; the old 16-bit bit-and-shift multiplier
; copied from old code, won't compile as-is

; inner loop for imul16
; bitnum < 8: 25 or 41 cycles
; bitnum >= 8: 30 or 46 cycles
.macro bitmul16 arg1, arg2, result, bitnum
    .local zero
    .local one
    .local next

    ; does 16-bit adds
    ; arg1 and arg2 are treated as unsigned
    ; negative signed inputs must be flipped first

    ; 7 cycles up to the branch

    ; check if arg1 has 0 or 1 bit in this place
    ; 5 cycles either way
    .if bitnum < 8
        lda arg1                 ; 3 cyc
        and #(1 << (bitnum))       ; 2 cyc
    .else
        lda arg1 + 1             ; 3 cyc
        and #(1 << ((bitnum) - 8)) ; 2 cyc
    .endif
    bne one ; 2 cyc

zero: ; 18 cyc, 23 cyc
    lsr result + 3 ; 5 cyc
    jmp next       ; 3 cyc

one: ; 32 cyc, 37 cyc
    ; 16-bit add on the top bits
    clc            ; 2 cyc
    lda result + 2 ; 3 cyc
    adc arg2       ; 3 cyc
    sta result + 2 ; 3 cyc
    lda result + 3 ; 3 cyc
    adc arg2 + 1   ; 3 cyc
    ror a          ; 2 cyc - get a jump on the shift
    sta result + 3 ; 3 cyc
next:
    ror result + 2 ; 5 cyc
    ror result + 1 ; 5 cyc
    .if bitnum >= 8
        ; we can save 5 cycles * 8 bits = 40 cycles total by skipping this byte
        ; when it's all uninitialized data
        ror result ; 5 cyc
    .endif

.endmacro

; 5 to 25 cycles
.macro check_sign arg
    ; Check sign bit and flip argument to postive,
    ; keeping a count of sign bits in the X register.
    .local positive
    lda arg + 1   ; 3 cyc
    bpl positive  ; 2 cyc
    neg16 arg     ; 18 cyc
    inx           ; 2 cyc
positive:
.endmacro

; 518 - 828 cyc
.macro imul16 dest, arg1, arg2
    copy16 FR0, arg1  ; 12 cyc
    copy16 FR1, arg2  ; 12 cyc
    jsr imul16_func   ; 470-780 cyc
    copy32 dest, FR2  ; 24 cyc
.endmacro


.macro shift_round_16 arg, shift
    .repeat shift
        shl32 arg
    .endrepeat
    round16 arg
.endmacro

.macro imul16_round dest, arg1, arg2, shift
    copy16 FR0, arg1  ; 12 cyc
    copy16 FR1, arg2  ; 12 cyc
    jsr imul16_func   ; 470-780 cyc
    shift_round_16 FR2, shift
    copy16 dest, FR2 + 2  ; 12 cyc
.endmacro

; min 470 cycles
; max 780 cycles
.proc imul16_func
    arg1 = FR0   ; 16-bit arg (clobbered)
    arg2 = FR1   ; 16-bit arg (clobbered)
    result = FR2 ; 32-bit result

    ldx #0          ; 2 cyc
    ; counts the number of sign bits in X
    check_sign arg1 ; 5 to 25 cyc
    check_sign arg2 ; 5 to 25 cyc
    
    ; zero out the 32-bit temp's top 16 bits
    lda #0          ; 2 cyc
    sta result + 2  ; 3 cyc
    sta result + 3  ; 3 cyc
    ; the bottom two bytes will get cleared by the shifts

    ; unrolled loop for maximum speed, at the cost
    ; of a larger routine
    ; 440 to 696 cycles
    .repeat 16, bitnum
        ; bitnum < 8: 25 or 41 cycles
        ; bitnum >= 8: 30 or 46 cycles
        bitmul16 arg1, arg2, result, bitnum
    .endrepeat

    ; In case of mixed input signs, return a negative result.
    cpx #1              ; 2 cyc
    bne positive_result ; 2 cyc
    neg32 result        ; 34 cyc
positive_result:

    rts ; 6 cyc
.endproc