Here's one historic bit of code in QuickDraw that author Bill Atkinson re-implemented from Xerox:
LISA_OS/LIBS/LIBQD/libqd-BITBLT.TEXT.unix.txt
;--------------------------------------------------------------
;
; PROCEDURE BitBlt(srcBits,dstBits: BitMap;
; srcRect,dstRect: Rect;
; mode: INTEGER; pat: Pattern);
;
; TRANSFERS A RECTANGULAR BLOCK OF BITS WITH NO CLIPPING AT ALL.
; MODE SPECIFIES THE COMBINATION MODE AND WHETHER THE SOURCE SHOULD COME
; FROM THE SOURCE BITMAP OR FROM A REPEATING PATTERN.
;
; COPYRIGHT APPLE COMPUTER INC.
; WRITTEN BY BILL ATKINSON
;
; POSITION INDEPENDENT AND RE-ENTRANT, CLOBBERS ONLY A0.
The name derives from the
BitBLT routine for the
Xerox Alto, computer, standing for
bit-boundary block transfer. Dan Ingalls, Larry Tesler, Bob Sproull, and Diana Merry first coded this function at Xerox PARC in November 1975 for the Smalltalk-72 system. In the Smalltalk language, BitBLT was slow. So Dan Ingalls later reimplemented it in microcode to make it blazingly fast.
This one snippet of code, perhaps more than any other, is what amazed Steve Jobs and his cadre of coders when they
visited Xerox PARC in 1979. Until that visit, the key Lisa graphical feature of overlapping windows seemed intractable. Atkinson, head of graphics on Lisa, had no idea how to solve the problem. But once he saw it in action a Xerox Alto, Atkinson knew there
was a solution. He reverse engineered what Xerox had done, inventing his own superior concept called "regions". Atkinson then coded routines to efficiently draw text and graphics within these regions, which represented rectangular screen areas that need to be redrawn or left untouched.
From
https://www-user.tu-chemnitz.de/~heha/petzold/ch14d.htm
This history of BitBlt is legendary, originating in the Xerox SmallTalk project.
Bitblt (pronounced "bit blit") stands for "bit-block transfer." The BLT originated as an assembly language instruction that did memory block transfers on the DEC PDP-10. The term "bitblt" was first used in graphics in connection with the SmallTalk system designed at the Xerox Palo Alto Research Center (PARC). In SmallTalk, all graphics output operations are based around the bitblt. Among programmers, "blt" is sometimes used as a verb, as in "Then I wrote some code to blt the happy face to the screen and play a wave file."
From
https://ethw.org/Milestones:Apple_Macintosh_Computer
Bill Atkinson invented QuickDraw [Ref-5: '545 patent], first for the Apple Lisa and then in an expanded form for the Macintosh. This hugely success application was inspired by Bill’s visit to Xerox PARC in 1979 - see Oral History of Andy Hertzfeld and Bill Atkinson via URL at bottom of page. As Bill was at least initially unaware that the BitBlt graphics code for the Xerox Alto (originally written by Dan Ingalls) was done in microcode, he didn't realize the difficulty of getting high-performance graphics built for the Motorola 68000.
This file is a true collector's item, as CMH's release is, I believe, the first time Atkinsons' code has been made public.