From fbe89665a2eda2b1e77953e91c961458ae64b6cc Mon Sep 17 00:00:00 2001
From: Shogo Ohta <shogo@xcoo.jp>
Date: Fri, 15 Dec 2023 11:11:23 +0900
Subject: [PATCH] wip

---
 src/cljam/io/cram/codecs/rans4x8.clj | 83 ++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)
 create mode 100644 src/cljam/io/cram/codecs/rans4x8.clj

diff --git a/src/cljam/io/cram/codecs/rans4x8.clj b/src/cljam/io/cram/codecs/rans4x8.clj
new file mode 100644
index 00000000..6d7b83df
--- /dev/null
+++ b/src/cljam/io/cram/codecs/rans4x8.clj
@@ -0,0 +1,83 @@
+(ns cljam.io.cram.codecs.rans4x8
+  {:clj-kondo/ignore [:missing-docstring]}
+  (:require [cljam.io.util.byte-buffer :as bb]
+            [cljam.io.cram.itf8 :as itf8]))
+
+(defn read-frequencies0 [bb]
+  (loop [sym (long (bb/read-ubyte bb))
+         rle 0
+         freqs (transient {})]
+    (let [freqs' (assoc! freqs sym (itf8/decode-itf8 bb))]
+      (if (pos? rle)
+        (recur (inc sym) (dec rle) freqs')
+        (let [sym' (long (bb/read-ubyte bb))
+              rle' (if (= sym' (inc sym))
+                     (long (bb/read-ubyte bb))
+                     rle)]
+          (if (zero? sym')
+            (persistent! freqs')
+            (recur sym' rle' freqs')))))))
+
+(defn cumulative-frequencies [freqs]
+  (loop [i 0
+         sum 0
+         cum-freqs (transient [])]
+    (if (< i 256)
+      (let [f (get freqs i 0)]
+        (recur (inc i) (+ sum (long f)) (conj! cum-freqs sum)))
+      (persistent! (conj! cum-freqs sum)))))
+
+(defn lookup-symbol ^long [cum-freqs ^long f]
+  (loop [s 0]
+    (if (>= f (long (nth cum-freqs (inc s))))
+      (recur (inc s))
+      s))
+  #_(loop [l 0
+           r (count cum-freqs)]
+      (if (< l r)
+        (let [m (+ l (quot (- r l) 2))
+              f' (long (nth cum-freqs m))]
+          (if (<= f f')
+            (recur l m)
+            (recur (inc m) r)))
+        l)))
+
+(defn advance-step ^long [^long c ^long f ^long state]
+  (-> (* f (bit-shift-right state 12))
+      (+ (bit-and state 0xfff))
+      (- c)))
+
+(defn renormalize-state ^long [bb ^long state]
+  (loop [state state]
+    (if (< state 0x800000)
+      (recur (+ (bit-shift-left state 8)
+                (long (bb/read-ubyte bb))))
+      state)))
+
+(defn decode-rans0 [bb ^long n-out]
+  (let [freqs (read-frequencies0 bb)
+        cum-freqs (cumulative-frequencies freqs)
+        out (byte-array n-out)]
+    (loop [i 0
+           states (into [] (map (fn [_] (bb/read-uint bb))) (range 4))]
+      (if (< i n-out)
+        (let [j (rem i 4)
+              state (long (nth states j))
+              f (bit-and state 0xfff)
+              sym (lookup-symbol cum-freqs f)
+              state' (->> state
+                          (advance-step (nth cum-freqs sym) (get freqs sym 0))
+                          (renormalize-state bb))]
+          (aset out i (byte sym))
+          (recur (inc i) (assoc states j state')))
+        out))))
+
+(defn decode-rans1 [_in _n-out])
+
+(defn decode-rans [bb]
+  (let [order (long (bb/read-ubyte bb))
+        _n-in (bb/read-uint bb)
+        n-out (bb/read-uint bb)]
+    (if (zero? order)
+      (decode-rans0 bb n-out)
+      (decode-rans1 bb n-out))))