# Copyright 2022 Gabriel Jensen.
# This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
# If a copy of the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
.globl zap_utf8enc
zap_utf8enc:
# rdi: Current input codepoint.
# rsi: Current output octet.
# rax: Current codepoint.
# rdx: Temporary.
# Iterate over the input:
.loop:
movl (%rdi),%eax
cmpl $0xFFFF,%eax
jg .oct4
cmpl $0x7FF,%eax
jg .oct3
cmpl $0x7F,%eax
jg .oct2 # Otherwise, only one octet is needed.
# One octet:
.oct1:
# Octet #0:
movb %al,(%rsi) # No conversion needed:
incq %rsi
# Test if we have reached the null-terminator:
testb %al,%al
jz .done
jmp .cnt
# Two octets:
.oct2:
/* Octet #0: */
movl %eax,%edx
shrl $0x6,%edx
orb $0xC0,%dl
movb %dl,(%rsi)
incq %rsi
# Octet #1:
movl %eax,%edx
andb $0x3F,%dl
orb $0x80,%dl
movb %dl,(%rsi)
incq %rsi
jmp .cnt
# Three octets:
.oct3:
# Octet #0:
movl %eax,%edx
shrl $0xC,%edx
orb $0xE0,%dl
movb %dl,(%rsi)
incq %rsi
# Octet #1:
movl %eax,%edx
shrl $0x6,%edx
andb $0x3F,%dl
orb $0x80,%dl
movb %dl,(%rsi)
incq %rsi
# Octet #2:
movl %eax,%edx
andb $0x3F,%dl
orb $0x80,%dl
movb %dl,(%rsi)
incq %rsi
jmp .cnt
# Four octets:
.oct4:
# Octet #0:
movl %eax,%edx
shrl $0x12,%edx
orb $0xF0,%dl
movb %dl,(%rsi)
incq %rsi
# Octet #1:
movl %eax,%edx
shrl $0xC,%edx
andb $0x3F,%dl
orb $0x80,%dl
movb %dl,(%rsi)
incq %rsi
# Octet #2:
movl %eax,%edx
shrl $0x6,%edx
andb $0x3F,%dl
orb $0x80,%dl
movb %dl,(%rsi)
incq %rsi
# Octet #3:
movl %eax,%edx
andb $0x3F,%dl
orb $0x80,%dl
movb %dl,(%rsi)
incq %rsi
# Continue to the next codepoint:
.cnt:
addq $0x4,%rdi
jmp .loop
# Done:
.done:
ret
