fix another boundary case and hopefully improve performance

The fix: the if we found ourselves at a charstart with only one
character to read, readsize would be zero, handle that correctly.

Performance: originally I read just the first byte of the next
character, which meant as many extra read calls as there are
characters left to read after the initial read.  So O(Nleft) reads
where Nleft is the number of characters left to read after the initial
read.

Now read as many bytes as there are characters left to read, which
should mean the number of reads comes down to O(log(Nleft**2)) I think
(but don't ask me to justify that.)

1 files changed, 17 insertions, 4 deletions

diff --git a/sv.c b/sv.c
index e2bed05753..34dc534503 100644
--- a/sv.c
+++ b/sv.c
@@ -7720,13 +7720,26 @@ S_sv_gets_read_record(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
 		}
 
 		if (charcount < recsize) {
-		    /* read the rest of the current character, and maybe the
-		       beginning of the next, if we need it */
-		    STRLEN readsize = (charstart ? 0 : skip - (bend - bufp))
-			+ (charcount + 1 < recsize);
+		    STRLEN readsize;
 		    STRLEN bufp_offset = bufp - buffer;
 		    SSize_t morebytesread;
 
+		    /* originally I read enough to fill any incomplete
+		       character and the first byte of the next
+		       character if needed, but if there's many
+		       multi-byte encoded characters we're going to be
+		       making a read call for every character beyond
+		       the original read size.
+
+		       So instead, read the rest of the character if
+		       any, and enough bytes to match at least the
+		       start bytes for each character we're going to
+		       read.
+		    */
+		    if (charstart)
+			readsize = recsize - charcount;
+		    else 
+			readsize = skip - (bend - bufp) + recsize - charcount - 1;
 		    buffer = SvGROW(sv, append + bytesread + readsize + 1) + append;
 		    bend = buffer + bytesread;
 		    morebytesread = PerlIO_read(fp, bend, readsize);