Removes common indentation from a collection of lines. Written in response to http://jjinux.blogspot.com/2012/09/dart-algorithm-to-unindent-code.html

core.clj

(ns dedent-text.core
  (:use [clojure.string :only [blank?]]))

(defn all-equal?
  "Returns the value if all values are equal, otherwise nil."
  [& values]
  (when (apply = values) (first values)))

(defn count-common-indentation
  "Returns length of common indentation of strings."
  [& strings]
  (count (take-while #(and (not (nil? %)) (blank? (str %)))
                     (apply map all-equal? strings))))

(defn dedent-lines
  "Returns a lazy seq of strs with the common indentation removed from lines.
  Only considers indentation of nonblank lines, and sets blank lines to an
  empty string."
  [lines]
  (if (empty? lines)
    lines
    (let [non-blank (remove blank? lines)
          common-indent (apply count-common-indentation non-blank)]
      (for [line lines]
        (if (blank? line)
          ""
          (subs line common-indent))))))

core_test.clj

(ns dedent-text.core-test
  (:use clojure.test
        dedent-text.core))

(deftest dedent-lines-test
  (testing "unindenting code"
    (is (= (dedent-lines ["  1"
                          "  2"])
           ["1"
            "2"])))

  (testing "unindenting code where the first line is indented the most"
    (is (= (dedent-lines ["\t    1"
                          "\t  2"
                          "\t    3"])
           ["  1"
            "2"
            "  3"])))

  (testing "doing nothing for unindented code"
    (is (= (dedent-lines ["1"
                          "2"
                          "3"])
           ["1"
            "2"
            "3"])))

  (testing "handling empty lists"
    (is (= (dedent-lines [])
           [])))

  (testing "not trying to handle inconsistent indentation"
    (is (= (dedent-lines ["\t1"
                          "  2"
                          "    3"
                          "        4"])
           ["\t1"
            "  2"
            "    3"
            "        4"])))

  (testing "handling really awkward short lines"
    (is (= (dedent-lines ["    1"
                          "2"])
           ["    1"
            "2"])))

  (testing "handling blank lines and lines with only indentation"
    (is (= (dedent-lines ["  1"
                          ""
                          " "
                          "    2"])
           ["1"
            ""
            ""
            "  2"]))))