Source Code Tips and Tricks (LowRes NX)

nathanielbabiak 2020-12-04 21:50 (Edited)

In this post "small" and "large" refer to the number of tokens required.

Non-zero Evaluations

Non-zero values (int and float) evaluate to true, these are smaller and faster:

Replace IF X<>0 THEN with IF X THEN
Replace IF X<>0 AND Y<>0 THEN with IF X*Y THEN
Replace IF X=0 OR Y=0 THEN with IF X*Y=0 THEN

There's two non-zero evaluation tricks if you're sure X and Y will always have the same sign:

Replace IF X<>0 OR Y<>0 THEN with IF X+Y THEN
Replace IF X=0 AND Y=0 THEN with IF X+Y=0 THEN

There's a few more non-zero evaluation tricks when X (and Y) are integers:

Replace IF X AND %001=%001 THEN with IF X AND %001 THEN
Replace IF X AND %010=%010 THEN with IF X AND %010 THEN
Replace IF X AND %100=%100 THEN with IF X AND %100 THEN
Replace IF X<=-Y OR X>=Y THEN with IF X\Y THEN
Replace IF X>-Y AND X<Y THEN with IF X\Y=0 THEN

Keywords from C

This switch-case structure is small but slow:

IF X=7 THEN
  '...
ELSE IF X=6 THEN
  '...
ELSE IF X=5 THEN
  '...
ELSE IF X=4 THEN
  '...
ELSE IF X=3 THEN
  '...
ELSE IF X=2 THEN
  '...
ELSE IF X THEN
  'X=1
  '...
ELSE
  'X=0
  '...
END IF

This switch-case structure is fast but large:

IF X AND %100 THEN
  IF X AND %010 THEN
    IF X AND %001 THEN
      'X=7
      '...
    ELSE
      'X=6
      '...
    END IF
  ELSE
    IF X AND %001 THEN
      'X=5
      '...
    ELSE
      'X=4
      '...
    END IF
  END IF
ELSE
  IF X AND %010 THEN
    IF X AND %001 THEN
      'X=3
      '...
    ELSE
      'X=2
      '...
    END IF
  ELSE
    IF X AND %001 THEN
      'X=1
      '...
    ELSE
      'X=0
      '...
    END IF
  END IF
END IF

The continue keyword can also be implemented:

FOR X=0 TO 7
  CALL FOO
  'IF TEST1 THEN "CONTINUE"
  IF NOT TEST1 THEN
    CALL BAR
    CALL ZOT
  END IF
  'NO CODE ALLOWED HERE, "END IF" AND "NEXT" MUST BE ADJACENT.
NEXT X

Small Declaration

To return a value from a subprograms in a simple variable, the variable name must be declared prior. If an algorithm needs more than 3 new variable names, this is smaller than declaring one variable per-line. It's slower only because of overhead, otherwise the same speed O(n).

DECLARE:
DATA 0, 0, 0, 0, 0, 0, 0, 0
LOAD DECLARE
READ Y0, Y1, Y2, Y3, Y4, Y5, Y6, Y7

Single-command if-then statements are smaller (and sometimes faster too) when written on a single line like this:

IF TEST1 THEN CALL FOO
IF TEST1 THEN CALL FOO ELSE CALL BAR
IF TEST1 THEN CALL FOO ELSE IF TEST2 THEN CALL BAR ELSE CALL ZOT

Loops

This loop is small but slow:

FOR X=0 TO 7
  TEXT 0,X,"-"
NEXT X

This unrolled loop is fast but large:

TEXT 0,0,"-"
TEXT 0,1,"-"
TEXT 0,2,"-"
TEXT 0,3,"-"
TEXT 0,4,"-"
TEXT 0,5,"-"
TEXT 0,6,"-"
TEXT 0,7,"-"

This is a sample loop:

DIM LIST(XHI-1)
FOR X=0 TO XHI-1
  CALL FOO(LIST(X),B*X+C)
NEXT X

The sample loop calculates the subtraction twice before entering the loop, then again at each iteration of the loop. This loop only calculates it once, it's faster and larger:

XMAX=XHI-1
DIM LIST(XMAX)
FOR X=0 TO XMAX
  CALL FOO(LIST(X),B*X+C)
NEXT X

The sample loop calculates math inside the loop. This loop uses a separate variable to track the math, it's faster and larger:

XMAX=XHI-1
DIM LIST(XMAX)
Y=C
FOR X=0 TO XMAX
  CALL FOO(LIST(X),Y)
  ADD Y,B
NEXT X

The separate-variable loop uses instructions inside the loop to track the math. This loop uses the for-variable itself, it's faster and larger (and also the code doesn't work, but I hope you get the point):

X=0
XMAX=XHI-1
DIM LIST(XMAX)
YMAX=B*XMAX+C
FOR Y=C TO YMAX STEP B
  CALL FOO(LIST(X),Y)
  INC X
NEXT Y

Note the step variable B, if replaced by a calculation, would be calculated at each iteration of the loop, even if B is negative (e.g. -3 is a calculation of "negation" on the constant 3).

How To

Source Code Tips and Tricks

Non-zero Evaluations

Keywords from C

Small Declaration

Loops