Prototype of pure POSIX terminal features decoder

decode_term_features

#!/bin/dash

__tf_decode() {
  local TABLE="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
  local COUNT=0
  local ACCUMULATOR=0
  local SKIP=0
  local ENCODED="$1"
  local REMAINING="$ENCODED"
  local FEATURES="8 VERSION 1 24BIT_BASIC 1 24BIT_FULL 1 CLIPBOARD_WRITE 1 DECSLRM 1 MOUSE_BASIC 1 MOUSE_ALT_SCROLL 1 UTF8 1 DECSCUSR_BASIC 1 DECSCUSR_BAR 1 AMBIGUOUS_FULLWIDTH 4 UNICODE_VERSION 1 RTL_LEVEL1 1 RTL_LEVEL2 1 TITLE_SETTING 1 BRACKETED_PASTE 1 FOCUS_REPORTING 1 CWD 1 BCE 1 UNDERLINE_STYLING 1 UNDERLINE_STYLE_DOUBLE 1 UNDERLINE_STYLE_CURLY 1 UNDERLINE_STYLE_DOTTED 1 UNDERLINE_STYLE_DASHED 1 STRIKETHROUGH 1 OVERLINE 1 SYNC 1 HYPERLINKS 1 NOTIFICATIONS 1 SIXEL 1 ITALICS 1 INVERSE_7 0 EOF"
  local BITS_NEEDED="${FEATURES%% *}"; FEATURES="${FEATURES#* }"
  local FEATURE_NAME="${FEATURES%% *}"; FEATURES="${FEATURES#* }"
  local VALUE=0

  while [ -n "$REMAINING" ]; do
    # Read the next byte from the base64-encoded input.
    local REST="${REMAINING#?}"
    CHAR="${REMAINING%"$REST"}"
    REMAINING="$REST"
    if [ "$CHAR" = $'\n' ]; then
      continue
    fi

    if [ "$CHAR" = '=' ]; then
      ACCUMULATOR=$(( ACCUMULATOR << 6 ))
      SKIP=$(( SKIP + 1 ))
      continue
    fi
    # Look up the byte in our table, setting CHAR to its 1-based index.
    CHAR=${TABLE#*$CHAR}
    CHAR=$(( ${#TABLE}-${#CHAR} ))
    if [ "$CHAR" = 0 ]; then
      continue
    fi

    # Accrue six bits from CHAR into the accumulator.
    ACCUMULATOR=$(( ACCUMULATOR << 6 | (CHAR - 1) ))

    COUNT=$(( COUNT + 1 ))
    if [ "$COUNT" -ne "4" ]; then
      continue
    fi

    # We have decoded 24 bits of data. Process it.
    LIMIT=$(( 8 * SKIP ))
    local SHIFT=16
    while [ $SHIFT -ge $LIMIT ]; do
      # Place the most significant unused byte from accumulator in CHAR.
      CHAR=$(( ACCUMULATOR >> SHIFT & 255 ))
      local BIT=0
      local UNSHIFT=0

      # Iterate over each bit in CHAR.
      while [ $BIT -lt 8 ]; do
        # OR the bit's value into VALUE.
        local IS_SET=$(( (1 << BIT) & CHAR ))
        VALUE=$(( VALUE + (IS_SET >> UNSHIFT) ))
        BITS_NEEDED=$(( BITS_NEEDED - 1 ))

        if [ $BITS_NEEDED = 0 ]; then
          # We have read enough bits for the next feature. Output it.
          export TF_"$FEATURE_NAME=$VALUE"

          # Shift in the next feature.
          BITS_NEEDED="${FEATURES%% *}"; FEATURES="${FEATURES#* }"
          FEATURE_NAME="${FEATURES%% *}"; FEATURES="${FEATURES#* }"
          VALUE=0
          UNSHIFT=$((BIT + 1))

          if [ $BITS_NEEDED = 0 ]; then
            # 0 bits signals no more features are expected.
            return
          fi
        fi

        BIT=$(( BIT + 1 ))
      done
      SHIFT=$(( SHIFT - 8 ))
    done

    # Prepare to handle four more bytes of input.
    ACCUMULATOR=0
    COUNT=0
  done
}

# Version 1. All features set to 1.
TEST1=$(echo -n 01FFFFFF FF03 | xxd -r -p - | base64)
__tf_decode $TEST1
env | grep TF_

# Version 255. All features set to 0.
TEST2=$(echo -n FF000000 0000 | xxd -r -p - | base64)
__tf_decode $TEST2
env | grep TF_

# Version 170, unicode=10, every other feature set to 1
TEST3=$(echo -n AAAAAAAA AA02 | xxd -r -p - | base64)
__tf_decode $TEST3
env | grep TF_

# Version 85, unicode=5, every other feature set to 1
TEST4=$(echo -n 55555555 5501 | xxd -r -p - | base64)
__tf_decode $TEST4
env | grep TF_