Oh, the signed division wasn't working? It worked for me when I tested it and it was faster than regular division (sometimes a lot faster)

This should fix it. There was a line of code checking for the parity of a result instead of whether it was positive or negative. Essentially, I used jp pe, instead of jp p,.

Oops.

That is excellent, thanks !

In particular, do you mean empty lines, or is it on lines that actually have data? I only encountered a problem with that on newlines, but if you could give an example of it failing on lines with data, that could be helpful (before I release another update).

EDIT: Fixed a problem for when HL=8000h.
I have this routine that is larger, but faster than my other division routines:
;===============================================================
HL_Div_BC_Signed:
;===============================================================
;Performs HL/BC
;Speed: 1350-55a-2b
;         b is the number of set bits in the result
;         a is the number of leading zeroes in the absolute value of HL, minus 1
;         add 24 if HL is negative
;         add 19 if BC is negative
;         add 28 if the result is negative
;Size:    68 bytes
;Inputs:
;     HL is the numerator
;     BC is the denominator
;Outputs:
;     DE is the quotient
;     HL is the remainder
;     BC is not changed
;Destroys:
;     A
;===============================================================
     ld a,h
     xor b
     push af
;absHL
     xor b
     jp p,$+9
     xor a \ sub l \ ld l,a
     sbc a,a \ sub h \ ld h,a
;absBC:
     bit 7,b
     jr z,$+8
     xor a \ sub c \ ld c,a
     sbc a,a \ sub b \ ld b,a

     ld de,0
     adc hl,hl
     jr z,EndSDiv
     ld a,16

     add hl,hl
     dec a
     jp nc,$-2
     ex de,hl
     jp jumpin
Loop1:
     add hl,bc     ;--
Loop2:
     dec a         ;4
     jr z,EndSDiv  ;12|23
     sla e         ;--
     rl d          ;--
jumpin:            ;
     adc hl,hl     ;15
     sbc hl,bc     ;15
     jr c,Loop1    ;23-2b     ;b is the number of bits in the absolute value of the result.
     inc e         ;--
     jp Loop2      ;--
EndSDiv:
     pop af \ ret p
     xor a \ sub e \ ld e,a
     sbc a,a \ sub d \ ld d,a
     ret
So at its slowest, it is 1396 clock cycles and at its fastest (for non-trivial division) it is can get as low as 572 t-states for non-trivial division.

EDIT: I think I messed up the clock cycle count (but not by much). Anyways, here is a cleaner version that is smaller and more predictable. 1315-2b t-states where b is the number of bits set in the absolute value of the result:
EDIT2: The timings above are correct, now.
;===============================================================
HL_Div_BC_Signed:
;===============================================================
;Performs HL/BC
;Speed:   1315-2b cycles
;         **same cycles as the regular HL_Div_BC
;         add 24 if HL is negative
;         add 24 if BC is negative
;         add 28 if only one is negative
;Size:    60 bytes
;Inputs:
;     HL is the numerator
;     BC is the denominator
;Outputs:
;     DE is the quotient
;     HL is the remainder
;     BC is not changed
;     A is 0
;     z flag is set
;     c flag is reset
;===============================================================
     ld a,h
     xor b
     push af
;absHL
     xor b
     jp p,$+9
     xor a \ sub l \ ld l,a
     sbc a,a \ sub h \ ld h,a
;absBC:
     xor b
     jp p,$+9
     xor a \ sub c \ ld c,a
     sbc a,a \ sub b \ ld b,a

     add hl,hl
     ld a,15
     ld de,0
     ex de,hl
     jp jumpin
;89+15*80+26 = 1315-2b
Div_Loop_1:
     add hl,bc   ;--
Loop2:
     dec a       ;4
     jr z,EndSDiv ;12|7
jumpin:
     sla e       ;8
     rl d        ;8
     adc hl,hl   ;15
     sbc hl,bc   ;15
     jr c,Loop1  ;23-2b
     inc e       ;--
     jp Loop2    ;--
EndSDiv:
     pop af \ ret p
     xor a \ sub e \ ld e,a
     sbc a,a \ sub d \ ld d,a
     ret


Awesome, have you seen my GrpRead, TPROG, and LblRW programs, too ? Those are also pretty powerful for BASIC games like RPGs. The first is like CopyProg for groups (it allows you to access data from the groups and extract variables, read lines from variables, and recall pictures from groups). The other is like a simplified CopyProg that is designed specifically for loading archived subprograms to RAM and cleaning them up (it is also tiny). LblRW is rather epic since it allows you to edit data inside your program by overwriting lines or reading lines starting at a label. This lets you keep user data directly in the program (for example, you can make a label called Lbl SV and store the user name and any string data to the lines following). I used that to store monster data, item data such as prices and descriptions, and all sorts of similar code that way.

The easiest way to do it, probably, is to just use a regular division routine and check for the sign of the input:
     ld a,d
     xor h       ;now the sign flag is set if the result needs to be negative or positive.
     push af
     xor h       ;now a=d again, but we have the sign flag set appropriately
     jp p,$+9    ;if the sign is minus, negate DE
       xor a
       sub e
       ld e,a
       sbc a,a  ;c flag is set, so this makes A=$FF
       sub d
       ld d,a
     ld a,h
     or a
     jp p,$+9
       xor a
       sub l
       ld l,a
       sbc a,a  ;c flag is set, so this makes A=$FF
       sub h
       ld h,a
     call HL_Div_DE    ;return the result in HL
     pop af
     ret p
     xor a
     sub l
     ld l,a
     sbc a,a
     sub h
     ld h,a
     ret
In the worst case, it is 141 t-states + however long it takes for the regular division.

141 t-states extra if one input is negative
137 t-states extra if both are negative
89 t-states extra if both inputs are positive

Oh, the BASIC raycaster is more like a pseudo-3D thing, but the topic is here. I based that off of Lodev's tutorial and code by Matrefeytontias. I'm not all that good at 3D stuff, but I found some math about 3D rotations and whatnot in my Linear Algebra book. I have some difficulty figuring out the right way to do projections, still.