From 8d53a185a487d76dec9a07dcde854d04921933db Mon Sep 17 00:00:00 2001
From: falsycat <me@falsy.cat>
Date: Wed, 9 Apr 2025 00:18:06 +0900
Subject: [PATCH] add Merged() util

---
 src/hncore/Merged.zig | 72 +++++++++++++++++++++++++++++++++++++++++++
 src/hncore/root.zig   |  2 ++
 2 files changed, 74 insertions(+)
 create mode 100644 src/hncore/Merged.zig

diff --git a/src/hncore/Merged.zig b/src/hncore/Merged.zig
new file mode 100644
index 0000000..724a6a9
--- /dev/null
+++ b/src/hncore/Merged.zig
@@ -0,0 +1,72 @@
+const std = @import("std");
+
+/// Returns a type having fields that type A and type B have.
+/// Fields from B are more preferred than from A in case that field names are duplicated.
+pub fn Merged(comptime A: type, comptime B: type) type {
+    const at = @typeInfo(A).@"struct";
+    const bt = @typeInfo(B).@"struct";
+
+    return @Type(.{
+        .@"struct" = .{
+            .layout   = .auto,
+            .fields   = mergeFields(at.fields, bt.fields),
+            .is_tuple = false,
+            .decls    = &.{},
+        },
+    });
+}
+fn mergeFields(
+    comptime a: []const std.builtin.Type.StructField,
+    comptime b: []const std.builtin.Type.StructField) []std.builtin.Type.StructField {
+    var ret: [a.len+b.len]std.builtin.Type.StructField = undefined;
+    var len: usize = 0;
+
+    inline for (b) |f| {
+        ret[len] = f;
+        ret[len].is_comptime = false;
+        len += 1;
+    }
+    inline for (a) |f| {
+        inline for (b) |fb| {
+            if (std.mem.eql(u8, f.name, fb.name)) {
+                break;
+            }
+        } else {
+            ret[len] = f;
+            ret[len].is_comptime = false;
+            len += 1;
+        }
+    }
+    return ret[0..len];
+}
+
+/// Same to `Merged(@TypeOf(a), @TypeOf(b))` but returns a merged value, not type.
+pub fn merge(a: anytype, b: anytype) Merged(@TypeOf(a), @TypeOf(b)) {
+    var ret: Merged(@TypeOf(a), @TypeOf(b)) = undefined;
+
+    const af = @typeInfo(@TypeOf(a)).@"struct".fields;
+    const bf = @typeInfo(@TypeOf(b)).@"struct".fields;
+
+    inline for (bf) |f| {
+        @field(ret, f.name) = @field(b, f.name);
+    }
+    inline for (af) |f| {
+        inline for (bf) |fb| {
+            if (std.mem.eql(u8, f.name, fb.name)) {
+                break;
+            }
+        } else {
+            @field(ret, f.name) = @field(a, f.name);
+        }
+    }
+    return ret;
+
+}
+
+test "merging 2 structs" {
+    const merged = merge(.{.a = 0, .b = 1, .c = 2,}, .{.b = 44, .d = 53,});
+    try std.testing.expect(merged.a == 0);
+    try std.testing.expect(merged.b == 44);
+    try std.testing.expect(merged.c == 2);
+    try std.testing.expect(merged.d == 53);
+}
diff --git a/src/hncore/root.zig b/src/hncore/root.zig
index 1a3d019..e58974e 100644
--- a/src/hncore/root.zig
+++ b/src/hncore/root.zig
@@ -2,6 +2,8 @@ pub const Mindmap = @import("./Mindmap.zig");
 pub const Node    = @import("./Node.zig");
 pub const Project = @import("./Project.zig");
 
+pub const merge  = @import("./Merged.zig").merge;
+
 test {
     @import("std").testing.refAllDecls(@This());
 }