path: root/2025/day6.asm

; Advent of Code 2025 - Day 6

bits 64
default rel

global _start

%include "utils/io.asm"

%define RAW_SIZE   1048576
%define MAX_W      8192
%define MAX_H      64
%define GRID_SIZE  (MAX_W * MAX_H)

section .data
    input_path db "input/day6.txt", 0

    label1     db "Part 1: ", 0
    label2     db "Part 2: ", 0

section .bss
    raw           resb RAW_SIZE
    grid          resb GRID_SIZE   ; height x width, space-filled

    raw_len       resq 1
    width         resq 1
    height        resq 1

    blk_c0        resq 1           ; current block: columns [blk_c0, blk_c1)
    blk_c1        resq 1
    op            resq 1           ; 0 = add, 1 = mul, -1 = none

    part1         resq 1
    part2         resq 1

section .text

_start:
    mov     rdi, input_path
    lea     rsi, [raw]
    mov     rdx, RAW_SIZE
    call    read_file
    mov     [raw_len], rax

    ; width = length of the first line
    xor     rcx, rcx
width_scan:
    cmp     rcx, [raw_len]
    jge     width_done
    mov     al, [raw + rcx]
    cmp     al, 10
    je      width_done
    cmp     al, 13
    je      width_done
    inc     rcx
    jmp     width_scan
width_done:
    mov     [width], rcx

    ; height = number of lines (newlines, plus one if the last line has no newline)
    xor     rcx, rcx
    xor     r8, r8
height_scan:
    cmp     rcx, [raw_len]
    jge     height_tail
    mov     al, [raw + rcx]
    cmp     al, 10
    jne     height_next
    inc     r8
height_next:
    inc     rcx
    jmp     height_scan
height_tail:
    mov     rax, [raw_len]
    test    rax, rax
    jz      height_store
    mov     dl, [raw + rax - 1]
    cmp     dl, 10
    je      height_store
    inc     r8
height_store:
    mov     [height], r8

    ; space-fill the grid, then copy the raw bytes into it
    mov     rax, [height]
    imul    rax, [width]
    mov     rcx, rax
    lea     rdi, [grid]
    mov     al, ' '
    rep     stosb

    xor     r8, r8   ; raw index
    xor     r9, r9   ; row
    xor     r10, r10 ; col
fill_loop:
    cmp     r8, [raw_len]
    jge     fill_done
    mov     al, [raw + r8]
    inc     r8
    cmp     al, 10
    je      fill_newline
    cmp     al, 13
    je      fill_loop
    cmp     r10, [width]
    jge     fill_loop
    mov     rdx, r9
    imul    rdx, [width]
    add     rdx, r10
    mov     [grid + rdx], al
    inc     r10
    jmp     fill_loop
fill_newline:
    inc     r9
    xor     r10, r10
    jmp     fill_loop
fill_done:

    ; walk the columns, carving out blocks between all-space separators
    xor     r15, r15   ; column cursor
block_scan:
    cmp     r15, [width]
    jge     blocks_done
    mov     rdi, r15
    call    col_is_sep
    test    rax, rax
    jnz     block_sep
    mov     [blk_c0], r15
block_extend:
    cmp     r15, [width]
    jge     block_end
    mov     rdi, r15
    call    col_is_sep
    test    rax, rax
    jnz     block_end
    inc     r15
    jmp     block_extend
block_end:
    mov     [blk_c1], r15
    call    process_block
    jmp     block_scan
block_sep:
    inc     r15
    jmp     block_scan
blocks_done:

print_results:
    mov     rcx, label1
    call    print_string
    mov     rax, [part1]
    call    print_int64
    call    print_newline

    mov     rcx, label2
    call    print_string
    mov     rax, [part2]
    call    print_int64
    call    print_newline

    mov     rax, SYS_EXIT
    xor     rdi, rdi
    syscall

; col_is_sep: is every row of column RDI a space?
;   Output: RAX = 1 if the column is a separator, else 0
;   Clobbers: RAX, RDX, R8
col_is_sep:
    xor     r8, r8
.loop:
    cmp     r8, [height]
    jge     .sep
    mov     rax, r8
    imul    rax, [width]
    add     rax, rdi
    movzx   edx, byte [grid + rax]
    cmp     dl, ' '
    jne     .not
    inc     r8
    jmp     .loop
.sep:
    mov     rax, 1
    ret
.not:
    xor     rax, rax
    ret

; process_block: fold the block [blk_c0, blk_c1) for both parts.
;   Part 1 reads each row as a number; part 2 reads each column as a number.
;   The bottom row supplies the operator. Clobbers RAX, RCX, RDX, R8-R14.
process_block:
    ; find the operator on the bottom row within the block
    mov     rax, [height]
    dec     rax
    imul    rax, [width]
    mov     r11, rax   ; offset of the operator row
    mov     qword [op], -1
    mov     rcx, [blk_c0]
.find_op:
    cmp     rcx, [blk_c1]
    jge     .op_done
    mov     rdx, r11
    add     rdx, rcx
    mov     al, [grid + rdx]
    cmp     al, '+'
    je      .set_add
    cmp     al, '*'
    je      .set_mul
    inc     rcx
    jmp     .find_op
.set_add:
    mov     qword [op], 0
    jmp     .op_done
.set_mul:
    mov     qword [op], 1
.op_done:
    cmp     qword [op], 0
    jl      .ret   ; no operator -> not a real problem

    ; part 1: one number per row
    mov     r12, [op]
    xor     r13, r13   ; accumulator
    test    r12, r12
    jz      .p1_acc
    mov     r13, 1
.p1_acc:
    xor     r14, r14   ; count of numbers folded
    xor     r10, r10   ; row
.p1_rows:
    mov     rax, [height]
    dec     rax
    cmp     r10, rax
    jge     .p1_done
    mov     rax, r10
    imul    rax, [width]
    mov     r11, rax   ; row offset
    xor     r8, r8     ; number
    xor     r9, r9     ; saw a digit?
    mov     rcx, [blk_c0]
.p1_cols:
    cmp     rcx, [blk_c1]
    jge     .p1_fold
    mov     rdx, r11
    add     rdx, rcx
    movzx   eax, byte [grid + rdx]
    cmp     al, '0'
    jl      .p1_col_next
    cmp     al, '9'
    jg      .p1_col_next
    sub     al, '0'
    movzx   rax, al
    imul    r8, r8, 10
    add     r8, rax
    mov     r9, 1
.p1_col_next:
    inc     rcx
    jmp     .p1_cols
.p1_fold:
    test    r9, r9
    jz      .p1_row_next
    test    r12, r12
    jnz     .p1_mul
    add     r13, r8
    jmp     .p1_counted
.p1_mul:
    imul    r13, r8
.p1_counted:
    inc     r14
.p1_row_next:
    inc     r10
    jmp     .p1_rows
.p1_done:
    test    r14, r14
    jz      .p2
    add     [part1], r13

    ; part 2: one number per column
.p2:
    mov     r12, [op]
    xor     r13, r13
    test    r12, r12
    jz      .p2_acc
    mov     r13, 1
.p2_acc:
    xor     r14, r14
    mov     rcx, [blk_c0]   ; column
.p2_cols:
    cmp     rcx, [blk_c1]
    jge     .p2_done
    xor     r8, r8   ; number
    xor     r9, r9   ; saw a digit?
    xor     r10, r10 ; row
.p2_rows:
    mov     rax, [height]
    dec     rax
    cmp     r10, rax
    jge     .p2_fold
    mov     rax, r10
    imul    rax, [width]
    add     rax, rcx
    movzx   edx, byte [grid + rax]
    cmp     dl, '0'
    jl      .p2_row_next
    cmp     dl, '9'
    jg      .p2_row_next
    sub     dl, '0'
    movzx   rdx, dl
    imul    r8, r8, 10
    add     r8, rdx
    mov     r9, 1
.p2_row_next:
    inc     r10
    jmp     .p2_rows
.p2_fold:
    test    r9, r9
    jz      .p2_col_next
    test    r12, r12
    jnz     .p2_mul
    add     r13, r8
    jmp     .p2_counted
.p2_mul:
    imul    r13, r8
.p2_counted:
    inc     r14
.p2_col_next:
    inc     rcx
    jmp     .p2_cols
.p2_done:
    test    r14, r14
    jz      .ret
    add     [part2], r13
.ret:
    ret