WebAssembly (WAT) implementation of atob / btoa

atob_btoa.wat

(module
  ;; Memory: 1 page = 64 KiB
  (memory $memory 1)

  ;; Exported error flag (0 = no error, 1 = error)
  (global $error_flag (mut i32) (i32.const 0))

  ;; Base64 encoding table
  (data (i32.const 0) "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")

  ;; Base64 decoding table
  (data (i32.const 64) "\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"
                      "\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x3e\xff\xff\xff\x3f"
                      "\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\xff\xff\xff\x00\xff\xff"
                      "\xff\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e"
                      "\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\xff\xff\xff\xff\xff"
                      "\xff\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27\x28"
                      "\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\xff\xff\xff\xff\xff")

  (export "memory" (memory $memory))
  (export "error_flag" (global $error_flag))
  (export "btoa" (func $btoa))
  (export "atob" (func $atob))

  ;; Encode (btoa)
  (func $btoa
    (param $input i32)
    (param $len i32)
    (result i32)

    (local $i i32)
    (local $o i32)
    (local $buffer i32)
    (local $char i32)

    (global.set $error_flag (i32.const 0))
    (local.set $o (i32.const 1024))

    (block $exit
      (loop $encode
        (br_if $exit (i32.ge_u (local.get $i) (local.get $len)))

        (local.set $buffer (i32.const 0))
        (local.set $buffer (i32.shl (i32.load8_u (i32.add (local.get $input) (local.get $i))) (i32.const 16)))
        (local.set $i (i32.add (local.get $i) (i32.const 1)))

        (if (i32.lt_u (local.get $i) (local.get $len))
          (then
            (local.set $buffer (i32.or (local.get $buffer) (i32.shl (i32.load8_u (i32.add (local.get $input) (local.get $i))) (i32.const 8))))
            (local.set $i (i32.add (local.get $i) (i32.const 1)))
          )
        )

        (if (i32.lt_u (local.get $i) (local.get $len))
          (then
            (local.set $buffer (i32.or (local.get $buffer) (i32.load8_u (i32.add (local.get $input) (local.get $i)))))
            (local.set $i (i32.add (local.get $i) (i32.const 1)))
          )
        )

        (local.set $char (i32.and (i32.shr_u (local.get $buffer) (i32.const 18)) (i32.const 63)))
        (i32.store8 (local.get $o) (i32.load8_u (i32.add (i32.const 0) (local.get $char))))
        (local.set $o (i32.add (local.get $o) (i32.const 1)))

        (local.set $char (i32.and (i32.shr_u (local.get $buffer) (i32.const 12)) (i32.const 63)))
        (i32.store8 (local.get $o) (i32.load8_u (i32.add (i32.const 0) (local.get $char))))
        (local.set $o (i32.add (local.get $o) (i32.const 1)))

        (local.set $char (i32.and (i32.shr_u (local.get $buffer) (i32.const 6)) (i32.const 63)))
        (i32.store8 (local.get $o) (i32.load8_u (i32.add (i32.const 0) (local.get $char))))
        (local.set $o (i32.add (local.get $o) (i32.const 1)))

        (local.set $char (i32.and (local.get $buffer) (i32.const 63)))
        (i32.store8 (local.get $o) (i32.load8_u (i32.add (i32.const 0) (local.get $char))))
        (local.set $o (i32.add (local.get $o) (i32.const 1)))

        (br $encode)
      )
    )

    (local.get $o)
  )

  ;; Decode (atob)
  (func $atob
    (param $input i32)
    (param $len i32)
    (result i32)

    (local $i i32)
    (local $o i32)
    (local $buffer i32)
    (local $valid i32)

    (global.set $error_flag (i32.const 0))
    (local.set $o (i32.const 1024))

    (block $exit
      (loop $decode
        (br_if $exit (i32.ge_u (local.get $i) (local.get $len)))

        (local.set $valid (i32.load8_u (i32.add (i32.const 64) (i32.load8_u (i32.add (local.get $input) (local.get $i))))))

        (if (i32.eq (local.get $valid) (i32.const 255))
          (then
            (global.set $error_flag (i32.const 1))
            (br $exit)
          )
        )

        (local.set $buffer (i32.shl (local.get $buffer) (i32.const 6)))
        (local.set $buffer (i32.or (local.get $buffer) (local.get $valid)))
        (local.set $i (i32.add (local.get $i) (i32.const 1)))

        (if (i32.eq (i32.and (local.get $i) (i32.const 3)) (i32.const 0))
          (then
            (i32.store8 (local.get $o) (i32.shr_u (local.get $buffer) (i32.const 16)))
            (local.set $o (i32.add (local.get $o) (i32.const 1)))
            (i32.store8 (local.get $o) (i32.and (i32.shr_u (local.get $buffer) (i32.const 8)) (i32.const 255)))
            (local.set $o (i32.add (local.get $o) (i32.const 1)))
            (i32.store8 (local.get $o) (i32.and (local.get $buffer) (i32.const 255)))
            (local.set $o (i32.add (local.get $o) (i32.const 1)))
          )
        )

        (br $decode)
      )
    )

    (local.get $o)
  )
)