Solution to the latest cassidoo challenge

longest.go

package longest

func longestPrefix(words []string) string {
	if len(words) == 0 {
		return ""
	}

	common := make([]rune, 0, len(words[0]))
	for i, letter := range words[0] {
		for _, word := range words[0:] {
			if letter != rune(word[i]) {
				return string(common)
			}
		}
		common = append(common, letter)
	}

	return string(common)
}

longest_test.go

package longest

import "testing"

func TestLongestPrefix(t *testing.T) {
	params := []struct {
		words    []string
		expected string
	}{
		{[]string{"cranberry", "crawfish", "crap"}, "cra"},
		{[]string{"parrot", "poodle", "fish"}, ""},
		{[]string{}, ""},
		{[]string{"fore", "fore", "fore"}, "fore"},
	}

	for _, param := range params {
		actual := longestPrefix(param.words)

		if param.expected != actual {
			t.Logf("given: %s\nexpected: %s\nactual: %s\n", param.words, param.expected, actual)
			t.Fail()
		}
	}
}