From c2e4639d2ebefbeb7f8b7e1826ae2391c6876a5a Mon Sep 17 00:00:00 2001
From: Leo Howell <leo@lwh.jp>
Date: Sat, 14 Nov 2009 20:24:57 +0900
Subject: Merge some files

---
 lpg/libqr/galois.c | 126 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 126 insertions(+)
 create mode 100644 lpg/libqr/galois.c

(limited to 'lpg/libqr/galois.c')

diff --git a/lpg/libqr/galois.c b/lpg/libqr/galois.c
new file mode 100644
index 0000000..e21ce22
--- /dev/null
+++ b/lpg/libqr/galois.c
@@ -0,0 +1,126 @@
+#include <assert.h>
+#include <stdlib.h>
+
+#include <qr/bitstream.h>
+#include "galois.h"
+
+/* Calculate the residue of a modulo m */
+unsigned long gf_residue(unsigned long a,
+                         unsigned long m)
+{
+        unsigned long o = 1;
+        int n = 1;
+
+        /* Find one past the highest bit of the modulus */
+        while (m & ~(o - 1))
+                o <<= 1;
+
+        /* Find the highest n such that O(m * x^n) <= O(a) */
+        while (a & ~(o - 1)) {
+                o <<= 1;
+                ++n;
+        }
+
+        /* For each n, try to reduce a by (m * x^n) */
+        while (n--) {
+                o >>= 1;
+
+                /* o is the highest bit of (m * x^n) */
+                if (a & o)
+                        a ^= m << n;
+        }
+
+        return a;
+}
+
+static unsigned int gf_mult(unsigned int a, unsigned int b)
+{
+        /* Reduce modulo x^8 + x^4 + x^3 + x^2 + 1
+         * using the peasant's algorithm
+         */
+        const unsigned int m = 0x11D;
+        unsigned int x = 0;
+        int i;
+
+        for (i = 0; i < 8; ++i) {
+                x ^= (b & 0x1) ? a : 0;
+                a = (a << 1) ^ ((a & 0x80) ? m : 0);
+                b >>= 1;
+        }
+
+        return x & 0xFF;
+}
+
+static unsigned int * make_generator(int k)
+{
+        unsigned int * g;
+        unsigned int a;
+        int i, j;
+
+        g = calloc(k, sizeof(*g));
+        if (!g)
+                return 0;
+
+        g[0] = 1; /* Start with g(x) = 1 */
+        a = 1;    /* 2^0 = 1 */
+
+        for (i = 0; i < k; ++i) {
+                /* Multiply our poly g(x) by (x + 2^i) */
+                for (j = k - 1; j > 0; --j)
+                        g[j] = gf_mult(g[j], a) ^ g[j-1];
+                g[0] = gf_mult(g[0], a);
+
+                a = gf_mult(a, 2);
+        }
+
+        return g;
+}
+
+struct qr_bitstream * rs_generate_words(struct qr_bitstream * data,
+                                     size_t data_words,
+                                     size_t rs_words)
+{
+        struct qr_bitstream * ec = 0;
+        unsigned int * b = 0;
+        unsigned int * g;
+        size_t n = rs_words;
+        int i, r;
+
+        assert(qr_bitstream_remaining(data) >= data_words * 8);
+
+        ec = qr_bitstream_create();
+        if (!ec)
+                return 0;
+
+        if (qr_bitstream_resize(ec, n * 8) != 0)
+                goto fail;
+
+        b = calloc(n, sizeof(*b));
+        if (!b)
+                goto fail;
+
+        g = make_generator(n);
+        if (!g)
+                goto fail;
+
+        /* First, prepare the registers (b) with data bits */
+        for (i = 0; i < data_words; ++i) {
+                unsigned int x = b[n-1] ^ qr_bitstream_read(data, 8);
+                for (r = n-1; r > 0; --r)
+                        b[r] = b[r-1] ^ gf_mult(g[r], x);
+                b[0] = gf_mult(g[0], x);
+        }
+
+        /* Read off the registers */
+        for (r = 0; r < n; ++r)
+                qr_bitstream_write(ec, b[(n-1)-r], 8);
+
+        free(g);
+        free(b);
+        return ec;
+fail:
+        free(b);
+        qr_bitstream_destroy(ec);
+        return 0;
+}
+
-- 
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