string.format: handle "}}" and "{-0}" correctly (#46)

Fixes #18

2 files changed, 38 insertions, 6 deletions

diff --git a/library.go b/library.go
index 1da0547..3503dc3 100644
--- a/library.go
+++ b/library.go
@@ -1571,13 +1571,29 @@ func string_format(fnname string, recv_ Value, args Tuple, kwargs []Tuple) (Valu
 	var buf bytes.Buffer
 	index := 0
 	for {
-		// TODO(adonovan): replace doubled "}}" with "}" and reject single '}'.
+		literal := format
 		i := strings.IndexByte(format, '{')
+		if i >= 0 {
+			literal = format[:i]
+		}
+
+		// Replace "}}" with "}" in non-field portion, rejecting a lone '}'.
+		for {
+			j := strings.IndexByte(literal, '}')
+			if j < 0 {
+				buf.WriteString(literal)
+				break
+			}
+			if len(literal) == j+1 || literal[j+1] != '}' {
+				return nil, fmt.Errorf("single '}' in format")
+			}
+			buf.WriteString(literal[:j+1])
+			literal = literal[j+2:]
+		}
+
 		if i < 0 {
-			buf.WriteString(format)
-			break
+			break // end of format string
 		}
-		buf.WriteString(format[:i])
 
 		if i+1 < len(format) && format[i+1] == '{' {
 			// "{{" means a literal '{'
@@ -1632,7 +1648,7 @@ func string_format(fnname string, recv_ Value, args Tuple, kwargs []Tuple) (Valu
 			}
 			arg = args[index]
 			index++
-		} else if num, err := strconv.Atoi(name); err == nil {
+		} else if num, err := strconv.Atoi(name); err == nil && !strings.HasPrefix(name, "-") {
 			// positional argument
 			if auto {
 				return nil, fmt.Errorf("cannot switch from automatic field numbering to manual field specification")
@@ -1652,13 +1668,17 @@ func string_format(fnname string, recv_ Value, args Tuple, kwargs []Tuple) (Valu
 				}
 			}
 			if arg == nil {
-				// Skylark does not support Python's x.y or a[i] syntaxes.
+				// Skylark does not support Python's x.y or a[i] syntaxes,
+				// or nested use of {...}.
 				if strings.Contains(name, ".") {
 					return nil, fmt.Errorf("attribute syntax x.y is not supported in replacement fields: %s", name)
 				}
 				if strings.Contains(name, "[") {
 					return nil, fmt.Errorf("element syntax a[i] is not supported in replacement fields: %s", name)
 				}
+				if strings.Contains(name, "{") {
+					return nil, fmt.Errorf("nested replacement fields not supported")
+				}
 				return nil, fmt.Errorf("keyword %s not found", name)
 			}
 		}
diff --git a/testdata/string.sky b/testdata/string.sky
index 1df8ee9..f92dadf 100644
--- a/testdata/string.sky
+++ b/testdata/string.sky
@@ -162,8 +162,13 @@ assert.fails(lambda: "%c" % -1, "requires a valid Unicode code point")
 assert.eq("a{}b".format(123), "a123b")
 assert.eq("a{}b{}c{}d{}".format(1, 2, 3, 4), "a1b2c3d4")
 assert.eq("a{{b".format(), "a{b")
+assert.eq("a}}b".format(), "a}b")
+assert.eq("a{{b}}c".format(), "a{b}c")
 assert.eq("a{x}b{y}c{}".format(1, x=2, y=3), "a2b3c1")
 assert.fails(lambda: "a{z}b".format(x=1), "keyword z not found")
+assert.fails(lambda: "{-1}".format(1), "keyword -1 not found")
+assert.fails(lambda: "{-0}".format(1), "keyword -0 not found")
+assert.fails(lambda: '{0,1} and {1}'.format(1, 2), "keyword 0,1 not found")
 assert.fails(lambda: "a{123}b".format(), "tuple index out of range")
 assert.fails(lambda: "a{}b{}c".format(1), "tuple index out of range")
 assert.eq("a{010}b".format(0,1,2,3,4,5,6,7,8,9,10), "a10b") # index is decimal
@@ -173,6 +178,13 @@ assert.eq("a{!r}c".format("b"), r'a"b"c')
 assert.eq("a{x!r}c".format(x='b'), r'a"b"c')
 assert.fails(lambda: "{x!}".format(x=1), "unknown conversion")
 assert.fails(lambda: "{x!:}".format(x=1), "unknown conversion")
+assert.fails(lambda: '{a.b}'.format(1), "syntax x.y is not supported")
+assert.fails(lambda: '{a[0]}'.format(1), "syntax a\[i\] is not supported")
+assert.fails(lambda: '{ {} }'.format(1), "nested replacement fields not supported")
+assert.fails(lambda: '{{}'.format(1), "single '}' in format")
+assert.fails(lambda: '{}}'.format(1), "single '}' in format")
+assert.fails(lambda: '}}{'.format(1), "unmatched '{' in format")
+assert.fails(lambda: '}{{'.format(1), "single '}' in format")
 
 # str.split, str.rsplit
 assert.eq("a.b.c.d".split("."), ["a", "b", "c", "d"])