In coolbutuseless/minitypes: Vector Type Conversions

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  library(minitypes)
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minitypes

minitypes is a collection of type conversion tools for R atomic vectors.

This package is built upon R's type conversion tools and aims to:

• have a consistent interface for converting between types
• add handling for types not handled by R explicitly e.g. uint8

Supported Types:

• raw byte
  • This is R's builtin raw byte format.
• chr
  • Ascii/utf8 representation.
• 1 bit:
  • logical - R's built-in logical type e.g. c(T, T, F)
  • bit - compact bit representation (from the bit package)
  • bitstring - character representaction of bits e.g. 110
• 8 bit
  • uint8 - R's built-in integer type. Checked to be within range [0 - 255]
  • int8 - R's built-in integer type. Checked to be within range [-128, 127]
  • hex8 - 2 character hexadecimal representation of a uint8
• 16 bit
  • uint16 - R's built-in integer type. Checked to be within range [0 - 2^16-1]
  • int16 - R's built-in integer type. Checked to be within range [-2^15, 2^15-1]
  • hex16 - 4 character hexadecimal representation of a uint16
• 24 bit
  • uint24 - R's built-in integer type. Checked to be within range [0 - 2^24-1]
  • hex24 - 6 character hexadecimal representation of a uint24
• 32 bit
  • int32 - R's built-in integer type

Installation

You can install minitypes from github with:

# install.packages("devtools")
devtools::install_github("coolbutuseless/minitypes")

Promotion Rules

• types representing single bits will be padded out to be
  • a multiple of 8 bits when converting to raw
  • a multiple of 32 bits when converting to int32
  • the side to pad and the value (T/F) to pad with can be specified
• types representing 8-bits can be promoted to 16-bit and 32-bit types as long as the correct number of bytes exist e.g.
  • can convert 4 uint8 values to int32
  • can NOT convert 3 raw values to uint16

Examples - Functions to convert supported types to/from raw

r <- as.raw(c(1, 2, 0, 255))

raw_to_uint8(r)

raw_to_uint16(r, endian = 'little')

int32_to_raw(1L, endian = 'big')

raw_to_bitstring(r)

logical_to_raw(c(T, F, F), pad = 'left')

convert_type() - conversion between any two supported types

All supported types can be converted to/from the raw type. Thus any two supported types can be converted between each other by using raw as the intermediary.

However, for complex endian/bit-order conversions (or cases where raw intermediary would lose information about the conversion), it is probably safer to do it manually as a two-step process.

convert_type("1111111100010011", src_type='bitstring', dst_type='uint8')

convert_type(c(1L, 2L, 3L), src_type = 'int32', dst_type = 'uint8', endian = 'big')

Examples - Specialised conversion functions

minitypes includes some functions for direct conversion between two supported types.

This may be for efficiency reasons, but is often because going through an intermediate raw step would lose information needed for the complete conversion.

bitstring_to_logical("100 011")

int32_to_bitstring(1L, endian = 'big')

int32_to_bitstring(1L, endian = 'little')

int32_to_bitstring(1L, endian = 'little', first_bit = 'lsb')

Helper functions

• raw_reverse_bits() - reverse the bits in a raw byte
• raw_swap_endian() - swap the order of a group of bytes (nbytes at a time) within a raw vector

Useless tricks - NA_integer_ & NA_character_ bit representation

NA_integer_ and NA_character_ are just specific bit patterns within each of those types.

convert_type(NA_integer_, 'int32', 'bitstring', endian = 'big', first_bit = 'msb')

convert_type(NA_character_, 'chr', 'bitstring', endian = 'big', first_bit = 'msb')

Worked Example - Parsing a colour table from a 'gif'

• Read in a GIF file as raw bytes
• Manually parse the GIF header (see e.g. GIF format)
• GIF data is stored little endian
• In most GIFs there's a global colour table with 255 colours (defined as a block of 255 RGB triplets)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# A scarlet tanager!
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gif_filename <- "figures/bird.gif"
gif          <- readBin(gif_filename, 'raw', n = file.size(gif_filename))

head(gif, 50)

r glue::glue("![]({gif_filename})")

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Subset out the header components and convert to the correct type
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
raw_to_chr(gif[1:6])  # "GIF89a" (text)

raw_to_uint16(gif[7:8], endian = 'little')  # width (integer)

raw_to_uint16(gif[9:10], endian = 'little')  # height (integer)

# Is a global colour table present?
colour_table_info <- raw_to_uint8(gif[11])  
if (bitwAnd(colour_table_info, 128L)) { print("GCT present") }

# read 255 RGB triplets has 6-char-hex (i.e. hex24) starting at position 14
colour_table <- raw_to_hex24(gif[seq_len(255*3) + 13L], endian = 'big')
colour_table <- sort(paste0('#', colour_table))

head(colour_table)

image(x = 1, y = 1:255, z = matrix(1:255, nrow = 1), col = colour_table, axes = FALSE, ann = FALSE)

coolbutuseless/minitypes documentation built on May 27, 2019, 9:55 a.m.