equals.js

const fs = require("fs");
const cp = require("child_process");

function equals(a, b) {
  const compare = toUTF8(b).map(c => `
    call readch
    cmp byte ptr [r8], ${c}
    jne fail
`);
  const code = `.intel_syntax noprefix

read:
    mov rax, 0
    mov rdi, 0
    mov rsi, r8
    mov rdx, 1
    syscall
    ret
readch:
    call read
    cmp rax, 0
    jle fail
    ret

main: .globl main
    lea r8, [rsp - 8];
    sub rsp, 16

${compare.join('\n')}

    call read
    cmp rax, 0
    jne fail

    mov rdi, 0
    jmp done
fail:
    mov rdi, 1
done:
    mov rax, 60
    syscall`;

  fs.writeFileSync("source.s", code);
  cp.execSync("gcc -o compare source.s");
  return cp.spawnSync("./compare", [], { input: toUTF8(a) }).status === 0;
}

startswith.js

const fs = require("fs");
const cp = require("child_process");

function startsWith(a, b) {
  const compare = toUTF8(b).map(c => `
    call read
    cmp byte ptr [r8], ${c}
    jne fail
`);
  const code = `.intel_syntax noprefix

read:
    mov rax, 0
    mov rdi, 0
    mov rsi, r8
    mov rdx, 1
    syscall
    cmp rax, 0
    jle fail
    ret

main: .globl main
    lea r8, [rsp - 8];
    sub rsp, 16

${compare.join('\n')}

    mov rdi, 0
    jmp done
fail:
    mov rdi, 1
done:
    mov rax, 60
    syscall`;

  fs.writeFileSync("source.s", code);
  cp.execSync("gcc -o compare source.s");
  return cp.spawnSync("./compare", [], { input: toUTF8(a) }).status === 0;
}