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norio_nomura 2/1/2019 10:39 AM
String.Indexはオフセットが同じでも、対象となるStringにより中身が変わってきます。 コメントに構造が記されてます。 https://github.com/apple/swift/blob/master/stdlib/public/core/StringIndex.swift#L17-L33 /* String's Index has the following layout: ┌──────────┬───────────────────┬────────────────┬──────────┐ │ b63:b16 │ b15:b14 │ b13:b8 │ b7:b0 │ ├──────────┼───────────────────┼────────────────┼──────────┤ │ position │ transcoded offset │ grapheme cache │ reserved │ └──────────┴───────────────────┴────────────────┴──────────┘ - grapheme cache: A 6-bit value remembering the distance to the next grapheme boundary - position aka `encodedOffset`: An offset into the string's code units - transcoded offset: a sub-scalar offset, derived from transcoding The use and interpretation of both `reserved` and `grapheme cache` is not part of Index's ABI; it should be hidden behind non-inlinable calls. However, the position of the sequence of 14 bits allocated is part of Index's ABI, as well as the default value being `0`. */ (edited)
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