Merge pull request #935 from proppy:gcd

PiperOrigin-RevId: 638888730

Author: copybara-github

xls/examples/BUILD

@@ -1011,3 +1011,15 @@ xls_dslx_verilog(
     library = "passthrough_dslx",
     verilog_file = "passthrough.sv",
 )
+
+xls_dslx_library(
+    name = "gcd_dslx",
+    srcs = ["gcd.x"],
+)
+
+xls_dslx_test(
+    name = "gcd_test",
+    size = "small",
+    srcs = ["gcd.x"],
+    deps = [":gcd_dslx"],
+)

xls/examples/gcd.x

@@ -0,0 +1,68 @@
+// Copyright 2023 The XLS Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import std;
+
+// https://en.wikipedia.org/wiki/Greatest_common_divisor#Euclidean_algorithm
+fn gcd_euclidean<N: u32, DN: u32 = {N * u32:2}>(a: uN[N], b: uN[N]) -> uN[N] {
+    let (gcd, _) = for (i, (a, b)) in range(u32:0, DN) {
+        if (b == uN[N]:0) {
+            (a, b)
+        } else {
+            (b, std::iterative_div_mod(a, b).1)
+        }
+    }((a, b));
+    gcd
+}
+
+fn gcd_binary_match<N: u32>(a: uN[N], b: uN[N], d: uN[N]) -> (uN[N], uN[N], uN[N]) {
+    match (a[0:1], b[0:1]) {
+      (u1:0, u1:1) => (b, a >> 1, d),
+      (u1:1, u1:0) => (a, b >> 1, d),
+      (u1:0, u1:0) => (a >> 1, b >> 1, d+uN[N]:1),
+      (u1:1, u1:1) => ((a - b) >> 1, b, d),
+      _ => fail!("unsupported_case", (uN[N]:0, uN[N]:0, uN[N]:0)),
+    }
+}
+
+// https://en.wikipedia.org/wiki/Greatest_common_divisor#Binary_GCD_algorithm
+fn gcd_binary<N: u32, DN: u32 = {N * u32:2}>(a: uN[N], b: uN[N]) -> uN[N] {
+    let (a, _, d) = for (i, (a, b, d)) in range(u32:0, DN) {
+        if (a == b) {
+            (a, b, d)
+        } else if (a < b) {
+            gcd_binary_match(b, a, d)
+        } else {
+            gcd_binary_match(a, b, d)
+        }
+    }((a, b, uN[N]:0));
+    (a << d)
+}
+
+#[test]
+fn gcd_euclidean_test() {
+    assert_eq(u8:6, gcd_euclidean(u8:48, u8:18));
+    assert_eq(u8:6, gcd_euclidean(u8:18, u8:48));
+}
+
+#[test]
+fn gcd_binary_test() {
+    assert_eq(u8:6, gcd_binary(u8:48, u8:18));
+    assert_eq(u8:6, gcd_binary(u8:18, u8:48));
+}
+
+#[quickcheck(test_count=50000)]
+fn prop_gcd_equal(a: u32, b: u32) -> bool {
+    gcd_euclidean<u32:32>(a, b) == gcd_binary<u32:32>(a, b)
+}