util: add a CRC calculator

src/main/scala/util/CRC.scala

@@ -0,0 +1,35 @@
+// See LICENSE.SiFive for license details.
+
+package util
+
+import Chisel._
+
+object CRC
+{
+  // A divisor of 0x1d5 is interpretted to be x^8 + x^7 + x^6 + x^4 + x^2 + 1
+  // Let n be the highest term in the divisor; n=8 in 0x1d5.
+  // Then, this function calculates c mod d, returning an n-bit UInt.
+  // coefficient.width must be <= width
+  def apply(divisor: BigInt, coefficient: UInt, width: Integer): UInt = {
+    require (divisor > 0 && divisor.testBit(0))
+    require (width > 0)
+    assert (coefficient >> width === UInt(0))
+    val n = log2Floor(divisor)
+    val m = width
+    if (m <= n) return coefficient
+
+    // Initialize the reduction matrix
+    val array = Array.tabulate(m) { BigInt(1) << _ }
+    // Reduce the matrix of terms larger than n
+    for {
+      i <- (n until m).reverse
+      j <- 0 to n
+      if divisor.testBit(j)
+    } array(i-(n-j)) ^= array(i)
+    // Construct the circuit
+    Cat(Seq.tabulate(n) { i => (UInt(array(i)) & coefficient).xorR } .reverse)
+  }
+
+  // Find more great CRC polynomials here: https://users.ece.cmu.edu/~koopman/crc/
+  val CRC_16F_4_2 = BigInt(0x1a2eb) // HD=4 for <32751 bits and HD=6 for <93 bits
+}