1 files changed, 296 insertions, 0 deletions
diff --git a/2021/18/pairs/parser.go b/2021/18/pairs/parser.go
new file mode 100644
index 0000000..08183e4
--- /dev/null
+++ b/2021/18/pairs/parser.go
@@ -0,0 +1,296 @@
+package pairs
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"strconv"
+)
+
+// Token represents a lexical token.
+type Token int
+
+const (
+	// Special tokens
+	ILLEGAL Token = iota
+	EOF
+	WHITESPACE
+
+	// Literals
+	INT // integers
+
+	// Misc characters
+	COMMA // ,
+	OPEN_BRACKET
+	CLOSE_BRACKET
+)
+
+type Pair struct {
+	HasSubPairs bool
+	Value       int
+	Left, Right *Pair
+}
+
+func (p Pair) String() string {
+	if p.HasSubPairs {
+		return fmt.Sprintf("[%s, %s]", p.Left.String(), p.Right.String())
+	}
+	return fmt.Sprintf("%d", p.Value)
+}
+
+func (p Pair) Magnitude() int {
+	if p.HasSubPairs {
+		return 3*p.Left.Magnitude() + 2*p.Right.Magnitude()
+	}
+	return p.Value
+}
+
+func (p1 *Pair) Add(p2 *Pair) *Pair {
+	p := &Pair{
+		HasSubPairs: true,
+		Left:        p1,
+		Right:       p2,
+	}
+	p.Reduce()
+	return p
+}
+
+func (p Pair) DeepCopy() *Pair {
+	if p.HasSubPairs {
+		return &Pair{
+			HasSubPairs: true,
+			Left:        p.Left.DeepCopy(),
+			Right:       p.Right.DeepCopy(),
+		}
+	}
+	return &Pair{Value: p.Value}
+}
+
+func (p *Pair) Reduce() {
+	for {
+		if exploded, _, _ := p.TryExplode(0); exploded {
+			continue
+		}
+		if p.TrySplit() {
+			continue
+		}
+		break
+	}
+}
+
+func (p *Pair) TrySplit() bool {
+	if p.HasSubPairs {
+		if p.Left.TrySplit() {
+			return true
+		}
+		return p.Right.TrySplit()
+	}
+	if p.Value >= 10 {
+		p.HasSubPairs = true
+		p.Left = &Pair{Value: p.Value / 2}
+		p.Right = &Pair{Value: p.Value/2 + p.Value%2}
+		p.Value = 0
+		return true
+	}
+	return false
+}
+
+func (p *Pair) TryExplode(depth int) (bool, *int, *int) {
+	if !p.HasSubPairs {
+		return false, nil, nil
+	}
+	if depth >= 4 {
+		p.HasSubPairs = false
+		p.Value = 0
+		l := p.Left.Value
+		r := p.Right.Value
+		p.Left = nil
+		p.Right = nil
+		return true, &l, &r
+	}
+	depth++
+	if exploded, l, r := p.Left.TryExplode(depth); exploded {
+		if r != nil {
+			r = p.Right.TryAddToLeftMostNumber(r)
+		}
+		return exploded, l, r
+	}
+	if exploded, l, r := p.Right.TryExplode(depth); exploded {
+		if l != nil {
+			l = p.Left.TryAddToRightMostNumber(l)
+		}
+		return exploded, l, r
+	}
+	return false, nil, nil
+}
+
+func (p *Pair) TryAddToLeftMostNumber(n *int) *int {
+	if p.HasSubPairs {
+		return p.Left.TryAddToLeftMostNumber(n)
+	}
+	p.Value += *n
+	return nil
+}
+
+func (p *Pair) TryAddToRightMostNumber(n *int) *int {
+	if p.HasSubPairs {
+		return p.Right.TryAddToRightMostNumber(n)
+	}
+	p.Value += *n
+	return nil
+}
+
+func isWhitespace(ch rune) bool {
+	return ch == ' ' || ch == '\t' || ch == '\n'
+}
+
+func isDigit(ch rune) bool {
+	return (ch >= '0' && ch <= '9')
+}
+
+var eof = rune(0)
+
+// Scanner represents a lexical scanner.
+type Scanner struct {
+	r *bufio.Reader
+}
+
+// NewScanner returns a new instance of Scanner.
+func NewScanner(r io.Reader) *Scanner {
+	return &Scanner{r: bufio.NewReader(r)}
+}
+
+// read reads the next rune from the bufferred reader.
+// Returns the rune(0) if an error occurs (or io.EOF is returned).
+func (s *Scanner) read() rune {
+	ch, _, err := s.r.ReadRune()
+	if err != nil {
+		return eof
+	}
+	return ch
+}
+
+// unread places the previously read rune back on the reader.
+func (s *Scanner) unread() { _ = s.r.UnreadRune() }
+
+// Scan returns the next token and literal value.
+func (s *Scanner) Scan() (tok Token, lit string) {
+	// Read the next rune.
+	ch := s.read()
+
+	// If we see whitespace then consume all contiguous whitespace.
+	for isWhitespace(ch) {
+		ch = s.read()
+	}
+
+	// If we see a digit then consume as an int.
+	if isDigit(ch) {
+		return s.scanInt(ch)
+	}
+
+	// Otherwise read the individual character.
+	switch ch {
+	case eof:
+		return EOF, ""
+	case ',':
+		return COMMA, string(ch)
+	case '[':
+		return OPEN_BRACKET, string(ch)
+	case ']':
+		return CLOSE_BRACKET, string(ch)
+	}
+
+	return ILLEGAL, string(ch)
+}
+
+// scanInt consumes the current rune and all contiguous digit runes.
+func (s *Scanner) scanInt(read rune) (tok Token, lit string) {
+	// Create a buffer and read the current character into it.
+	var buf bytes.Buffer
+	buf.WriteRune(read)
+
+	// Read every subsequent ident character into the buffer.
+	// Non-ident characters and EOF will cause the loop to exit.
+	for {
+		if ch := s.read(); ch == eof {
+			break
+		} else if !isDigit(ch) {
+			s.unread()
+			break
+		} else {
+			_, _ = buf.WriteRune(ch)
+		}
+	}
+
+	// Otherwise return as a regular identifier.
+	return INT, buf.String()
+}
+
+// Parser represents a parser.
+type Parser struct {
+	s   *Scanner
+	buf struct {
+		tok Token  // last read token
+		lit string // last read literal
+		n   int    // buffer size (max=1)
+	}
+}
+
+// NewParser returns a new instance of Parser.
+func NewParser(r io.Reader) *Parser {
+	return &Parser{s: NewScanner(r)}
+}
+
+// scan returns the next token from the underlying scanner.
+// If a token has been unscanned then read that instead.
+func (p *Parser) scan() (tok Token, lit string) {
+	// If we have a token on the buffer, then return it.
+	if p.buf.n != 0 {
+		p.buf.n = 0
+		return p.buf.tok, p.buf.lit
+	}
+
+	// Otherwise read the next token from the scanner.
+	tok, lit = p.s.Scan()
+
+	// Save it to the buffer in case we unscan later.
+	p.buf.tok, p.buf.lit = tok, lit
+
+	return
+}
+
+// unscan pushes the previously read token back onto the buffer.
+func (p *Parser) unscan() { p.buf.n = 1 }
+
+func (p *Parser) Parse() (pair *Pair, err error) {
+	pair = &Pair{}
+	tok, lit := p.scan()
+	switch tok {
+	case OPEN_BRACKET:
+		pair.HasSubPairs = true
+		pair.Left, err = p.Parse()
+		if err != nil {
+			return nil, err
+		}
+		if tok, lit := p.scan(); tok != COMMA {
+			return nil, fmt.Errorf("found %q, expected COMMA", lit)
+		}
+		pair.Right, err = p.Parse()
+		if err != nil {
+			return nil, err
+		}
+		if tok, lit := p.scan(); tok != CLOSE_BRACKET {
+			return nil, fmt.Errorf("found %q, expected CLOSE_BRACKET", lit)
+		}
+	case INT:
+		if v, err := strconv.Atoi(lit); err != nil {
+			return nil, fmt.Errorf("failed to atoi an INT %s : %w", lit, err)
+		} else {
+			pair.Value = v
+		}
+	default:
+		return nil, fmt.Errorf("unexpected %q, expected OPEN_BRACKER or INT", lit)
+	}
+	return pair, nil
+}