From f64d28168b6b6a9dcad65db2f1dfab44ca45b25e Mon Sep 17 00:00:00 2001
From: hugova <hugova@kth.se>
Date: Thu, 17 Jul 2025 13:49:09 +0200
Subject: [PATCH] make line_down draw from other end of line. Reduses one CMP
 instruction

---
 wip-hugo/routines/line/line_down.s | 42 +++++++++++++++++-------------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/wip-hugo/routines/line/line_down.s b/wip-hugo/routines/line/line_down.s
index b0cf736..6c2070d 100644
--- a/wip-hugo/routines/line/line_down.s
+++ b/wip-hugo/routines/line/line_down.s
@@ -73,27 +73,34 @@ selfmod:
         LDA V + 1
         STA case_1 + 7
 end_selfmod:
+        LDA X_end
+        STA X_pos
+        LDA Y_end
+        STA Y_pos
+
         JSR pixel_calc ;;only used first pixel. after this relative position is abused
         STA byte_to_paint
         ;; X = X_end - X_pos
         LDX dx
+        Sub_16 btp_mem_pos, btp_mem_pos + 1, #$00, #$00,! ;; Y has always a offset of at least 1 = C + $0000
+        INY
 for_x:
         ;; Paints A to address in |btp_mem_pos* + Y|
         ;; Y is pixel position in the chunk. Therefor it may be that Y = 0, 1, 2, 3, 4, ,5 ,6 ,7.
         LDA byte_to_paint ;A byte containing a single 1. Coresponds to X position in the chunk.
         ORA (btp_mem_pos), Y
         STA (btp_mem_pos), Y
-increment_pixel_x:
-        LSR byte_to_paint ; Rotates the pixel one bit to the left ON THE SCREEN.
-        BCC increment_pixel_x_end; We need to move to the next chunk
-move_8px_left:
-        ;; Restores byte to paint to #%10000000
-        ROR byte_to_paint
+decrement_pixel_x:
+        ASL byte_to_paint ; Rotates the pixel one bit to the left ON THE SCREEN.
+        BCC decrement_pixel_x_end; We need to move to the next chunk
+move_8px_right:
+        ;; Restores byte to paint to #%00000001
+        ROL byte_to_paint
         ;; Next chunk is 8 addresses away. Look in pixel_draw for more detail.
         ;; -8.
-        ;; C = 1 therefore you se 07
-        Add_16 btp_mem_pos, btp_mem_pos + 1, #$08, #$00, !
-increment_pixel_x_end:
+        ;; C = 0
+        Sub_16 btp_mem_pos, btp_mem_pos + 1, #$07, #$00, !
+decrement_pixel_x_end:
         DEX
         BEQ end ;We keep track on when to stop line draw with the X registry.
         ;;If D <  %00000010 00000000:  case_2
@@ -105,20 +112,19 @@ case_1:
         ;;      C =1 so we can use !
         ;;      A = >D
         Sub_16_A D, D + 1, #>V, #<V, !
-increment_y_pos:
-        INY ; Increment Y pos inside the buffer
-        CPY #$08
+decrement_y_pos:
+        DEY ; Increment Y pos inside the buffer
         BNE for_x
-move_8px_down: ; Z=1 --> C=1
-        LDY #$00
+move_8px_up: ; Z=1 --> C=1
+        LDY #$08
         ;; Switch to chunk bellow
         ; C = 1
-        ; So we subtract  #$3F, #$01 +C
-        Add_16 btp_mem_pos, btp_mem_pos + 1, #$3F, #$01, !; +320
+        ; So we subtract  #$40, #$01
+        Sub_16 btp_mem_pos, btp_mem_pos + 1, #$40, #$01, !; +320
         JMP for_x
-increment_y_pos_end:
+decrement_y_pos_end:
 case_2:
-        Add_16 D, D + 1, #>dy_2, #<dy_2, ! ;D = D + 2*dy
+        Add_16 D, D + 1, #>dy_2, #<dy_2,!  ;D = D + 2*dy
         JMP for_x
 end:
         RTS