test(zigux): restack lane16 hweight bench replay runner

zigux/tests/phase1_hweight_bench_replay.zig

@@ -0,0 +1,50 @@
+const std = @import("std");
+const hweight = @import("hweight");
+
+const iterations_hweight = 100_000;
+
+const HweightReplay = struct {
+    checksum: u64,
+    first_counts: [4]u32,
+    last_count: u32,
+};
+
+fn runHweightReplay() HweightReplay {
+    var checksum: u64 = 0;
+    var first_counts: [4]u32 = undefined;
+    var last_count: u32 = 0;
+    var idx: usize = 0;
+    while (idx < iterations_hweight) : (idx += 1) {
+        const value: u32 = @truncate(0xf0f0_a5a5 ^ @as(u32, @intCast(idx)));
+        const count = hweight.swHweight32(value);
+        checksum +%= count;
+        if (idx < first_counts.len) {
+            first_counts[idx] = count;
+        }
+        if (idx + 1 == iterations_hweight) {
+            last_count = count;
+        }
+    }
+    return .{
+        .checksum = checksum,
+        .first_counts = first_counts,
+        .last_count = last_count,
+    };
+}
+
+test "phase1 hweight bench replay keeps the rolling popcount witness explicit" {
+    const replay = runHweightReplay();
+    try std.testing.expectEqual([4]u32{ 16, 15, 17, 16 }, replay.first_counts);
+    try std.testing.expectEqual(@as(u32, 16), replay.last_count);
+    try std.testing.expectEqual(@as(u64, 1_648_432), replay.checksum);
+}
+
+test "phase1 hweight bench replay matches the parked bench checksum packet" {
+    var checksum: u64 = 0;
+    var idx: usize = 0;
+    while (idx < iterations_hweight) : (idx += 1) {
+        const value: u32 = @truncate(0xf0f0_a5a5 ^ @as(u32, @intCast(idx)));
+        checksum +%= hweight.swHweight32(value);
+    }
+    try std.testing.expectEqual(@as(u64, 1_648_432), checksum);
+}

zigux/tests/phase1_hweight_bench_replay_build.zig

@@ -0,0 +1,39 @@
+const std = @import("std");
+
+pub fn build(b: *std.Build) void {
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    const root_module = b.createModule(.{
+        .root_source_file = b.path("phase1_hweight_bench_replay.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+    const hweight_module = b.createModule(.{
+        .root_source_file = b.path("../../tools/lib/hweight.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+
+    root_module.addImport("hweight", hweight_module);
+
+    const replay_tests = b.addTest(.{
+        .name = "phase1-hweight-bench-replay",
+        .root_module = root_module,
+    });
+    const run_replay = b.addRunArtifact(replay_tests);
+
+    const replay_step = b.step(
+        "phase1-hweight-bench-replay",
+        "Run the focused Phase 1 hweight bench replay from zigux/tests",
+    );
+    replay_step.dependOn(&run_replay.step);
+
+    const test_step = b.step(
+        "test",
+        "Run the focused Phase 1 hweight bench replay from zigux/tests",
+    );
+    test_step.dependOn(&run_replay.step);
+
+    b.default_step.dependOn(test_step);
+}