commit 09953646eb8e112fece131957c9730c62ddde019
Author: Christophe Grand <christophe@cgrand.net>
Date:   Thu Sep 3 12:39:22 2015 +0200

    init

diff --git a/README.md b/README.md
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+# xforms
+
+More transducers and reduciing functions for Clojure!
+
+## Usage
+
+```clj
+=> (require '[net.cgrand.xforms :as x])
+```
+
+`str` and `str!` are two reducing functions to build Strings and StringBuilders in linear time.
+
+```clj
+=> (quick-bench (reduce str (range 256)))
+             Execution time mean : 58,714946 µs
+=> (quick-bench (reduce x/str (range 256)))
+             Execution time mean : 11,609631 µs
+```
+
+`by-key` and `reduce` are two new transducers. Here is an example usage:
+
+```clj
+;; reimplementing group-by
+(defn my-group-by [kfn coll]
+  (into {} (by-key kfn (reduce conj)) coll))
+
+;; let's go transient!
+(defn my-group-by [kfn coll]
+  (into {} (by-key kfn (reduce (completing conj! persistent!))) coll))
+
+=> (quick-bench (group-by odd? (range 256)))
+             Execution time mean : 29,356531 µs
+=> (quick-bench (my-group-by odd? (range 256)))
+             Execution time mean : 20,604297 µs
+```
+
+
+
+## License
+
+Copyright © 2015 Christophe Grand
+
+Distributed under the Eclipse Public License either version 1.0 or (at
+your option) any later version.
diff --git a/project.clj b/project.clj
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+(defproject net.cgrand/xforms "0.1.0-SNAPSHOT"
+  :description "Extra transducers for Clojure"
+  #_#_:url "http://example.com/FIXME"
+  :license {:name "Eclipse Public License"
+            :url "http://www.eclipse.org/legal/epl-v10.html"}
+  :dependencies [[org.clojure/clojure "1.7.0"]])
diff --git a/src/net/cgrand/xforms.clj b/src/net/cgrand/xforms.clj
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+(ns net.cgrand.xforms
+  "Extra transducers for Clojure"
+  {:author "Christophe Grand"}
+  (:refer-clojure :exclude [reduce for partition str])
+  (:require [clojure.core :as clj]))
+
+(defmacro for
+  "Like clojure.core/for with the first expression being replaced by % (or _). Returns a transducer."
+  [[binding %or_ & seq-exprs] body]
+  (assert (and (symbol? %or_) (#{"%" "_"} (name %or_)))
+    "The second element of the comprehension vector must be % or _.")
+  (let [rf (gensym 'rf)
+        acc (gensym 'acc)
+        body
+        (clj/reduce (fn [body [expr binding]]
+                      (case binding
+                        :let `(let ~expr ~body)
+                        :when `(if ~expr ~body ~acc)
+                        :while `(if ~expr ~body (reduced ~acc))
+                        `(reduce (fn [~acc ~binding] ~body) ~acc ~expr))) 
+          `(~rf ~acc ~body)
+          (clj/partition 2 (rseq (vec seq-exprs))))]
+    `(fn [~rf]
+       (fn
+         ([] (~rf))
+         ([~acc] (~rf ~acc))
+         ([~acc ~binding] ~body)))))
+
+(defn reduce
+  "A transducer that reduces a collection to a 1-item collection consisting of only the reduced result."
+  ([f]
+    (fn [rf]
+     (let [vacc (volatile! (f))]
+       (fn
+         ([] (rf))
+         ([acc] (rf (rf acc (f (unreduced @vacc)))))
+         ([acc x]
+           (if (reduced? (vswap! vacc f x))
+             (reduced acc)
+             acc))))))
+  ([f init]
+    (reduce (fn ([] init) ([acc] (f acc)) ([acc x] (f acc x))))))
+
+(defmacro ^:private or-instance? [class x y]
+  (let [xsym (gensym 'x_)]
+    `(let [~xsym ~x]
+       (if (instance? ~class ~xsym) ~(with-meta xsym {:tag class}) ~y))))
+
+(defn str!
+  "Like xforms/str but returns a StringBuilder."
+  ([] (StringBuilder.))
+  ([sb] (or-instance? StringBuilder sb (StringBuilder. (clj/str sb)))) ; the instance? checks are for compatibility with str in case of seeded reduce/transduce.
+  ([sb x] (.append (or-instance? StringBuilder sb (StringBuilder. (clj/str sb))) x)))
+
+(def str
+  "Reducing function to build strings in linear time. Acts as replacement for clojure.core/str in a reduce/transduce call."
+  (completing str! clj/str))
+
+;; for both map entries and vectors 
+(defn- key' [kv] (nth kv 0))
+(defn- val' [kv] (nth kv 1))
+
+(defn- noprf "The noop reducing function" ([acc] acc) ([acc _] acc))
+
+(defn by-key
+  "Returns a tranducer that applies the transducer xform independently for items partitioned by kfn.
+   (This docstring is a hard nut to crack.)"
+  ([xform] (by-key key' val' vector xform))
+  ([kfn xform] (by-key kfn identity vector xform))
+  ([kfn vfn xform] (by-key kfn vfn vector xform))
+  ([kfn vfn pair xform]
+    (fn [rf]
+      (let [make-rf (if pair
+                      (fn [k] (fn ([acc] (rf acc)) ([acc v] (rf acc (pair k v)))))
+                      (constantly rf))
+            m (volatile! (transient {}))]
+        (fn self
+          ([] (rf))
+          ([acc] (clj/reduce (fn [acc krf] (krf acc)) acc (vals (persistent! @m))))
+          ([acc x]
+            (let [k (kfn x)
+                  krf (or (get @m k) (doto (xform (make-rf x)) (->> (vswap! m assoc! k))))
+                  acc (krf acc (vfn x))]
+              (when (reduced? acc)
+                (vswap! m assoc! k noprf))
+              (unreduced acc))))))))
+
+
+