In response to @laboon's Twitter post concerning a predicate that determines whether a text string contains only one kind of open or close curly braces (https://twitter.com/BillLaboon/status/313085206347276288), I wanted to explore a more efficient approach—one that doesn't involve regular expressions. I may have the predicate inverted here; min…

brace_xor_test.go

package main

import (
	"strings"
	"testing"
)

func doesntContainRune(s string, r rune) bool {
	return !strings.ContainsRune(s, r)
}

func ContainsOneButNotOther(s string, a rune, b rune) bool {
	// Alternately, use strings.IndexAny() and check which matched.
	for i, r := range s {
		switch r {
		case a:
			return doesntContainRune(s[i+1:], b)
		case b:
			return doesntContainRune(s[i+1:], a)
		}
	}
	return false
}

func ContainsOnlyOneKindOfBrace(s string) bool {
	// Alternately, use strings.IndexAny() and check which matched.
	return ContainsOneButNotOther(s, '{', '}')
}

func checkPredicate(t *testing.T,
	s string, expected bool,
	clarification string) {
	if expected != ContainsOnlyOneKindOfBrace(s) {
		t.Errorf("%q contains %s and should count as %t.",
			s, clarification, expected)
	}
}

func TestNoBraces(t *testing.T) {
	checkPredicate(t,
		"Hello.", false,
		"no braces")
}

func TestTwoBraces(t *testing.T) {
	checkPredicate(t,
		"Hello {there}.", false,
		"two braces")
}

func TestOneOpenBrace(t *testing.T) {
	checkPredicate(t,
		"Hello {there.", true,
		"one open brace and no close braces")
}

func TestOneCloseBrace(t *testing.T) {
	checkPredicate(t,
		"Hello }there.", true,
		"one close brace and no open braces")
}

func TestTwoOpenBraces(t *testing.T) {
	checkPredicate(t,
		"Hello {there{.", true,
		"two open braces and no close braces")
}

func TestTwoCloseBraces(t *testing.T) {
	checkPredicate(t,
		"Hello }there}.", true,
		"two close braces and no open braces")
}

func TestTwoOpenAndCloseCloseBraces(t *testing.T) {
	checkPredicate(t,
		"Hello} {{there}.", false,
		"two open and two close braces")
}