add Packbits compression

README.md

@@ -34,7 +34,7 @@ This table lists photometric interpretations and sample formats which are suppor
 | None     | ✓        | ✓        |
 | LZW      | ✓        | ✓        |
 | Deflate  | ✓        | ✓        |
-| PackBits | ✓        |          |
+| PackBits | ✓        | ✓        |
 
 
 ## Not yet supported

src/encoder/compression/mod.rs

@@ -3,10 +3,12 @@ use std::io::{self, Write};
 
 mod deflate;
 mod lzw;
+mod packbits;
 mod uncompressed;
 
 pub use self::deflate::{Deflate, DeflateLevel};
 pub use self::lzw::Lzw;
+pub use self::packbits::Packbits;
 pub use self::uncompressed::Uncompressed;
 
 /// An algorithm used for compression
@@ -30,6 +32,7 @@ pub enum Compressor {
     Uncompressed(Uncompressed),
     Lzw(Lzw),
     Deflate(Deflate),
+    Packbits(Packbits),
 }
 
 impl Default for Compressor {
@@ -45,6 +48,7 @@ impl CompressionAlgorithm for Compressor {
             Compressor::Uncompressed(algorithm) => algorithm.write_to(writer, bytes),
             Compressor::Lzw(algorithm) => algorithm.write_to(writer, bytes),
             Compressor::Deflate(algorithm) => algorithm.write_to(writer, bytes),
+            Compressor::Packbits(algorithm) => algorithm.write_to(writer, bytes),
         }
     }
 }

src/encoder/compression/packbits.rs

@@ -0,0 +1,213 @@
+use crate::{encoder::compression::*, tags::CompressionMethod};
+use std::io::{BufWriter, Error, ErrorKind, Write};
+
+/// Compressor that uses the Packbits[^note] algorithm to compress bytes.
+///
+/// [^note]: PackBits is often ineffective on continuous tone images,
+///          including many grayscale images. In such cases, it is better
+///          to leave the image uncompressed.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct Packbits;
+
+impl Compression for Packbits {
+    const COMPRESSION_METHOD: CompressionMethod = CompressionMethod::PackBits;
+
+    fn get_algorithm(&self) -> Compressor {
+        Compressor::Packbits(*self)
+    }
+}
+
+impl CompressionAlgorithm for Packbits {
+    fn write_to<W: Write>(&mut self, writer: &mut W, bytes: &[u8]) -> Result<u64, io::Error> {
+        // Inspired by https://github.com/skirridsystems/packbits
+
+        const MIN_REPT: u8 = 3; // Minimum run to compress between differ blocks
+        const MAX_BYTES: u8 = 128; // Maximum number of bytes that can be encoded in a header byte
+
+        // Encoding for header byte based on number of bytes represented.
+        fn encode_diff(n: u8) -> u8 {
+            n - 1
+        }
+        fn encode_rept(n: u8) -> u8 {
+            let var = 256 - (n - 1) as u16;
+            var as u8
+        }
+
+        fn write_u8<W: Write>(writer: &mut W, byte: u8) -> Result<u64, Error> {
+            writer.write(&[byte]).map(|byte_count| byte_count as u64)
+        }
+
+        let mut bufwriter = BufWriter::new(writer);
+        let mut bytes_written = 0u64; // The number of bytes written into the writer
+        let mut offset: Option<u64> = None; // The index of the first byte written into the writer
+
+        let mut src_index: usize = 0; // Index of the current byte
+        let mut src_count = bytes.len(); //The number of bytes remaining to be compressed
+
+        let mut in_run = false; // Indication whether counting of similar bytes is performed
+        let mut run_index = 0u8; // Distance into pending bytes that a run starts
+
+        let mut bytes_pending = 0u8; // Bytes looked at but not yet output
+        let mut pending_index = 0usize; // Index of the first pending byte
+
+        let mut curr_byte: u8; // Byte currently being considered
+        let mut last_byte: u8; // Previous byte
+
+        // Need at least one byte to compress
+        if src_count == 0 {
+            return Err(Error::new(ErrorKind::WriteZero, "write zero"));
+        }
+
+        // Prime compressor with first character.
+        last_byte = bytes[src_index];
+        src_index += 1;
+        bytes_pending += 1;
+
+        while src_count - 1 != 0 {
+            src_count -= 1;
+            curr_byte = bytes[src_index];
+            src_index += 1;
+            bytes_pending += 1;
+
+            if in_run {
+                if (curr_byte != last_byte) || (bytes_pending > MAX_BYTES) {
+                    offset.get_or_insert(write_u8(&mut bufwriter, encode_rept(bytes_pending - 1))?);
+                    write_u8(&mut bufwriter, last_byte)?;
+                    bytes_written += 2;
+
+                    bytes_pending = 1;
+                    pending_index = src_index - 1;
+                    run_index = 0;
+                    in_run = false;
+                }
+            } else {
+                if bytes_pending > MAX_BYTES {
+                    // We have as much differing data as we can output in one chunk.
+                    // Output MAX_BYTES leaving one byte.
+                    offset.get_or_insert(write_u8(&mut bufwriter, encode_diff(MAX_BYTES))?);
+                    bufwriter.write(&bytes[pending_index..pending_index + MAX_BYTES as usize])?;
+                    bytes_written += 1 + MAX_BYTES as u64;
+
+                    pending_index += MAX_BYTES as usize;
+                    bytes_pending -= MAX_BYTES;
+                    run_index = bytes_pending - 1; // A run could start here
+                } else if curr_byte == last_byte {
+                    if (bytes_pending - run_index >= MIN_REPT) || (run_index == 0) {
+                        // This is a worthwhile run
+                        if run_index != 0 {
+                            // Flush differing data out of input buffer
+                            offset.get_or_insert(write_u8(&mut bufwriter, encode_diff(run_index))?);
+                            bufwriter
+                                .write(&bytes[pending_index..pending_index + run_index as usize])?;
+                            bytes_written += 1 + run_index as u64;
+                        }
+                        bytes_pending -= run_index; // Length of run
+                        in_run = true;
+                    }
+                } else {
+                    run_index = bytes_pending - 1; // A run could start here
+                }
+            }
+            last_byte = curr_byte;
+        }
+
+        // Output the remainder
+        if in_run {
+            bytes_written += 2;
+            offset.get_or_insert(write_u8(&mut bufwriter, encode_rept(bytes_pending))?);
+            write_u8(&mut bufwriter, last_byte)?;
+        } else {
+            bytes_written += 1 + bytes_pending as u64;
+            offset.get_or_insert(write_u8(&mut bufwriter, encode_diff(bytes_pending))?);
+            bufwriter.write(&bytes[pending_index..pending_index + bytes_pending as usize])?;
+        }
+
+        Ok(bytes_written)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::encoder::compression::tests::TEST_DATA;
+    use std::io::Cursor;
+
+    #[test]
+    fn test_packbits_single_byte() {
+        // compress single byte
+        const UNCOMPRESSED_DATA: [u8; 1] = [0x3F];
+        const EXPECTED_COMPRESSED_DATA: [u8; 2] = [0x00, 0x3F];
+
+        let mut compressed_data = Vec::<u8>::new();
+        let mut writer = Cursor::new(&mut compressed_data);
+        Packbits::default()
+            .write_to(&mut writer, &UNCOMPRESSED_DATA)
+            .unwrap();
+        assert_eq!(compressed_data, EXPECTED_COMPRESSED_DATA);
+    }
+
+    #[test]
+    fn test_packbits_rept() {
+        // compress buffer with repetitive sequence
+        const UNCOMPRESSED_DATA: &'static [u8] =
+            b"This strrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrring hangs.";
+        const EXPECTED_COMPRESSED_DATA: &'static [u8] = b"\x06This st\xD1r\x09ing hangs.";
+
+        let mut compressed_data = Vec::<u8>::new();
+        let mut writer = Cursor::new(&mut compressed_data);
+        Packbits::default()
+            .write_to(&mut writer, &UNCOMPRESSED_DATA)
+            .unwrap();
+        assert_eq!(compressed_data, EXPECTED_COMPRESSED_DATA);
+    }
+
+    #[test]
+    fn test_packbits_large_rept_nonrept() {
+        // compress buffer with large repetitive and non-repetitive sequence
+        let mut data = b"This st".to_vec();
+        for _i in 0..158 {
+            data.push(b'r');
+        }
+        data.extend_from_slice(b"ing hangs.");
+        for i in 0..158 {
+            data.push(i);
+        }
+
+        const EXPECTED_COMPRESSED_DATA: [u8; 182] = [
+            0x06, 0x54, 0x68, 0x69, 0x73, 0x20, 0x73, 0x74, 0x81, 0x72, 0xE3, 0x72, 0x7F, 0x69,
+            0x6E, 0x67, 0x20, 0x68, 0x61, 0x6E, 0x67, 0x73, 0x2E, 0x00, 0x01, 0x02, 0x03, 0x04,
+            0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10, 0x11, 0x12,
+            0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F, 0x20,
+            0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2A, 0x2B, 0x2C, 0x2D, 0x2E,
+            0x2F, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3A, 0x3B, 0x3C,
+            0x3D, 0x3E, 0x3F, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4A,
+            0x4B, 0x4C, 0x4D, 0x4E, 0x4F, 0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
+            0x59, 0x5A, 0x5B, 0x5C, 0x5D, 0x5E, 0x5F, 0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66,
+            0x67, 0x68, 0x69, 0x6A, 0x6B, 0x6C, 0x6D, 0x6E, 0x6F, 0x70, 0x71, 0x72, 0x73, 0x74,
+            0x75, 0x27, 0x76, 0x77, 0x78, 0x79, 0x7A, 0x7B, 0x7C, 0x7D, 0x7E, 0x7F, 0x80, 0x81,
+            0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8A, 0x8B, 0x8C, 0x8D, 0x8E, 0x8F,
+            0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9A, 0x9B, 0x9C, 0x9D,
+        ];
+
+        let mut compressed_data = Vec::<u8>::new();
+        let mut writer = Cursor::new(&mut compressed_data);
+        Packbits::default()
+            .write_to(&mut writer, data.as_slice())
+            .unwrap();
+        assert_eq!(compressed_data, EXPECTED_COMPRESSED_DATA);
+    }
+
+    #[test]
+    fn test_packbits() {
+        // compress teststring
+        const EXPECTED_COMPRESSED_DATA: &'static [u8] =
+            b"\x3CThis is a string for checking various compression algorithms.";
+
+        let mut compressed_data = Vec::<u8>::new();
+        let mut writer = Cursor::new(&mut compressed_data);
+        Packbits::default()
+            .write_to(&mut writer, &TEST_DATA)
+            .unwrap();
+        assert_eq!(compressed_data, EXPECTED_COMPRESSED_DATA);
+    }
+}

tests/encode_images_with_compression.rs

@@ -150,3 +150,8 @@ fn encode_decode_with_deflate() {
     encode_decode_with_compression(Deflate::with_level(DeflateLevel::Balanced));
     encode_decode_with_compression(Deflate::with_level(DeflateLevel::Best));
 }
+
+#[test]
+fn encode_decode_with_packbits() {
+    encode_decode_with_compression(Packbits::default());
+}