From a1dd57402151e289a6297b42f5a3bafe4769730f Mon Sep 17 00:00:00 2001
From: ocsmit <osmith2@ncsu.edu>
Date: Thu, 20 Jul 2023 09:41:36 -0400
Subject: [PATCH] Add encoding routines

---
 src/encoding.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++
 src/encoding.h | 19 ++++++++++++++++
 tests/tests.c  | 12 ++++++++++
 3 files changed, 91 insertions(+)
 create mode 100644 src/encoding.c
 create mode 100644 src/encoding.h

diff --git a/src/encoding.c b/src/encoding.c
new file mode 100644
index 0000000..2a1fe56
--- /dev/null
+++ b/src/encoding.c
@@ -0,0 +1,60 @@
+/**
+ * @file
+ * @brief Routines for encoding and decoding integers
+ */
+
+#include "encoding.h"
+
+#include <stdint.h>
+#include <stdio.h>
+
+
+uint32_t count_trailing_zeros(unsigned int v)
+{
+    // modified from:
+    // http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup
+    static const uint32_t Mod37BitPosition[] = {
+      32, 0, 1, 26, 2, 23, 27, 0, 3, 16, 24, 30, 28, 11, 0, 13, 4,
+      7, 17, 0, 25, 22, 31, 15, 29, 10, 12, 6, 0, 21, 14, 9, 5,
+      20, 8, 19, 18
+    };
+
+    // map a bit value mod 37 to its position
+    return Mod37BitPosition[(-v & v) % 37];
+}
+
+
+unsigned int unary_encode(uint32_t k)
+{
+    // code 1 . 0 k times (e.g. 3 := 1 . 000
+    return (1 << k);
+
+}
+
+unsigned int gamma_encode(uint32_t k)
+{
+    uint32_t length, offset;
+
+    // floor(log_2(k))
+    length = (uint32_t) log2(k);
+    offset = k - (1 << length);
+
+    // Unary coded offset . length 
+    // e.g. 13 (l = 3, o = 5) := 101.1000
+    return (offset << (length+1) | ((1 << length)));
+}
+
+unsigned int gamma_decode(unsigned int k)
+{
+    uint32_t l = 0;
+    uint32_t o = 0;
+
+    l = count_trailing_zeros(k);
+    k >>= l;
+
+    // Mask for l bits
+    o = (1 << l) - 1 & (k >> 1);
+    return (1 << l) + o;
+}
+
+
diff --git a/src/encoding.h b/src/encoding.h
new file mode 100644
index 0000000..fecda75
--- /dev/null
+++ b/src/encoding.h
@@ -0,0 +1,19 @@
+#ifndef ENCODING_H_
+#define ENCODING_H_
+
+#include <stdint.h>
+#include <math.h>
+
+
+
+
+uint32_t count_trailing_zeros(unsigned int v);
+
+unsigned int unary_encode(uint32_t k);
+
+unsigned int gamma_encode(uint32_t k);
+
+unsigned int gamma_decode(unsigned int k);
+
+#endif // !ENCODING_H_
+
diff --git a/tests/tests.c b/tests/tests.c
index a4ed48a..e395807 100644
--- a/tests/tests.c
+++ b/tests/tests.c
@@ -1,9 +1,12 @@
 #include "tests.h"
 #include "assert.h"
+#include <stdint.h>
 #include <stdio.h>
 #include "../src/bitarr.h"
 #include "../src/bitops.h"
 #include "../src/bitarr_io.h"
+#include "../src/encoding.h"
+
 
 
 
@@ -132,6 +135,15 @@ TEST("Read from disk")
 
 BitArray_free(bit_arr);
 
+TEST("Gamma encoding")
+{
+    uint32_t og_int = 13;
+    unsigned int gc = gamma_encode(og_int);
+    gc = gamma_decode(gc);
+    assert(og_int == gc);
+    printf("✔ Gamma coding\n");
+}
+
 
 
 
-- 
2.45.2