strconv: handle very large inputs

Fixes #2642.

R=remyoudompheng, r, r
CC=golang-dev
https://golang.org/cl/5639052

src/pkg/strconv/atof.go

@@ -52,10 +52,10 @@ func special(s string) (f float64, ok bool) {
 	return
 }
 
-// TODO(rsc): Better truncation handling.
 func (b *decimal) set(s string) (ok bool) {
 	i := 0
 	b.neg = false
+	b.trunc = false
 
 	// optional sign
 	if i >= len(s) {
@@ -88,8 +88,12 @@ func (b *decimal) set(s string) (ok bool) {
 				b.dp--
 				continue
 			}
-			b.d[b.nd] = s[i]
+			if b.nd < len(b.d) {
-			b.nd++
+				b.d[b.nd] = s[i]
+				b.nd++
+			} else if s[i] != '0' {
+				b.trunc = true
+			}
 			continue
 		}
 		break

src/pkg/strconv/atof_test.go

@@ -9,6 +9,7 @@ import (
 	"math/rand"
 	"reflect"
 	. "strconv"
+	"strings"
 	"testing"
 	"time"
 )
@@ -117,6 +118,20 @@ var atoftests = []atofTest{
 
 	// A very large number (initially wrongly parsed by the fast algorithm).
 	{"4.630813248087435e+307", "4.630813248087435e+307", nil},
+
+	// A different kind of very large number.
+	{"22.222222222222222", "22.22222222222222", nil},
+	{"2." + strings.Repeat("2", 4000) + "e+1", "22.22222222222222", nil},
+
+	// Exactly halfway between 1 and math.Nextafter(1, 2).
+	// Round to even (down).
+	{"1.00000000000000011102230246251565404236316680908203125", "1", nil},
+	// Slightly lower; still round down.
+	{"1.00000000000000011102230246251565404236316680908203124", "1", nil},
+	// Slightly higher; round up.
+	{"1.00000000000000011102230246251565404236316680908203126", "1.0000000000000002", nil},
+	// Slightly higher, but you have to read all the way to the end.
+	{"1.00000000000000011102230246251565404236316680908203125" + strings.Repeat("0", 10000) + "1", "1.0000000000000002", nil},
 }
 
 type atofSimpleTest struct {

src/pkg/strconv/decimal.go

@@ -12,12 +12,11 @@
 package strconv
 
 type decimal struct {
-	// TODO(rsc): Can make d[] a bit smaller and add
+	d     [800]byte // digits
-	// truncated bool;
+	nd    int       // number of digits used
-	d   [800]byte // digits
+	dp    int       // decimal point
-	nd  int       // number of digits used
+	neg   bool
-	dp  int       // decimal point
+	trunc bool // discarded nonzero digits beyond d[:nd]
-	neg bool
 }
 
 func (a *decimal) String() string {
@@ -145,8 +144,12 @@ func rightShift(a *decimal, k uint) {
 	for n > 0 {
 		dig := n >> k
 		n -= dig << k
-		a.d[w] = byte(dig + '0')
+		if w < len(a.d) {
-		w++
+			a.d[w] = byte(dig + '0')
+			w++
+		} else if dig > 0 {
+			a.trunc = true
+		}
 		n = n * 10
 	}
 
@@ -242,7 +245,11 @@ func leftShift(a *decimal, k uint) {
 		quo := n / 10
 		rem := n - 10*quo
 		w--
-		a.d[w] = byte(rem + '0')
+		if w < len(a.d) {
+			a.d[w] = byte(rem + '0')
+		} else if rem != 0 {
+			a.trunc = true
+		}
 		n = quo
 	}
 
@@ -251,11 +258,18 @@ func leftShift(a *decimal, k uint) {
 		quo := n / 10
 		rem := n - 10*quo
 		w--
-		a.d[w] = byte(rem + '0')
+		if w < len(a.d) {
+			a.d[w] = byte(rem + '0')
+		} else if rem != 0 {
+			a.trunc = true
+		}
 		n = quo
 	}
 
 	a.nd += delta
+	if a.nd >= len(a.d) {
+		a.nd = len(a.d)
+	}
 	a.dp += delta
 	trim(a)
 }
@@ -286,6 +300,10 @@ func shouldRoundUp(a *decimal, nd int) bool {
 		return false
 	}
 	if a.d[nd] == '5' && nd+1 == a.nd { // exactly halfway - round to even
+		// if we truncated, a little higher than what's recorded - always round up
+		if a.trunc {
+			return true
+		}
 		return nd > 0 && (a.d[nd-1]-'0')%2 != 0
 	}
 	// not halfway - digit tells all
@@ -293,7 +311,6 @@ func shouldRoundUp(a *decimal, nd int) bool {
 }
 
 // Round a to nd digits (or fewer).
-// Returns receiver for convenience.
 // If nd is zero, it means we're rounding
 // just to the left of the digits, as in
 // 0.09 -> 0.1.
@@ -309,7 +326,6 @@ func (a *decimal) Round(nd int) {
 }
 
 // Round a down to nd digits (or fewer).
-// Returns receiver for convenience.
 func (a *decimal) RoundDown(nd int) {
 	if nd < 0 || nd >= a.nd {
 		return
@@ -319,7 +335,6 @@ func (a *decimal) RoundDown(nd int) {
 }
 
 // Round a up to nd digits (or fewer).
-// Returns receiver for convenience.
 func (a *decimal) RoundUp(nd int) {
 	if nd < 0 || nd >= a.nd {
 		return