Use a multistage lookup table for grapheme segmentation

kitty/char-props.h

@@ -0,0 +1,53 @@
+/*
+ * char-props.h
+ * Copyright (C) 2025 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPL3 license.
+ */
+
+#pragma once
+
+#include "data-types.h"
+
+typedef union CharProps {
+    struct {
+        uint8_t grapheme_break : 4;
+        uint8_t indic_conjunct_break : 2;
+        uint8_t is_extended_pictographic: 1;
+        uint8_t is_invalid: 1;
+        uint8_t shifted_width : 3;
+        uint8_t is_non_rendered : 1;
+        uint8_t : 4;
+    };
+    uint16_t val;
+} CharProps;
+static_assert(sizeof(CharProps) == sizeof(uint16_t), "Fix the ordering of CharProps");
+
+typedef struct GraphemeSegmentationState {
+    int last_char_prop;
+
+    /* True if the last character ends a sequence of Indic_Conjunct_Break
+values:  consonant {extend|linker}*  */
+    bool incb_consonant_extended;
+    /* True if the last character ends a sequence of Indic_Conjunct_Break
+values:  consonant {extend|linker}* linker  */
+    bool incb_consonant_extended_linker;
+    /* True if the last character ends a sequence of Indic_Conjunct_Break
+values:  consonant {extend|linker}* linker {extend|linker}*  */
+    bool incb_consonant_extended_linker_extended;
+
+    /* True if the last character ends an emoji modifier sequence
+       \p{Extended_Pictographic} Extend*.  */
+    bool emoji_modifier_sequence;
+    /* True if the last character was immediately preceded by an
+       emoji modifier sequence   \p{Extended_Pictographic} Extend*.  */
+    bool emoji_modifier_sequence_before_last_char;
+
+    /* Number of consecutive regional indicator (RI) characters seen
+       immediately before the current point.  */
+    size_t ri_count;
+} GraphemeSegmentationState;
+
+CharProps char_props_for(char_type ch);
+void grapheme_segmentation_reset(GraphemeSegmentationState *s);
+bool grapheme_segmentation_step(GraphemeSegmentationState *s, CharProps ch);