[GitHub] [arrow] maartenbreddels commented on a change in pull request #7449: ARROW-9133: [C++] Add utf8_upper and utf8_lower

[GitBox <gi...@apache.org>]

maartenbreddels commented on a change in pull request #7449:
URL: https://github.com/apache/arrow/pull/7449#discussion_r446934057



##########
File path: cpp/src/arrow/compute/kernels/scalar_string.cc
##########
@@ -30,6 +31,124 @@ namespace internal {
 
 namespace {
 
+// lookup tables
+std::vector<uint32_t> lut_upper_codepoint;
+std::vector<uint32_t> lut_lower_codepoint;
+std::once_flag flag_case_luts;
+
+constexpr uint32_t REPLACEMENT_CHAR =
+    '?';  // the proper replacement char would be the 0xFFFD codepoint, but that can
+          // increase string length by a factor of 3
+constexpr int MAX_CODEPOINT_LUT = 0xffff;  // up to this codepoint is in a lookup table
+
+static inline void utf8_encode(uint8_t*& str, uint32_t codepoint) {
+  if (codepoint < 0x80) {
+    *str++ = codepoint;
+  } else if (codepoint < 0x800) {
+    *str++ = 0xC0 + (codepoint >> 6);
+    *str++ = 0x80 + (codepoint & 0x3F);
+  } else if (codepoint < 0x10000) {
+    *str++ = 0xE0 + (codepoint >> 12);
+    *str++ = 0x80 + ((codepoint >> 6) & 0x3F);
+    *str++ = 0x80 + (codepoint & 0x3F);
+  } else if (codepoint < 0x200000) {
+    *str++ = 0xF0 + (codepoint >> 18);
+    *str++ = 0x80 + ((codepoint >> 12) & 0x3F);
+    *str++ = 0x80 + ((codepoint >> 6) & 0x3F);
+    *str++ = 0x80 + (codepoint & 0x3F);
+  } else {
+    *str++ = codepoint;
+  }
+}
+
+static inline bool utf8_is_continuation(const uint8_t codeunit) {
+  return (codeunit & 0xC0) == 0x80;  // upper two bits should be 10
+}
+
+static inline uint32_t utf8_decode(const uint8_t*& str, int64_t& length) {
+  if (*str < 0x80) {  //
+    length -= 1;
+    return *str++;
+  } else if (*str < 0xC0) {  // invalid non-ascii char
+    length -= 1;
+    str++;
+    return REPLACEMENT_CHAR;

Review comment:
       Would you mind we keep it as is for now, and maybe discuss this separately? Happy to take this to the mailing list.




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