From 8eb5ff855b2e00dcfac19d354a7b1e8c26a5395a Mon Sep 17 00:00:00 2001
From: hugova <hugova@kth.se>
Date: Sat, 26 Jul 2025 19:42:57 +0200
Subject: [PATCH] fix most of by one errors of linedraw.

---
 wip-hugo/routines/line/line.s                | 29 +++++++--------
 wip-hugo/routines/line/line_down.s           |  1 +
 wip-hugo/routines/line/line_down_inv.s       |  1 +
 wip-hugo/routines/line/line_test_extensive.s | 39 ++++++++++++++++++--
 wip-hugo/routines/line/line_up.s             |  1 +
 wip-hugo/routines/line/line_up_inv.s         |  1 +
 6 files changed, 52 insertions(+), 20 deletions(-)

diff --git a/wip-hugo/routines/line/line.s b/wip-hugo/routines/line/line.s
index 9768771..89a50de 100644
--- a/wip-hugo/routines/line/line.s
+++ b/wip-hugo/routines/line/line.s
@@ -5,26 +5,24 @@
         .include "line.inc"
 
 ;; Fix line that is too long
-        LDA Y_pos
-        CMP #$C8 ;;y_max = $C8
-        BCC do_not_fix_y_pos
-        LDA #$C8
+        LDA #$C8 ;y_max = $C8
+        CMP Y_pos
+        BCS do_not_fix_y_pos
         STA Y_pos
 do_not_fix_y_pos:
-        LDA Y_end
-        CMP #$C8 ;;y_max = $C8
-        BCC do_not_fix_y_end
         LDA #$C8
+        CMP Y_pos ;y_max = $C8
+        BCS do_not_fix_y_end
         STA Y_end
 do_not_fix_y_end:
 
-
         ;;dx
         SEC
         LDA X_end
         SBC X_pos
         BCS dx_no_underflow;; X_end >= X_pos
         EOR #$ff ; Fix bit underflow
+        ADC #$01
         STA dx
         ;; line_* expect X_pos < X_end and now its not the case.
         ;; Lets move them around
@@ -36,28 +34,27 @@ do_not_fix_y_end:
         LDY Y_end
         STX Y_end
         STY Y_pos
-
-
 dx_no_underflow:
         STA dx
+
         SEC
-        LDA Y_pos
-        SBC Y_end
-        STA dy
-        BCC down ;normal  Y_pos < Y_end
+        LDA Y_end
+        SBC Y_pos
+        BCS down ;normal  Y_pos < Y_end
 up:; Y_pos > Y_end
+        EOR #$ff ; Fix bit underflow
+        ADC #$01
         STA dy
+
         CMP dx
         BCC shallow; dy < dx
 steep:
         jsr line_up_inv
         RTS
 shallow: ;dy =< dx
-        lda dx
         jsr line_up
         RTS
 down:
-        EOR #$ff ; Fix bit underflow
         STA dy
         CMP dx
         BCC shallow_; dy < dx
diff --git a/wip-hugo/routines/line/line_down.s b/wip-hugo/routines/line/line_down.s
index b81a095..6641a0a 100644
--- a/wip-hugo/routines/line/line_down.s
+++ b/wip-hugo/routines/line/line_down.s
@@ -67,6 +67,7 @@ end_selfmod:
         STA byte_to_paint
         ;; X = X_end - X_pos
         LDX dx
+        INX ; okay if it overflow to $00 because it will go back to $ff in loop.
         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:
diff --git a/wip-hugo/routines/line/line_down_inv.s b/wip-hugo/routines/line/line_down_inv.s
index 465ffdf..4446379 100644
--- a/wip-hugo/routines/line/line_down_inv.s
+++ b/wip-hugo/routines/line/line_down_inv.s
@@ -55,6 +55,7 @@ end_selfmod:
         INY
         Sub_16 btp_mem_pos, btp_mem_pos + 1, #$00, #$00, !
         LDX dy
+        INX
         CLC
 for_y: ; C =0
         LDA byte_to_paint
diff --git a/wip-hugo/routines/line/line_test_extensive.s b/wip-hugo/routines/line/line_test_extensive.s
index f24537c..8438af5 100644
--- a/wip-hugo/routines/line/line_test_extensive.s
+++ b/wip-hugo/routines/line/line_test_extensive.s
@@ -8,6 +8,8 @@
         X_pos_ = $0E
         Y_end_ = $0F
         X_end_ = $0C
+
+full_angle_test:
         LDA #$d2
         STA X_pos_
         LDA #$62
@@ -36,14 +38,43 @@
 
 jsr clear_screen
 
+full_angle_test_pos_end_swapped:
+LDA #$d2
+STA X_end_
+LDA #$62
+STA Y_end_
+LDA #$ff
+STA X_pos_
+LDA #$0
+STA Y_pos_
+
+;; Full angle test
+@loop:
+LDA Y_end_
+STA Y_end
+LDA X_end_
+STA X_end
+LDA X_pos_
+STA X_pos
+LDA Y_pos_
+STA Y_pos
+
+jsr line
+INC Y_pos_
+LDA Y_pos_
+CMP #$ff
+bne @loop
+
+jsr clear_screen
+
 long_line_test_a:
-        LDA #$00
+        LDA #$01
         STA X_pos_
         LDA #$60
         STA Y_pos_
         LDA #$ff
         STA X_end_
-        LDA #$0
+        LDA #$00
         STA Y_end_
 @loop:
         LDA Y_pos_
@@ -64,7 +95,7 @@ long_line_test_a:
         jsr clear_screen
 
 long_line_test_b:
-        LDA #$00
+        LDA #$01
         STA X_pos_
         LDA #$00
         STA Y_pos_
@@ -90,7 +121,7 @@ long_line_test_b:
 jsr clear_screen
 
 long_line_test_b_pos_end_swapped:
-        LDA #$00
+        LDA #$01
         STA X_end_
         LDA #$00
         STA Y_end_
diff --git a/wip-hugo/routines/line/line_up.s b/wip-hugo/routines/line/line_up.s
index 0edae87..24f1ae1 100644
--- a/wip-hugo/routines/line/line_up.s
+++ b/wip-hugo/routines/line/line_up.s
@@ -50,6 +50,7 @@ end_selfmod:
         ;LDY #$01
         INY
         LDX dx
+        INX
 for_x:
         LDA byte_to_paint
         ORA (btp_mem_pos), Y
diff --git a/wip-hugo/routines/line/line_up_inv.s b/wip-hugo/routines/line/line_up_inv.s
index 5ad7940..a99b39b 100644
--- a/wip-hugo/routines/line/line_up_inv.s
+++ b/wip-hugo/routines/line/line_up_inv.s
@@ -52,6 +52,7 @@ end_selfmod:
         Sub_16 btp_mem_pos, btp_mem_pos + 1, #$00, #$00,!
         CLC
         LDX dy
+        INX
 for_y:
         LDA byte_to_paint
         ORA (btp_mem_pos), Y