From 1383c70ddc85a30f21b202e91c7e297858ed197e Mon Sep 17 00:00:00 2001
From: Rikard Nordgren <hrn@posteo.net>
Date: Tue, 14 Apr 2026 19:36:47 +0200
Subject: [PATCH] Update sin_FAC1

---
 STATUS.md                            |  2 +-
 src/basic/math_trans/e26b.sin_FAC1.s | 88 ++++++++++++++++------------
 tools/similarity.c                   |  2 +-
 3 files changed, 51 insertions(+), 41 deletions(-)

diff --git a/STATUS.md b/STATUS.md
index 421b706d..4bf4dc02 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -263,7 +263,7 @@ Floating point mathematical routines - not official, but well known and broadly
 | `$E09A`   | `rnd_A`                | DONE     |                                                    |
 | `$E0BE`   | `rnd_generate`         | DONE     |                                                    |
 | `$E264`   | `cos_FAC1`             | DONE     | From the Microsoft BASIC, original name: COS       |
-| `$E26B`   | `sin_FAC1`             | PARTIAL  | not fully tested yet                               |
+| `$E26B`   | `sin_FAC1`             | DONE     | From the Microsoft BASIC, original name: SIN       |
 | `$E2B4`   | `tan_FAC1`             | PARTIAL  | not fully tested yet                               |
 | `$E30E`   | `atn_FAC1`             | DONE     | From the Microsoft BASIC, original name: ATN       |
 
diff --git a/src/basic/math_trans/e26b.sin_FAC1.s b/src/basic/math_trans/e26b.sin_FAC1.s
index 93ce1660..3986d322 100644
--- a/src/basic/math_trans/e26b.sin_FAC1.s
+++ b/src/basic/math_trans/e26b.sin_FAC1.s
@@ -2,9 +2,22 @@
 ;; #LAYOUT# *   BASIC_0 #TAKE
 ;; #LAYOUT# *   *       #IGNORE
 
-;
+; This file is under the MIT license, it contains code released by Microsoft Corporation.
+; See LICENSE for more information.
+
 ; Math package - sine of FAC1 in radians
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named SIN.
+;
+; USE IDENTITIES TO GET FAC IN QUADRANTS I OR IV.
+; THE FAC IS DIVIDED BY 2*PI AND THE INTEGER PART IS IGNORED
+; BECAUSE SIN(X+2*PI)=SIN(X). THEN THE ARGUMENT CAN BE COMPARED
+; WITH PI/2 BY COMPARING THE RESULT OF THE DIVISION
+; WITH PI/2/(2*PI)=1/4.
+; IDENTITIES ARE THEN USED TO GET THE RESULT IN QUADRANTS
+; I OR IV. AN APPROXIMATION POLYNOMIAL IS THEN USED TO
+; COMPUTE SIN(X).
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 210 - XXX address does not match
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
@@ -17,45 +30,42 @@
 
 sin_FAC1:
 
-    jsr mov_FAC1_FAC2               ; FAC1 <- X / (2*pi)     (X2)
-    ldy #>const_DOUBLE_PI
-    lda #<const_DOUBLE_PI
-    jsr mov_MEM_FAC1
-    jsr div_FAC2_FAC1
+        jsr mov_r_FAC1_FAC2
+        lda #<const_DOUBLE_PI       ; Get pointer to divisor
+        ldy #>const_DOUBLE_PI
+        ldx FAC2_sign               ; Get sign of result
+        jsr div_FAC2_MEM
+        jsr mov_r_FAC1_FAC2         ; Get result into FAC2
+        jsr int_FAC1
+        lda #$00
+        sta ARISGN                  ; Always have the same sign
+        jsr sub_FAC2_FAC1           ; Keep only the fractional part
+        lda #<const_QUARTER         ; Get pointer to 1/4
+        ldy #>const_QUARTER
+        jsr sub_MEM_FAC1            ; Compute 1/4 - FAC1
+        lda FAC1_sign               ; Save sign for later
+        pha
+        bpl SIN1                    ; First quadrant
+        jsr add_HALF_FAC1           ; Add 1/2 to FAC1
+        lda FAC1_sign               ; Sign is negative?
+        bmi SIN2
+        lda TANSGN                  ; Quadrants II and III come here
+        eor #$FF
+        sta TANSGN
+SIN1:
+        jsr toggle_sign_FAC1        ; If positive, negate it
+SIN2:
+        lda #<const_QUARTER         ; Get pointer to 1/4
+        ldy #>const_QUARTER
+        jsr add_MEM_FAC1            ; Add it in
+        pla                         ; Get original quadrant
+        bpl SIN3
+        jsr toggle_sign_FAC1        ; If negative, negate result
+SIN3:
+        lda #<poly_sin
+        ldy #>poly_sin
+        jmp poly1_FAC1              ; Do approximation polynomial
 
-    jsr mov_FAC1_FAC2               ; FAC2 <- X2
-    jsr int_FAC1                    ; FAC1 <- INT(X2)
-    jsr sub_FAC2_FAC1               ; FAC1 <- X2 - INT(X2)   (F)
-
-    ldy #>const_QUARTER
-    lda #<const_QUARTER
-    jsr FCOMP
-    bmi sin_next                    ; FAC1 < 0.25
-
-    ldy #>sin_three_quarters
-    lda #<sin_three_quarters
-    jsr FCOMP
-    bmi sin_q23                     ; FAC1 < 0.75
-
-    jsr mov_FAC1_FAC2               ; FAC1 <- FAC1 - 1.0
-    ldy #>const_ONE
-    lda #<const_ONE
-    jsr mov_MEM_FAC1
-    jsr sub_FAC2_FAC1
-    +bra sin_next
-
-sin_q23:
-    ldy #>const_HALF
-    lda #<const_HALF
-    jsr sub_MEM_FAC1                ; FAC1 <- 0.5 - FAC1
-
-    ; FALLTHROUGH
-    
-sin_next:
-    ldy #>poly_sin
-    lda #<poly_sin
-    jmp poly1_FAC1
-    
 
 } else {
     jmp do_NOT_IMPLEMENTED_error
diff --git a/tools/similarity.c b/tools/similarity.c
index 4f24c5eb..7d4ba857 100644
--- a/tools/similarity.c
+++ b/tools/similarity.c
@@ -32,7 +32,7 @@ unsigned short microsoft_basic_chunks[NUM_CHUNKS * 2] = {
     0xBDB3, 0xBDC1,
     0xBDDD, 0xBF51,
     0xBFB4, 0xBFEC,
-    0xE264, 0xE26A,
+    0xE264, 0xE2B3,
     0xE2EF, 0xE37A,
 };