Kev1n8 commented on code in PR #12044:
URL: https://github.com/apache/datafusion/pull/12044#discussion_r1727098779
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datafusion/functions/src/unicode/substr.rs:
##########
@@ -89,29 +94,193 @@ impl ScalarUDFImpl for SubstrFunc {
}
}
+/// Extracts the substring of string starting at the start'th character, and
extending for count characters if that is specified. (Same as substring(string
from start for count).)
+/// substr('alphabet', 3) = 'phabet'
+/// substr('alphabet', 3, 2) = 'ph'
+/// The implementation uses UTF-8 code points as characters
pub fn substr(args: &[ArrayRef]) -> Result<ArrayRef> {
match args[0].data_type() {
DataType::Utf8 => {
let string_array = args[0].as_string::<i32>();
- calculate_substr::<_, i32>(string_array, &args[1..])
+ string_substr::<_, i32>(string_array, &args[1..])
}
DataType::LargeUtf8 => {
let string_array = args[0].as_string::<i64>();
- calculate_substr::<_, i64>(string_array, &args[1..])
+ string_substr::<_, i64>(string_array, &args[1..])
}
DataType::Utf8View => {
let string_array = args[0].as_string_view();
- calculate_substr::<_, i32>(string_array, &args[1..])
+ string_view_substr(string_array, &args[1..])
}
- other => exec_err!("Unsupported data type {other:?} for function
substr"),
+ other => exec_err!(
+ "Unsupported data type {other:?} for function substr,\
+ expected Utf8View, Utf8 or LargeUtf8."
+ ),
}
}
-/// Extracts the substring of string starting at the start'th character, and
extending for count characters if that is specified. (Same as substring(string
from start for count).)
-/// substr('alphabet', 3) = 'phabet'
-/// substr('alphabet', 3, 2) = 'ph'
-/// The implementation uses UTF-8 code points as characters
-fn calculate_substr<'a, V, T>(string_array: V, args: &[ArrayRef]) ->
Result<ArrayRef>
+// Return the exact byte index for [start, end), set count to -1 to ignore
count
+fn get_true_start_count(input: &str, start: usize, count: i64) -> (usize,
usize) {
+ let (mut st, mut ed) = (input.len(), input.len());
+ let mut start_counting = false;
+ let mut cnt = 0;
+ for (char_cnt, (byte_cnt, _)) in input.char_indices().enumerate() {
+ if char_cnt == start {
+ st = byte_cnt;
+ if count != -1 {
+ start_counting = true;
+ } else {
+ break;
+ }
+ }
+ if start_counting {
+ if cnt == count {
+ ed = byte_cnt;
+ break;
+ }
+ cnt += 1;
+ }
+ }
+ (st, ed)
+}
+
+// The decoding process refs the trait at: arrow/arrow-data/src/byte_view.rs:44
+// From<u128> for ByteView
+fn string_view_substr(
+ string_view_array: &StringViewArray,
+ args: &[ArrayRef],
+) -> Result<ArrayRef> {
+ let mut builder = StringViewBuilder::new();
+ // Copy all blocks from input
+ for block in string_view_array.data_buffers() {
+ builder.append_block(block.clone());
+ }
+
+ let start_array = as_int64_array(&args[0])?;
+
+ match args.len() {
+ 1 => {
+ for (idx, (raw, start)) in string_view_array
+ .views()
+ .iter()
+ .zip(start_array.iter())
+ .enumerate()
+ {
+ if let Some(start) = start {
+ let length = *raw as u32;
+ let start = (start - 1).max(0);
+
+ // Operate according to the length of bytes
+ if length == 0 {
+ builder.append_null();
+ } else if length > 12 {
+ let view = ByteView::from(*raw);
+
+ // Safety:
+ // 1. idx < string_array.views.size()
+ // 2. builder is guaranteed to have corresponding
blocks
+ unsafe {
+ let str = string_view_array.value_unchecked(idx);
+ let (start, end) =
+ get_true_start_count(str, start as usize, -1);
+ builder.append_view_unchecked(
+ view.buffer_index,
+ view.offset + start as u32,
+ // guarantee that end-offset >= 0 for end <=
str.len()
+ (end - start) as u32,
+ );
+ }
+ } else {
+ // Safety:
+ // (1) original bytes are valid utf-8,
+ // (2) we do not slice on utf-8 codepoint
+ unsafe {
+ let bytes =
+ StringViewArray::inline_value(raw, length as
usize);
Review Comment:
Maybe we could add a function like `append_view_u128_unchecked(view: u128)`
in arrow/src/builder/generic_bytes_view_builder.rs to simply add a view with a
given `u128`. Then the whole process would be:
1. Get the str of the view by `value_unchecked`, then get the [start, end)
2. sub_view = ~~if end-start>12 substr_large_view() else
substr_small_view()~~ make_view()
3. call `appned_view_u128_unchecked(sub_view)` on the builder
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