terminal: add isTextMime helper for plain text MIME type names

src/terminal/clipboard.zig

@@ -1,3 +1,5 @@
+const std = @import("std");
+
 /// The clipboard destination for a write.
 pub const Location = enum(c_int) {
     standard = 0,
@@ -6,6 +8,21 @@ pub const Location = enum(c_int) {
     _,
 };
 
+/// MIME types that name plain text across the platforms terminals run
+/// on. We accept the union everywhere since serving text under any of these
+/// names is harmless.
+pub fn isTextMime(mime: []const u8) bool {
+    const names: []const []const u8 = &.{
+        "text/plain",
+        "text/plain;charset=utf-8",
+        "UTF8_STRING",
+        "TEXT",
+        "STRING",
+    };
+    for (names) |n| if (std.mem.eql(u8, mime, n)) return true;
+    return false;
+}
+
 /// A single representation of clipboard data.
 ///
 /// The MIME type and data are borrowed and only valid for the duration of a
@@ -34,3 +51,11 @@ pub const WriteResult = enum(c_int) {
     io_error = 5,
     _,
 };
+
+test isTextMime {
+    const testing = std.testing;
+    try testing.expect(isTextMime("text/plain"));
+    try testing.expect(isTextMime("UTF8_STRING"));
+    try testing.expect(!isTextMime("image/png"));
+    try testing.expect(!isTextMime("."));
+}