Day3 part 1

README.md

@@ -19,4 +19,14 @@ Also without the hint on reddit I think it would have taken me a long time to fi
 
 ## Day2
 Part 1 is mostly about parsing the input, I went for a quick and dirty 'split' based solution. No to be used in prod, that would be very fragile !
+
 Part 2 was straightforward, we didn't even need to worry about interger overflows.
+
+## Day 3
+The most straightforward solution is probably to iterate over the grid, down to bottom and left to right to find the number and determines which ones are close to symbols.
+
+Out of curiosity I'm going to try a solution where I zip the current line with the lines below and above (to find the nearby symbols) and then 'split' the line to find the symbols.
+
+Initially I thought it would be a lot more complex than the iterative approach but it turns out to be quite short.
+
+It may be interesting to try to implement the iterative solution with a state monad.

src/main/scala/day2.scala

@@ -1,6 +1,5 @@
 import zio._
 import zio.Console._
-import scala.io.BufferedSource
 
 object Day2 extends ZIOAppDefault {
 

src/main/scala/day3.scala

@@ -0,0 +1,53 @@
+import zio._
+import zio.Console._
+
+object Day3 extends ZIOAppDefault {
+
+  def padSchematics(schematics: List[String]) =
+    val s2 = schematics.map("." + _ + ".")
+    val pad = "." * s2(0).length
+    pad :: s2.appended(pad)
+
+  def horizontalZip(pSchematics: List[String]): List[List[(Char, Char, Char)]] =
+    val n = pSchematics.length - 2
+    val psr1 = pSchematics.take(n)
+    val psr2 = pSchematics.tail.take(n)
+    val psr3 = pSchematics.tail.tail
+    ((psr1 zip psr2) zip psr3) map ((a, b) =>
+      val bt = List(a._1, a._2, b)
+      bt.transpose.map { case c1 :: c2 :: c3 :: _ =>
+        (c1, c2, c3)
+      }
+    )
+
+  def isDigit(c: Char) = '0' <= c && c <= '9'
+
+  def isPart(c: Char): Boolean = !(isDigit(c) || c == '.')
+
+  def isPart(col: (Char, Char, Char)): Boolean = isPart(col._1) || isPart(col._2) || isPart(col._3)
+
+  def findPartsNumber(zSchematics: List[(Char, Char, Char)]): List[Int] =
+    zSchematics
+      .foldLeft((List[Int](), "", false)) {
+        case ((l, digits, isPartNumber), col) if isDigit(col._2) =>
+          (l, digits.appended(col._2), isPartNumber || isPart(col))
+        case ((l, digits, isPartNumber), col) =>
+          (
+            if (isPartNumber || isPart(col)) && digits != "" then l.appended(digits.toInt)
+            else l,
+            "",
+            isPart(col)
+          )
+      }
+      ._1
+
+  def part1(schematics: List[String]) =
+    val zs = horizontalZip(padSchematics(schematics))
+    zs.flatMap(findPartsNumber).sum
+
+  def run =
+    for {
+      v <- Day1.readFile("day3_input.txt")
+      _ <- printLine(s"part1=${part1(v)}")
+    } yield ()
+}

src/test/scala/day3_test.scala

@@ -0,0 +1,34 @@
+import org.junit.Test
+import org.junit.Assert._
+
+class Day3Test {
+
+  val schematics = List(
+    "467..114..",
+    "...*......",
+    "..35..633.",
+    "......#...",
+    "617*......",
+    ".....+.58.",
+    "..592.....",
+    "......755.",
+    "...$.*....",
+    ".664.598.."
+  )
+
+  @Test def testPadSchematics(): Unit =
+    assertEquals(List("...", ".#.", "..."), Day3.padSchematics(List("#")))
+
+  @Test def testHorizontalZip(): Unit =
+    assertEquals(
+      List(List(('1', '3', '5'), ('2', '4', '6')), List(('3', '5', '7'), ('4', '6', '8'))),
+      Day3.horizontalZip(List("12", "34", "56", "78"))
+    )
+
+  @Test def testFindPartsNumber(): Unit =
+    val hz = Day3.horizontalZip(List(schematics(5), schematics(6), schematics(7)))(0)
+    assertEquals(List(592), Day3.findPartsNumber(hz))
+
+  @Test def testPart1(): Unit =
+    assertEquals(4361, Day3.part1(schematics));
+}