vt: only apply VS-15/16 to valid sequences

At compile time, build a lookup table from the Unicode data file
'emoji-variation-sequences.txt'.

At run-time, when we detect a VS-15/16 sequence, do a lookup in this
table, and enforce the variation selector iff the sequence is valid.

Closes #1742

scripts/generate-emoji-variation-sequences.py

@@ -0,0 +1,95 @@
+#!/usr/bin/env python3
+
+import argparse
+import sys
+
+
+class Codepoint:
+    def __init__(self, start: int, end: None|int = None):
+        self.start = start
+        self.end = start if end is None else end
+        self.vs15 = False
+        self.vs16 = False
+
+    def __repr__(self) -> str:
+        return f'{self.start:x}-{self.end:x}, vs15={self.vs15}, vs16={self.vs16}'
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('input', type=argparse.FileType('r'))
+    parser.add_argument('output', type=argparse.FileType('w'))
+    opts = parser.parse_args()
+
+    codepoints: list[Codepoint] = []
+
+    for line in opts.input:
+        line = line.rstrip()
+        if not line:
+            continue
+        if line[0] == '#':
+            continue
+
+        cp, vs, _ = line.split(' ', maxsplit=2)
+        cp = int(cp, 16)
+        vs = int(vs, 16)
+
+        assert vs == 0xfe0e or vs == 0xfe0f
+
+        if len(codepoints) == 0 or codepoints[-1].start != cp:
+            codepoints.append(Codepoint(cp))
+        else:
+            assert codepoints[-1].start == cp
+
+        if vs == 0xfe0e:
+            codepoints[-1].vs15 = True
+        else:
+            codepoints[-1].vs16 = True
+
+
+    compacted_codepoints: list[Codepoint] = []
+    for i, cp in enumerate(codepoints):
+        assert cp.end == cp.start
+
+        if i == 0:
+            compacted_codepoints.append(cp)
+            continue
+
+        last_cp = compacted_codepoints[-1]
+        if last_cp.end == cp.start - 1 and last_cp.vs15 == cp.vs15 and last_cp.vs16 == cp.vs16:
+            compacted_codepoints[-1].end = cp.start
+        else:
+            compacted_codepoints.append(cp)
+
+    opts.output.write('#pragma once\n')
+    opts.output.write('#include <stdint.h>\n')
+    opts.output.write('#include <stdbool.h>\n')
+    opts.output.write('\n')
+    opts.output.write('struct emoji_vs {\n')
+    opts.output.write('    uint32_t start;\n')
+    opts.output.write('    uint32_t end;\n')
+    opts.output.write('    bool vs15:1;\n')
+    opts.output.write('    bool vs16:1;\n')
+    opts.output.write('} __attribute__((packed));\n')
+    opts.output.write('_Static_assert(sizeof(struct emoji_vs) == 9, "unexpected struct size");\n')
+    opts.output.write('\n')
+    opts.output.write('#if defined(FOOT_GRAPHEME_CLUSTERING)\n')
+    opts.output.write('\n')
+
+    opts.output.write(f'static const struct emoji_vs emoji_vs[{len(compacted_codepoints)}] = {{\n')
+
+    for cp in compacted_codepoints:
+        opts.output.write('    {\n')
+        opts.output.write(f'        .start = 0x{cp.start:X},\n')
+        opts.output.write(f'        .end = 0x{cp.end:x},\n')
+        opts.output.write(f'        .vs15 = {"true" if cp.vs15 else "false"},\n')
+        opts.output.write(f'        .vs16 = {"true" if cp.vs16 else "false"},\n')
+        opts.output.write('    },\n')
+
+    opts.output.write('};\n')
+    opts.output.write('\n')
+    opts.output.write('#endif  /* FOOT_GRAPHEME_CLUSTERING */\n')
+
+
+if __name__ == '__main__':
+    sys.exit(main())