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Postings from April 2020
I thought that doing a readchars on a filehandle, seeking backwards
the width of the char in bytes and then doing another read
would always get the same character. That works for ascii-range
characters (1-byte in utf-8 encoding) but not multi-byte "wide"
characters (commonly 3-bytes in utf-8).
The question then, is why do I need a $nudge of 3 for wide chars, but
not ascii-range ones?
use v6;
use Test;
my $tmpdir = IO::Spec::Unix.tmpdir;
my $file = "$tmpdir/scratch_file.txt";
my $unichar_str = "\x[1200]\x[2D80]\x[4DFC]\x[AAAA]\x[2CA4]\x[2C8E]"; # ሀⶀ䷼ꪪⲤⲎ
my $ascii_str = "ABCDEFGHI";
subtest {
my $nudge = 3;
test_read_and_read_again($unichar_str, $file, $nudge);
}, "Wide unicode chars: $unichar_str";
subtest {
my $nudge = 0;
test_read_and_read_again($ascii_str, $file, $nudge);
}, "Ascii-range chars: $ascii_str";
# write given string to file, then read the third character twice and check
sub test_read_and_read_again($str, $file, $nudge = 0) {
spurt $file, $str;
my $fh = $file.IO.open;
$fh.readchars(2); # skip a few
my $chr_1 = $fh.readchars(1);
my $width = $chr_1.encode('UTF-8').bytes; # for our purposes, always 1 or 3
my $step_back = $width + $nudge;
$fh.seek: -$step_back, SeekFromCurrent;
my $chr_2 = $fh.readchars(1);
is( $chr_1, $chr_2,
"read, seek back, and read again gets same char with nudge of $nudge" );
}
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