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parser.go
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/
parser.go
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package calc
import (
"errors"
"io"
)
type Parser struct {
s *Scanner
buf struct {
tok Token
n int
}
}
func NewParser(r io.Reader) *Parser {
return &Parser{s: NewScanner(r)}
}
func (p *Parser) Scan() (Token, error) {
if p.buf.n != 0 {
p.buf.n = 0
return p.buf.tok, nil
}
tok, err := p.s.Scan()
if err != nil {
return Token{}, err
}
p.buf.tok = tok
return tok, nil
}
func (p *Parser) ScanIgnoreWhitespace() (Token, error) {
tok, err := p.Scan()
if err != nil {
return Token{}, err
}
if tok.Type == Whitespace {
return p.Scan()
}
return tok, nil
}
func (p *Parser) Unscan() {
p.buf.n = 1
}
func (p *Parser) Parse() (Stack, error) {
stack := Stack{}
for {
tok, err := p.ScanIgnoreWhitespace()
if errors.Is(err, io.EOF) {
break
} else if err != nil {
return Stack{}, err
} else if tok.Type == Operator && tok.Value == "-" {
lastTok := stack.Peek()
nextTok, err := p.ScanIgnoreWhitespace()
if err != nil {
return Stack{}, err
}
if (lastTok.Type == Operator || lastTok.Value == "" || lastTok.Type == Lparen) && nextTok.Type == Number {
stack.Push(Token{Number, "-" + nextTok.Value})
} else {
stack.Push(tok)
p.Unscan()
}
} else {
stack.Push(tok)
}
}
return stack, nil
}