From f169c1bbd9863966a94a07b3972518259710a6b2 Mon Sep 17 00:00:00 2001
From: Julien Dessaux
Date: Thu, 16 Dec 2021 20:49:45 +0100
Subject: Added solutions for 16th day: Packet Decoder

---
 2021/16/bits/scanner.go | 217 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 217 insertions(+)
 create mode 100644 2021/16/bits/scanner.go

(limited to '2021/16/bits')

diff --git a/2021/16/bits/scanner.go b/2021/16/bits/scanner.go
new file mode 100644
index 0000000..296705d
--- /dev/null
+++ b/2021/16/bits/scanner.go
@@ -0,0 +1,217 @@
+package bits
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+)
+
+type Bits struct {
+	Version   byte
+	TypeID    byte
+	Value     int
+	Operators []*Bits
+	Len       int
+}
+
+// Scanner represents a lexical scanner.
+type Scanner struct {
+	r            *bufio.Reader
+	buff         byte
+	readBits     byte
+	parsingDepth int
+}
+
+// NewScanner returns a new instance of Scanner.
+func NewScanner(r io.Reader) *Scanner {
+	return &Scanner{r: bufio.NewReader(r)}
+}
+
+// read reads the next bits from the bufferred reader.
+// Returns the rune(0) if an error occurs (or io.EOF is returned).
+func (s *Scanner) readBit() (bool, error) {
+	var ret bool
+	s.readBits++
+	switch s.readBits {
+	case 2:
+		ret = s.buff&0b0100 != 0
+	case 3:
+		ret = s.buff&0b0010 != 0
+	case 4:
+		ret = s.buff&0b0001 != 0
+	default:
+		var err error
+		s.buff, err = s.r.ReadByte()
+		if err != nil || s.buff == '\n' {
+			return false, err
+		}
+		if s.buff <= '9' {
+			s.buff -= '0'
+		} else {
+			s.buff = s.buff - 'A' + 10
+		}
+		if err != nil {
+			return false, err
+		}
+		s.readBits = 1
+		ret = s.buff&0b1000 != 0
+	}
+	if ret {
+		fmt.Print("1")
+	} else {
+		fmt.Print("0")
+	}
+	return ret, nil
+}
+
+// Scan returns the next Bits packet
+func (s *Scanner) Scan() *Bits {
+	var bits Bits
+	for i := 0; i < 3; i++ {
+		bits.Version <<= 1
+		b, err := s.readBit()
+		if err != nil {
+			return nil
+		}
+		if b {
+			bits.Version++
+		}
+	}
+	fmt.Println("Version:", bits.Version)
+	for i := 0; i < 3; i++ {
+		bits.TypeID <<= 1
+		b, err := s.readBit()
+		if err != nil {
+			return nil
+		}
+		if b {
+			bits.TypeID++
+		}
+	}
+	bits.Len += 6
+	fmt.Println("TypeID:", bits.TypeID)
+	if bits.TypeID == 4 {
+		for {
+			keepGoing, err := s.readBit()
+			if err != nil {
+				return nil
+			}
+			var buff byte
+			for i := 0; i < 4; i++ {
+				buff <<= 1
+				b, err := s.readBit()
+				if err != nil {
+					return nil
+				}
+				if b {
+					buff++
+				}
+			}
+			bits.Len += 5
+			bits.Value = bits.Value<<4 + int(buff)
+			if !keepGoing {
+				break
+			}
+		}
+		fmt.Println("Value:", bits.Value)
+	} else { // operator
+		b, err := s.readBit()
+		if err != nil {
+			return nil
+		}
+		bits.Len++
+		if b {
+			var buff int
+			for i := 0; i < 11; i++ {
+				buff <<= 1
+				b, err := s.readBit()
+				if err != nil {
+					return nil
+				}
+				if b {
+					buff++
+				}
+			}
+			bits.Len += 11
+			fmt.Println("SubPackets type 1, nb of subpackets:", buff)
+			s.parsingDepth++
+			for i := 0; i < buff; i++ {
+				subBits := s.Scan()
+				if subBits == nil {
+					return nil
+				}
+				bits.Operators = append(bits.Operators, subBits)
+				bits.Len += subBits.Len
+			}
+			s.parsingDepth--
+		} else {
+			var buff int
+			for i := 0; i < 15; i++ {
+				buff <<= 1
+				b, err := s.readBit()
+				if err != nil {
+					return nil
+				}
+				if b {
+					buff++
+				}
+			}
+			bits.Len += 15
+			fmt.Println("SubPackets type 0 of len:", buff)
+			s.parsingDepth++
+			for buff > 0 {
+				subBits := s.Scan()
+				if subBits == nil {
+					return nil
+				}
+				bits.Operators = append(bits.Operators, subBits)
+				bits.Len += subBits.Len
+				buff -= subBits.Len
+				fmt.Println("remaining bits:", buff)
+			}
+			s.parsingDepth--
+		}
+	}
+	if s.parsingDepth == 0 {
+		bits.Len += 4 - int(s.readBits)
+		s.readBits = 0
+	}
+	switch bits.TypeID {
+	case 0:
+		for _, sub := range bits.Operators {
+			bits.Value += sub.Value
+		}
+	case 1:
+		bits.Value = 1
+		for _, sub := range bits.Operators {
+			bits.Value *= sub.Value
+		}
+	case 2:
+		bits.Value = bits.Operators[0].Value
+		for _, sub := range bits.Operators {
+			if bits.Value > sub.Value {
+				bits.Value = sub.Value
+			}
+		}
+	case 3:
+		bits.Value = bits.Operators[0].Value
+		for _, sub := range bits.Operators {
+			if bits.Value < sub.Value {
+				bits.Value = sub.Value
+			}
+		}
+	case 5:
+		if bits.Operators[0].Value > bits.Operators[1].Value {
+			bits.Value = 1
+		}
+	case 6:
+		if bits.Operators[0].Value < bits.Operators[1].Value {
+			bits.Value = 1
+		}
+	case 7:
+		if bits.Operators[0].Value == bits.Operators[1].Value {
+			bits.Value = 1
+		}
+	}
+	return &bits
+}
-- 
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