Utilize Z3

README.md

@@ -160,6 +160,7 @@ as the value which passed the given day/phase combination
 #### 2024
 * Day 11 The fact that the order of stones didn't actually matter helped to allow us to just keep a list of how many we had.
 * Day 13 Being able to solve the basic with shortest path while needing math for part two is always fun re-learning linear algebra. And refactoring always speeds things up.
+* Day 17 Another good example of how powerful Z3 can really be.
 
 ## Takeaways
 

src/main/kotlin/me/peckb/aoc/_2024/calendar/day17/Day17.kt

@@ -1,5 +1,11 @@
 package me.peckb.aoc._2024.calendar.day17
 
+import com.microsoft.z3.BitVecExpr
+import com.microsoft.z3.BitVecNum
+import com.microsoft.z3.BitVecSort
+import com.microsoft.z3.Context
+import com.microsoft.z3.Expr
+import com.microsoft.z3.Status
 import javax.inject.Inject
 import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
 import kotlin.math.pow
@@ -16,7 +22,64 @@ class Day17 @Inject constructor(
   fun partTwo(filename: String) = generatorFactory.forFile(filename).read { input ->
     val computer = Computer.fromInput(input)
 
-    findProgram(computer, computer.operations.map{ it.toLong() })
+    val recursive = findProgram(computer, computer.operations.map{ it.toLong() })
+    val maths = uzeZ3(computer.operations)
+
+    if (recursive != maths) throw IllegalStateException("Uh oh!")
+
+    maths
+  }
+
+  private fun uzeZ3(operations: List<Int>): Long {
+    var result: Long = -1
+
+    Context().use { ctx ->
+      val bits = 64 // the answer will fit inside a long
+
+      fun valueOf(value: Int) = ctx.mkBV(value, bits)
+
+      fun variableOf(name: String) = ctx.mkBVConst(name, bits)
+
+      operator fun Expr<BitVecSort>.rem(t: Expr<BitVecSort>) = ctx.mkBVSMod(this, t)
+
+      infix fun Expr<BitVecSort>.xor(t: Expr<BitVecSort>) = ctx.mkBVXOR(this, t)
+
+      infix fun Expr<BitVecSort>.equalTo(t: Expr<BitVecSort>) = ctx.mkEq(this, t)
+
+      infix fun Expr<BitVecSort>.shr(t: Expr<BitVecSort>) = ctx.mkBVLSHR(this, t)
+
+      val eight = valueOf(8)
+      val five  = valueOf(5)
+      val six   = valueOf(6)
+      val three = valueOf(3)
+      val zero  = valueOf(0)
+
+      val a = variableOf("a")
+
+      var aReg = a
+      var bReg: BitVecExpr
+      var cReg: BitVecExpr
+
+      with(ctx.mkOptimize()) {
+        operations.forEach { op ->
+          bReg = aReg % eight
+          bReg = bReg xor five
+          cReg = aReg shr bReg
+          bReg = bReg xor six
+          bReg = bReg xor cReg
+          Add(bReg % eight equalTo valueOf(op))
+          aReg = aReg shr three
+        }
+        Add(aReg equalTo zero)
+        MkMinimize(a)
+
+        if (Check() == Status.SATISFIABLE) {
+          result = (model.evaluate(a, true) as BitVecNum).long
+        }
+      }
+    }
+
+    return result
   }
 
   private fun findProgram(computer: Computer, target: List<Long>): Long {