Complete Day 16 Packet Decoder implementation for both parts

_2021/AoC2021.Tests.ps1

@@ -119,4 +119,41 @@ Describe 'Day 5 Hydro Venture' {
             Should -Invoke Write-Warning -Exactly 1 -Scope It -ParameterFilter { $message -eq $expected }
         }
     }
+}
+
+Describe 'Day 16 Packet Decoder' {
+    BeforeEach { Remove-Item "$PSScriptRoot/input16" -ErrorAction 'SilentlyContinue' }
+
+    It 'Given example input, it solves Part 1 correctly for literal value' {
+        Out-File -InputObject 'D2FE28' "$PSScriptRoot/input16"
+        Mock Write-Warning { Write-Output $message }
+        $result = & "$PSScriptRoot/Day 16 Packet Decoder.ps1" 
+        $result.Count | Should -Be 2
+        Should -Invoke Write-Warning -Exactly 1 -Scope It -ParameterFilter { $message -eq 'Part 1 - Version sum: 6' }
+        Should -Invoke Write-Warning -Exactly 1 -Scope It -ParameterFilter { $message -eq 'Part 2 - Evaluated value: 2021' }
+    }
+
+    It 'Given example input, it solves Part 1 correctly for operator packet' {
+        Out-File -InputObject '8A004A801A8002F478' "$PSScriptRoot/input16"
+        Mock Write-Warning { Write-Output $message }
+        $result = & "$PSScriptRoot/Day 16 Packet Decoder.ps1" 
+        $result.Count | Should -Be 2
+        Should -Invoke Write-Warning -Exactly 1 -Scope It -ParameterFilter { $message -eq 'Part 1 - Version sum: 16' }
+    }
+
+    It 'Given example input, it solves Part 2 correctly for sum operation' {
+        Out-File -InputObject 'C200B40A82' "$PSScriptRoot/input16"
+        Mock Write-Warning { Write-Output $message }
+        $result = & "$PSScriptRoot/Day 16 Packet Decoder.ps1" 
+        $result.Count | Should -Be 2
+        Should -Invoke Write-Warning -Exactly 1 -Scope It -ParameterFilter { $message -eq 'Part 2 - Evaluated value: 3' }
+    }
+
+    It 'Given example input, it solves Part 2 correctly for product operation' {
+        Out-File -InputObject '04005AC33890' "$PSScriptRoot/input16"
+        Mock Write-Warning { Write-Output $message }
+        $result = & "$PSScriptRoot/Day 16 Packet Decoder.ps1" 
+        $result.Count | Should -Be 2
+        Should -Invoke Write-Warning -Exactly 1 -Scope It -ParameterFilter { $message -eq 'Part 2 - Evaluated value: 54' }
+    }
 }

_2021/Day 16 Packet Decoder.ps1

@@ -1,4 +1,14 @@
-<# --- Day 16: Packet Decoder ---#>
+<# --- Day 16: Packet Decoder ---
+As you leave the cave and reach open waters, you receive a transmission from the Elves back on the ship.
+
+The transmission was sent using the Buoyancy Interchange Transmission System (BITS), 
+a method of packing numeric expressions into a binary sequence. 
+
+Part 1: Decode the structure of your hexadecimal-encoded BITS transmission; 
+what do you get if you add up the version numbers in all packets?
+
+Part 2: What do you get if you evaluate the expression represented by your hexadecimal-encoded BITS transmission?
+#>
 
 $year, $day = 2021, 16
 
@@ -18,38 +28,138 @@ $binary = $string.ToCharArray().foreach({ [Convert]::ToString([Convert]::ToByte(
 # Every packet begins with a standard header: the first three bits encode the packet version, and the next three bits encode the packet type ID. These two values are numbers; all numbers encoded in any packet are represented as binary with the most significant bit first. 
 # For example, a version encoded as the binary sequence 100 represents the number 4.
 
-function decode_package ($package)
+function decode_package ($package, $position = 0)
 {
-  Write-Debug "Decoding ${package}"
-  $version = [convert]::ToInt16($package.Substring(0, 3), 2)
-  Write-Debug $version
-  $type_ID = [convert]::ToInt16($package.Substring(3, 3), 2)
-  Write-Debug $type_ID
+  Write-Debug "Decoding ${package} at position ${position}"
+  $version = [convert]::ToInt16($package.Substring($position, 3), 2)
+  $version_sum = $version
+  Write-Debug "Version: $version"
+  $type_ID = [convert]::ToInt16($package.Substring($position + 3, 3), 2)
+  Write-Debug "Type ID: $type_ID"
+  $current_pos = $position + 6
+
   if ($type_ID -eq 4)
   {
     # literal value
-    $pointer = 1
     $value = ''
     do
     {
-      $pointer += 5
-      $value += $package.Substring($pointer + 1, 4)
-    } until ($package.Substring($pointer, 1) -eq '0')
-    return [convert]::ToInt16($value, 2)
+      $group = $package.Substring($current_pos, 5)
+      $continue_bit = $group.Substring(0, 1)
+      $value += $group.Substring(1, 4)
+      $current_pos += 5
+    } while ($continue_bit -eq '1')
+
+    $literal_value = [convert]::ToInt64($value, 2)
+    Write-Debug "Literal value: $literal_value"
+    return @{
+      Version = $version
+      TypeID = $type_ID
+      Value = $literal_value
+      VersionSum = $version_sum
+      BitsConsumed = $current_pos - $position
+    }
   }
   else
   {
     # operator
-    if ($package.Substring($pointer, 1) -eq '0')
+    $length_type_ID = $package.Substring($current_pos, 1)
+    $current_pos += 1
+    $sub_packets = @()
+
+    if ($length_type_ID -eq '0')
     {
       # the next 15 bits are a number that represents the total length in bits of the sub-packets contained by this packet.
+      $sub_packets_length = [convert]::ToInt16($package.Substring($current_pos, 15), 2)
+      $current_pos += 15
+      $sub_packets_end = $current_pos + $sub_packets_length
+
+      Write-Debug "Operator with length type 0, sub-packets length: $sub_packets_length"
+
+      while ($current_pos -lt $sub_packets_end)
+      {
+        $sub_packet = decode_package $package $current_pos
+        $sub_packets += $sub_packet
+        $version_sum += $sub_packet.VersionSum
+        $current_pos += $sub_packet.BitsConsumed
+      }
     }
     else
     {
       # the next 11 bits are a number that represents the number of sub-packets immediately contained by this packet.
+      $num_sub_packets = [convert]::ToInt16($package.Substring($current_pos, 11), 2)
+      $current_pos += 11
+
+      Write-Debug "Operator with length type 1, number of sub-packets: $num_sub_packets"
+
+      for ($i = 0; $i -lt $num_sub_packets; $i++)
+      {
+        $sub_packet = decode_package $package $current_pos
+        $sub_packets += $sub_packet
+        $version_sum += $sub_packet.VersionSum
+        $current_pos += $sub_packet.BitsConsumed
+      }
     }
 
+    return @{
+      Version = $version
+      TypeID = $type_ID
+      LengthTypeID = $length_type_ID
+      SubPackets = $sub_packets
+      VersionSum = $version_sum
+      BitsConsumed = $current_pos - $position
+    }
   }
 }
 
-decode_package $binary
+function evaluate_packet ($packet)
+{
+  if ($packet.TypeID -eq 4)
+  {
+    # Literal value
+    return $packet.Value
+  }
+  else
+  {
+    # Operator packet - evaluate sub-packets first
+    $sub_values = $packet.SubPackets | ForEach-Object { evaluate_packet $_ }
+
+    switch ($packet.TypeID)
+    {
+      0 { # sum
+        return ($sub_values | Measure-Object -Sum).Sum
+      }
+      1 { # product
+        $product = 1
+        $sub_values | ForEach-Object { $product *= $_ }
+        return $product
+      }
+      2 { # minimum
+        return ($sub_values | Measure-Object -Minimum).Minimum
+      }
+      3 { # maximum
+        return ($sub_values | Measure-Object -Maximum).Maximum
+      }
+      5 { # greater than
+        return [int]($sub_values[0] -gt $sub_values[1])
+      }
+      6 { # less than
+        return [int]($sub_values[0] -lt $sub_values[1])
+      }
+      7 { # equal to
+        return [int]($sub_values[0] -eq $sub_values[1])
+      }
+      default {
+        throw "Unknown type ID: $($packet.TypeID)"
+      }
+    }
+  }
+}
+
+# Parse the packet and calculate version sum for Part 1
+$packet = decode_package $binary
+Write-Warning "Part 1 - Version sum: $($packet.VersionSum)"
+
+# Evaluate the packet for Part 2
+$result = evaluate_packet $packet
+Write-Warning "Part 2 - Evaluated value: $result"