Add expansion package

pkg/expansion/expand.go

@@ -0,0 +1,119 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package expansion provides functions find and replace $(FOO) style variables in strings.
+package expansion
+
+import (
+	"bytes"
+)
+
+const (
+	operator        = '$'
+	referenceOpener = '('
+	referenceCloser = ')'
+)
+
+// syntaxWrap returns the input string wrapped by the expansion syntax.
+func syntaxWrap(input string) string {
+	return string(operator) + string(referenceOpener) + input + string(referenceCloser)
+}
+
+// MappingFuncFor returns a mapping function for use with Expand that
+// implements the expansion semantics defined in the expansion spec; it
+// returns the input string wrapped in the expansion syntax if no mapping
+// for the input is found.
+func MappingFuncFor(context ...map[string]string) func(string) string {
+	return func(input string) string {
+		for _, vars := range context {
+			val, ok := vars[input]
+			if ok {
+				return val
+			}
+		}
+
+		return syntaxWrap(input)
+	}
+}
+
+// Expand replaces variable references in the input string according to
+// the expansion spec using the given mapping function to resolve the
+// values of variables.
+func Expand(input string, mapping func(string) string) string {
+	var buf bytes.Buffer
+	checkpoint := 0
+	for cursor := 0; cursor < len(input); cursor++ {
+		if input[cursor] == operator && cursor+1 < len(input) {
+			// Copy the portion of the input string since the last
+			// checkpoint into the buffer
+			buf.WriteString(input[checkpoint:cursor])
+
+			// Attempt to read the variable name as defined by the
+			// syntax from the input string
+			read, isVar, advance := tryReadVariableName(input[cursor+1:])
+
+			if isVar {
+				// We were able to read a variable name correctly;
+				// apply the mapping to the variable name and copy the
+				// bytes into the buffer
+				buf.WriteString(mapping(read))
+			} else {
+				// Not a variable name; copy the read bytes into the buffer
+				buf.WriteString(read)
+			}
+
+			// Advance the cursor in the input string to account for
+			// bytes consumed to read the variable name expression
+			cursor += advance
+
+			// Advance the checkpoint in the input string
+			checkpoint = cursor + 1
+		}
+	}
+
+	// Return the buffer and any remaining unwritten bytes in the
+	// input string.
+	return buf.String() + input[checkpoint:]
+}
+
+// tryReadVariableName attempts to read a variable name from the input
+// string and returns the content read from the input, whether that content
+// represents a variable name to perform mapping on, and the number of bytes
+// consumed in the input string.
+//
+// The input string is assumed not to contain the initial operator.
+func tryReadVariableName(input string) (string, bool, int) {
+	switch input[0] {
+	case operator:
+		// Escaped operator; return it.
+		return input[0:1], false, 1
+	case referenceOpener:
+		// Scan to expression closer
+		for i := 1; i < len(input); i++ {
+			if input[i] == referenceCloser {
+				return input[1:i], true, i + 1
+			}
+		}
+
+		// Incomplete reference; return it.
+		return string(operator) + string(referenceOpener), false, 1
+	default:
+		// Not the beginning of an expression, ie, an operator
+		// that doesn't begin an expression.  Return the operator
+		// and the first rune in the string.
+		return (string(operator) + string(input[0])), false, 1
+	}
+}