Make printf %c work on Unicode codepoints (#236)

interp/functions.go

@@ -415,15 +415,16 @@ func (p *interp) sprintf(format string, args []value) (string, error) {
 			n, isStr := a.isTrueStr()
 			if isStr {
 				s := p.toString(a)
-				if len(s) > 0 {
-					c = []byte{s[0]}
+				_, size := utf8.DecodeRuneInString(s)
+				if size > 0 {
+					c = []byte(s[:size])
 				} else {
 					c = []byte{0}
 				}
 			} else {
-				// Follow the behaviour of awk and mawk, where %c
-				// operates on bytes (0-255), not Unicode codepoints
-				c = []byte{byte(n)}
+				c = make([]byte, utf8.UTFMax)
+				size := utf8.EncodeRune(c, rune(n))
+				c = c[:size]
 			}
 			v = c
 		}

interp/interp_test.go

@@ -85,14 +85,14 @@ NR==3, NR==5 { print NR }
 	{`BEGIN { printf "%.1g", 42 }  # !windows-gawk`, "", "4e+01", "", ""}, // for some reason gawk gives "4e+001" on Windows
 	{`BEGIN { printf "%d", 12, 34 }`, "", "12", "", ""},
 	{`BEGIN { printf "%d" }`, "", "", "format error: got 0 args, expected 1", "not enough arg"},
-	// Our %c handling is mostly like awk's, except for multiples
-	// 256, where awk is weird, and we're like mawk
 	{`BEGIN { printf "%c", 0 }`, "", "\x00", "", ""},
 	{`BEGIN { printf "%c", 127 }`, "", "\x7f", "", ""},
-	{`BEGIN { printf "%c", 128 }  # !gawk`, "", "\x80", "", ""},
-	{`BEGIN { printf "%c", 255 }  # !gawk`, "", "\xff", "", ""},
-	{`BEGIN { printf "%c", 256 }  # !awk !gawk`, "", "\x00", "", ""},
+	{`BEGIN { printf "%c", 128 }  # !windows-gawk`, "", "\u0080", "", ""},
+	{`BEGIN { printf "%c", 255 }  # !windows-gawk`, "", "ÿ", "", ""},
+	{`BEGIN { printf "%c", 256 }  # !windows-gawk`, "", "Ā", "", ""},
+	{`BEGIN { printf "%c", 4660 }  # !windows-gawk`, "", "\u1234", "", ""},
 	{`BEGIN { printf "%c", "xyz" }`, "", "x", "", ""},
+	{`BEGIN { printf "%c %c %c", "Ā", "ĀĀĀ", "Āx" }  # !windows-gawk`, "", "Ā Ā Ā", "", ""},
 	{`BEGIN { printf "%c", "" }  # !awk`, "", "\x00", "", ""},
 	{`BEGIN { printf }  # !awk !posix - doesn't error on this`, "", "", "parse error at 1:16: expected printf args, got none", "printf: no arguments"},
 	{`BEGIN { printf("%%%dd", 4) }`, "", "%4d", "", ""},