bug: Fixing day9 solution

Author: truggeri

src/day9/day9.go

@@ -1,28 +1,28 @@
 package day9
 
 import (
-	"strconv"
+	"math"
 )
 
 const ZERO_RUNE = '0'
 const EMPTY_RUNE = '.'
-const EMPTY_STRING = string(EMPTY_RUNE)
 
-func Solve(input string) int {
+type file struct {
+	empty bool
+	digit uint
+}
+
+func Solve(input string) uint64 {
 	return checksum(compress(expand(input)))
 }
 
-func expand(input string) string {
-	var result string
+func expand(input string) []file {
+	result := make([]file, 0)
 	blankSpace := false
-	id := 0
+	var id uint = 0
 	for _, fileSize := range input {
 		for i := 0; i < runeToInt(fileSize); i++ {
-			if blankSpace {
-				result += EMPTY_STRING
-			} else {
-				result += strconv.Itoa(id)
-			}
+			result = append(result, file{empty: blankSpace, digit: id})
 		}
 		if !blankSpace {
 			id += 1
@@ -32,11 +32,15 @@ func expand(input string) string {
 	return result
 }
 
-func compress(input string) string {
-	result := []byte(input)
+func runeToInt(r rune) int {
+	return int(r - ZERO_RUNE)
+}
+
+func compress(input []file) []file {
+	result := input
 	ptr, tailPtr := 0, len(input)-1
 	for {
-		if result[tailPtr] != EMPTY_RUNE || tailPtr <= 0 {
+		if !result[tailPtr].empty || tailPtr <= 0 {
 			break
 		}
 		tailPtr--
@@ -46,34 +50,34 @@ func compress(input string) string {
 		if ptr >= tailPtr {
 			break
 		}
-		if result[ptr] != EMPTY_RUNE {
+		if !result[ptr].empty {
 			ptr++
 			continue
 		}
 		result[ptr] = result[tailPtr]
-		result[tailPtr] = EMPTY_RUNE
+		result[tailPtr] = file{empty: true}
 		ptr++
 		for {
 			tailPtr--
-			if result[tailPtr] != EMPTY_RUNE {
+			if !result[tailPtr].empty {
 				break
 			}
 		}
 	}
-	return string(result)
+	return result
 }
 
-func checksum(input string) int {
-	result := 0
+func checksum(input []file) uint64 {
+	result := uint64(0)
 	for i, d := range input {
-		if d == EMPTY_RUNE {
+		if d.empty {
 			break
 		}
-		result += i * runeToInt(d)
+		val := uint64(i * int(d.digit))
+		if math.MaxUint64-val < result {
+			panic("overflow")
+		}
+		result += val
 	}
 	return result
 }
-
-func runeToInt(r rune) int {
-	return int(r - ZERO_RUNE)
-}