Implement letter frequency function for lab3

Includes tests

03_letters/ailyntang/letters.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"sort"
+	"strings"
+	"io"
+	"os"
+)
+
+var out io.Writer = os.Stdout
+
+func main() {
+	fmt.Fprintln(out, strings.Join(sortLetters(letters("aba")), "\n"))
+}
+
+func letters(s string) map[rune]int {
+	count := make(map[rune]int)
+	for _, r := range s {
+		count[r]++
+	}
+	return count
+}
+
+func sortLetters(m map[rune]int) []string {
+	output := []string{}
+	for rune, count := range m {
+		output = append(output, string(rune) + ":" + strconv.Itoa(count))
+	}
+	sort.Strings(output)
+	return output
+}

03_letters/ailyntang/letters_test.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestInputsToLetters(t *testing.T) {
+	assert := assert.New(t)
+
+	assert.Equal(map[rune]int{}, letters(""), "empty string")
+	assert.Equal(map[rune]int{'F': 1}, letters("F"), "letter")
+	assert.Equal(map[rune]int{'\n': 1}, letters("\n"), "new line")
+	assert.Equal(map[rune]int{'*': 1}, letters("*"), "symbol")
+	assert.Equal(map[rune]int{'✌': 1}, letters("✌"), "emoticon")
+}
+
+func TestOutputToLetters(t *testing.T) {
+	assert := assert.New(t)
+	longString := "################################################"
+
+	assert.Equal(map[rune]int{'#': 48}, letters(longString), "long string")
+	assert.Equal(map[rune]int{'H': 1, 'h': 1, 'e': 4, 'r': 2, ' ': 2, '!': 1, '2': 1}, letters("Here here 2!"), "mixed input")
+}
+
+func TestSortLetters(t *testing.T) {
+	assert := assert.New(t)
+
+	longInput := "5555555555555555bbbbbbbbbbbbbbbbbbb✌✌✌✌✌✌✌✌✌✌✌????????????"
+	output1 := []string{" :8", "T:1", "a:1", "b:1", "c:1", "d:1", "e:3", "f:1", "g:1", "h:2", "i:1", "j:1", "k:1", "l:1"}
+	output2 := []string{"m:1", "n:1", "o:4", "p:1", "q:1", "r:2", "s:1", "t:1", "u:2", "v:1", "w:1", "x:1", "y:1", "z:1"}
+	outputManyChars := append(output1, output2...)
+
+	assert.Equal([]string{"a:1", "b:1", "c:1", "d:1", "e:1"}, sortLetters(letters("edcba")), "unsorted input")
+	assert.Equal([]string{"5:16", "?:12", "b:19", "✌:11"}, sortLetters(letters(longInput)), "long input")
+	assert.Equal(outputManyChars, sortLetters(letters("The quick brown fox jumps over the lazy dog")), "many characters")
+}
+
+func TestMainOutput(t *testing.T) {
+	var buf bytes.Buffer
+	out = &buf
+
+	main()
+
+	assert.Equal(t, "a:2\nb:1\n", buf.String())
+}