(update => strArrToNumArr(update.split(','))).filter(update => update.length)
+
+const output = UPDATES.reduce((acc, update) => {
+  const lastIndex = update.length - 1
+  const middleIndex = lastIndex - lastIndex / 2
+  if (followsRules(update)) {
+    acc.partOne += update[middleIndex]
+  } else {
+    const fixedUpdate = fixUpdate(update)
+    acc.partTwo += fixedUpdate[middleIndex]
+  }
+  return acc
+}, { partOne: 0, partTwo: 0 })
+
+console.log(`Part 1 output: ${output.partOne}`)
+console.log(`Part 2 output: ${output.partTwo}`)