lib: enum func

src/lib/enum.zig

@@ -0,0 +1,85 @@
+const std = @import("std");
+
+/// Create an enum type with the given keys that is C ABI compatible
+/// if we're targeting C, otherwise a Zig enum with smallest possible
+/// backing type.
+///
+/// In all cases, the enum keys will be created in the order given.
+/// For C ABI, this means that the order MUST NOT be changed in order
+/// to preserve ABI compatibility. You can set a key to null to
+/// remove it from the Zig enum while keeping the "hole" in the C enum
+/// to preserve ABI compatibility.
+///
+/// C detection is up to the caller, since there are multiple ways
+/// to do that. We rely on the `target` parameter to determine whether we
+/// should create a C compatible enum or a Zig enum.
+///
+/// For the Zig enum, the enum value is not guaranteed to be stable, so
+/// it shouldn't be relied for things like serialization.
+pub fn Enum(
+    target: Target,
+    keys: []const ?[:0]const u8,
+) type {
+    var fields: [keys.len]std.builtin.Type.EnumField = undefined;
+    var fields_i: usize = 0;
+    for (keys, 0..) |key_, key_i| {
+        const key: [:0]const u8 = key_ orelse switch (target) {
+            .c => std.fmt.comptimePrint("__unused_{d}", .{key_i}),
+            .zig => continue,
+        };
+
+        fields[fields_i] = .{
+            .name = key,
+            .value = fields_i,
+        };
+        fields_i += 1;
+    }
+
+    return @Type(.{ .@"enum" = .{
+        .tag_type = switch (target) {
+            .c => c_int,
+            .zig => std.math.IntFittingRange(0, fields_i - 1),
+        },
+        .fields = fields[0..fields_i],
+        .decls = &.{},
+        .is_exhaustive = true,
+    } });
+}
+
+pub const Target = union(enum) {
+    c,
+    zig,
+};
+
+test "zig" {
+    const testing = std.testing;
+    const T = Enum(.zig, &.{ "a", "b", "c", "d" });
+    const info = @typeInfo(T).@"enum";
+    try testing.expectEqual(u2, info.tag_type);
+}
+
+test "c" {
+    const testing = std.testing;
+    const T = Enum(.c, &.{ "a", "b", "c", "d" });
+    const info = @typeInfo(T).@"enum";
+    try testing.expectEqual(c_int, info.tag_type);
+}
+
+test "abi by removing a key" {
+    const testing = std.testing;
+    // C
+    {
+        const T = Enum(.c, &.{ "a", "b", null, "d" });
+        const info = @typeInfo(T).@"enum";
+        try testing.expectEqual(c_int, info.tag_type);
+        try testing.expectEqual(3, @intFromEnum(T.d));
+    }
+
+    // Zig
+    {
+        const T = Enum(.zig, &.{ "a", "b", null, "d" });
+        const info = @typeInfo(T).@"enum";
+        try testing.expectEqual(u2, info.tag_type);
+        try testing.expectEqual(2, @intFromEnum(T.d));
+    }
+}