1 files changed, 77 insertions, 0 deletions
diff --git a/src/combine.rs b/src/combine.rs
new file mode 100644
index 0000000..4dd0ee0
--- /dev/null
+++ b/src/combine.rs
@@ -0,0 +1,77 @@
+const GF2_DIM: usize = 32;
+
+fn gf2_matrix_times(mat: &[u32; GF2_DIM], mut vec: u32) -> u32 {
+    let mut sum = 0;
+    let mut idx = 0;
+    while vec > 0 {
+        if vec & 1 == 1 {
+            sum ^= mat[idx];
+        }
+        vec >>= 1;
+        idx += 1;
+    }
+    return sum;
+}
+
+fn gf2_matrix_square(square: &mut [u32; GF2_DIM], mat: &[u32; GF2_DIM]) {
+    for n in 0..GF2_DIM {
+        square[n] = gf2_matrix_times(mat, mat[n]);
+    }
+}
+
+pub(crate) fn combine(mut crc1: u32, crc2: u32, mut len2: u64) -> u32 {
+    let mut row: u32;
+    let mut even = [0u32; GF2_DIM]; /* even-power-of-two zeros operator */
+    let mut odd = [0u32; GF2_DIM]; /* odd-power-of-two zeros operator */
+
+    /* degenerate case (also disallow negative lengths) */
+    if len2 <= 0 {
+        return crc1;
+    }
+
+    /* put operator for one zero bit in odd */
+    odd[0] = 0xedb88320; /* CRC-32 polynomial */
+    row = 1;
+    for n in 1..GF2_DIM {
+        odd[n] = row;
+        row <<= 1;
+    }
+
+    /* put operator for two zero bits in even */
+    gf2_matrix_square(&mut even, &odd);
+
+    /* put operator for four zero bits in odd */
+    gf2_matrix_square(&mut odd, &even);
+
+    /* apply len2 zeros to crc1 (first square will put the operator for one
+       zero byte, eight zero bits, in even) */
+    loop {
+        /* apply zeros operator for this bit of len2 */
+        gf2_matrix_square(&mut even, &odd);
+        if len2 & 1 == 1 {
+            crc1 = gf2_matrix_times(&even, crc1);
+        }
+        len2 >>= 1;
+
+        /* if no more bits set, then done */
+        if len2 == 0 {
+            break;
+        }
+
+        /* another iteration of the loop with odd and even swapped */
+        gf2_matrix_square(&mut odd, &even);
+        if len2 & 1 == 1 {
+            crc1 = gf2_matrix_times(&odd, crc1);
+        }
+        len2 >>= 1;
+
+        /* if no more bits set, then done */
+        if len2 == 0 {
+            break;
+        }
+    }
+
+    /* return combined crc */
+    crc1 ^= crc2;
+    return crc1;
+}