fmt: implement precs for %q.

Also fix a bug: precision was in terms of bytes; should be runes.
Fixes #1652.

R=rsc, bradfitzgo, r2, bradfitzwork
CC=golang-dev
https://golang.org/cl/4280086

diff --git a/src/pkg/fmt/fmt_test.go b/src/pkg/fmt/fmt_test.go
index 4d308ac342dea8371f877f1d4363244b837bc3d2..3766c838a91009c44fb0ed188eaaf7a488fd163f 100644 (file)
--- a/src/pkg/fmt/fmt_test.go
+++ b/src/pkg/fmt/fmt_test.go
@@ -139,7 +139,17 @@ var fmttests = []struct {
        {"%5s", "abc", "  abc"},
        {"%2s", "\u263a", " \u263a"},
        {"%-5s", "abc", "abc  "},
+       {"%-8q", "abc", `"abc"   `},
        {"%05s", "abc", "00abc"},
+       {"%08q", "abc", `000"abc"`},
+       {"%5s", "abcdefghijklmnopqrstuvwxyz", "abcdefghijklmnopqrstuvwxyz"},
+       {"%.5s", "abcdefghijklmnopqrstuvwxyz", "abcde"},
+       {"%.5s", "日本語日本語", "日本語日本"},
+       {"%.5s", []byte("日本語日本語"), "日本語日本"},
+       {"%.5q", "abcdefghijklmnopqrstuvwxyz", `"abcde"`},
+       {"%.3q", "日本語日本語", `"\u65e5\u672c\u8a9e"`},
+       {"%.3q", []byte("日本語日本語"), `"\u65e5\u672c\u8a9e"`},
+       {"%10.1q", "日本語日本語", `  "\u65e5"`},
 
        // integers
        {"%d", 12345, "12345"},

diff --git a/src/pkg/fmt/format.go b/src/pkg/fmt/format.go
index caaa7ac1a8af36ed2fb0ac13ef69911f33971954..f9d2b4fcaf6b177425d769c7f2fc62aef9ccc7dc 100644 (file)
--- a/src/pkg/fmt/format.go
+++ b/src/pkg/fmt/format.go
@@ -235,13 +235,24 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
        f.pad(buf[i:])
 }
 
-// fmt_s formats a string.
-func (f *fmt) fmt_s(s string) {
-       if f.precPresent {
-               if f.prec < len(s) {
-                       s = s[0:f.prec]
+// truncate truncates the string to the specified precision, if present.
+func (f *fmt) truncate(s string) string {
+       if f.precPresent && f.prec < utf8.RuneCountInString(s) {
+               n := f.prec
+               for i := range s {
+                       if n == 0 {
+                               s = s[:i]
+                               break
+                       }
+                       n--
                }
        }
+       return s
+}
+
+// fmt_s formats a string.
+func (f *fmt) fmt_s(s string) {
+       s = f.truncate(s)
        f.padString(s)
 }
 
@@ -275,6 +286,7 @@ func (f *fmt) fmt_sX(s string) {
 
 // fmt_q formats a string as a double-quoted, escaped Go string constant.
 func (f *fmt) fmt_q(s string) {
+       s = f.truncate(s)
        var quoted string
        if f.sharp && strconv.CanBackquote(s) {
                quoted = "`" + s + "`"