add zigzag coding

src/ZigZag.zig

···
+pub fn encode(value: isize) usize {
+    if (value < 0) return @as(usize, @intCast(value * -1)) * 2 - 1;
+    return @as(usize, @intCast(value)) * 2;
+}
+
+pub fn decode(value: usize) isize {
+    if (value & 1 == 1) return -@as(isize, @intCast(value / 2)) - 1;
+    return @as(isize, @intCast(value / 2));
+}
+
+test "zigzag.encode val = 10" {
+    const testing = @import("std").testing;
+
+    const encoded = encode(10);
+    try testing.expectEqual(20, encoded);
+}
+
+test "zigzag.encode val = -42" {
+    const testing = @import("std").testing;
+
+    const encoded = encode(-42);
+    try testing.expectEqual(83, encoded);
+}
+
+test "zigzag.decode val = 20" {
+    const testing = @import("std").testing;
+
+    const decoded = decode(20);
+    try testing.expectEqual(10, decoded);
+}
+
+test "zigzag.decode val = 83" {
+    const testing = @import("std").testing;
+
+    const decoded = decode(83);
+    try testing.expectEqual(-42, decoded);
+}

src/root.zig

···
 const std = @import("std");
 
 pub const Golomb = @import("Golomb.zig");
+pub const ZigZag = @import("ZigZag.zig");
 
 test {
     std.testing.refAllDecls(@This());