1 files changed, 45 insertions, 7 deletions
diff --git a/src/fmt/format.go b/src/fmt/format.go
index 8aeffd7b2..355b73262 100644
--- a/src/fmt/format.go
+++ b/src/fmt/format.go
@@ -49,9 +49,14 @@ type fmt struct {
 	plus        bool
 	sharp       bool
 	space       bool
-	unicode     bool
-	uniQuote    bool // Use 'x'= prefix for %U if printable.
-	zero        bool
+	// For the format %#v, we set this flag and
+	// clear the plus flag, since it is in effect
+	// a different, flagless format set at the top level.
+	// TODO: plusV could use this too.
+	sharpV   bool
+	unicode  bool
+	uniQuote bool // Use 'x'= prefix for %U if printable.
+	zero     bool
 }
 
 func (f *fmt) clearflags() {
@@ -63,6 +68,7 @@ func (f *fmt) clearflags() {
 	f.plus = false
 	f.sharp = false
 	f.space = false
+	f.sharpV = false
 	f.unicode = false
 	f.uniQuote = false
 	f.zero = false
@@ -199,10 +205,36 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	// block but it's not worth the duplication, so ua has 64 bits.
 	i := len(buf)
 	ua := uint64(a)
-	for ua >= base {
-		i--
-		buf[i] = digits[ua%base]
-		ua /= base
+	// use constants for the division and modulo for more efficient code.
+	// switch cases ordered by popularity.
+	switch base {
+	case 10:
+		for ua >= 10 {
+			i--
+			next := ua / 10
+			buf[i] = byte('0' + ua - next*10)
+			ua = next
+		}
+	case 16:
+		for ua >= 16 {
+			i--
+			buf[i] = digits[ua&0xF]
+			ua >>= 4
+		}
+	case 8:
+		for ua >= 8 {
+			i--
+			buf[i] = byte('0' + ua&7)
+			ua >>= 3
+		}
+	case 2:
+		for ua >= 2 {
+			i--
+			buf[i] = byte('0' + ua&1)
+			ua >>= 1
+		}
+	default:
+		panic("fmt: unknown base; can't happen")
 	}
 	i--
 	buf[i] = digits[ua]
@@ -314,11 +346,17 @@ func (f *fmt) fmt_sbx(s string, b []byte, digits string) {
 
 // fmt_sx formats a string as a hexadecimal encoding of its bytes.
 func (f *fmt) fmt_sx(s, digits string) {
+	if f.precPresent && f.prec < len(s) {
+		s = s[:f.prec]
+	}
 	f.fmt_sbx(s, nil, digits)
 }
 
 // fmt_bx formats a byte slice as a hexadecimal encoding of its bytes.
 func (f *fmt) fmt_bx(b []byte, digits string) {
+	if f.precPresent && f.prec < len(b) {
+		b = b[:f.prec]
+	}
 	f.fmt_sbx("", b, digits)
 }