refactor: clean up repository layout

1 files changed, 234 insertions, 0 deletions

diff --git a/2025/day3.asm b/2025/day3.asm
new file mode 100644
index 0000000..8330755
--- /dev/null
+++ b/2025/day3.asm
@@ -0,0 +1,234 @@
+; Advent of Code 2025 - Day 3
+
+bits 64
+default rel
+
+global _start
+
+%include "utils/io.asm"
+
+section .data
+    input_path db "input/day3.txt", 0
+
+    label1 db "Part 1: ", 0
+    label2 db "Part 2: ", 0
+
+section .bss
+    buffer       resb 4096
+    line_buffer   resb 65536
+    line_len      resd 1
+
+    sum1         resq 1
+    sum2         resq 1
+
+    work_digits   resb 64  ; stack for greedy subsequence
+
+section .text
+
+_start:
+    mov     rax, SYS_OPEN
+    mov     rdi, input_path
+    mov     rsi, O_RDONLY
+    xor     rdx, rdx
+    syscall
+
+    mov     r12, rax  ; fd
+    mov     dword [line_len], 0
+
+read_loop:
+    mov     rax, SYS_READ
+    mov     rdi, r12
+    mov     rsi, buffer
+    mov     rdx, 4096
+    syscall
+
+    cmp     rax, 0
+    jle     eof
+
+    mov     rdi, buffer
+    mov     rbp, rax
+
+scan_chunk:
+    cmp     rbp, 0
+    je      read_loop
+
+    mov     al, [rdi]
+
+    cmp     al, 13
+    je      skip_cr
+
+    cmp     al, 10
+    je      end_line
+
+    mov     edx, [line_len]
+    cmp     edx, 65535
+    jae     skip_store
+    mov     [line_buffer+rdx], al
+    inc     edx
+    mov     [line_len], edx
+
+skip_store:
+    inc     rdi
+    dec     rbp
+    jmp     scan_chunk
+
+skip_cr:
+    inc     rdi
+    dec     rbp
+    jmp     scan_chunk
+
+end_line:
+    mov     edx, [line_len]
+    mov     byte [line_buffer+edx], 0
+
+    cmp     edx, 0
+    je      clear_line
+
+    push    rdi
+    mov     rsi, line_buffer
+    call    handle_line
+    pop     rdi
+
+clear_line:
+    mov     dword [line_len], 0
+    inc     rdi
+    dec     rbp
+    jmp     scan_chunk
+
+eof:
+    ; possible last partial line
+    mov     edx, [line_len]
+    cmp     edx, 0
+    jle     print_results
+
+    mov     byte [line_buffer+edx], 0
+    mov     rsi, line_buffer
+    call    handle_line
+
+print_results:
+    mov     rcx, label1
+    call    print_string
+    mov     rax, [sum1]
+    call    print_int64
+    call    print_newline
+
+    mov     rcx, label2
+    call    print_string
+    mov     rax, [sum2]
+    call    print_int64
+    call    print_newline
+
+    mov     rax, SYS_CLOSE
+    mov     rdi, r12
+    syscall
+
+    mov     rax, SYS_EXIT
+    xor     rdi, rdi
+    syscall
+
+handle_line:
+    mov     rdi, rsi
+    xor     ecx, ecx  ; ECX = 0
+.length_loop:
+    mov     al, [rdi+rcx]
+    cmp     al, 0
+    je      .length_done
+    inc     ecx
+    jmp     .length_loop
+.length_done:
+    mov     r15d, ecx  ; r15d = n
+
+    ; part 1: k = 2
+    mov     ecx, r15d      ; ECX = n
+    mov     edi, 2         ; K = 2
+    call    get_max_for_k  ; RAX = best 2-digit value
+    add     qword [sum1], rax
+
+    ; part 2: k = 12, only if n >= 12
+    cmp     r15d, 12
+    jl      .skip_part2
+
+    mov     ecx, r15d
+    mov     edi, 12
+    call    get_max_for_k  ; RAX = best 12-digit value
+    add     qword [sum2], rax
+
+.skip_part2:
+    ret
+
+; Greedy lexicographically largest subsequence of length K
+;
+; Input:
+;   RSI -> digits string
+;   ECX = n (length)
+;   EDI = k
+; Output:
+;   RAX = numeric value of chosen k digits
+; Clobbers: R8..R11, EAX, EDX
+get_max_for_k:
+    ; drops_remaining = n - k
+    mov     eax, ecx
+    sub     eax, edi
+    mov     r8d, eax  ; r8d = drops
+
+    xor     r9d, r9d    ; r9d = stack_size
+    xor     r10d, r10d  ; r10d = i
+
+.loop:
+    cmp     r10d, ecx
+    jge     .after_input
+
+    ; current digit in AL
+    mov     r11, r10
+    mov     al, [rsi + r11]
+
+.pop:
+    cmp     r9d, 0
+    jle     .push
+    cmp     r8d, 0
+    jle     .push
+
+    mov     edx, r9d
+    dec     edx
+    mov     r11d, edx
+    mov     bl, [work_digits + r11]
+
+    cmp     bl, al
+    jge     .push
+
+    dec     r9d
+    dec     r8d
+    jmp     .pop
+.push:
+    mov     r11d, r9d
+    mov     [work_digits + r11], al
+    inc     r9d
+    inc     r10d
+    jmp     .loop
+
+.after_input:
+    ; trim if stack_size > k
+.trim:
+    cmp     r9d, edi
+    jle     .convert
+    dec     r9d
+    jmp     .trim
+.convert:
+    xor     rax, rax
+    xor     r11d, r11d
+.convert_loop:
+    cmp     r11d, edi
+    jge     .done
+
+    mov     rdx, r11
+    mov     dl, [work_digits + rdx]
+    sub     dl, '0'
+    movzx   rdx, dl
+
+    imul    rax, rax, 10
+    add     rax, rdx
+
+    inc     r11d
+    jmp     .convert_loop
+.done:
+    ret