; =============================================================================
; fs.asm - a tiny RAM filesystem in WRAMX ($D000-$DFFF, 4 KiB).
;
; 8 fixed slots of 512 bytes: name[8] (first byte 0 = free) + len[2] + data[502].
; Slot i base = $D000 + i*512  ->  high = $D0 + i*2, low = $00.
; Because WRAMX is always mapped (DMG) and the caller's buffers live in the
; mapped $A000 window, the kernel can copy between them directly.
; =============================================================================
INCLUDE "include/gbos.inc"

SECTION "fs", ROM0

; -----------------------------------------------------------------------------
; FsSlotBase - in: A = slot (0..7)  out: HL = slot base.  (A clobbered)
; -----------------------------------------------------------------------------
FsSlotBase::
    add a                       ; slot*2
    add HIGH(FS_BASE)           ; + $D0
    ld h, a
    ld l, 0
    ret

; -----------------------------------------------------------------------------
; FsNameEq - compare the <=8-byte NUL-terminated names at DE and HL.
;   out: Z set if equal.  preserves DE, HL.
; -----------------------------------------------------------------------------
FsNameEq::
    push de
    push hl
    ld b, 8
.cmp
    ld a, [de]
    cp [hl]
    jr nz, .ne
    or a                        ; equal byte; if NUL, strings ended equal
    jr z, .eq
    inc de
    inc hl
    dec b
    jr nz, .cmp
.eq
    pop hl
    pop de
    xor a                       ; Z set
    ret
.ne
    pop hl
    pop de
    ld a, 1
    and a                       ; Z clear
    ret

; -----------------------------------------------------------------------------
; FsFind - in: DE = name  out: A = slot (0..7) or $FF.
; -----------------------------------------------------------------------------
FsFind::
    ld c, 0
.l
    ld a, c
    call FsSlotBase             ; HL = base
    ld a, [hl]                  ; name[0]
    or a
    jr z, .next                 ; free slot
    call FsNameEq               ; DE vs HL
    jr z, .found
.next
    inc c
    ld a, c
    cp FS_MAX_FILES
    jr c, .l
    ld a, $FF
    ret
.found
    ld a, c
    ret

; -----------------------------------------------------------------------------
; FsAllocSlot - out: A = free slot or $FF.
; -----------------------------------------------------------------------------
FsAllocSlot::
    ld c, 0
.l
    ld a, c
    call FsSlotBase
    ld a, [hl]
    or a
    jr z, .found
    inc c
    ld a, c
    cp FS_MAX_FILES
    jr c, .l
    ld a, $FF
    ret
.found
    ld a, c
    ret

; -----------------------------------------------------------------------------
; FsSetName - copy the name at DE into slot wFsSlot (<=8 bytes, incl. NUL).
; -----------------------------------------------------------------------------
FsSetName::
    ld a, [wFsSlot]
    call FsSlotBase             ; HL = base (= name)
    push de
    ld b, 8
.cp
    ld a, [de]
    ld [hl+], a
    or a
    jr z, .done
    inc de
    dec b
    jr nz, .cp
.done
    pop de
    ret

; -----------------------------------------------------------------------------
; OFPtr - in: A = fd  out: HL = &open-file entry.  preserves DE.
; -----------------------------------------------------------------------------
OFPtr::
    push de
    ld h, 0
    ld l, a
    ld d, h
    ld e, l                     ; DE = fd
    add hl, hl                  ; 2
    add hl, hl                  ; 4
    add hl, de                  ; 5*fd
    ld de, wOFTable
    add hl, de
    pop de
    ret

; =============================================================================
; sys_open(DE = name, B = mode 0=read/1=write) -> A = fd or $FF
; =============================================================================
sys_open::
    ld a, b
    ld [wFsMode], a
    call FsFind                 ; A = slot or $FF (DE = name)
    ld [wFsSlot], a
    ld a, [wFsMode]
    or a
    jr nz, .write
    ; --- read mode: must exist ---
    ld a, [wFsSlot]
    cp $FF
    jr z, .fail
    jr .alloc
.write
    ; --- write mode: create or truncate ---
    ld a, [wFsSlot]
    cp $FF
    jr nz, .trunc
    call FsAllocSlot
    cp $FF
    jr z, .fail
    ld [wFsSlot], a
    call FsSetName              ; DE still = name
.trunc
    ld a, [wFsSlot]
    call FsSlotBase
    ld a, l
    add FS_LEN
    ld l, a                     ; HL = &len
    xor a
    ld [hl+], a
    ld [hl], a                  ; len = 0
.alloc
    ; --- allocate an open-file entry ---
    ld c, 0
.of
    ld a, c
    call OFPtr                  ; HL = &OF[c]
    ld a, [hl]
    or a
    jr z, .gotof
    inc c
    ld a, c
    cp OF_MAX
    jr c, .of
.fail
    ld a, $FF
    ret
.gotof
    ld a, 1
    ld [hl+], a                 ; inuse
    ld a, [wFsSlot]
    ld [hl+], a                 ; slot
    ld a, [wFsMode]
    ld [hl+], a                 ; mode
    xor a
    ld [hl+], a                 ; pos lo
    ld [hl], a                  ; pos hi
    ld a, c
    ret

; =============================================================================
; sys_close(B = fd)
; =============================================================================
sys_close::
    ld a, b
    cp OF_MAX
    jr nc, .done
    call OFPtr
    xor a
    ld [hl], a                  ; inuse = 0
.done
    xor a
    ret

; =============================================================================
; sys_getb(B = fd) -> A = byte, CF set on EOF
; =============================================================================
sys_getb::
    ld a, b
    cp OF_MAX
    jr nc, .eof
    call OFPtr                  ; HL = OF entry
    ld a, [hl]
    or a
    jr z, .eof
    ld a, l
    ld [wFsOFPtr], a
    ld a, h
    ld [wFsOFPtr+1], a
    inc hl                      ; OF_SLOT
    ld a, [hl]
    ld [wFsSlot], a
    inc hl                      ; OF_MODE
    inc hl                      ; OF_POS lo
    ld a, [hl+]
    ld e, a
    ld a, [hl]
    ld d, a                     ; DE = pos
    ; file length -> BC
    ld a, [wFsSlot]
    call FsSlotBase             ; HL = base
    ld a, l
    add FS_LEN
    ld l, a
    ld a, [hl+]
    ld c, a
    ld a, [hl]
    ld b, a                     ; BC = len
    ; pos >= len ?  EOF
    ld a, e
    sub c
    ld a, d
    sbc b
    jr nc, .eof
    ; addr = base + FS_DATA + pos
    ld a, [wFsSlot]
    call FsSlotBase             ; HL = base (low = 0)
    ld a, FS_DATA
    add e
    ld l, a
    ld a, h
    adc d
    ld h, a                     ; HL = &data[pos]
    ld a, [hl]
    ld [wFsByte], a
    inc de                      ; pos++
    ; store pos back
    ld a, [wFsOFPtr]
    add OF_POS
    ld l, a
    ld a, [wFsOFPtr+1]
    adc 0
    ld h, a
    ld a, e
    ld [hl+], a
    ld a, d
    ld [hl], a
    ld a, [wFsByte]
    and a                       ; CF = 0 (success)
    ret
.eof
    scf
    ret

; =============================================================================
; sys_putb(B = fd, E = byte)   -- byte is in E, not A: the syscall dispatch
; clobbers A (it indexes the jump table with it).
; =============================================================================
sys_putb::
    ld a, e
    ld [wFsByte], a
    ld a, b
    cp OF_MAX
    jr nc, .done
    call OFPtr
    ld a, [hl]
    or a
    jr z, .done
    ld a, l
    ld [wFsOFPtr], a
    ld a, h
    ld [wFsOFPtr+1], a
    inc hl
    ld a, [hl]
    ld [wFsSlot], a
    inc hl
    inc hl                      ; OF_POS lo
    ld a, [hl+]
    ld e, a
    ld a, [hl]
    ld d, a                     ; DE = pos
    ; pos >= FS_MAX_DATA ? full
    ld a, e
    sub LOW(FS_MAX_DATA)
    ld a, d
    sbc HIGH(FS_MAX_DATA)
    jr nc, .done
    ; addr = base + FS_DATA + pos
    ld a, [wFsSlot]
    call FsSlotBase
    ld a, FS_DATA
    add e
    ld l, a
    ld a, h
    adc d
    ld h, a
    ld a, [wFsByte]
    ld [hl], a                  ; write byte
    inc de                      ; pos++
    ; len = pos  (sequential append)
    ld a, [wFsSlot]
    call FsSlotBase
    ld a, l
    add FS_LEN
    ld l, a
    ld a, e
    ld [hl+], a
    ld a, d
    ld [hl], a
    ; store pos back
    ld a, [wFsOFPtr]
    add OF_POS
    ld l, a
    ld a, [wFsOFPtr+1]
    adc 0
    ld h, a
    ld a, e
    ld [hl+], a
    ld a, d
    ld [hl], a
.done
    xor a
    ret

; =============================================================================
; sys_list(B = slot, DE = namebuf[8]) -> A = 1 if used (name copied), else 0
; =============================================================================
sys_list::
    ld a, b
    cp FS_MAX_FILES
    jr nc, .no
    call FsSlotBase             ; HL = base (name)
    ld a, [hl]
    or a
    jr z, .no
    ld b, 8
.cp
    ld a, [hl+]
    ld [de], a
    inc de
    dec b
    jr nz, .cp
    ld a, 1
    ret
.no
    xor a
    ret

; =============================================================================
; sys_remove(DE = name) -> A = 0 ok / $FF not found
; =============================================================================
sys_remove::
    call FsFind
    cp $FF
    jr z, .no
    call FsSlotBase
    xor a
    ld [hl], a                  ; name[0] = 0 -> free
    xor a
    ret
.no
    ld a, $FF
    ret

; =============================================================================
; fs_init - mark all slots free, clear the OF table, seed a "readme" file.
; =============================================================================
fs_init::
    ld c, 0
.clr
    ld a, c
    call FsSlotBase
    xor a
    ld [hl], a                  ; name[0] = 0
    inc c
    ld a, c
    cp FS_MAX_FILES
    jr c, .clr
    ; zero the open-file table
    ld hl, wOFTable
    ld b, OF_MAX * OF_SIZE
    xor a
.zof
    ld [hl+], a
    dec b
    jr nz, .zof
    ; seed slot 0 = "readme"
    xor a
    ld [wFsSlot], a
    ld de, .rname
    call FsSetName
    ; copy content -> data
    xor a
    call FsSlotBase             ; HL = base
    ld a, FS_DATA
    ld l, a                     ; HL = &data (base low = 0)
    ld de, .rtext
    ld bc, .rtext_end - .rtext
.ccpy
    ld a, [de]
    ld [hl+], a
    inc de
    dec bc
    ld a, b
    or c
    jr nz, .ccpy
    ; set len
    xor a
    call FsSlotBase
    ld a, l
    add FS_LEN
    ld l, a
    ld a, LOW(.rtext_end - .rtext)
    ld [hl+], a
    ld a, HIGH(.rtext_end - .rtext)
    ld [hl], a
    ret
.rname
    db "readme", 0
.rtext
    db "welcome to gbos!", $0D, $0A
    db "files live in RAM.", $0D, $0A
.rtext_end