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-rw-r--r--util/utf8.c66
1 files changed, 66 insertions, 0 deletions
diff --git a/util/utf8.c b/util/utf8.c
new file mode 100644
index 00000000..802fd010
--- /dev/null
+++ b/util/utf8.c
@@ -0,0 +1,66 @@
+#include <stdint.h>
+#include "util/utf8.h"
+
+static bool in_range(char x, uint8_t low, uint8_t high) {
+ uint8_t v = (uint8_t)x;
+ return low <= v && v <= high;
+}
+
+bool is_utf8(const char *string) {
+ /* Returns true iff the string is 'well-formed', as defined by
+ * Unicode Standard 15.0.0. See Chapter 3, D92 and Table 3.7.
+ *
+ * UTF-8 strings are sequences of code points encoded in one of the
+ * following ways. The first byte determines the pattern.
+ *
+ * 00..7F
+ * C2..DF 80..BF
+ * E0 A0..BF 80..BF
+ * E1..EC 80..BF 80..BF
+ * ED 80..9F 80..BF
+ * EE..EF 80..BF 80..BF
+ * F0 90..BF 80..BF 80..BF
+ * F1..F3 80..BF 80..BF 80..BF
+ * F4 80..8F 80..BF 80..BF
+ */
+ uint8_t range_table[9][8] = {
+ {0x00, 0x7F},
+ {0xC2, 0xDF, 0x80, 0xBF},
+ {0xE0, 0xE0, 0xA0, 0xBF, 0x80, 0xBF},
+ {0xE1, 0xEC, 0x80, 0xBF, 0x80, 0xBF},
+ {0xED, 0xED, 0x80, 0x9F, 0x80, 0xBF},
+ {0xEE, 0xEF, 0x80, 0xBF, 0x80, 0xBF},
+ {0xF0, 0xF0, 0x90, 0xBF, 0x80, 0xBF, 0x80, 0xBF},
+ {0xF1, 0xF3, 0x80, 0xBF, 0x80, 0xBF, 0x80, 0xBF},
+ {0xF4, 0xF4, 0x80, 0x8F, 0x80, 0xBF, 0x80, 0xBF},
+ };
+ int lengths[9] = {
+ 1, 2, 3, 3, 3, 3, 4, 4, 4
+ };
+
+ while (string[0]) {
+ bool accept = false;
+ for (int i = 0; i < 9; i++) {
+ if (!in_range(string[0], range_table[i][0],
+ range_table[i][1])) {
+ continue;
+ }
+ for (int j = 1; j < lengths[i]; j++) {
+ if (!in_range(string[j], range_table[i][2 * j],
+ range_table[i][2 * j + 1])) {
+ // Early exit is necessary to avoid
+ // reading past the null terminator
+ return false;
+ }
+ }
+ string += lengths[i];
+ accept = true;
+ break;
+ }
+ if (!accept) {
+ return false;
+ }
+ }
+
+ return true;
+}