challenge1.go

// Challenge 1 June 29th 2020: implement a binary operation parser.
// It must be implemented in Go, and the only import allowed is "strings".
// For bonus points, implement the scanner functions.
// The solution must start with comments with the following information:
// Your name, any open-source project links (on github, gitlab, anything publicly accessible).
// We are looking for elegance of implementation.
// Test cases are not necessary, we will test code automatically, so keep the method signatures.
// Email solutions to jae+challenge@tendermint.com.
// For questions, email there or respond to @jaekwon's latest tweets.
// NOTE Sharing your solution, or hints, will disqualify you and everyone involved from future challenges.
// Tweet, if you love toxicity: https://twitter.com/jaekwon/status/1276780601761320961
package challenge1

import "strings"

var sp = " "
var prec5 = strings.Split("*  /  %  <<  >>  &  &^", sp)
var prec4 = strings.Split("+ - | ^", sp)
var prec3 = strings.Split("== != < <= > >=", sp)
var prec2 = strings.Split("&&", sp)
var prec1 = strings.Split("||", sp)
var precs = [][]string{prec1, prec2, prec3, prec4, prec5}

// Splits a Go expression into left and right parts.
// Returns idx=-1 if not a binary operator.
// Precedence    Operator
//     5             *  /  %  <<  >>  &  &^
//     4             +  -  |  ^
//     3             ==  !=  <  <=  >  >=
//     2             &&
//     1             ||
//
// Examples:
//  - "5 * 2":       left="5 ", op="*", right=" 2", ok=true
//  - " 5*2 ":       left=" 5", op="*", right="2 ", ok=true
//  - "1*2+ 3":      left="1*2", op="+", right=" 3", ok=true
//  - "1*2+(3+ 4)":  left="1*2", op="+", right="(3+ 4)", ok=true
//  - "'foo'+'bar'": left="'foo'", op="+", right="'bar'", ok=true     
//  - "'x'":         ok=false
//
// Never panics.
func chopBinaryOperator(expr string) (left, op, right string, ok bool) {
	// XXX Implement this.
	// The methods for "scanner" are provided below and omitted with '...'.
	// For bonus points, implement all the scanner methods (with method signatures as is).
}

// You can assume that the scanner functions below are implemented.
type runestate int

const (
	runestateCode           runestate  = 0
	runestateRune           scanestate = 1
	runestateStringQuote    runestate  = 2
	runestateStringBacktick runestate  = 3
)

type scanner struct {
	str       string
	rnz       []rune
	idx       int
	runestate
	curly     int
	round     int
	square    int
}

// returns a new scanner.
func newScanner(str string) *scanner {
	rnz := make([]rune, 0, len(str))
	for _, r := range str {
		rnz = append(rnz, r)
	}
	return &scanner{
		str: str,
		rnz: rnz,
	}
}

// Peeks the next n runes and returns a string.  returns a shorter string if
// there are less than n runes left.
// e.g. if the next rules are '+' followed by '=', followed by end of file,
// peek(2) returns "+=", and so would peek(3) because eof.
func (ss *scanner) peek(n int) string {
...
}

// Advance a single rune, e.g. by incrementing ss.curly if ss.rnz[ss.idx] is
// '{' before advancing.  If ss.runestate is runestateRune or runestateQuote,
// advances escape sequences to completion so ss.idx may increment more than
// one.  Returns true if done.
func (ss *scanner) advance() bool {
...
}

// returns true if no runes left to advance.
func (ss *scanner) done() bool {
	return ss.idx == len(ss.rnz)
}

// returns true if outside the scope of any
// parentheses, brackets, strings, or rune literals.
func (ss *scanner) out() bool {
	return ss.runestate == runestateCode &&
		ss.curly == 0 &&
		ss.round == 0 &&
		ss.square == 0
}

// Advances runes, while checking that each passes `check`.  if error, panics
// with info including `back` runes back.
func (ss *scanner) eatRunes(back int, eat int, check func(rune) bool) {
...
}

// increments idx until escape sequence is complete.
// returns true if done.
func (ss *scanner) advanceEscapeSequence() bool {
...
}