beer-protocols: add the graphics placeholder diacritic table

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Iaefa912063b068ea8500091e5fba0ec66a6a6964

crates/beer-protocols/src/graphics.rs

@@ -139,6 +139,51 @@ impl GraphicsCommand {
     }
 }
 
+/// The Unicode placeholder character. A cell holding this code point displays a
+/// slice of an image, with the row/column carried by combining diacritics and
+/// the image id by the cell's foreground colour (see [`diacritic_value`]). This
+/// lets images survive multiplexers and editors that only forward text.
+pub const PLACEHOLDER: char = '\u{10EEEE}';
+
+/// The ordered combining marks the placeholder protocol uses to encode a row,
+/// column, or high image-id byte: the value is the mark's index in this list.
+/// Taken verbatim from kitty's `rowcolumn-diacritics.txt`.
+#[rustfmt::skip]
+const DIACRITICS: [u32; 297] = [
+    0x305, 0x30D, 0x30E, 0x310, 0x312, 0x33D, 0x33E, 0x33F, 0x346, 0x34A, 0x34B, 0x34C,
+    0x350, 0x351, 0x352, 0x357, 0x35B, 0x363, 0x364, 0x365, 0x366, 0x367, 0x368, 0x369,
+    0x36A, 0x36B, 0x36C, 0x36D, 0x36E, 0x36F, 0x483, 0x484, 0x485, 0x486, 0x487, 0x592,
+    0x593, 0x594, 0x595, 0x597, 0x598, 0x599, 0x59C, 0x59D, 0x59E, 0x59F, 0x5A0, 0x5A1,
+    0x5A8, 0x5A9, 0x5AB, 0x5AC, 0x5AF, 0x5C4, 0x610, 0x611, 0x612, 0x613, 0x614, 0x615,
+    0x616, 0x617, 0x657, 0x658, 0x659, 0x65A, 0x65B, 0x65D, 0x65E, 0x6D6, 0x6D7, 0x6D8,
+    0x6D9, 0x6DA, 0x6DB, 0x6DC, 0x6DF, 0x6E0, 0x6E1, 0x6E2, 0x6E4, 0x6E7, 0x6E8, 0x6EB,
+    0x6EC, 0x730, 0x732, 0x733, 0x735, 0x736, 0x73A, 0x73D, 0x73F, 0x740, 0x741, 0x743,
+    0x745, 0x747, 0x749, 0x74A, 0x7EB, 0x7EC, 0x7ED, 0x7EE, 0x7EF, 0x7F0, 0x7F1, 0x7F3,
+    0x816, 0x817, 0x818, 0x819, 0x81B, 0x81C, 0x81D, 0x81E, 0x81F, 0x820, 0x821, 0x822,
+    0x823, 0x825, 0x826, 0x827, 0x829, 0x82A, 0x82B, 0x82C, 0x82D, 0x951, 0x953, 0x954,
+    0xF82, 0xF83, 0xF86, 0xF87, 0x135D, 0x135E, 0x135F, 0x17DD, 0x193A, 0x1A17, 0x1A75, 0x1A76,
+    0x1A77, 0x1A78, 0x1A79, 0x1A7A, 0x1A7B, 0x1A7C, 0x1B6B, 0x1B6D, 0x1B6E, 0x1B6F, 0x1B70, 0x1B71,
+    0x1B72, 0x1B73, 0x1CD0, 0x1CD1, 0x1CD2, 0x1CDA, 0x1CDB, 0x1CE0, 0x1DC0, 0x1DC1, 0x1DC3, 0x1DC4,
+    0x1DC5, 0x1DC6, 0x1DC7, 0x1DC8, 0x1DC9, 0x1DCB, 0x1DCC, 0x1DD1, 0x1DD2, 0x1DD3, 0x1DD4, 0x1DD5,
+    0x1DD6, 0x1DD7, 0x1DD8, 0x1DD9, 0x1DDA, 0x1DDB, 0x1DDC, 0x1DDD, 0x1DDE, 0x1DDF, 0x1DE0, 0x1DE1,
+    0x1DE2, 0x1DE3, 0x1DE4, 0x1DE5, 0x1DE6, 0x1DFE, 0x20D0, 0x20D1, 0x20D4, 0x20D5, 0x20D6, 0x20D7,
+    0x20DB, 0x20DC, 0x20E1, 0x20E7, 0x20E9, 0x20F0, 0x2CEF, 0x2CF0, 0x2CF1, 0x2DE0, 0x2DE1, 0x2DE2,
+    0x2DE3, 0x2DE4, 0x2DE5, 0x2DE6, 0x2DE7, 0x2DE8, 0x2DE9, 0x2DEA, 0x2DEB, 0x2DEC, 0x2DED, 0x2DEE,
+    0x2DEF, 0x2DF0, 0x2DF1, 0x2DF2, 0x2DF3, 0x2DF4, 0x2DF5, 0x2DF6, 0x2DF7, 0x2DF8, 0x2DF9, 0x2DFA,
+    0x2DFB, 0x2DFC, 0x2DFD, 0x2DFE, 0x2DFF, 0xA66F, 0xA67C, 0xA67D, 0xA6F0, 0xA6F1, 0xA8E0, 0xA8E1,
+    0xA8E2, 0xA8E3, 0xA8E4, 0xA8E5, 0xA8E6, 0xA8E7, 0xA8E8, 0xA8E9, 0xA8EA, 0xA8EB, 0xA8EC, 0xA8ED,
+    0xA8EE, 0xA8EF, 0xA8F0, 0xA8F1, 0xAAB0, 0xAAB2, 0xAAB3, 0xAAB7, 0xAAB8, 0xAABE, 0xAABF, 0xAAC1,
+    0xFE20, 0xFE21, 0xFE22, 0xFE23, 0xFE24, 0xFE25, 0xFE26, 0x10A0F, 0x10A38, 0x1D185, 0x1D186, 0x1D187,
+    0x1D188, 0x1D189, 0x1D1AA, 0x1D1AB, 0x1D1AC, 0x1D1AD, 0x1D242, 0x1D243, 0x1D244,
+];
+
+/// The value a placeholder diacritic encodes (its position in the table), or
+/// `None` if `c` is not one of the protocol's row/column diacritics.
+pub fn diacritic_value(c: char) -> Option<u32> {
+    let cp = c as u32;
+    DIACRITICS.iter().position(|&d| d == cp).map(|i| i as u32)
+}
+
 /// Parse the control-data field of a graphics command (everything between
 /// `ESC _ G` and the `;` that precedes the payload). Unknown keys and malformed
 /// pairs are ignored; an empty field yields the default command.
@@ -333,4 +378,16 @@ mod tests {
         assert_eq!(c.action, Action::Transmit);
         assert_eq!(c.width, 8);
     }
+
+    #[test]
+    fn placeholder_diacritics_encode_positions() {
+        // The first two table entries are values 0 and 1 (the doc's examples).
+        assert_eq!(diacritic_value('\u{0305}'), Some(0));
+        assert_eq!(diacritic_value('\u{030D}'), Some(1));
+        // The last entry is value 296.
+        assert_eq!(diacritic_value('\u{1D244}'), Some(296));
+        // A non-diacritic is rejected.
+        assert_eq!(diacritic_value('a'), None);
+        assert_eq!(DIACRITICS.len(), 297);
+    }
 }