Below are functions written in Scala and Clojure to do simple replacement of templates in Strings. The input to each function is a String containing templates of the form {key} and a map from Symbol/Keyword to replacement value.
For example:
Scala:
replaceTemplates("This is a {test}", Map('test -> "game"))
Clojure:
(replace-templates "This is a {test}" {:test "game"})
will return "This is a game".
The input map uses Symbols/Keywords so that I don't have to deal with corner cases where the templates in the Strings contain braces.
Unfortunately, the algorithm is not very efficient.
Here is the Scala code:
def replaceTemplates(text: String,
templates: Map[Symbol, String]): String = {
val builder = new StringBuilder(text)
@tailrec
def loop(key: String,
keyLength: Int,
value: String): StringBuilder = {
val index = builder.lastIndexOf(key)
if (index < 0) builder
else {
builder.replace(index, index + keyLength, value)
loop(key, keyLength, value)
}
}
templates.foreach {
case (key, value) =>
val template = "{" + key.name + "}"
loop(template, template.length, value)
}
builder.toString
}
and here is the Clojure code:
(defn replace-templates
"Return a String with each occurrence of a substring of the form {key}
replaced with the corresponding value from a map parameter.
@param str the String in which to do the replacements
@param m a map of keyword->value"
[text m]
(let [sb (StringBuilder. text)]
(letfn [(replace-all [key key-length value]
(let [index (.lastIndexOf sb key)]
(if (< index 0)
sb
(do
(.replace sb index (+ index key-length) value)
(recur key key-length value)))))]
(doseq [[key value] m]
(let [template (str "{" (name key) "}")]
(replace-all template (count template) value))))
(.toString sb)))
Here is a test case (Scala code):
replaceTemplates("""
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
elit nisi, egestas et tincidunt eget, {foo} mattis non erat. Aenean ut
elit in odio vehicula facilisis. Vestibulum quis elit vel nulla
interdum facilisis ut eu sapien. Nullam cursus fermentum
sollicitudin. Donec non congue augue. {bar} Vestibulum et magna quis
arcu ultricies consectetur auctor vitae urna. Fusce hendrerit
facilisis volutpat. Ut lectus augue, mattis {baz} venenatis {foo}
lobortis sed, varius eu massa. Ut sit amet nunc quis velit hendrerit
bibendum in eget nibh. Cras blandit nibh in odio suscipit eget aliquet
tortor placerat. In tempor ullamcorper mi. Quisque egestas, metus eu
venenatis pulvinar, sem urna blandit mi, in lobortis augue sem ut
dolor. Sed in {bar} neque sapien, vitae lacinia arcu. Phasellus mollis
blandit commodo.
""", Map('foo -> "HELLO", 'bar -> "GOODBYE", 'baz -> "FORTY-TWO"))
and the output:
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque
elit nisi, egestas et tincidunt eget, HELLO mattis non erat. Aenean ut
elit in odio vehicula facilisis. Vestibulum quis elit vel nulla
interdum facilisis ut eu sapien. Nullam cursus fermentum
sollicitudin. Donec non congue augue. GOODBYE Vestibulum et magna quis
arcu ultricies consectetur auctor vitae urna. Fusce hendrerit
facilisis volutpat. Ut lectus augue, mattis FORTY-TWO venenatis HELLO
lobortis sed, varius eu massa. Ut sit amet nunc quis velit hendrerit
bibendum in eget nibh. Cras blandit nibh in odio suscipit eget aliquet
tortor placerat. In tempor ullamcorper mi. Quisque egestas, metus eu
venenatis pulvinar, sem urna blandit mi, in lobortis augue sem ut
dolor. Sed in GOODBYE neque sapien, vitae lacinia arcu. Phasellus mollis
blandit commodo.
The algorithm transverses the input map and for each pair, does a replacement in the input String, temporarily held in a StringBuilder. For each key/value pair, we search for the last occurrence of the key (enclosed in braces) and replace it with the value, until there are no more occurrences.
Does it make any performance difference if we use .lastIndexOf versus .indexOf in the StringBuilder?
How can the algorithm be improved? Is there a more idiomatic way to write the Scala and/or Clojure code?
UPDATE: See my follow-up.
UPDATE 2: Here is a better Scala implementation; O(n) in the length of the String. Note that I modified the Map to be [String, String] instead of [Symbol, String] on the recommendation of several people. (thanks mikera, kotarak):
/**
* Replace templates of the form {key} in the input String with values from the Map.
*
* @param text the String in which to do the replacements
* @param templates a Map from Symbol (key) to value
* @returns the String with all occurrences of the templates replaced by their values
*/
def replaceTemplates(text: String,
templates: Map[String, String]): String = {
val builder = new StringBuilder
val textLength = text.length
@tailrec
def loop(text: String): String = {
if (text.length == 0) builder.toString
else if (text.startsWith("{")) {
val brace = text.indexOf("}")
if (brace < 0) builder.append(text).toString
else {
val replacement = templates.get(text.substring(1, brace)).orNull
if (replacement != null) {
builder.append(replacement)
loop(text.substring(brace + 1))
} else {
builder.append("{")
loop(text.substring(1))
}
}
} else {
val brace = text.indexOf("{")
if (brace < 0) builder.append(text).toString
else {
builder.append(text.substring(0, brace))
loop(text.substring(brace))
}
}
}
loop(text)
}
UPDATE 3: Here are a set of Clojure test cases (Scala versions are left as an exercise :-)):
(use 'clojure.test)
(deftest test-replace-templates
(is (= ; No templates
(replace-templates "this is a test" {:foo "FOO"})
"this is a test"))
(is (= ; One simple template
(replace-templates "this is a {foo} test" {:foo "FOO"})
"this is a FOO test"))
(is (= ; Two templates, second at end of input string
(replace-templates "this is a {foo} test {bar}" {:foo "FOO" :bar "BAR"})
"this is a FOO test BAR"))
(is (= ; Two templates
(replace-templates "this is a {foo} test {bar} 42" {:foo "FOO" :bar "BAR"})
"this is a FOO test BAR 42"))
(is (= ; Second brace-enclosed item is NOT a template
(replace-templates "this is a {foo} test {baz} 42" {:foo "FOO" :bar "BAR"})
"this is a FOO test {baz} 42"))
(is (= ; Second item is not a template (no closing brace)
(replace-templates "this is a {foo} test {bar" {:foo "FOO" :bar "BAR"})
"this is a FOO test {bar"))
(is (= ; First item is enclosed in a non-template brace-pair
(replace-templates "this is {a {foo} test} {bar" {:foo "FOO" :bar "BAR"})
"this is {a FOO test} {bar")))
(run-tests)
I think the best algorithm you can build is O(n) in the length of the input string and would go something like:
Converting to Scala/Clojure left as an exercise :-)