Implemented speller

The solution is efficient, but unreadable
Since the performance is not the priority in this task, a prettier solution must be implemented

speller/speller.go

@@ -2,6 +2,87 @@
 
 package speller
 
-func Spell(n int64) string {
-	return ""
+import (
+	"strings"
+)
+
+var numToDigits = []string{
+	"", "one", "two", "three", "four", "five",
+	"six", "seven", "eight", "nine",
+}
+
+var teens = []string{
+	"ten", "eleven", "twelve", "thirteen", "fourteen",
+	"fifteen", "sixteen", "seventeen", "eighteen", "nineteen",
+}
+
+var decs = []string{
+	"", "", "twenty", "thirty", "forty", "fifty",
+	"sixty", "seventy", "eighty", "ninety",
+}
+
+var large = []string{
+	"billion", "million", "thousand",
+}
+
+func digitsReversedSlice(n int64) []uint8 {
+	digits := make([]uint8, 12)
+	for i := 0; i < len(digits); i, n = i+1, n/10 {
+		digits[i] = uint8(n % 10)
+	}
+	return digits
+}
+
+func Spell(n int64) string {
+	if n == 0 {
+		return "zero"
+	}
+	sign := n < 0
+	if sign {
+		n = -n
+	}
+	num, digits := strings.Builder{}, digitsReversedSlice(n)
+	// 35 is the optimal number which gives approx needed
+	// space for the number in 1 allocation
+	num.Grow(len(digits) * 35)
+	if sign {
+		num.WriteString("minus ")
+	}
+	largeIndex, iters := 0, 0
+	for i := len(digits) - 1; i >= 0; i, largeIndex = i-3, largeIndex+1 {
+		first, second, third := digits[i-2], digits[i-1], digits[i]
+		if first == 0 && second == 0 && third == 0 {
+			continue
+		}
+		if iters != 0 {
+			num.WriteRune(' ')
+		}
+		hundreds := numToDigits[third]
+		if hundreds != "" {
+			num.WriteString(hundreds)
+			num.WriteString(" hundred")
+			if second != 0 || first != 0 {
+				num.WriteRune(' ')
+			}
+		}
+		if second > 1 {
+			num.WriteString(decs[second])
+			if first > 0 {
+				num.WriteRune('-')
+				num.WriteString(numToDigits[first])
+			}
+		} else if second == 1 {
+			num.WriteString(teens[first])
+		} else {
+			if first > 0 {
+				num.WriteString(numToDigits[first])
+			}
+		}
+		if largeIndex < len(large) {
+			num.WriteRune(' ')
+			num.WriteString(large[largeIndex])
+		}
+		iters++
+	}
+	return num.String()
 }